Acta Energetica Electrical Power Engineering Quarterly no. 01/2011 by ENERGA SA

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Electrical Power Engineering Quarterly

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act

nergetica featuring

NUCLEAR POWER PLANTS DURING BLACKOUT Ireneusz Grządzielski Krzysztof Sroka

SHUNT COMPENSATOR AS CONTROLLED REACTIVE POWER SOURCES Robert Kowalak Robert Małkowski

RADIOACTIVE WASTE AND BURNT NUCLEAR FUEL MANAGEMENT DURING CONSTRUCTION OF THE NUCLEAR POWER PLANT IN ŻARNOWIEC Tomasz Minkiewicz

PROBLEMS WITH OPERATION OF MEDIUM VOLTAGE POWER DISTRIBUTION NETWORKS IN THE ASPECT OF RELIABILITY OF POWER SYSTEM PROTECTION Wiesław Nowak Szczepan Moskwa Rafał Tarko

DEVELOPMENT OF HIGH AND EXTRA-HIGH VOLTAGE COMPACT OVERHEAD POWER LINES Waldemar Skomudek

COORDINATION OF REGULATION SYSTEMS FOR GENERATORS AND TRANSFORMERS IN AN INDUSTRIAL COMBINED HEAT AND POWER PLANT Zbigniew Szczerba

INFLUENCE OF ECONOMIC, ORGANIZATION AND LEGAL FACTORS ON ENERGY SECURITY IN THE COUNTRY Artur Wilczynski

EARTHING MEASUREMENTS FOR POWER LINE TOWERS Stanisław Wojtas

Most of the power transmission lines with a voltage of 400 kV and 220 were built in the 1970s and ‘80s. The same goes for HV transformers and switchgear - the percentage number of devices that have been operating for 30-40 years is significant. PSE Operator SA reported that the number of power line facilities that are older than the planned period of operation is equal to around 41 percent in the transmission network, and about 35 percent in distribution networks. These are very high numbers. It is claimed that most of the facilities with a voltage of 400 kV can be operated for a further dozen or so years, while the power lines with a voltage of 220 kV require much earlier modernization (mainly replacement with the new 400 kV lines - which is planned). The situation is even more dramatic if we take into account the fact that the pace of developing power networks (including the modernization of existing lines) is slow. For example, 83 km of extra-high voltage lines were rebuilt over the past five years, whereas the needs for 2020 have been determined at 2600 km. The slow pace of reconstruction (development) of power networks is caused mainly by the applicable law, which prefers ownership to energy security. It should be assumed that the right of way in the power sector, preparation of which is in progress, will resolve this problem, i.e. it will intensify the reconstruction of the power network. Additionally, the above is accompanied by the development of renewable energy sources, connected to 110 kV networks and MV networks, leading to depletion of network transmission capacity (because of the grid current operation rules). Apart from the development of power networks, the intensification of their use appears to be the solution to the problem. It can be implemented in various ways, including through technical and organizational measures. Most of the articles presented in this issue of Acta Energetica touch upon this issue. We have also included nuclear power plants. Enjoy reading.

Zbigniew Lubośny Editor-in-Chief of Acta Energetica

Ireneusz Grządzielski / Poznań University of Technology Krzysztof Sroka / Poznań University of Technology

Authors / Biographies

Ireneusz Grządzielski Poznań / Poland

Krzysztof Sroka Poznań / Poland

Graduated from the Faculty of Electrical Engineering in Poznań University of Technology (1973). Degree of PhD obtained at the same faculty (1982). Currently works as an adjunct professor at the Institute of Electrical Power, the Faculty of Electrical Engineering of Poznań University of Technology. The range of his academic interests include issues related to defence and reconstruction of power systems in cases of catastrophic failure, operation of power systems in transient states, connection of diffuse, especially wind sources to power systems.

Graduated from the Faculty of Electrical Engineering in Poznań University of Technology (1976). PhD obtained at the same faculty (1986). Currently works as an assistant professor at the Institute of Electrical Power, the Faculty of Electrical Engineering of Poznań University of Technology.

Nuclear Power Plants During Blackout

NUCLEAR POWER PLANTS DURING BLACKOUT Ireneusz Grządzielski / Poznań University of Technology Krzysztof Sroka / Poznań University of Technology

1. INTRODUCTION The Polish power system requires complex changes in the generating sector. They arise from two factors: • a significantly exploited generating base – more than 45 percent of generating sources has been operating for over thirty years • the need to significantly reduce CO2 emissions. Because currently 96 percent of electricity is generated in power plants using hard coal or brown coal, 2 percent comes from power plants using natural gas, and another 2 percent from renewable energy sources, there is an urgent need for diversification of primary energy used in the electricity generation process. The document “Polish Energy Policy by 2030” assumes that hard coal will still be the primary energy source in Poland, but the share of renewable energy sources in the electricity generation process will increase significantly. Also, nuclear power engineering will also become important. This change in structure of generating sources may considerably affect the operation of the power system and significantly determine the solutions for scenarios of defence and reconstruction of the power system in the event of catastrophic failure. According to the power development plans, nuclear power could reach even up to 10 000 MW of capacity in new power plants by 2030, so, already at the planning and design stage, it is worth taking into account the safety aspect of their operation and apply solutions that will enable their proper operation under the threat of system failures. In the absence of direct experience in operation of nuclear power plants in Poland, the attempt to present a preliminary assessment of nuclear power plant functioning in blackout conditions was based on experiences from previous failures in the countries with a significant nuclear power plant generation. The source of such observations was especially the failure on 14 August 2003 in the United States and Canada. On the basis of this experience, we made an attempt to define the characteristic features of nuclear power plants and formulate the requirements to be met by these plants in the power system in the context of a possible blackout.

2. EXPERIENCE IN SYSTEM FAILURE IN THE COUNTRIES WITH NUCLEAR POWER PLANT GENERATION Experience from the system failure in the United States and Canada in 2003 [6] 263 plants (531 generation units) were shut down as a result of the blackout on 14 August 2003 in the U.S. and Canada, primarily during the second phase of the failure (during voltage and frequency cascade), but without causing any signiﬁcant damage to these facilities. At the time of failure, nine nuclear power plants in the U.S. and seven plants in Canada were forcibly shut down due to voltage failure in the external network (LOOP - loss of offsite power). Four other Canadian nuclear power plants were automatically disconnected from the network due to its interference, but could still continue to operate at reduced power level and return to the power system

Abstract This article attempts to present a preliminary assessment of nuclear power plant operation in the event of catastrophic failure. This analysis was conducted using experiences from the previous catastrophic failures in the countries with a signiﬁcant nuclear power plant generation.

On the basis of these experiences, we made an attempt to deﬁne the characteristic features of nuclear power plants and formulate the requirements to be met by these plants in the power system in the context of a possible blackout.

Ireneusz Grządzielski / Poznań University of Technology Krzysztof Sroka / Poznań University of Technology

after the network was rebuilt. One Canadian and six American nuclear power plans suffered signiﬁcant electrical damage, but their technical condition allowed system restoration and generation of electricity. Nine American nuclear power plants were shut down within 60 seconds as a result of interferences in the network. All the plants were shut down automatically by the activation of security systems of the reactor or turbine set in response to conditions in the network, rather than by manual operation by the staff. Emergency shut-down of these plants took place following the rules of safety and good technical conditions of technological devices and systems. The deactivated power plants remained in safe condition until restart. The deactivation time for nuclear power plants in the U.S. is presented in tab. 1. Tab. 1. Deactivation time for nuclear power plants in the U.S.

Reactor shut-down

Generator shut-down

Perry

Power plant

16:10:25

16:10:42

Fermi 2

16:10:53

Oyster Creek

16:10:58

16:10:57

Nine Mile1

16:11:00

16:11:09

Indian Point 2

16:11:00

16:11:21

Indian Point 3

16:11:00

16:11:32

FitzPatrick

16:11:04

Ginna

16:11:36

16:12:17

Nine Mile2

16:11:48

16:11:52

Nuclear Working Group (NWG), which investigated the course of failure on 14 August 2003, found no evidence that the shut-down of American and Canadian nuclear power plants caused blackout or contributed to its spread. All the plants responded properly to the conditions in the power network. Transition processes in the network caused the operational parameters of generators, turbines and reactors to reach the safety limits. Then the relevant security functions shut down the plants. The plants were disconnected from the network only in an automatic way. The reasons for shutting down the plants were the following: • the generator turned off as a result of voltage and frequency fluctuations in the system • the turbine turned off as a result of frequency fluctuations in the system • the reactor turned off as a result of activated control system for low pressure in the turbine, caused by frequency fluctuations in the system • disconnection of the generator from the system, automatic reactor power reduction as a result of significant loss of electrical power caused by frequency fluctuations in the system • disconnection of the generator from the system and automatic reactor power reduction as a result of power swings in the system • reactor power reduction to the level of 60 percent. (the reactor was ready to continue providing electricity to the system on the command of the system operator). Characteristic features of a nuclear power plant in terms of blackout threats are as follows: • The electrical system of nuclear block - it basically works similar to conventional power blocks, both in terms of power output circuit as well as the auxiliary supply (primary from the main generator and secondary from the reserve and start-up bridge); a significant difference is that in the case of an unacceptable voltage, the switches of power supply in auxiliary switching stations are automatically opened, and selected sections of the switching station are immediately powered by quick emergency power generators. • The reaction of nuclear power plant to voltage changes - in automatic mode of the controller of the main generator voltage, the generator will react to changes in voltage, changing the reactive power generation. Undervoltage protection ensures safe operation of the switching station in a nuclear power plant and its individual elements of technological system (auxiliary equipment). It is particularly used in pressurized water reactors (PWR) to protect the water pump that cools the reactor. • Safety features in nuclear power plants - both the generator and the turbine have a safety system similar to that used in a conventional plant using solid fuel. Generally, the reactor safety systems are

Nuclear Power Plants During Blackout

designed to protect the reactor fuel system from damage and to protect the reactor cooling system from excessive pressure and temperature. It should be stressed that specific interactions exist between particular safety features. Canadian plants require special attention as they are equipped with CANDU-type reactors. In contrast to the pressurized water reactors (PWR) used in the U.S., which use enriched uranium as fuel while ordinary water acts as a coolant and moderator, the CANDU reactor uses natural uranium and heavy water as a coolant and moderator. Therefore, the CANDU reactor may be operated with a significant reduction in the load. A much smaller number of control rods in the reactor enable safe power reduction even to 60 percent. The consequence of a significant reduction in power is that the reactor is not “poisoned” and can operate at reduced power for up to two days. In PWR the power has to be reduced to zero. These unique properties of CANDU reactors can reduce the full power to 60 percent, also when the reactor is disconnected from the system and it is necessary to maintain readiness for several days. When resynchronization with the system is complete, the load of 60 percent can be obtained after several minutes whereas full power can be restored after 25 hours. The specific design of the CANDU reactor is also the reason for using a different reactor protection system. Most often such a reactor has two separate, independent and different systems for shutting down the reactor in the case of disturbances in the power system. The first system uses a large number of emergency cadmium rods, which are placed in the reactor core causing absorption of neutrons. The second system uses a high-pressure injection of gadolinium nitrate, also absorbing neutrons. The disconnection of generating unit from the system causes a safe discharge of reactor power. At the same time, the activity of necessary systems is secured by powering them with batteries and emergency generators. In the case of a reduction of the reactor load to zero, its restoration to full load takes about two days and after this time it is possible to activate the turbine and synchronize the unit with the system. These properties of CANDU reactors were used for a quick restart of some nuclear power plants in Canada. The reconstruction of generating capacities of nuclear power plants after the failure on 14 August proceeded as follows: American power plants • 17 August - four power plants began generating electricity • 18 August - two power plants • 20 August - one power plant • 21 August - one power plant • 22 August - one power plant • after removal of the failure caused by the problems with equipment operation was restored in two power plants Canadian power plants • 7 hours after the incident - four units were synchronized with the network • 17 and 18 August - three units • 22-25 August - the remaining units. Failure in Scandinavia in 2003 [4] The failure occurred in the area of southern Sweden and Denmark Zealand on 23 September 2003, i.e. five days before the large failure in Italy. The system failure began with an emergency shut-down of the Oskarshamn nuclear power plant block with a capacity of 1,175 MW at 12.30. Automatic activation of reserves in water power plants began the process of stabilizing the frequency; however, a double fault in the busbar in Horred station, which outputs power from Ringhals power plant, occurred at 12.35 due to mechanical damage to the disconnector. During a short circuit, two nuclear power blocks with a total power of 1,750 MW fell out. In the next 90 seconds from the short circuit, other transmission lines in the station began to fall out, and the system in southern Sweden, in which there was a significant power shortage resulting in the blackout, was separated. The failure in Sweden shows that the generation of electricity from nuclear power plants is characterized by high unit power of particular generation sources. In the event of failure this results in a significant power deficit in the system. Simulation studies conducted in Sweden during the failure analysis showed that in such cases the power system should meet the n-2 safety criterion.

Ireneusz Grządzielski / Poznań University of Technology Krzysztof Sroka / Poznań University of Technology

3. REQUIREMENTS FOR NUCLEAR POWER PLANTS DURING THE RESTORATION OF POWER SYSTEM The primary purpose of any power plant, including in particular nuclear power plants, de-energized after a catastrophic failure in the power system, should be a quick restoration of power supply to the auxiliary equipment of this power plant from an offsite source. As a consequence, the safe operation of the reactor is ensured and the plant can be restarted. Detailed instructions for operating the plant specify in detail time schedules for emergency switch-off and activation of the generating unit, and the conditions that must be met so that these procedures can be carried out. Due to these requirements, the restoration of nuclear power plant to operation in the power system can be much longer than in the case of other types of power plants. Such a statement is given in the NERC Electric System Restoration [6]: “Nuclear generating units require special treatment. Guidelines of Nuclear Regulatory Commission generally do not allow a hot restart. Nuclear units that have been disconnected from the network in a safe manner due to emergency can be restored to operation within about 24 hours, but it is more likely that their generating capacity is restored within 48 hours after an emergency shut-down. Therefore, whereas the restoration of voltage from the external network to a nuclear power plant requires speed, the restoration of power supply to the consumers has to take place without the participation of this nuclear power plant.” For these reasons, the restoration of power supply to all consumers in the subsystems, in which nuclear power plants have a signiﬁcant share, may not be possible even for several days. Nuclear power plant in emergency shut-down conditions When a blackout occurs, the system operator has to be informed whether the nuclear power plant has been safely shut-down and whether it is in a technical condition allowing maintaining its security. The safe survival of the plant after such emergency shut-down will be determined by an effective powering of certain auxiliary devices. Conventional power plants require the operation of turbine rotators, lube and seal oil pumps, with the operation of rotary air heaters, etc. In nuclear power plants the above conditions are even more important for safe survival. Therefore, nuclear power plants are equipped with emergency power sources, mostly diesel units, which ensure the operation of cooling systems of the reactor core in event of an emergency in such a way that the reactor and turbine can be safely shut-down when all the offsite power sources are lost. In general, a nuclear power plant may remain in a state of emergency shut-down over a long period of time. However, the technical requirements of power plants allow remaining in the state of shut-down in the absence of offsite power sources for a limited time only. Therefore, the mode in which the generating unit is under threat during the blackout, is a function of the length of time that will be needed to restore the power supply from offsite sources. Ultimately, the required time of restitution will be treated as the start-up time to the synchronization of the power plant with the power system. Nuclear power plants are designed to cope with the loss of offsite power (LOOP – loss of offsite power) using emergency sources, mainly diesel generators. Even though the control rods are inserted into the reactor core, stopping cleavage reaction, the decay of radioactive isotopes produced during the reactor operation is accompanied by generation of heat, which must be received from the core for several weeks. The lack of cooling can damage the fuel elements and release radioactive material into the environment. In order to remove decay heat over a long period, the supply from the external network or activation of emergency sources is required. Thus, the LOOP event is considered as a potential precursor of a much more serious situation. The risk of damaging the reactor core rises with increasing frequency of occurrence or duration of LOOP. The transmission system operator should determine how long the nuclear power plant may remain without an offsite power supply (e.g. amount of fuel for continuous operation of emergency generator). The transmission system operator’s highest priority task is the restoration of the external voltage sources for the nuclear power plant. If the nuclear power plant is to be used for the system restoration, it must be immediately provided with an external voltage source. Due to their large capacity and remote locations, nuclear power plants are usually connected to the power system through high voltage networks. For this reason, a nuclear power plant can not be supplied with energy from external sources prior to restoration of high voltage networks. Therefore, the reconstruction of high voltage networks is particularly important for the full restoration of a system that includes a nuclear power plant.

Nuclear Power Plants During Blackout

The restoration of high voltage networks can take many hours, which in turn prevents quick connection of the nuclear power plant to the system. Conditions for restoring generating capacity of nuclear power plants The nuclear power plant may be in one of four standard states: • operation • stop after an emergency shut-down • hot reserve • cold reserve. The optimal mode for restart is the hot reserve state (including a stop after an emergency shut-down). The operator (DIRE) of the plant should specify in detail the conditions that must be met for the power plant to remain in this mode, and the length of time the plant can remain in such a state. He should also specify the time for restart and synchronization with the network in the above states. Such information should be taken into consideration when developing the strategy for restoring the entire system. The summary below shows typical actions to be taken before connecting the nuclear power plant to network after losing voltage in the switching station: • Voltage supplied from an external network to the switching station in nuclear power plant must be normal and stable. Nuclear power plants are not designed to be activated without an external supply. • Busbars of the auxiliary switching station in the plant have to be powered from a substation, and the emergency diesel generator must be turned off at this point. • Basic auxiliary equipment in the plant, such as pumps for reactor cooling and recirculating water pumps must be running. • All the technical conditions must be met. Technical conditions for each nuclear power plant are determined as part of its licence. They specify which equipment has to be functional and which parameters of technological process must be met before the reactor can become operational. Example requirements, which were also imposed after the events of 14 August, include: filling of fuel tanks for diesel units, filling of condensate tanks, and implementation of the compulsory reactor coolant circulation. Control tests should be conducted in accordance with the applicable technical specification (e.g. the efficiency of thermal neutron detectors should be checked). In particular: • the safety system must be set to the sate that enables operation • pressure and temperature of the reactor cooling system must be set according to the conditions of its operation • calculation of the reactor criticality should be made in order to determine the control rod setting necessary to achieve criticality at which the chain reaction becomes self-sustaining. When the reactor is deactivated, the concentration of certain products that absorb neutrons increases. The concentration of boron in the coolant in the primary circuit of pressurized water reactors should be adjusted to the level resulting from the criticality calculations. At the end of the fuel cycle, the nuclear power plant may not be able to adjust the concentration of boron or control rod settings required to operate. In such a situation, activation will be possible after reducing the absorption of neutrons; this requires more than 24 hours from shut-down. A delay of one or more days before the nuclear power plant can be run after a normal shut-down may be necessary. Power plant shut-downs are transient states for power plant equipment, which can cause abnormal operating conditions. Therefore, careful operation is required to provide good technical conditions for this equipment before it can be activated; also, the procedure of activation itself has to be correct. Connecting nuclear units to the power system A typical sequence of connecting the generating units is as follows: • hydroelectric power stations • gas turbines • conventional power plants (fossil-fired units) • nuclear units. A large part of all non-nuclear units is restored to the system within 24 hours. Quick connection of large units to the system is not desirable for two reasons. Large units tend to have high minimum requirements.

Ireneusz Grządzielski / Poznań University of Technology Krzysztof Sroka / Poznań University of Technology

Therefore, it is not appropriate to connect those units before the system can provide an adequate additional load for these units. The process of providing additional load can cause frequency ﬂuctuations, which can lead to activation of safety features and another breakdown of the system. Moreover, the ability to connect these units to the system is limited by the activation of high voltage lines.

BIBLIOGRAPHY 1. Adamski G., Jenkins R., Gill P. , Nuclear Plant Requirements During Power System Restoration, IEEE Transactions on Power Systems, vol. 10, no. 3, August 1995, pp. 1486–1491. 2. Grządzielski I., Marszałkiewicz K., Sroka K., Kuczyński R., Układy wyspowe wokół dużych jednostek wytwórczych jako podstawowy element scenariuszy odbudowy KSE. Energetyka, Zeszyt tematyczny no. XVII: Blackout a Krajowy System Elektroenergetyczny – Rola dużych jednostek wytwórczych w realizacji planów obrony i odbudowy KSE, 2008, pages 152–158. 3. Markov Y., Reshetov V., Stroev V., Voropai N., Blackout Prevention in the United States, Europe and Russia, Proceedings of the IEEE, vol. 93, no. 11, November 2005. 4. Power failure in Eastern Denmark and Southern Sweden on 23 September 2003, Final report on the course of events, Elkraft System Report, November 4th, 2003, www.elkraft-system.dk. 5. Rychlak J., Kuczyński R., Regulacyjne Usługi Systemowe – środki techniczne obrony i odbudowy KSE, II Konferencja Naukowo-Techniczna: Blackout a Krajowy System Elektroenergetyczny, Poznań, April 2007, Energetyka, Zeszyt tematyczny no. X/2007. 6. U.S.-Canada Power System Outage Task Force: Final Report on the August 14, 2003, Blackout in the United States and Canada, April 2004.

Robert Kowalak / Gdańsk University of Technology Robert Małkowski / Gdańsk University of Technology

Authors / Biographies

Robert Kowalak Gdańsk / Poland

Robert Małkowski Gdańsk / Poland

He graduated from the Faculty of Electrical and Control Engineering at Gdańsk University of Technology (2000). He received his PhD at the same faculty (2005). Presently, he is working as a lecturer in the Power Engineering Department of the Faculty of Electrical and Control Engineering at Gdańsk University of Technology. His professional interests include: high-voltage power electronics systems (FACTS, HVDC), modelling the operation of power electronics systems in a power system, cooperation of power supply systems with traction power systems.

He graduated from the Faculty of Electrical and Control Engineering at Gdańsk University of Technology (1999). In 2003, he received his PhD. He is currently employed as an assistant professor at the Department of Electrical Engineering at Gdańsk University of Technology. The scope of his scientiﬁc interests covers issues related to wind energy, catastrophic failures of power systems, as well as to adjustment of voltage levels and distribution of reactive power in power systems.

Shunt Compensator as Controlled Reactive Power Sources

SHUNT COMPENSATOR AS CONTROLLED REACTIVE POWER SOURCES Robert Kowalak / Gdańsk University of Technology Robert Małkowski / Gdańsk University of Technology

1. INTRODUCTION Problems with maintaining adequate levels of voltage in the power system nodes have been occurring practically since the first power systems were started. Increasing requirements regarding both the supply reliability and quality of supplied power force using more modern (faster, more reliable, with a broader range of applications) devices. This trend also concerns the devices used to control voltage or compensate reactive power. In order to cover the additional demand for reactive power and maintain the ability to control voltage within the target range, various sources of reactive power, e.g. shunt compensators, are used. In recent decades there has been significant progress in terms of equipment designed to improve the stability of voltage in power systems. This is mainly due to the development of power supply systems in the world, which requires seeking better ways of adjusting and controlling power flows and voltage levels. An increasing importance in this field has been gained by FACTS (Flexible Alternating Current Transmission Systems). The main feature of these systems that distinguishes them from other solutions is the high speed of operation at high control dynamics [1, 9]. This article contains a concise description of selected control features of shunt power systems such as SVC (Static Var Compensator) – static compensators of reactive power, STATCOM-type systems (Static Compensator) – static reactive power generators and systems that combine both these solutions, which are referred to as SVC based on STATCOM. So far, such systems have not been used in the National Power System (NPS). Given the need to improve the voltage safety of NPS, as well as the increasing requirements for energy quality, more interest in these systems should be expected.

2. THE ROLE OF SHUNT COMPENSATORS IN POWER SYSTEMS The world’s first FACTS compensator system for voltage of over 100 kV was launched in 1977 in the United States. It was a SVC for controlling voltage on 138 kV busbars in the node that caused huge problems with maintaining voltage in the right value range [3]. The first STATCOM was developed in Japan in 1991 [21]. Thanks to the applied technical solution, STATCOM is considered as one of the best power devices used in power systems for controlling voltage and reactive power levels. These devices are often referred to as the “younger brother” of SVCs, because they play the same roles in the system. Power compensator systems are designed primarily for carrying out the process of voltage and/or reactive power control at the connection point. These systems may also operate according to other criteria (fig. 1).

Abstract Based on the analysis of technical solutions used around the world, we present basic design features of a shunt power system. ,The article discusses the advantages

and disadvantages in terms of using these systems as controlled sources of reactive power in power systems.

Robert Kowalak / Gdańsk University of Technology Robert Małkowski / Gdańsk University of Technology

The criteria for control used in FACTS shun compensators Voltage control Maintaining the reference voltage in connection node. This is the fundamental criterion for operation of these types of compensators. Power factor control Criterion used mainly in industrial plants in order to ensure that power factor is maintained within a certain range of values Reactive power control Criterion possible for implementation in order to maintain the value of reactive power at a certain level, not used in practice Damping of power oscillations Criterion whose task is to eliminate the oscillations of power e.g. after short circuits. Fig. 1. Control criteria used in FACTS shunt compensators

Thanks to their speed and continuous control, the role of shunt power compensators in the system is not limited to providing an additional source/receipt of reactive power and the associated voltage control process. They may also improve the quality of voltage in a power supply system. This refers to limiting rapid changes and voltage dips caused by connection processes or irregular receipts (e.g. steel mills, traction substations, chemical plants).

3. USED COMPENSATOR SOLUTIONS 3.1. Introduction The basic division of shunt compensators is shown in ﬁg. 2. Shunt compensators

Electromechanical

Static

Power Fig. 2. The basic division of shunt compensators

Conventional

Shunt Compensator as Controlled Reactive Power Sources

Electromechanical compensators are primarily regulated synchronous machines that receive or supply reactive power. Currently, they are rarely found in the power system. Static compensators can be divided into conventional and power compensators. The main disadvantage of conventional compensators apart from the discrete control method is the fact that they use mechanical power switches. These switches have fairly long switching times and a limited number of connections (due to wear of mechanical components and contacts), which prevents the control process from being performed in fast-changing states. This group of compensators includes capacitors that are switched using electromechanical switches (MSC – Mechanically Switched Capacitor) and reactors that are also switched using similar switches (MSR – Mechanically Switched Reactor). Currently, compensators of this type are most widely used in the NPS. Shunt power compensators are the most modern group of compensators. They enable continuous control, both in steady and fast-changing states.

3.2. SVC compensators The largest group of shunt power compensators consists of the compensators belonging to the SVC group. They are characterised by a modular structure, so it is possible to have many types of these devices; the features of each type depend on the components used. Depending on individual needs, the use of configurations for discrete and continuous control is possible. Analysis of technical solutions in SVC encountered around the world allows us to divide them into several types: • TSC (Thyristor Switched Capacitor) is a capacitor switched using thyristor. Systems of this type consist of one or more cooperating three-phase sections of TSC, where each section includes capacitors, thyristor switches that are switched on or off depending on the total reactive power supplied by the entire device. • TSR or TCR. TSR and TCR are systems with induction components only. They consist of a TSR (Thyristor Switched Reactor) or TCR (Thyristor Controlled Reactor) section; TSR includes thyristor switched reactors, whereas TCR are reactors with thyristor controlled induction. A TSR-type compensator is composed of several three-phase TSR sections whose thyristor switches are switched on or off (discrete control) depending on reactive power that is to be received by the entire device from the system. A TCR-type compensator has a similar structure, but the basic difference between these devices lies in the fact that TCR has no ability to provide smooth control of inductance. • TCR-FC. These devices consist of two types of components. The first one is a TCR module that receives reactive power, and the second is FC (Fixed Capacitor), which include also higher harmonic filters. They are an essential element when it comes to the work of TCR. FC is a source of reactive power. • TCR-TSC-FC. These compensators consist of three groups of components. The first group consists of thyristor controlled reactors. The second group consists of TSC, which is the primary source of reactive power. The third group is higher harmonic filter (treated as fixed capacities – FC), which are an additional source of reactive power. Their presence in this system is necessary due to the need to eliminate the interferences caused by TCR. However, additional filters that do not come from the same compensator and can eliminate other interferences can be used. The discussed solution is identified as a typical structure of SVC. Fig. 3 presents the structure of this type of system with voltage controller, consisting of one branch of TCR, and one branch of TSC and higher harmonic filters.

Robert Kowalak / Gdańsk University of Technology Robert Małkowski / Gdańsk University of Technology

HV � U�T� I�k�

TR� RU�

MV TCR�

��

UT�z�

TSC�

on off

��

� � �

��

�

Fig. 3. Structure of SVC compensator, type TCR-TSC-FC: USS – susceptance control system, RU – voltage controller, TR – transformer WN/SN, α – thyristor ignition angle TCR, UTz – reference voltage, UT – controlled voltage, Ik – compensator current

• TSR-TSC. Compensators of this type consist of two groups of elements. The ﬁrst group consists of thyristor switched inductors, while the second group consists of TSC. The system may provide only discrete control. Presented division of SVC is based on the divisions used by manufacturers of these systems [2, 4, 12, 13, 14, 17 and 18].

3.3. STATCOM – type compensators The second group of systems includes STATCOM-type compensators. These systems are characterized by a compact structure. Having the same available value of reactive power as SVC, STATCOM occupies much less space. In addition, they have better dynamics. Despite their properties, these systems have not supplanted SVC. One of the reasons is the installation cost – despite continuous technological development, STATCOM is still more expensive compared with SVC. In terms of design, there are two basic types of STATCOM. The ﬁrst consists of VSI systems Voltage Source Inverter), in which the inverter is used as voltage converter. In this system, the capacitor is the inverter load. The more popular of the two possible methods for inverter control is the method of pulse phase modulation. In this case, it is required to maintain constant voltage in the capacitor which is the inverter load on the DC side. Fig. 4 presents the structure of such a system with voltage controller. HV � UT�� I�T�

TR�

Uk�� Ik��

MV �

��

�

UD� C�

�

UWT�

Uk��,��

RU�

UT� z�

U�DC�, ID� C�

�

Fig. 4. Structure of STATCOM compensator constructed on the basis of voltage converter VSI: TR – transformer WN/SN, RU – voltage controller, UWT – thyristor control system, α – inverter control signal, UTz – reference voltage UT – controlled voltage, IT – compensator current, Uk – inverter voltage, Ik – inverter current, UDC – circuit voltage DC, IDC – circuit current DC

Shunt Compensator as Controlled Reactive Power Sources

VSI-type STATCOMs have been used in power systems as devices designed to cooperate with wind farms, irregular receipts (e.g. steel mills, traction substations), and also to control the voltage in the power system nodes [10, 11, 15, 16, 19 and 20]. The second type of STATCOM is a device based on the current CSI (Current Source Inverter). This type of system has been used in power systems yet.

3. 4. Hybrid compensators The most recent group of shunt power compensators is a hybrid combination of SVC and STATCOM. Because of their construction, it is often called a STATCOM-based SVC. This is due to the fact that the structure of this system is based on the structure of SVC; however, thyristor controlled reactors (TCR) have been replaced with STATCOM (ﬁg. 5). WN�

��

UT Ik

TR�

SN Filters

�

TSC�

STATCOM�

�

� ��

on off

�

��

UDC

�

��

UT z

�

� � � �

U DC I DC

�

Fig. 5. Structure of STATCOM-based SVC: TR – HV/MV transformer, Controller – system controller, α – STATCOM inverter control signal, UTz – reference voltage, UT – controlled voltage, Ik – compensator current, UDC – circuit voltage DC, IDC – circuit current DC

Replacing TCR in SVC with STATCOM with the same rated power makes the range of reactive power generated in the entire system larger; at the same time the ability to consume this power has not changed. Furthermore, STATCOM can perform faster control than TCR and bring less interference to the supply system. However, despite many advantages, similar to STATCOM, the main drawback of these systems is the high price, so for now, their number in power systems remains low.

4. VOLTAGE CONTROL USING SHUNT POWER SYSTEMS The quality and range of voltage control in a shunt compensator connecting node is dependent both on the control algorithm and available value of reactive power. In steady states, the control properties describe the external characteristics well. External characteristics of voltage and power in the discussed systems are presented in ﬁg. 6. a)

b) U

U Umax

A B

cap.

ind.

cap.

Umin ind.

Fig. 6. Static voltage and power characteristics: a) SVC – with voltage controller, b) STATCOM – with voltage controller

Robert Kowalak / Gdańsk University of Technology Robert Małkowski / Gdańsk University of Technology

Both systems are equipped with a voltage controller. In the control range, this allows obtaining the voltage characteristics of low gradient, corresponding to the assumed statics (usually 1-10 percent). In the linear range, both compensators behave similarly. The main difference is present in the case of too high or too low voltage. In SVC, the available power is changed in the voltage square as follows: • for capacitive part the changes in voltage correspond to parabola: Q = Bmax × U2

(1)

• for inductive part: Q = Bmin × U2

(2)

Where: Bmax – corresponds to capacitive susceptance occurring when all components of a capacitor bank are switched on and the reactors are switched off Bmin – corresponds to inductive susceptance occurring when all components of a capacitor bank are switched off and the reactors are switched on The controlled value in STATCOM is current. After reaching the limit values (points A and B), the current is kept at a constant level I = const, until voltage limiters become active (Umax , Umin ). Therefore, reactive power described by the relation: Q=I×U

(3)

will vary in direct proportion to voltage value. The properties described above are reflected in practice. Especially during operation outside the control range, mostly at voltages that differ significantly from rated conditions. Behaviours of various types of compensators are well illustrated in the curves. The following figures present example curves determined for a 400 kV node (fig. 7 and 8). General load / without compensator General load / MSC or TSC General load / SVC General load / STATCOM

Fig. 7. Effect of various installed types of compensators on the shape of the curves

Shunt Compensator as Controlled Reactive Power Sources

Switching capacitor banks of MSC or TSC

Switching capacitor banks of SVC

, ,

General load / without compensator General load / MSC or TSC General load / SVC General load / STATCOM

Fig. 8. Magniﬁed fragment of ﬁg. 7, covering the control range in compensators

Two curves were compared: for the nodes without compensator and with compensators, type: MSC, SVC and STATCOM. The curve for MSC also corresponds to the behaviour of TSC-type SVC. The curve described as SVC concerns the TSR-TSC-FC-type SVC. The FC-TCR-type SVC behaves in a very similar way (apart from interferences associated with connecting further sections of TSC). The curves were obtained for compensator systems with rated power of 200 MVA each. In terms of control, the results of operation of SVC and STATCOM are comparable. Voltage surges visible in waveforms for MSC are related to connection of further capacitor banks. It should be noted that MSC was designed to prevent a drop in voltage below 3 percent of the reference value, while SVC and STATCOM worked with a reference droop at 3 percent. The above assumption provided similar control ranges for all systems. When voltage was decreased to a value at which the process of voltage control in all systems was completed (MSC – switching on all the capacitor banks, SVC – switching off the reactor of TCR and switching on all TSC components, STATCOM – maximum current in the generation of reactive power), the impact on particular systems on the power system were changed. At voltage levels above 85 percent of the reference voltage (but below the control zone), SVC, STATCOM and MSC behaved very similarly. Nonetheless, small differences in favour of SVC and MSC in relation to STATCOM can be observed. However, STATCOM proves to be better for lower voltages. This is due to the fact that in capacitors (MSC and SVC behave similarly outside the control range), their generated reactive power depends on the voltage square, while in STATCOM its ability to generate reactive power depends linearly on the voltage (see relations 1, 2 and 3).

5. SUMMARY Power compensators belong to the group of compensators that enable fast automatic voltage control in the system. An important feature of these devices, especially in emergency situations, is their ability to maintain control also in fast-changing states. Such compensators should be considered as a very good solution for increasing voltage security in the system [6, 7 , 8]. Although these solutions are more expensive than conventional compensator systems, their properties make them worth considering for use in NPS. Control properties of power compensators can also be successfully used in distribution networks, e.g. to improve the quality of voltage in networks with high occurrence of wind farms.

Robert Kowalak / Gdańsk University of Technology Robert Małkowski / Gdańsk University of Technology

As demonstrated by voltage emergencies in recent years, it is becoming necessary to use additional sources of reactive power in the NPS. It is likely that conventional compensators will be installed more frequently due to the costs. However, shunt power compensators may be an alternative worth considering.

BIBLIOGRAPHY 1. Acha E., Fuerte-Esquivel C.R., Ambriz-Perez H., Angeles-Comacho C., FACTS Modelling and Simulaton in Power Networks, John Wiley & Sons, LTD, 2004. 2. Faruque M.O., Dinahavi V., Santoso S., Adapa R., Review of Electromagnetic Transient Models for Non-VSC FACTS, IEEE Transactions on Power Delivery, vol. 20, no. 2, April 2005. 3. Hingorani N.G., Flexible ac transmission, IEEE SPECTRUM April 1993. 4. Kodsi S.K.M., Cañizares C.A., Kazerani M., Rective current control through SVC for load power factor correction, Electric Power System Research 76, 2006. 5. Announcement of PSE-Operator SA on the final report from the tests regarding voltage breakdown on 26 June 2006 and the measures taken to prevent emergencies in the future. 6. Kowalak R., Małkowski R., Zajczyk R., Zbroński A., Instalowanie kompensatorów w sieci przesyłowej KSE, Konferencja Naukowo-Techniczna „Problematyka mocy biernej w sieciach dystrybucyjnych i przesyłowych”, Wisła, 7–8 December 2010. 7. Kowalak R., Szczeciński P. , Zajczyk R., Wpływ układów SVC na rozwój awarii napięciowej. Energetyka, zeszyt tematyczny nr XVII, październik 2008 (jako materiały konferencyjne III Międzynarodowej Konferencji Naukowej „Blackout a krajowy system elektroenergetyczny” 2008, Rosnówko near Poznań, 8–10 October 2008). 8. Kowalak R., Zajczyk R., Wpływ kompensatorów energoelektronicznych zainstalowanych w określonych punktach KSE na awarię napięciową, Energetyka, zeszyt tematyczny nr XX, 2010 (jako materiały konferencyjne IV Międzynarodowej Konferencji Naukowej „Blackout a krajowy system elektroenergetyczny” 2010, Rosnówko near Poznań, 16-18 June 2010). 9. Machowski J., Elastyczne systemy przesyłowe – FACTS, Przegląd Elektrotechniczny 7/2002. 10. Information materials, ABB Advanced Power Electronics, ABB Switzerland Ltd., Advanced Power Electronics, 3BH-S237242 ZAB E01, acquired on: July 2010. 11. Information materials: ABB STATCOM For flexibility in power systems, ABB Power Systems AB, A02-0165E, acquired on: July 2010. 12. Information materials, AMSCTM SVC Static Var Compensator, American Superconductor Corporation, 2008. 13. Information Materials, Modelling of SVC in Power System Studies, ABB Power Systems AB, information NR 500-026E, April 1996. 14. Information materials, Power Transmission and Distribution, Discover the World of FACTS Technology, Technical Compendium, SIEMENS AG Power Transmission and Distribution High Voltage Division, No. E50001-U131-A99-X-7600. 15. Information materials, STATCOM solutions for Wind Farm, ABB Switzerland Ltd., Advanced Power Electronics, 3BHT490587R0001, 2008. 16. Information materials, STATCOM, ABB Switzerland Ltd., Advanced Power Electronics, 3BHT490522R0001, 2006. 17. Information materials, SVC Configuration Optimisation, Nokian Capacitors Ltd., EN-TH18-03/2007, 2007. 17. Information materials, SVC Configuration Optimisation, Nokian Capacitors Ltd., EN-TH18-03/2007, 2007 18. Information materials, SVC Static Var Compensator, ABB Power Technologies AB, A02–0100E, acquired on: July 2010. 19. Information materials, Using Dynamic Reactive Compensation to Mitigate Voltage Sags at a Micron Technology Semiconductor Manufacturing Facility, American Superconductor Corporation, MCRN_CS_0610, 2010. 20. Oskoui A., Mathew B., Hasler J.P., Oliveira M., Larsson T., Petersson A., John E., Holly STATCOM – FACTS to replace critical generation, operation experience, materials acquired from the company ABB: July 2010. 21. Strzelecki R., Benysek G., Układy STATCOM i ich rola w systemie elektroenergetycznym, Międzynarodowa Konferencja Naukowo-Techniczna „Nowoczesne urządzenia zasilające w energetyce”, Kozienice, March 2004.

Tomasz Minkiewicz / Gdańsk University of Technology

Authors / Biograﬁe

Tomasz Minkiewicz Gdańsk / Poland He graduated from the Faculty of Electrical and Control Engineering at Gdańsk University of Technology (2009). He is a student of Doctoral Studies at his ﬁrst faculty, and is employed as an assistant at the Department of Electrical Engineering at Gdańsk University of Technology. His professional interests include the current status and development of nuclear energy in Poland and in the world.

Radioactive Waste and Burnt Nuclear Fuel Management During Construction of the Nuclear Power Plant in Żarnowiec

RADIOACTIVE WASTE AND BURNT NUCLEAR FUEL MANAGEMENT DURING CONSTRUCTION OF THE NUCLEAR POWER PLANT IN ŻARNOWIEC Tomasz Minkiewicz / Gdańsk University of Technology

1. INTRODUCTION Żarnowiec Nuclear Power Plant was built in 1982-1990. The area of former Kartoszyno village at Żarnowieckie Lake in the northern part of Pomeranian Voivodeship (60 km from Gdansk, 10 km from the Baltic Sea) was selected for the plant location. Four power units equipped with VVER-440 reactors were to be installed at the location. Each unit was supposed to have electric power of 465 MW and thermal power of 1375 MW. Total gross electric power of the entire power plant was to be about 1860 MW. The location of Żarnowiec NPP was divided into two zones: controlled zones I and II. Zone II, referred to as dirty, included buildings where the staff could deal with radioactive contamination – the area for management of radioactive waste and burnt nuclear fuel (reactor building, radioactive waste management, incineration of waste and burnt fuel buildings, workshops, laboratories and laundries). The proposed arrangement of buildings is shown in ﬁg. 1. The red frames indicate the facilities in which the staff could have direct contact with radioactive materials. The overall land use plan for Żarnowiec NPP is presented in documents regarding the power plant being built at the time, e.g. [1], which includes ﬁg. 1.

Fig. 1. Żarnowiec NPP land use plan [1]

B1 – reactor building D2 – radioactive waste management building D4 – burnt nuclear fuel building D5 – radioactive waste incineration building D6 – low-level waste repository

Abstract This article offers an insight into aspects relating to management of radioactive waste (solid, liquid and gaseous waste) and burnt nuclear fuel (RW and BNF) at the time when the Żarnowiec nuclear power plant was

designed and being built.,The article also describes the current state of management of RW and BNF in Poland and Europe, drawing appropriate conclusions.

Tomasz Minkiewicz / Gdańsk University of Technology

As is clear from the fig., transition between the zones was to be possible only through sanitary airlocks (fully controlled; employers were required to dress in special clothing, shoes and to use dosimeters). The level of body and clothing contamination had to be checked in the building of maintenance services (fig. 1 - F1D), and the burnt fuel building (fig. 1 - D4). Additional protection would also be provided by control of contamination of staff personal clothing as well as of road and rail transport at the entrance (gate) to Żarnowiec Nuclear Power Plant. The entire area of Żarnowiec NPP, as well as the surrounding locations, were to be covered by the dosimetry control system (control in individual rooms, facilities, and surroundings of the NPP; continuous measurements of factor activity and integrity of technological equipment; activity control of waste discharged from the NPP; control of internal contamination of personnel and a selected group of population). There were plans to install ten automatic measuring stations that would read the level of radioactive contamination with radioactive aerosols, the activity of precipitation and the natural gamma radiation. Three types of radioactive waste were to be produced in the plant: liquid, solid and gaseous waste, of which about 99 percent of radioactive material is contained in fuel elements, while the remaining part of waste is produced mainly due to fuel clad leakage and fission products that manage to get to the coolant. During the construction of Żarnowiec NPP, the Sejm of the People’s Republic of Poland planned that, according to the energy development program, the Central Radioactive Waste Repository (CRWR) would be built by the year 2000. The repository was supposed to be located outside the Żarnowiec NPP and take all the solid radioactive waste from all operating nuclear power plants (up to 2000, the installed capacity in Polish nuclear units was to be at the level of 9860 MW - 16 locations were proposed). If necessary, there was also a possibility of building a temporary storage of low-level radioactive waste in the area of Żarnowiec NPP.

2. SOLID WASTE MANAGEMENT Solid waste that was to be produced during the plant operation was divided according to the rate of gamma radiation exposure dose. The formerly used unit of exposure dose was roentgen per hour (R/h); now it is ampere per kilogram (A/kg) in the SI. The division was as follows: • low-level waste (2.15 – 2.15×103 pA/kg) • medium-level waste (2.15×103 – 7.17×104 pA/kg) • high-level waste (over 7.17×104 pA/kg). For comparison, the average gamma dose rate in air is 0.72 pA/kg in Kraków, and 0.85 pA/kg in Zakopane. There are places around the world where the dose rate is much higher than the average dose in Poland. An example is the Capitol and the Library of Congress in the U.S., where the radiation emitted by walls is 2.15 × 103 pA/kg - this is an upper limit for gamma dose rate in low-level radioactive waste [6, 7]. Low-level waste include used work and protective clothing, rags, construction elements and small auxiliary devices, laboratory equipment, tools, but also radioactive deposits that may accumulate on the surfaces of equipment and premises (due to leakage from the primary circuit and deposition of aerosols). The share of solid waste in the overall volume of waste is quite low. These wastes were to be kept in solid radioactive waste storage located in the radioactive waste management building (D2), and ultimately in the CRWR. Medium-level wastes (ﬁlter inserts in ventilation systems, sections of pipes, ﬁttings in contact with the reactor coolant, thermal insulation of the primary circuit, etc.) were to be stored in concrete chambers of radioactive waste management building, and ultimately in the CRWR. The volume of chambers was calculated for ten years of NPP operation. High-level radioactive waste, such as structural components of the reactor located in the neutron radiation zone, were to be stored in bunkers located in close proximity to the pool of used nuclear fuel. The volume of bunkers was calculated for the full operation life of the NPP. In most types of waste (mainly low-level), its processing was to be based primarily on cutting, pressing and incineration, which would allow reducing the volume. In the case of deposits, it was proposed to use the so-called decontamination, i.e. a process of removing and deactivating radioactive materials, resulting in the formation of liquid waste.

Radioactive Waste and Burnt Nuclear Fuel Management During Construction of the Nuclear Power Plant in Żarnowiec

Low-level waste does not require any special containers for transport and storage - drum-shaped containers made of galvanized steel with a capacity of 25-200 litres are used. Transport and storage of medium-level and high-level waste requires the use of additional shielding made of concrete, lead or multi-layered protective containers.

3. LIQUID WASTE MANAGEMENT Liquid wastes have the largest share in all the waste produced in nuclear power plants, both in terms of quantity and activity. The system of liquid radioactive waste management was to be located in the radioactive waste management building. It was supposed to be used for collecting waste, temporary storage and transport for further processing. Two forms of waste were to be obtained in the process. One of them were used ion exchangers, i.e. substances with ability to transform ions from solution into exchange ions, which form radioactive waste and sludge produced from treatment of radioactive sewage and as a result of treatment of water from the reactor cooling system. The second form of waste is evaporation residue. Treatment of liquid radioactive waste was to be based on the use of systems for solidification of evaporation residues with melted bitumen (a thick mixture of solid and liquid organic substances, mainly hydrocarbons, mostly asphalt) [2]. This process is used for concentrating and processing liquid waste into solid waste. The systems for carrying out the process enabled solidification of waste (with a concentration of 200 g/kg) and pulp (water mixture) of used ion exchangers. Concentrated radioactive sewage was to be fed to the bitumen system powered by steam at a pressure of 0.2 MPa. The system was to carry out water evaporation and precipitation of salt crystals with 5 percent of humidity. Bitumen and dry evaporation residue were to be mixed in the proportion of 1:0.8 (in the case of solidification of used ion exchanger pulp it was to be mixed with bitumen and the dry evaporation residue in the proportion of 1:3:1). The diagram of system for storing evaporation residue and used ion exchangers can be found in [1]. The prepared mixtures were to be kept in barrels and transported to the solidified waste storage. This storage was to be located in the radioactive waste management building and was supposed to be used for five years of normal operation of four VVER-440 nuclear units. A possible expansion of the storage was considered, which would have had increased the storage period to ten years.

4. GASEOUS WASTE MANAGEMENT The layout of gas waste management was to be based on treatment of radioactive gases produced during the operation of NPP (85Kr and 129I). These gases are escaping through leakages in fuel elements to the primary circuit, aerosols and vapours released from the water surface in fuel pools. The system was to include two heat exchangers, two moisture separators, a self-cleaning filter, iodine filter, two zeolite filters (working and reserve filters designed to remove nitrogen and phosphorus from the water, containing material for biological and chemical filtration), two (working and reserve) aerosol filters, two (working and reserve) electric heaters and a water reservoir. The system is described in detail in [1]. In addition, the systems for temporary storage of radioactive gases were designed, which was supposed to result in a reduction of activity due to radioactive decay. Purified air was to be transported to the ventilation stack. The level of gas purification was to be high enough not to allow the acceptable value of releases into the atmosphere to be exceeded.

5. BURNT NUCLEAR FUEL MANAGEMENT According to the plan for the fuel handling in Żarnowiec NPP, 104 fuel assemblies and 12 control assemblies were to be exchanged each year. During handling/shufﬂing of fuel, the fuel pool and reactor cavity, as well as the corridor linking both areas were to be ﬂooded with boron water, which was supposed to provide full biological protection (water layer of about 3.3 m).

Tomasz Minkiewicz / Gdańsk University of Technology

After removal of assemblies from the core, they were to be checked for cladding leaks (leaks caused by volatile fission products that go from fuel to coolant, resulting in activation of the coolant). If such a leak would be detected, then the assemblies with defective fuel elements were to be placed in one of 53 sealed trays per each nuclear power unit, and then transported to the used fuel pool. The pool was to be filled with boric acid solution with a concentration of no less than 12 g/kg and provided with a layer of no less than 3 m (to maintain the temperature to 323 K under normal conditions or to 343 K during emergency). Gases accumulated above the cooling water, produced during the used fuel storage were to be collected by an exhaust ventilation system. Used fuel was to be stored in the pool for three years (six years for storage of assemblies with used nuclear fuel above the standard amount), and then transported to the used fuel building D4 (fig. 1) when the reactor was deactivated. The building was to include four fuel pools (three primary and one backup), and each could accommodate 56 containers with 30 fuel assemblies each. The entire building was designed for storage of used nuclear fuel from all four planned VVER-440 nuclear units over a period of ten years (5040 assemblies of used fuel). After that time, the burnt fuel was to be transported to the USSR.

6. CURRENT STATUS OF RADIOACTIVE WASTE AND BURNT NUCLEAR FUEL MANAGEMENT At this time, Poland has no experience in the storage of burnt nuclear fuel from nuclear power plants, so we should use the experience of countries in which this process has been carried out for years. These may be, e.g. France with its plant for treatment and enrichment of nuclear waste of COGEMA Society and low-level and medium-level waste repository in La Hague in Normandy, Finland (repository in Olkiluoto) or Sweden (repository in Forsmark) which are European countries with a highly developed nuclear technology. The issue of storing the burnt nuclear fuel may occur 30-40 years after launching the first nuclear power plant in Poland. This is due to the fact that the current power plants are designed so as to store the radioactive waste produced throughout the operation in a completely safe way only at the location of the NPP. Radioactive waste management in Poland is currently managed by the National Radioactive Waste Repository (NRWR). According to the classification of the International Atomic Energy Agency, the aforementioned facility is a surface storage for final disposal of mainly short-lived, low- and medium-level waste. It is located in Różan (Masovian Voivodeship). It has been collecting radioactive low- and medium-level waste from the whole of Poland for fifty years. The waste is prepared in the Radioactive Waste Management Plant (ZUOP) in Otwock-Świerk. ZUOP has been working on improving the techniques for processing and solidification of radioactive waste (technology for solidification of used ion exchangers has been improved by the use of polyester resin as binder, the method of preparation for storage of radium sources has been modified by sealing them using a combination of artificial barriers, and it is planned to launch a new technology for concentrating liquid waste using the distillation process and membrane processes) [4]. Reports on the condition of radiological protection in the area of NRWR in Różan and in its vicinity are prepared every year and then are provided to the State Nuclear Power Safety and Radiological Protection Inspectorate, the National Atomic Energy Agency, the Governor of Masovia and the Municipal Council in Różan. Measurements of doses in this municipality do not differ from values in other areas of Poland, and according to the opinion of specialists from the Maria Skłodowska-Curie Oncology Centre in Warsaw, the municipality is characterized by one of the lowest mortality rates caused by cancer in Poland [3]. The European Commission has been working on a new directive on the management of radioactive waste and burnt nuclear fuel. The new directive will impose on individual countries an obligation to construct own storage sites for burnt nuclear fuel and its processing residue or to construct common repositories for several countries. A team has been appointed with the aim of establishing the European Repository Development Organization (ERDO). According to the SAPIERR II report, prepared by EU experts, the shared storage is much cheaper than individual storage of waste by each country. If the countries of the ERDO working group (including Poland) decide to establish one very large repository (located in the eastern part of Europe), the savings can amount to 15-25 billion EUR. Building smaller facilities for two or three countries can also reduce the costs of disposal of radioactive waste by several billion [5].

Radioactive Waste and Burnt Nuclear Fuel Management During Construction of the Nuclear Power Plant in Żarnowiec

7. SUMMARY The techniques used in designing Żarnowiec NPP and associated with radioactive waste management were developed and implemented mainly due to establishing the Central Radioactive Waste Repository (CRWR) in 1961, currently NRWR. One of the major issues associated with nuclear power engineering (except for the choice of technology and location) is currently the matter of storing low- and medium-level radioactive waste that come not only from nuclear power plants, but are also produced due to the use of radioisotopes in medicine, industry and research, during the production of open and sealed sources of radiation and the operation of a research reactor, which is used also for production of radioisotopes. In view of the fact that the only radioactive waste repository in Poland is beginning to fill up (its closure is scheduled for 2020), according to the schedule of the Polish Nuclear Energy Program (PNEP), the design works for a new repository are to begin in 2013, while construction will begin two years later; the launch is planned for 2018. Apart from the need to build a repository for low- and medium-level waste, an important issue related to radioactive waste management is the answer to the question whether in the future Poland should build a repository for high-level waste, transport used fuel to other countries, or build its own facility for processing burnt nuclear fuel. The best solution would be to build one large repository or facility for the recycling of nuclear fuel together with several other countries. However, it is certain that due to technical and social aspects, most countries would like to build such a facility outside their borders. Apart from the legal and organizational changes associated with radioactive waste management, it is necessary to educate qualified personnel and prepare an appropriate information campaign which will convince the public of the safety of nuclear facilities, including repositories for radioactive waste from nuclear power plants. Also, all possible steps should be taken to optimize the use of nuclear fuel in reactors, i.e. to reduce the amount of produced waste. Radioactive waste and burnt fuel management must ensure the safety of both people and the environment. Security must be maintained both in normal conditions and in emergencies. Both these conditions were met when designing the Żarnowiec NPP (increased safety margins, redundancy of safety systems, continuous measurement systems, etc.). Based on the experience from the construction of Żarnowiec NPP, fifty years of experience in the operation of NRWR in Różan and cooperation with the countries that have a lot more experience in the field (France, Finland, Sweden, and the United Kingdom), it is hoped that in the upcoming years Poland will develop a modern nuclear technology and a secure system for the processing and storage of radioactive waste and burnt nuclear fuel.

BIBLIOGRPHY 1. Elektrownia Jądrowa Żarnowiec. II Etap 2x465 MW. Wstępny raport bezpieczeństwa, część I, opis ogólny, praca zbiorowa, BSiPE Energoprojekt, Warszawa 1989. 2. Reński A., Elektrownie Jądrowe. Część II, Wydawnictwo Politechniki Gdańskiej, Gdańsk 1991. 3. Madaj K., Doświadczenia z 50 lat unieszkodliwiania odpadów promieniotwórczych w Polsce, II Szkoła Energetyki Jądrowej, Warszawa 2009. 4. Zakład Unieszkodliwiania Odpadów Promieniotwórczych, http://www.zuop.pl. 5. ICEM 2009, Shared, regional repositories: developing a practical implementation strategy, http://www.arius-world. org/pages/pdf_2009/02_ICEM_2009_SAPIERR.pdf. 6. Naniewicz J., Naturalne tło promieniowania i inne źródła – percepcja ryzyka, Instytut Hematologii i Transfuzjologii, III Szkoła Energetyki Jądrowej, Warszawa 2010, RTA Sp. z o.o. Warszawa. 7. Strupczewski A., Porównanie zagrożeń związanych z różnymi źródłami energii elektrycznej, Polskie Towarzystwo Nukleoniczne, March 2005.

Wiesław Nowak; Szczepan Moskwa / AGH University of Science and Technology in Kraków Rafał Tarko / AGH University of Science and Technology in Kraków

Authors / Biographies

Wiesław Nowak Kraków / Poland

Szczepan Mosk wa Kraków / Poland

A graduate of the AGH University of Science and Technology. He received the degree of MSc Engineer (1988), then the degree of PhD (1995) and postdoctoral degree (2006) in the ﬁeld of electrical engineering at the Faculty of Electrical Engineering, Automatics, Computer Science and Electronics, AGH University of Science and Technology. He has worked at the AGH University of Science and Technology since 1987, and now holds the position of associate professor. He specializes in electrical engineering, and his main scientiﬁc interests are related to computer modelling and analysis of dynamic states in power systems.

Received his MSc at the Faculty of Electrical Engineering, Automatics, Computer Science and Electronics, majoring in electrical engineering (2000). PhD received at the same Faculty (2007). He has been working at the Department of Electrical and Power Engineering of the AGH University of Science and Technology since 2000. The main areas of his professional activities relate to the strategies for operation of electrical equipment, reliability of electrical equipment and power systems.

Rafał Tarko Kraków / Poland He received his master’s degree in electrical engineering, specialization in electrical engineering, at the Faculty of Electrical Engineering, Automatics, Computer Science and Electronics at the AGH University of Science and Technology (2001). PhD received at the same Faculty (2007). He has been working at the Department of Electrical and Power Engineering at the AGH University of Science and Technology since 2001. His main research interests focus on the analysis of operational and electromagnetic stress related to transient states in power systems.

Explaoatation Problems of Medium Voltage Power Distribution Networks in the Aspect of Reliability of Power System Protection

EXPLAOATATION PROBLEMS OF MEDIUM VOLTAGE POWER DISTRIBUTION NETWORKS IN THE ASPECT OF RELIABILITY OF POWER SYSTEM PROTECTION Wiesław Nowak / AGH University of Science and Technology in Kraków Szczepan Moskwa / AGH University of Science and Technology in Kraków Rafał Tarko / AGH University of Science and Technology in Kraków

1. INTRODUCTION Operation of power systems is accompanied by various states of faults, preventing or impeding their normal work. Short-circuits are one of the basic types of faults, which could have both a local (primarily thermal and dynamic stress of the system elements and the risk of electrocution) and global character, such as a possible loss of power system stability. It should be noted that operational practice reveals that about 70-80 percent of faults are single-phase short circuits to earth. Detection and elimination of faults or other abnormal conditions occurring in the power system is the task of power system protection (PSP). One of the basic requirements for PSP is its reliability. Operation of PSP may be correct or incorrect; incorrect operations include both missing and unnecessary operations. On the contrary to the high voltage (HV) and extra-high voltage (EHV) transmission systems, the medium voltage (MV) distribution networks are operated as isolated neutral systems, resonant earthed neutral system, or resistance earthed neutral system. Regardless of the method of operation of MV networks, their common feature are relatively low ground current values (single-phase earth short circuit) as compared to the HV and EHV systems, which usually do not exceed several dozen amperes in the isolated and resonant earthed neutral system, and several hundred amperes in the resistance earthed neutral system. Although the currents of earth faults in medium voltage networks do not cause thermal and dynamic stress of electrical equipment and devices, such a fault should be eliminated as quickly as possible, mainly due to the risks of electrocution. However, the lack of adequate earthing of the neutral point implies many disadvantageous conditions and difﬁculties in the implementation of effective earth fault protection [e.g. 1, 2, and 3]. The above problems are the subject of this article, which presents the results of tests on two selected cases of incorrect operation of earth fault protection in MV power networks: 1. missing operation in the event of high-resistance earth fault 2. unnecessary operation in the event of ferroresonance.

2. ANALYSIS OF CONDITIONS FOR OPERATIONS IN GROUND FAULT PROTECTION IN CASE OF HIGH-RESISTANCE EARTH FAULTS The subject of the analysis was a isolated neutral system and voltage of 15 kV (ﬁg. 1), operated by one of the domestic distribution companies. In line LPi connected in the ﬁeld P06 of the 15 kV distribution board there was a disruption in the form of a multi-phase short circuit caused by a tree that fell on the line. This fault was effectively eliminated as a result of overload protection, but the line was reconnected by automatic reclosing.

Abstract The article addresses the issue of operating medium voltage distribution networks in the aspect of proper working of power system protection. Two documented cases of incorrect operations of ground fault protections are presented as well as the results of analysis based on the measurements made in distribution networks and

calculations using the EMTP-ATP simulation program. The conclusions of the conducted analysis are not only of an individual character, speciﬁc to the particular case of disruption, but also of general character for earth fault protection installed in medium voltage distribution networks.

Wiesław Nowak; Szczepan Moskwa / AGH University of Science and Technology in Kraków Rafał Tarko / AGH University of Science and Technology in Kraków

Fig. 1. Simpliﬁed diagram of a 15 kV system with isolated neutral point

Four hours after the incident, the Distribution Region Dispatcher received information about a broken line cable that was touching the ground. The energy services that arrived at the place reported that the ground under the broken line cable was burnt (the length of a dozen metres). The conductor was live all the time until the dispatcher remotely switched off the line. There was a risk of electrocution for people who might have happened to be close to the broken conductor. Based on preliminary analysis it was found out that during the phase-to-phase short circuit, the line conductor was burnt, broken and fallen down onto the ground, which did not activate the earth fault protection. It was assumed that the correct operation of the earth fault was prevented by excessive transition resistance at the contact between the phase conductor and the ground. The tests conducted for electrical features of the ground at the place of the incident showed that its resistivity ranged from 8.9 to 12.3 kΩm. These are typical values for dry gravel and sandy soils. In order to determine the possible transition resistance that can occur when a conductor breaks and falls to the ground, a 10-metre section of steel reinforced aluminium conductor (type AFL-6 70 mm2) has been used. The obtained values of transition resistance exceeded the value of 20 kΩ. It brings to the conclusion that due to the specific geoelectric features of the ground at the place of the disruption, resistances of several thousand ohms may occur in case of ground faults. In order to determine the voltage and current conditions for operation of ground fault protection in the discussed system, the Electromagnetic Transients Program (EMTP-ATP) was developed – fig. 2. Important relations between the following values from the transition resistance Rd at the site of the earth fault, have been analysed: • zero sequence U0 of phase voltages on busbars of a 15 kV distribution board powering the earthed line (open-delta voltage transformer forming a zero-sequence voltage filter) • zero sequence I0 of currents in the field supplying the earthed line (geometric sum of phase currents measured by the zero sequence current transformer or Holmgreen system, calculated to primary side).

Explaoatation Problems of Medium Voltage Power Distribution Networks in the Aspect of Reliability of Power System Protection

P06

LCC

0.099 km 0.466 km 2.4 km

LCC

3.6 km

7.4 km

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3.1 km

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2.8 km LCC

4.4 km LCC

LCC

5.6 km LCC

2.4 km

LAT

PL6

P06

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BCT

LCC

GROUP wyl

P12

P10

2.953 km 0.674 km LCC LCC

P08

0.561 km 21.43 km 2.731 km 54.76 km LCC LCC LCC LCC

P04

0.381 km 0.265 km 0.79 km 2.364 km LCC LCC LCC

P15

10.49 km 0.552 km 1.077 km LCC LCC LCC

P19

0.811 km 0.21 km 4.341 km LCC LCC LCC

P24

3.316 km 4.944 km 2.703 km LCC

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P18

0.133 km 1.255 km 8.853 km 32.27 km LCC

P16

3.477 km LCC

P14

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LCC

0.17 km

M M M

Fig. 2. A fragment of the analysed model of a 15 kV system for the EMTP-ATP program

PGW

Fig.3 shows the relations U0 = f(Rd ) and I0 = f(Rd ) obtained for three options of the system operation: 1. isolated neutral system 2. resonant earthed system, with reactance of 75 Ω (network compensation with mistuning S = 2%) 3. resistance earthed neutral system, with resistance of 56.8 Ω. b)

voltage U0, V

current l0, A

resistance of earth fault Rd, Ω.

Fig. 3. Relation of voltage U0 of zero sequence (a) and current I0 of zero sequence (b) depending on the earthing resistance Rd: PNI – isolated neutral point, PNR – neutral point earthed by a resistor, UK – compensating system

Wiesław Nowak; Szczepan Moskwa / AGH University of Science and Technology in Kraków Rafał Tarko / AGH University of Science and Technology in Kraków

The conducted analysis of input signals (U0, I0) in earth fault protections showed that when the values of transition resistance at the point of short circuit were over several hundred ohms, the installed EX-BEL digital protection system could not be activated; the system was equipped with a directional earth fault protection module with the following settings: voltage starting value U0 = 15 V; current starting value I0 = 6 A; activation time – 0.5 s. Regardless of the network structure and the operation of the neutral point in case of high-resistance earth faults, as in this test, the correct and reliable operation of the currently used earth-fault protections, and thus detection of an earth fault occurring under such circumstances, is practically impossible.

3. ANALYSIS OF FERRORESONANCE INFLUENCE ON EARTH FAULT PROTECTIONS The subject of the analysis was a circuit with a voltage of 30 kV, with isolated neutral system. A simpliﬁed diagram of the circuit is presented in ﬁg. 4.

Fig. 4. Simpliﬁed diagram of a 30 kV system with isolated neutral point

In a separate part of the system, stations GPZ-Z, EW-T and EW-Z include three 30 kV distribution boards connected with lines T1 and T2 as well as Z1 and Z2. Lines T1 and T2 are overhead lines about 8.9 km long, running as a double line on the common supporting structures. Line Z1 is also an overhead line about 5.5 km long, whereas line Z2 is a cable line about 5.1 km long. The system includes the distance digital protection, series 7SA511 7SA610 Siemens, equipped with a sensitive earth fault protection for the isolated networks. As the protection system records the voltage and current waveforms, it was able to find faults states associated with the switching off of earth faults in the analysed system. Such a case was an earth fault Z2, which was properly eliminated by the protection in the field of line Z2 in the distribution board EW-T. After line Z2 was switched off at the busbars of the 30 kV distribution board in the station GPZ-Z, an increase in phase voltage of about 50 kV was recorded and sustained for a long time; also line T2 was unnecessarily switched off. Voltage waveforms recorded during this fault bring to the conclusion that it was caused by the ferroresonance. Ferroresonance occurs when the ferromagnetic cores of power devices – especially voltage transformers – are working under conditions of saturation and the magnetizing inductance becomes in this case a nonlinear element (fig. 5). On the contrary to linear resonance, where the resonant frequency is precisely determined, ferroresonance may occur for a frequency depending on the system conditions. In practice, ferroresonance can be initiated even by a temporary saturation of the core. This may occur, for example, in case of switching operations or changing the value of supply voltage, e.g. due to an earth fault. Ferroresonance poses a significant threat of overvoltage to insulation systems, causes an electric shock risk in primary windings of earthed voltage transformers and increases the neutral point potential [e.g. 4, 5, and 6]. Increasing the potential of the neutral point also causes the appearance of zero sequence voltage, which can falsify the operation of earth fault protection. Based on the analysis conducted in a real system, the EMTP-ATP program was used for developing a model where conditions for inducing ferroresonance could be analysed and methods of suppressing it could be determined.

current, mA

Explaoatation Problems of Medium Voltage Power Distribution Networks in the Aspect of Reliability of Power System Protection

Fig. 5. Magnetization characteristics of transformers used in a 30 kV system

voltage, kV

The analysis required an accurate mapping of all network elements which might affect the voltage and current waveforms in transient states, and thus were likely to cause abnormal operation of power system protection. Particularly important elements in determining the occurrence of ferroresonance were voltage transformers installed on the busbars of 30 kV distribution board. The results of measurements for nonlinear current and voltage characteristics were used to develop relevant mathematical models. These characteristics were determined by saturation of the ferromagnetic core (ďŹ g. 5). The models of voltage transformers were prepared in the EMTP-ATP program and based both on the above characteristics and the tests on transformers during short-circuit. A sample model of an U30-1MOc-type transformer is shown in ďŹ g. 6 and 7. a)

Fig. 6. A model of U30-1MOc type transformer in the EMTP-ATP program: a) diagram of three-phase group b) transformer dialogue window

Wiesław Nowak; Szczepan Moskwa / AGH University of Science and Technology in Kraków Rafał Tarko / AGH University of Science and Technology in Kraków

KARD 3 3 4 4 5 5 6 6 6 37 38 39 41 42 42 43 44 44 KARG 1 -1 -1 -2 -1 -3 6 7 -1 5 3 -1 -2 2 3 -3 4 5 KBEG 3 9 3 9 3 9 27 33 3 3 3 3 3 21 27 3 21 27 KEND 8 14 8 14 8 14 32 38 8 8 8 8 8 26 32 8 26 32 KTEX 1 1 1 1 1 1 0 0 1 1 1 1 1 1 1 1 1 1 $ERASE /BRANCH C < n1 >< n2 ><ref1><ref2>< R >< L >< C > C < n1 >< n2 ><ref1><ref2>< R >< A >< B ><Leng><><>0 HA____MAG___ 20685.31143. MAG___TM____ 19446. .21362 MAG___TL____ 1.29E568355. C BUS1->BUS2->BUS3->BUS4-><-CURR<-FLUX<.......... 93MAG___ CUR___FLU___ 0.0 0.0 5.00027E-5 5. 1.00087E-4 10. 1.5066E-4 15. 2.0278E-4 20. 2.58485E-4 25. 3.21114E-4 30. 3.95636E-4 35. 4.88975E-4 40. 6.10336E-4 45. 7.71531E-4 50. 9.87304E-4 55. 0.00128 60. 0.00166 65. 0.00216 70. 0.00281 75. 0.00365 80. 0.00471 85. 0.00603 90. 0.00767 95. 0.00969 100. 0.01214 105. 0.01509 110. 0.01863 115. 0.02282 120. 0.02777 125. 0.03356 130. 0.04031 135. 0.04813 140. 9999 LN____ 1.E6 MN____ 1.E6 MAG___ 1.4E7 /SOURCE 14TM____ 1.E-20 50. 18 300.MA____MN____ 14TL____ 1.E-20 50. 18 519.615LA____LN____ $EOF User-supplied header cards follow. 16-Dec-08 ARG,HA____,MA____,MN____,LA____,LN____,CUR___,FLU___ NUM,CUR___,FLU___ DUM,MAG___,TM____,TL____ Fig. 7. The content of ﬁle vt2_mod.lib attached in dialogue window in ﬁg. 6b

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Explaoatation Problems of Medium Voltage Power Distribution Networks in the Aspect of Reliability of Power System Protection

voltage, UN, Kv

voltage, UC, Kv

voltage, UB, Kv voltage, UA, Kv

The analysis conﬁrmed the possibility of inducting ferroresonance as a result of switching off earth faults. Such a case was the earthing of cable line Z2. Fig. 8 shows the exact waveforms of phase voltages UA, UB, UC and neutral point voltage UN in the 30 kV distribution board in the GPZ-Z station, obtained after installing the earth fault for phase A of line Z2 at the moment t = 0.1 s, and then at the moment of swithing off the earthed line t = 0.3 s. This is accompanied by ferroresonance being the cause of oscillations of both the phase voltage and neutral point, which is at the same time the zero frequency U0 of phase voltages. As the function of sensitive earth-fault protection is implemented as a result of their stimulation by the zero sequence voltage. The ferroresonance oscillations occurring in the system are the reason of unnecessary PSP operations, which result in disconnection of unbroken line T2.

time, s

Fig. 8. Waveforms of phase voltages UA, UB, UC and neutral point voltage UN in distribution board of the 30 kV GPZ-Z station

time, s

Fig. 9. Waveforms of phase voltages UA, UB, UC and neutral point voltages UN in distribution board of the 30 kV GPZ-Z during suppression of ferroresonance

The analysis enabled to determine any potential disruptive states (earth fault) and states of normal connections which may lead to the ferroresonance, and therefore to incorrect operation of protection. Also, solutions aimed at suppressing ferroresonance in the analysed 30 kV system which could improve the protection operation were suggested. One of them is connecting an open-delta to the circuit; the delta is formed by additional windings of transformers (fig. 6a) with a suppression resistor. Sample simulation results for a resistor with a resistance of 10 � (connected at the time of 0.6 s, and then disconnected at the time of 0.9 s) confirm the efficiency of this solution (fig. 9). Another analysed solution involves equipment called VTGuard made by ABB, whose resistance actively adjusts to the working conditions [7]. In case of zero sequence with a low value resulting from the acceptable asymmetries during normal operation, the device represents a very high resistance and causes no heat load of the transformers or the device itself. If zero frequency exceeds the device dead zone, VTGuard becomes a resistor with a resistance value that effectively suppresses ferroresonance, a fact which was confirmed by both computer simulations and experiments. If the zero component present in the circuit of open delta lasts for a long time, which may result from, e.g. a large asymmetry in the network, caused by e.g. an earth fault, the device automatically switches into a high-ohm state without posing an unnecessary load to the transformers. When the cause of asymmetry is no longer active, the device automatically returns to initial state.

Wiesław Nowak; Szczepan Moskwa / AGH University of Science and Technology in Kraków Rafał Tarko / AGH University of Science and Technology in Kraków

36 4. SUMMARY

Ensuring the proper quality of electricity supply depends on mastering many technical issues. Earth fault protection in medium voltage distribution networks is part of them. Two cases of incorrect operations of earth-fault protections, documented by the operation services, were presented in the paper. However, the conclusions of the conducted analysis are not only of an individual character, specific to the particular event of disruption, but also of a general character for earth fault protection installed in medium voltage distribution networks. Regardless of structure and tape of power system, in case of high-resistance earth faults such as those presented in the paper, the correct and reliable operation of the currently used earth fault protections, and thus detection of earth fault occurring under such circumstances, is practically impossible. The above conclusion prompts one to look for effective and economically justified technical solutions of this problem. The specificity of medium voltage distribution networks, resulting from their service at non-solidly earthed neutral system, also implies the possibility of ferroresonance occurrence, which can significantly affect the operation of earth-fault protection. Even though the first publications on ferroresonance appeared at the beginning of the twentieth century, effective practical criteria for its occurrence and methods for its prevention have not yet been specified. Research activities in this area may be divided into groups such as the analysis of ferroresonance states (including since the 1990s, with the use of nonlinear dynamics and chaos theory), identification of ferroresonance impact on power systems and prevention against the occurrence of ferroresonance. Important elements of this research are computer models of power systems and dynamic phenomena occurring in them.

BIBLIOGRAPHY 1. Lorenc J., Admitancyjne zabezpieczenia ziemnozwarciowe, Poznań, Komitet Elektrotechniki Polskiej Akademii Nauk, Wydawnictwo Politechniki Poznańskiej 2007. 2. Nowak W., Moskwa S., Tarko R., Analiza warunków działania zabezpieczeń ziemnozwarciowych w przypadku wysokorezystancyjnych zwarć z ziemią. Materials from Scientiﬁc Conference titled High resistance short circuits with ground in MV overhead networks, SEP, Tarnów, 3 December 2008. 3. Nowak W., Tarko R., Moskwa S., Gawryał A., Cich W., Analiza warunków działania zabezpieczeń ziemnozwarciowych w sieci średniego napięcia, Archiwum Energetyki, volume XXXIX, 2009, no. 1, pp. 135–145. 4. Piasecki W., Florkowski M., Fulczyk M., Mahonen P., Luto M., Nowak W., Ferroresonance involving voltage transformers in medium voltage networks, 14th International Symposium on High Voltage Engineering ISH2005, Beijing, China, 2005, paper F-19. 5. Piasecki W., Florkowski M., Fulczyk M., Nowak W., Preventing the risk of ferroresonance involving Voltage Transformers in MV ungrounded networks, 3rd International Symposium on Modern Electric Power (MEPS’06) under auspices of IEEE, Poland, Wrocław, September 6–8, 2006, pp. 398–401. 6. Piasecki W., Florkowski M., Fulczyk M., Mahonen P., Luto M., Nowak W., Mitigating Ferroresonance in Voltage Transformers in Ungrounded MV Networks, IEEE Trans. on Power Delivery, vol. 22, 2007, no. 4, pp. 2362–2369.

Waldemar Skomudek / Opole University of Technology

Authors / Biographies

Waldemar Skomudek Opole / Poland Graduated from the Faculty of Electrical Engineering at Higher Engineering School in Opole. Over 25 years of experience in the energy sector. He received his PhD in 1998, and postdoctoral degree in the field of technical sciences in 2009. In addition, he completed doctoral studies and postgraduate studies „Accounting and Finance” (Opole University of Technology) and „Modern Financial Management” (WIFI Osterreich Institut in Vienna). In the technical field his research work is focused on the issue of assessing the risk and effectiveness of protection against the effects of wave (electromagnetic) phenomena that accompany atmospheric discharges and disruptive states in electricity networks. In the field of management he deals with the functioning of the electricity market in the country, job stability in NPS and the management of entities in the energy sub-sector in a competitive environment. He has published 85 scientific articles and papers, and is a co-author of two books and author of two monographs.

Development of High and Extra-high Voltage Compact Overhead Power Lines

DEVELOPMENT OF HIGH AND EXTRA-HIGH VOLTAGE COMPACT OVERHEAD POWER LINES Waldemar Skomudek / Opole University of Technology

1. INTRODUCTION The results of research, analysis and discussions carried out by the author of this paper in objectifying the threat to insulation systems caused by surges show that these specific electrical loads created during various types of transient states in power systems have a significant impact on making decisions on the method of operation and equipment in medium, high and extra-high voltage power networks [2, 3, 4 and 8]. The importance of surges justifies the need for their testing and analysis, which should be continued and developed with the progress of measuring and calculating capabilities, as well as in the field of new design solutions, in particular those affecting the reliability and quality of electricity supply. However, a possible alternative approach to the principles of insulation coordination and protection against surges in medium, high and extra-high voltage networks presented in publications [1, 8 and 9] indicates directions of modification, based mainly on: • increasing the role of natural reduction of particular surge types • optimization of surge insulation strength against surges (reducing the oversize of insulation systems) • rational approach to the insulation protection factor, including the use of modern equipment for limiting surges • unification of provisions contained in the standards relating to the issues of insulation coordination and protection against surges. An in-depth analysis of the proposed courses of modifications takes into account also the economic aspect of the investment process in the power sector.

2. ASSESSMENT OF INVESTMENT NEEDS The national power sector, as well as the companies belonging to this sector in other European Union countries, is undergoing profound changes. Since the electricity markets, dominated by competition, demopolisation and privatisation, were established in the 1990s, they have undergone substantial changes. The economy of this sector is becoming important. The analysis of macroeconomic statistics characterizing the electricity market in relation to economic growth determined by GDP1 shows that electricity consumption in Poland has been increasing over the last few years; it was temporarily slowed down only by the crisis at the end of 2009. Currently, the national electricity consumption is at the level equal to 2008. However, the observed consumption of electricity in the ﬁrst half of 2010 is over 4.7% higher than at the same period last year. Although the last two years brought a signiﬁcant re-

1 Gross domestic product – the aggregate value of ﬁnal goods and services produced within the particular country in a given time unit (e.g. one year).

Abstract A comprehensive analysis of the principles of insulation coordination indicates the possibility of modifying the existing procedure. Taking action in this direction would lead to measurable economic results, which, in light of current investment needs in power transmission and distribution network infrastructure, the ﬁnancial capabilities

of entities that manage this infrastructure and the existing legal and environmental barriers for their implementation (particularly in the case of line investments) would be a determining factor.

duction in fuel and electricity consumption in the Polish power sector, confirming the risks arising from the financial crisis, the aforementioned increase should be taken as a clear signal of economic recovery in the country. Based on the current and projected trends in GDP growth, the demand for gross electricity 2and simultaneous reduction of energy consumption in the economy, it will reach about 200 TWh in 2030, while in 2010 the national consumption of electricity amounted to almost 155 TWh3. However, given the current and future rates of electricity demand in network capacity analysis, we arrive at alarming conclusions, as the depreciation of both transmission and distribution power networks reaches 40-65% (fig. 1). ��

��

Waldemar Skomudek / Opole University of Technology

��

Fig. 1. Age structure of national overhead power lines

As a result, the rules of a competitive market based on balanced supply and demand for electricity reveals a signiﬁcant gap, also in overhead power lines. It is estimated that to close the gap, an increase in the length of transmission lines by about 2000 km and distribution lines by about 2800 km is needed in between 2011 and 2020. In addition, the age structure of existing overhead lines indicates the great need for modernization, which is estimated for about 9000 km4. The above scale of investment needs is motivation for seeking and implementing solutions that will allow avoiding interruptions in energy supply, and thus will increase energy security. Thus, each investment task will trigger the need for an in-depth economic analysis, which will optimize the spending of funds.

3. LEGAL AND ENVIRONMENTAL RESTRICTIONS Regulation of many legal acts, ranging from energy law, through construction law, public procurement law, spatial planning and development law to environmental protection law, plays an important role in implementation of electric power investments, especially line investments. Environmental restrictions dictated by localization conditions are particularly important. They are based on a complicated procedure of spatial planning, including: • introduction of investment to the local development plans • obtaining the-so called right of way, i.e. the right to use the area for construction of lines or stations, along with granting the decision on immediate enforceability • proceedings associated with environmental impact assessment of the planned project • obtaining social approval for the investment implementation.

2 Electricity obtained from the energy conversion process in the form of electrical current measured at the generator terminals. 3 Data according to McKinsey & Company, Global Insight, ARE. 4 The principle of cautious estimate was applied.

Development of High and Extra-high Voltage Compact Overhead Power Lines

The experience of the national and international power sector shows that most problems are caused by the proceedings related to environmental impact assessment of the project. Many of the problems that need to be solved in the course of investment are social in nature and are caused directly by the fact that the investment is implemented in a specific natural environment. The relationship between investor, local administration and local communities are governed by the law that imposes the obligation to implement the investment in such a way as to ensure public participation in environmental decision-making. In such a case, the concept of environment has both a natural and socio-environmental meaning. The specificity of line investments lies in the fact that the line route usually runs through much diversified areas, including in terms of ownership. Therefore, solutions that maximally limit interference in the environment and private property (e.g. small-sized stations, compact lines built on steel poles with narrow shafts, steel poles or concrete poles) are preferred in line construction. Hence, the selection of the proper route and type of line becomes an issue of crucial importance. The electromagnetic field that accompanies the operation of every power line and station is equally important. This field may be a nuisance to the environment. Particularly its impact on living organisms requires the use of solutions that reduce the negative impact to the lowest level possible when using reasonable technical means and expenditures. The maximum value and distribution of electric and magnetic field in the vicinity of overhead lines are mainly affected by the type and value of rated voltage, the distance of phase conductors from the ground, the gaps between the wires of particular phases, the geometric arrangement of wires and their height. The need to create a protection (technology) belt for the line, the width of which depends on the rated voltage and type of line is also a significant nuisance to the environment5. Therefore, the goal is to ensure that overhead lines are constructed using poles that are hardly noticeable and perfectly integrated into the landscape, and are additionally located in forest areas or woodlands as lines with very tall towers that allow running cables over tree-tops.

4. IMPLEMENTATION OF GROUNDS FOR MODIFICATION OF INSULATION COORDINATION The behaviour of overhead lines during lightning discharges depends on many factors, of which the most important are: • density of lightning discharges • resistance of insulation (insulation system) • resistance of pole earthing • protection by lightning arresters • cable system • height of cable lines. Analysis of principles and rules applied in determining the levels of surge amplitudes and strength of insulation (insulation system) described in the monograph [8] showed that the distance between phase cables and earthed pole structure may be optimized. Risk assessment for breakdown, expressed in threat to insulation, indicates also the possibility of optimizing the levels of protection and insulation. In both cases, optimization may bring technical and economic benefits. To determine a measurable efficiency of implementation of research, analysis and evaluation results in power lines, an attempt was made to assess the benefits that will result in: • minimizing the reserves of impulse insulation strength • coordination of distances between insulation on the pole • reduction of the protection belt width in the line route • natural reduction of lightning surges by reducing resistance and inductance of earthed poles.

5 The minimum distance from the trees (vertically) is 1.5 m + Del; according to PN-EN 50341-1 Del is the minimum distance in the air required to avoid a complete discharge between the phase conductor and earth potential objects during overvoltages with mild or steep front; for the highest voltage of devices Um = 123 kV the distance.

Waldemar Skomudek / Opole University of Technology

4.1. Optimization of impulse insulation strength In order to implement the economic projects of medium, high and extra-high voltage overhead power lines, the reserve of impulse insulation strength should be optimized. In this process, crucial importance is attributed to impulse voltages that have an impact on the selection of insulation distances of both external and internal lines and on the width of protection belt in the line route. Example values of minimum distances in the air for various values of standard withstand lightning impulse voltage (Uw) shown in fig. 2 were determined carefully, taking into consideration the operation experience and economy, and ensure a particular level of insulation. Therefore, assuming in further considerations minimum distances for overhead lines with a rated voltage of 110 kV for the extreme values of voltage Uw (fig. 2 - values on grey background), we may reduce the weight of the supporting structure by shortening the length of cross arms and insulators, which has an additional influence on reducing cable sag or pole height in adjusting the length of the insulator (insulator chain). Correction of the external and internal distances within the lines also has an influence on the width of the protection belt. To estimate the achievable level of economic benefits resulting from the coordination of insulation distances, calculations were made using the following data as basic assumptions: • unit length of the line is 100 km • the share of straight-line (P) and power (M) poles in the line is 70% and 30%, respectively • power poles (M) are indicated as M3 (or ON 150) • shortening of cross arms does not change the cost of other line elements • assumed construction costs as the percentage of the cost of materials (100% of the cost of materials) • the cost of line does not include the expenses on acquiring the rights to land • unit prices of the 110 kV line elements are assumed as of 1 October 2009.

��

Fig. 2. Presentation of the minimum insulation distance in the air for overhead lines with a rated voltage of 110 kV; values on the grey background show the differences in the minimum distances for extreme values of the rated withstand lightning impulse voltage

Development of High and Extra-high Voltage Compact Overhead Power Lines

��

�

��

�

��

�

� ��

��

The obtained calculation results are presented in fig. 3; the final iteration assumes that steel cross arms will be replaced with insulators, e.g. according to the suggestions included in fig. 4

��

Fig. 3. Effect of reduction of the pole cross arms per the level of unit cost of line segment

Analysis of the calculation results showed that reduction of the cross arm length that does not exceed 40 cm may reduce the line construction costs by 0.3-0.5%. However, the replacement of traditional cross arms on overhead line poles with insulator systems leads to further reduction of the line construction costs by 1.3-2.5%. Implementation of the above actions involves the possibility of reducing the surface area of protection belt along the line route by about 6%. It also leads to reducing the influence of the maximum values of electric field by about 19% and magnetic field by about 28% (assuming the maximum acceptable reduction of the length of pole cross arms in terms of impact strength). Therefore, it can be concluded that restriction of insulation distances (while maintaining the required levels of impulse insulation strength) leads to a measurable economic effect, while maximization of this effect takes into account the investment needs for overhead power lines existing in the country. The type of lines used (single-track or multi-track) is also of significance to the level of obtained benefits, as is the selected system for installing cables (steel cross arms with reduced length or insulator systems replacing steel cross arms).

Fig. 4. Selected examples of insulator systems used to hang the cables of high and extra-high voltage overhead lines: a) traditional system with possibility of reducing the length of cross arms, b) triangular system with vertical installation of insulators, c) triangular system with horizontal installation of insulators

Waldemar Skomudek / Opole University of Technology

4.2. Natural reduction of lightning surges One way to protect overhead power lines against surges is to prevent the surges generated in the line from causing damage to the line insulation. The results of surge propagation analysis in overhead lines described in publications [5, 6 and 7] show that modiﬁcations in this area are possible, and one of the directions for their practical use is to increase the role of natural reduction of lightning surges in overhead power lines. This can be achieved by changing the value of resistance and inductance of earthed supporting structures. To demonstrate the correctness of the above action, the phenomenon of direct lightning striking an overhead line pole was analysed. This incident creates the potential difference between the pole tip and the ground surface, which is the earth voltage of the pole, determined by the relation uws = ip × Rs

(1)

where Rs – resistance of the pole earthing. However, this relation does not include the inductive voltage drop. Assuming the likely parameters of lightning current (about 30-50 kA) and the value of the pole earthing resistance of 20 Ω, we can determine the peak voltage drop in the pole. It is high enough to make the difference between earth fault voltage in the pole and earth fault voltage in the return conductor6 usually exceed the value of withstand lightning impulse surge in the insulator (insulation system). This causes an earth fault. Because in this case the pole has a higher potential than the return conductor, a so-called reverse breakdown in the line insulation occurs. Reverse breakdowns may occur in insulation of one phase or simultaneously in the insulation of two or all phases. Because the rate of rise of lightning current is high, the pole inductance can not be ignored. In this case, di

the pole tip voltage is increased by the inductive drop of voltage L p . In such a situation, the pole tip voltage dt can be determined using the formula uws  uR  uL  Rs � ip  L

di p dt

(2)

Using formula (2), it is possible to determine the value of maximum resistance of pole earthing, which causes a reverse breakdown for speciﬁc values of lightning current and its rate of rise. After making simple transformations, the condition takes the following form

Rs 

U ws  L Ip

di p dt

(3)

If the pole tip voltage is replaced with the value of withstand lightning impulse surge of the insulator (insulation system) in relation (3) U, the condition can be written using the formula U w L

di p dt

(4)

Independent calculations for two types of steel poles: lattice towers and solid poles (so-called tubular poles) were made using relations (3) and (4). Three peak values and rates of rise for the lightning discharge current were selected, and unit inductance values for poles in the range of 0.5-1.2 µH/m were assumed for this calculation7; the extreme values relate to solid pole and lattice tower, respectively. The calculations were made assuming that the overhead line is equipped with a lightning arrester that connects in parallel the pole struck by 6 Earth fault voltage of return conductor consists of working voltage and lightning current-induced voltage. The voltage induced by lightning current in working cable in the line is determined by the relation 2.41. 7 Steel line pole inductance Ls = (0,5 ÷ 1,2) ˣ l, w μH [ 2]; it can be also determined using the formula for inductance of long coil (with the number of turns z = 1) L s 

�z 2 �R 2 , where: l - pole length, z - number of turns, R - long coil radius [10]. l

Development of High and Extra-high Voltage Compact Overhead Power Lines

lightning with the neighbouring poles. Under this assumption, which additionally took into account the occurrence of lightning arrester inductance and the poles of adjacent lines, it can be assumed that about 60% of the peak lightning current flows through the pole struck by lightning. Spread of the remaining 40% of the lightning current is symmetrical in both sides of the line. The calculation results are presented in fig. 5. The obtained results lead to the conclusion that the comparison of two values, i.e. the pole earth fault voltage Uws with the withstand lightning impulse surge in the insulator Uw allows determining the acceptable value of pole earthing resistance Rs, at which the expected values of parameters describing the lightning current (ipmax i dipmax / dt do not cause a breakdown (fig. 5a). Thus, at the specified value of pole earthing resistance (e.g. arising from technical capabilities), it is possible to assess whether there is a threat to insulation due to the occurrence of reverse breakdowns. It should also be noted that in the case under consideration the insulators are tensioned in an electric field of the line with the voltage that is the difference between the earth fault voltage in the pole, which consists of voltage in the earth electrode and induction voltage drop in the pole, and the earth fault voltage of the return conductor, consisting of working voltage and lightning current-induced voltage. Therefore, in order to reduce the earth fault voltage in the pole, and thus reduce the influence of field on the insulation, earth electrodes with low impact resistances in supporting structures and/or supporting structures with low inductance (e.g. steel tubular poles) should be used. An example of dependence of the pole earthing voltage on its inductance is shown in fig. 5b. Using appropriately low values of inductance in the line supporting structures, it can be demonstrated that increase of the level of withstand lightning impulse voltage is unnecessary, which leads tooptimization of the margin betweenthe primary insulation level and protection level. Such action reduces the cost of insulation and protection.

Standard withstand lightning impulse voltage, in kV

Earthing resistance, in Ω

Pole earthing voltage, in kV

Pole inductance, in μH

Fig. 5. Illustration of impact of the difference in pole earth voltage and voltage drop in its inductance on the pole earthing resistance (a) and the pole inductance on the earth voltage in the case of a direct lightning striking the pole (b)

Waldemar Skomudek / Opole University of Technology

Therefore, using the supporting structures with a low inductance, we can reduce the likelihood of reverse breakdowns and thus improve the reliability of these elements in the power network.

5. SUMMARY The assessment of the correctness of ranking of the current strength levels in overhead power lines under consideration, the estimation of the breakdown risk in a self-regenerating line insulation and the evaluation of safety rules applicable in these networks showed that there are reasonable grounds for modifying the rules of insulation coordination and surge protection. In particular, the possibility of lowering the level of protection using the current solutions in devices for surge protection was demonstrated. In practice, the choice of insulation and protection levels should also take into account the expected surge waveforms, the ageing of insulation, the impact of environmental factors and mutual location of protected and protecting devices. Although the actual electrical parameters for lightning surges generally differ (are more mild) from the values assumed for standard waveform (1.2/50 µs), the scope of these changes requires an individual assessment of each tested case, taking into account the requirements for electrical strength and breakdown risk. It should be stressed that the deliberations based on the results of research, analysis and evaluations are the reason to take further rational actions towards modifying the insulation coordination rules, leading to the achievement of economic effects. The expected economic effects will be invested in both technical solutions and environmental impact. But the common beneﬁt will result from the economy of implementation of power sector investment plans. The conducted analyses also justify the use of steel solid poles, i.e. the so-called tubular poles as an alternative to steel lattice towers in the network construction. Undoubtedly, the structures of tubular poles taking into account the factors optimizing their construction, replacement of the traditional steel cross arms with the composite insulation systems, and improving the reliability of overhead lines are solutions that are more and more often used by operators of transmission and distribution networks in the country and abroad.

BIBLIOGRAPHY 1. Flisowski Z., Technika wysokich napięć (ed. 5), WNT, Warszawa 2005. 2. Gacek Z., Technika wysokich napięć. Izolacja wysokonapięciowa w elektroenergetyce. Przepięcia i ochrona przed przepięciami (ed. 3), Draft of the Silesian University of Technology, no. 2137, Gliwice 1999. 3. Jakubowski J.L., Podstawy teorii przepięć w układach elektroenergetycznych, PWN, Warszawa 1968. 4. Kosztaluk R., Flisowski Z., Koordynacja izolacji polskich sieci wysokich napięć, Przegląd Elektrotechniczny, no. 2/ 1998, pp. 41–45. 5. Skomudek W., Computer analysis of overvoltage hazard due to lightning discharges in medium voltage overhead lines with covered conductors, Journal of Electrical Engineering, vol. 55, no. 5-6, Slovakia 2004, pp. 161–164. 6. Skomudek W., Assessment of overvoltage hazard for the polymer insulation of medium voltage electricity distribution cables. CIGRE Gen. Session 2008, rep. B1-201. 7. Skomudek W., The Comparative Analysis of Lightning Overvoltages in Distribution Lines on the Ground of Laboratory Tests and Measurements, Journal of Material Science, 3/2009. 8. Skomudek W., Analiza i ocena skutków przepięć w elektroenergetycznych sieciach średniego i wysokiego napięcia, Oﬁcyna Wydawnicza Politechniki Opolskiej, Opole 2008. 9. Skomudek W., Modyﬁkacja zasad koordynacji izolacji w sieciach wysokiego napięcia w aspekcie ekonomicznym, Przegląd Elektrotechniczny, no. 11b/2010. 10. Rawa H., Elektryczność i magnetyzm w technice, PWN, Warszawa 1994.

Rozwój elektroenergetycznych kompaktowych linii napowietrznych wysokich i najwyższych napięć

Zbigniew Szczerba / Gdańsk University of Technology

Authors / Biographies

Zbigniew Szczerba Gdańsk / Poland In 1952, he obtained the Engineer degree. Four years later, he obtained the MsC degree. In 1963, he graduated from PhD studies in the Faculty of Electrical Engineering at Gdańsk University of Technology. Among others, in the Power Engineering Department, he managed his team which designed numerous types of excitation systems and generator voltage regulators of power ranging from a few hundred kW for the shipbuilding industry to 500 MW. In the peak period, generators controlled by these regulators constituted 75% of the power provided by the national power system. In 1972, he was transferred to the Institute of Power Systems Automation in Wrocław where he took the post of Deputy Scientiﬁc Director. In 1977, he obtained the assistant professor degree. He took up the post of manager of the Power Engineering Unit in the faculty of Electrical Engineering at Gdańsk University of Technology. Soon afterwards, he obtained the associate professor degree and was twice chosen to be the dean of this department. In 1987-90, he worked as a visiting professor at the University of Technology in Oran (Algeria). Having returned to Poland, he organised the Power Systems Department in the present Faculty of Electrical and Control Engineering. Since 1991, he has had the full professor degree of Gdańsk University of Technology. In 1990-1996, he held the post of Vice-Rector for Academic Affairs. He is author or co-author of over 50 patents and over 200 scientiﬁc works most of which have been implemented in practice.

Coordination of Regulation Systems for Generators and Transformers in an Industrial Combined Heat and Power Plant

COORDINATION OF REGULATION SYSTEMS FOR GENERATORS AND TRANSFORMERS IN AN INDUSTRIAL COMBINED HEAT AND POWER PLANT Zbigniew Szczerba / Gdańsk University of Technology

1. INTRODUCTION All the national combined heat and power plants work with the national power system; usually they are connected to a 110 kV network. Generator – transformer units in combined heat and power plants are connected to 110 kV busbars, because the power for their own needs is small compared with the power transferred to the system. Power received for technological purposes from industrial heat and power plants is comparable to the generated power. Generators (usually there are several of them) in industrial heat and power plants work in parallel, connected to 6 kV busbars. 6 kV busbars are connected to the 110 kV station with at least two (usually several) 110/6 kV transformers. Sometimes these are three-coil transformers aimed at reducing fault currents on the side of 6 kV. An example diagram of an industrial heat and power plant is shown in ﬁg. 1. 110 kV

RT2

RT1

RT3

6 kV

RG1

RG2

RG3

Fig. 1. Example diagram of industrial heat and power plant

In accordance with the requirements of network operators, generators and transformers are equipped with control systems. The experience gained by the author in several industrial plants brings him to the conclusion that the generator control systems are often incorrectly adjusted, and the algorithms of transformer 110/6 kV controllers are wrong. The article presents a proposal for coordination of algorithms used in control systems.

2. GENERATOR CONTROL SYSTEMS Generally, all generators are equipped with systems for voltage regulation with current and power angle limiters. All control systems have a current compensation system, which is sometimes very simpliﬁed (depending on the controller supplier). Tab. 1 shows example data from the controllers in one of the industrial heat and power plants.

Abstract Generators (usually there are several of them) in industrial heat and power plants work in parallel, connected to 6 kV busbars. 6 kV busbars are connected to the 110 kV station with at least two (usually several) 110/6 kV transformers. Generator control systems are often incorrectly adjusted, and the algorithms of 110/6 kV transformer

controllers are wrong, making cooperation between the control systems in the plant impossible.The article presents a proposal for coordination of algorithms used in control systems.

Zbigniew Szczerba / Gdańsk University of Technology

Tab. 1. Generator control systems No.

Symbol

Sn [MVA]

Pn [MW]

Type

LPS

LPW

LKM

PSS

TG1

38.8

Digital

YES

Rk+jXk

YES

TG2

56.7

Digital

YES

TG3

40.0

Digital

YES

Sn, Pn – rated apparent and active power RN – generator controller LPS, LPW, LKM – limiter of stator and rotor current and power angle PSS – system stabilizer Zk – current compensation impedance Zk = Rk + jXk A current compensation system makes the value of obtained voltage conditional on the load value (mainly reactive power) and allows a stable distribution of reactive power between cooperating generators, even in the case of cooperation of directly connected (without reactors) sections of busbars. As shown in tab. 1, TG1 has a full current compensation system, whereas TG2 and TG4 are only dependent on reactive power (part Xk). TG1 has all limiters. TG2 and TG4 have no stator current limiters. The present settings of power angle limiters (reactive power consumption): Q ≥ 0 of all generators are incorrect because they do not use the possibility of reactive power consumption by generators. Current compensation settings are mutually uncoordinated, which is caused by the fact that generators are started by various teams at several-year intervals. A similar condition was found in many industrial heat and power plants.

Current use of control systems When adjusting the reference voltage of generators, the control room staff can control the proper distribution of reactive power, approximately proportional to the rated apparent power. With the adjusting cooperation of control room staff, the control systems are responsible for the following tasks: • maintaining the traditionally set reference voltage on 6 kV busbars • providing an approximate correct distribution of reactive power between cooperating generators. For the proper performance of the second task without constant intervention by the control room staff, it is necessary to set an equal – in relative units – current compensation of controllers in all generators. Correct properties Q-U of generator control systems, ensuring a proportionate or reference distribution of reactive power between generators In the task of controlling voltage, the modern generator controllers have the following properties U = f(Q): • Without current compensation Ug ≈ Ug with a very slight gradient resulting from finite value of amplification in the open loop of voltage control circuit. • With current compensation introducing a virtual replacement impedance between1 generator and controller. If generators do not run connected in parallel to the same busbar system, or their busbars are always separated by a reactor, they can operate with current compensation set to 0 (zero). If generators run connected in parallel to the same busbar system, or their busbars are not always separated by a reactor, two solutions may be used: 1. Assuming that the accuracy of digital controllers is very high and the aforementioned gradient without current compensation is sufficient, the current compensation should be set to 0 (zero). Also, the distribution of reactive power between generators without participation of control room staff should be observed. If the distribution of reactive power is approximately proportional to the apparent power 1 Some controller suppliers offer only simplified current compensation, which provides only virtual reactance.

Coordination of Regulation Systems for Generators and Transformers in an Industrial Combined Heat and Power Plant

in generators during changes in load, the ﬁrst solution should be used. Otherwise the second solution solution should be selected. 2. Current compensation should be set to e.g. Xk = 0.02 (2%), then the distribution of reactive power between generators without participation of control room staff should be observed. If the distribution of reactive power is approximately proportional to the apparent power in generators during changes in load, the above solution should be used. Despite the high accuracy of digital controllers, the unsystematic deviations in voltage transformers may require the use of the above solution. Due to the lack of or inaccurate information on the value of real strengthening in the open loop of a control system, an experimental procedure according to point 1 or 2 is necessary. In the unlikely event that at the value of Xk = 0.02 pu the distribution of reactive power is not approximately proportional to the apparent power in generators, the value Xk should be increased to Xk = 0.03 pu. The method for setting generator controllers described above should ensure that the voltage in busbars is maintained with high accuracy and without interference of transformer operators and controllers.

3. TRANSFORMER CONTROL SYSTEMS Current state Usually, all transformers are equipped with voltage controllers that operate tap switches. The used controllers come from different manufacturers, but in principle, according to the documentation of one of the producers, they are “designed to automatically control the voltage of lower side or the number of transformer (transmission) tap”. Such controllers are designed to be used in 110/15 kV transformers located in stations supplying the 15 kV distribution network, the so-called switching stations. Controllers have no components that ensure mutual coordination during parallel operation of transformers. Algorithms for operation of such controllers prevent cooperation with the generator control systems on the side of 6 kV. In industrial heat and power plants such transformer controllers are always off. The off state is correct because – probably because of poor project documentation – transformer voltage controllers with incorrect operation algorithms have been used. The algorithms used in controllers are for “voltage regulation” appropriate for e.g. 110/15 kV transformers powering medium-voltage distribution networks. The designer probably designed a “transformer with controller” without considering the speciﬁcity of three transformers connecting the 6 kV network in heat and power plant with the 110 kV network. Presently the tap switches of transformers in industrial heat and power plants are controlled manually by control room staff, which controls the appropriate exchange of reactive power with the 110 kV network and proper distribution of reactive power between cooperating transformers. Correct properties Q-U of 110/6 kV transformer control systems, ensuring a proportionate or reference distribution of reactive power between transformers and proper cooperation with generator control systems Assumptions Transformer regulators should control the tap switches in all 110/6 kV transformers without constant interference by the control room staff. The operation algorithm should ensure: • Proper cooperation between transformers and reactive power distribution in approximation proportional to the rated power in particular transformer coils • Maintaining the exchange of reactive power with the 110 kV network of an average value minimizing the tariff charges for readings of kvarh meters • That the acceptable tap switching frequency is not exceeded. Under normal conditions, the task of maintaining the set voltage on 6 kV busbars is performed by generators control systems.

Zbigniew Szczerba / Gdańsk University of Technology

Characteristics of transformer controllers 110/6 kV transformer controllers should have characteristics that ensure: • Maintaining the reference value of reactive power in range Qtrn,QtrM if the voltage on the 6 kv side is within Utrn,UtrM • Taking over the task of maintaining voltage on the 6 kV side after exhausting the generator capability • The assumed distribution of reactive power, proportional to rated power of respective transformer windings. Controllers offered by the better suppliers have components ensuring such cooperation. The characteristics proposed for transformer controllers are shown in fig. 2. Dead bands and time delays Dead bands, independent of variables U and Q, should ensure stable operation of the control system at the borders of the “O” area in fig.2. Time delays • For variable Q, they should ensure that the allowed frequency of tap switching is not not exceeded • For variable U, they should ensure that tap switches are controlled as quickly as possible. The need for action in order to maintain a safe voltage for the plant is rare and fast switching will not cause a significant increase in the number of switches. The priority ensuring a proper quality of electricity that determines the continuity of plant operation is obvious. Utr

+ +

UtrzM

Utrzm

Qtr Qmtr

QMtr

Fig. 2. Characteristics of transformer with controller

Qtr – transformer reactive power received from the 110 kV network Utr – transformer voltage Utrzm, UtrzM – lower and upper reference value of transformer voltage QMtr – reduction of reactive power consumption from the network Qmtr – reduction of reactive power supply to the network – the area where transformer controllers are not running + – increasing voltage on the 6 kV side - – reducing voltage on the 6 kV side 0 – absence of transmission switching.

4. GUIDELINES FOR CHANGES IN SETTINGS AND IN ALGORITHMS FOR GENERATOR CONTROL SYSTEMS IN HEAT AND POWER PLANTS AND 110/6 KV TRANSFORMERS Assumptions Synchronous generator and 110/6 kV transformer control systems in heat and power plants should cooperate and ensure: • Maintaining voltages that guarantee the proper operation of all receivers connected with the technological process in the plant

Coordination of Regulation Systems for Generators and Transformers in an Industrial Combined Heat and Power Plant

• Reducing consumption of reactive power from the 110 kV network for the purpose of minimizing the costs of purchasing electricity from the 110 kV network • The possibility of using the entire area of acceptable generator states, taking into account all limitations (stator current, rotor current, power angle, temperature of extreme stator packages) • Proper (approximately proportional) distribution of reactive power between cooperating generators • Proper (approximately proportional) distribution of reactive power between cooperating transformers • That the acceptable transformer switching frequency is not exceeded.

Characteristics of generators The following are important elements in modern generator controllers:

Ig = IP + jIQ

Zk = Rk + jXk

Ugz0

Ug Fig. 3. Equivalent diagram of generator with controller with Zk

• Current compensation system providing equivalent diagram in the area of steady states is shown in ﬁg.3. Ugz0 – reference voltage in generator at idle run Ug – generator voltage Zk = Rk + jXk – current compensation impedance. Current compensation impedance Zk = Rk + jX, with adjusted value in four quadrants of complex plane allows the introduction of virtual voltage measurement and almost any shape of equivalent circuit (steady state) in the controller in the diagram of a power subsystem of heat and power plant. • Stator current limiter ensuring that the acceptable value of stator current is not exceeded • Rotor current limiter ensuring that the acceptable value of rotor current is not exceeded • Power angle (reactive power consumption) limiter that ensures stable cooperation with the system and, sometimes, prevents the extreme parts of stator core from heating.

Ug δMax

Ugz0 IMax

Fig. 4. Characteristics of generator with controller

Qg – generator reactive power Ug – generator voltage Ugz0 – reference voltage in generator at idle run IMax – stator or rotator current limitation δMax – power angle limitation

Generator characteristics – on the plane Q, U – with control system containing the aforementioned elements show the relation U = f(Q) with limitations controlled by limiters, ﬁg. 4.

Zbigniew Szczerba / Gdańsk University of Technology

Combination of generator and transformer characteristics Combination of the characteristics of generator and transformer control systems, which implement the algorithms described above, gives the resultant characteristics, shown in ﬁg. 5 ��

δMax

Ugz0

UtrzM

�� reg. tr. ��

Utrzm

��

IMax

reg. tr.

0 �� Range Q g

Qg, Qtr, QΣ

Fig. 5. Combined characteristics of generators and transformers

Qg, Qtr,, QΣ– total reactive power of generators and transformers Ug – voltage in 6 kV busbars Ugz0 – reference voltage at idle run IMax – stator or rotator current limitation δMax – power angle limitation UtrzM – reference value of the highest voltage in transformers on the 6 kV side Utrzm – reference value of the lowest voltage in transformers on the 6 kV side – resultant characteristics of generators and transformers – fuzzy part of the resultant characteristics – the area where transformer controllers are not running

5. DESCRIPTION OF COOPERATION BETWEEN CONTROL SYSTEMS OF GENERATORS AND TRANSFORMERS Normal state In normal states, the generator and transformer controllers run autonomously. Generator regulators with the reference voltage occasionally adjusted by the control room staff ensure the correct distribution of reactive power between cooperating generators. Generator controllers maintain the reference voltage in 6 kV busbars (fig. 4 and 5) in the full range of reactive power generation or consumption from limitation of power angles to current reduction (within the acceptable states). Transformers controllers maintain reactive power consumption (fig. 2 and 5) within the limits ensuring avoidance of charges for readings of kvarh meters (the so-called reactive power). Abnormal state After reaching the boundaries of acceptable states, the generator controllers are not able to maintain the reference voltage on 6 kV busbars (fig. 3 and 5) as a result of working power angle limiters or current limiters. After exhausting the ability to maintain reference voltage by generators, the task of maintaining reference voltage is assumed by transformer controllers (fig. 2 and 5). Emergency During emergencies such as short circuits in 110 kV and 6 kV networks, short-term voltage dips, etc., the transformer control systems should not be running. Generator control systems must react quickly, according to the current characteristics, in order to maintain the stability of cooperation between the heat and power plant and the power system, as well as to ensure proper operation of power protection systems.

Coordination of Regulation Systems for Generators and Transformers in an Industrial Combined Heat and Power Plant

6. PROPOSED CONCEPT OF PRIMARY SYSTEM COORDINATING THE COOPERATION OF ALL GENERATOR CONTROLLERS IN HEAT AND POWER PLANTS AND 110/6 KV TRANSFORMERS Local demand for reactive power is met from the following sources: • Generators of heat and power plants • The 110 kV network (by 110/15 kV transformers) • Capacitor banks in the 6 kV network. The selection of characteristics and settings in control systems of generators and 110/6 kV transformers described above provides the correct voltage on 6 kV busbars and correct value of reactive power received from the 110 kV network. Capacitor banks are controlled manually from the control room. It is possible to develop a control algorithm for the primary system which includes all the mentioned sources; however, this algorithm should be based on technical and economic analysis: • Optimal voltage on 6 kV busbars, taking into account the voltage drops 6 kV busbars – receivers relation • Optimal use of capacitor banks • Optimal use of exchange of reactive power with the 110 kV network, not only for the purpose of avoiding charges • A proper selection of settings in 6/0.4 kV transmissions in transformers. The algorithm mentioned above can be included in the manual for the control room (to be implemented by the staff on duty), or implanted in the primary control circuit. Such systems are used in the country in production nodes, the so-called GCPN (Group Control of Production Node). Domestic suppliers offer such systems, which can execute algorithms required by the ordering parties.

7. SUMMARY AND CONCLUSIONS Generator controllers In general, the present generator controllers described above do not require replacement. Using appropriate settings, these controllers can implement the algorithms described above and properly cooperate with new transformer regulators. Transformer controllers The currently used controllers are designed to “automatically control the voltage of lower side or the number of transformer tap”. Typical controllers have no components that ensure mutual coordination during parallel operation of transformers. Algorithms for operation of these controllers prevent cooperation with the generator control systems. These controllers should be replaced with the controllers executing the algorithm described in the article. Transformer controllers are currently offered on the domestic market, and are used only for the purpose of voltage regulation. No controller executes the proposed algorithm. Domestic suppliers should offer controllers that execute the proposed algorithm for control of reactive power – in the speciﬁed range of 6 kV voltage value – and control of voltage at the borders of this range. Primary control system The decision to use a primary GCPN (Group Control of Production Node) system in a particular heat and power plant should be made after the preparation of technical and economic analysis that takes into account the beneﬁts of optimizing the voltage levels and reactive power management in the entire facility, including the plant. Domestic suppliers offer such systems, which can execute algorithms required by the ordering parties. BIBLIOGRAPHY 1. Work of the Department of Electrical Power Engineering at Gdańsk University of Technology carried out within the research project “Power System Security”, 2009. 2. Hellmann W., Szczerba Z., Regulacja Częstotliwości i Napięcia w Systemie Elektroenergetycznym, WNT 1978. 3. Machowski J. et al., Power System Dynamics – Stability and Control. J. Wiley, 2008. 4. Szczerba Z. et al., Poradnik Inżyniera Elektryka, rozdział 7, Systemy Elektroenergetyczne, WNT 2005.

Artur Wilczyński / Wrocław University of Technology

Authors / Biographies

Artur Wilczyński Wrocław / Poland Graduated from the Faculty of Electrical Engineering at Wrocław University of Technology (1971). Received his doctoral degree at the Institute of Electrica Power Engineering of Wrocław University of Technology (1977), and received a post-doctoral degree in economic sciences at the Faculty of Management and Computer Science of Wrocław University of Economics (1991). In 1998-1999 he worked on behalf of the head of the Power Research Group at the Institute of Electrical Power Engineering of Wrocław University of Technology and became the head of this Department in 2005. At the same time, he was employed as full-time professor at the Institute of Power System Automation in Wrocław in 1993-2001, where he served as the head of Department of Economics, Pricing and Forecasting in Power Engineering. In 2007 he became Professor of Engineering. Since 2011, he has had the full professor degree of Wrocław University of Technology. He has taken part in many research projects, including ones ﬁnanced by State Committee for Scientiﬁc Research, usually as project manager. He is the author or co-author of over 140 publications and 80 research reports.

The Inﬂuence of Economic, Organizational and Legal Factors on Energy Security in The Country

THE INFLUENCE OF ECONOMIC, ORGANIZATIONAL AND LEGAL FACTORS ON ENERGY SECURITY IN THE COUNTRY Artur Wilczyński / Wrocław University of Technology

1. THE IMPACT OF ECONOMIC MEASURE AND MARKET MECHANISMS ON SECURITY Economic and market factors affect many functional areas of management within the power sector, and are also important for the security of electricity supply. Some of these factors were the subject of analysis carried out under Research Project No. PBZ-MEiN-1/2.2006, titled “National Energy Security.” Important results of these analyses and indications of certain abnormalities are presented in this article, and a more detailed presentation of them is included in the study [18].

1.1. Electricity tariffs Tariffs belong to economic factors determining the status of energy security, defined as market and regulatory actions that form the scope and level of competition in the power sector. Tariff solutions promote the achievement of desired economic effects and improved of efficiency of energy use through market mechanisms. Electricity and transmission tariffs can effectively influence the level of energy security through the proper implementation of the functions assigned to them: • revenue function – related to ensuring adequate revenue from the sale of electricity, covering the costs of their own activities and gaining accumulative surplus, which enables the financing of necessary investments within in the power sector • information function – based on informing the electricity consumers about the possibilities of substituting this energy and changes in supply costs over time; with reliable provision of this information, the tariffs will appropriately stimulate the behaviour of consumers (in terms of shaping the energy input), which in turn should affect the improvement of energy security; the information function is often referred to as stimulation function. The conducted research and analysis shows that tariffs can be an effective tool for shaping customer behaviour. Assuming even a relatively small price elasticity of demand for electric energy and power, obtained through an appropriate internal structure of the tariff, a significant impact on the course of electrical load can be obtained, thus lowering the power consumption during peak period. Current tariffs provide poor motivation for consumers to rationally shape the load curve, reduce peak power consumptions, and do not encourage properly managing reactive power. This condition is affected by many factors, related both to development of energy tariff, as well as its transmission.

Abstract Discussions on the issue of energy security in the country should take into account all the basic functional areas within the sector, including the acquisition of energy resources, electricity generation, its transmission, distribution and use. They are a part of the technological chain of supply of electricity to end users. Important factors affecting the security of the power sector operation include: economic, organizational and legal factors. They were subject to analysis carried out under Research

Project No. PBZ-MEiN-1/2.2006, titled “National Energy Security”, implemented by the consortium of Gdańsk, Silesia, Warsaw and Wrocław Universities of Technology, in particular concerning task 7 – titled “The impact of economic, organizational and legal factors on energy security.” This article is a brief synthesis of the results of the above analysis conducted by various research centers in the country.

Artur Wilczyński / Wrocław University of Technology

Electricity tariffs Limited effectiveness of the tariff system is affected by [14]: • low innovation level of used electricity tariffs • defective internal structure of electricity tariffs, reflected by, for example, low range of fees for electricity, often with a tendency to minimize the differences at the levels of fees • frequent changes in the levels of fees for electricity, in particular, successive increases and reductions of fees, which are usually not a result of an intention to develop adequate electricity consumption • the fact that electricity consumers are not kept informed by suppliers on the available tariff options and benefits resulting from adjusting the demand for electricity to tariff signals. The analysis also covered reactive energy settlement [1, 15]. It is emphasized that the use of progressive fees for reactive power consumption is quite an effective tool for stimulating the economy with reactive power. Supplementing this method of settlement uses the approach that takes into account the turbulent energy consumption (already being tested in our country) and the consumption accompanied by distorted waveforms can result in a stronger influence on consumers, in order to eliminate the negative effects of power system operation. Transmission tariffs Apart from covering the justified costs of activity of a network company, the task of a transmission tariff is to provide an effective stimulation of the power network users, adequate to the specific nature and conditions of its operation, which include the levels of network losses, transmission limitations and security of the system. The transmission tariffs used in Poland are far from perfect. The structure of transmission tariffs should be changed so that they give reliable information on the costs of supplying electricity. The following changes are proposed [13, 16]: • separation of the transmission fee part that is directly related to the connection of a user (consumer and producer) to the power network • participation of producers in the transmission fee • it is suggested to depart from the group tariff and use a nodal tariff (and possibly a new option, i.e. a layer tariff) instead, which allows a proper representation of the network operation status • it is advised to transfer the costs of the quality fee to the balancing market area, i.e. the location where they actually arise • it is recommended to develop the concept of “Smart metering”. 1.2. The issue of network limitations The number of limitations in the national transmission network is significant, resulting from a poorly developed transmit network. One way that can be used to eliminate the limitations in a national transmission network is the use of settlement for the supply of electricity [17]. Solutions to eliminate the limitations include the following: • use of transmission tariffs that encourage appropriate behaviour of the network users • use of power distribution methods (e.g. load flow tracing method and incremental method) when calculating the transmission tariff • it is suggested to apply a coordinated redistribution method, which allows the maximum use of available resources (production sources, market participants) in order to minimize the costs of eliminating transmission limitations in inter-system connections • avoid long-term reservation of capacity, often leading to the elimination of other market participants. 1.3. Market solutions Market solutions were evaluated from the perspective of ensuring economic efficiency and requirements for energy security [5]. The analysis covered the so-called market dimension of energy security, understood as the entire set of legal, technical and economic activities, which should create the conditions for short-, medium– and long-term security. An extremely important aspect of these conditions is to ensure competitiveness, which means creating equal opportunities for all energy market participants. This involves ensuring the transparency of prices and costs, and adequate legal and economic systems. It is very important to monitor the market situ-

The Inﬂuence of Economic, Organizational and Legal Factors on Energy Security in The Country

ation, assessing the energy market operation. Studies [11, 12] pointed out the shortcomings found in current market solutions, indicated the reasons for lack of economic efficiency and inability to obtain efficient conditions for energy security without making extensive changes in the current market and regulatory mechanisms. From the point of view of energy security, a particularly important link is the balancing market. It was found that a comprehensive solution for a theoretically complete electricity market model, which would incorporate all the essential requirements, (economic efficiency, security, environmental issues, system limitations, real-time requirements, etc. ) has not yet been developed in the world. However, it was stressed that in the coming years it will be possible to develop such comprehensive solutions for an effective balancing mechanism, which will take into account the security requirements and diversified nature of the electricity market. It is recommended to work on an overall project for an electricity efficiency and diversified electricity market model. In particular, a prospective concept for the mechanism of Balancing Market should be developed. Establishing an efficient energy market requires clear provisions of the energy law, which would allow the evolutionary introduction of efficient market mechanisms, and also improve the security conditions. It is advised to continue research in the fields of: • changing the model and architecture of the electricity market – transition from energy market model to energy market model with production/transmission capacity, which would effectively stimulate investments in the sector • impact of information and communications technology – integrating the existing solutions with the latest trends.

2. IMPACT OF ORGANIZATIONAL RESOURCES ON SECURITY Power sector organization A power system is a set of closely related technical facilities, organizational systems and human teams, operating in developing market mechanisms. In order to ensure the safe operation of this system, cooperation between individual entities is essential, which is largely the result of the organizational factor. The influence of various organizational resources on energy security was analysed, including [7]: • impact of the current state of organizational resources • division of powers and responsibilities of various entities • impact of current solutions of the electricity market organization • relationships between entities operating in the energy market • impact of energy supply contracts and associated services • impact of training that simulate various types of failures. The analysis provides conclusions on the organizational status of individual energy market participants, the division of duties and competences as well as actions that can strengthen the energy security. Transformations in the power sector The organization of the electricity sector has been changing for many years. The aim of the reforms is to improve the efficiency of the domestic energy sector. However, as emphasized by the authors [6], the successive stages of transformation have not had a good influence on improving the efficiency and, indirectly, on the energy security in our country. The process of consolidation of the electricity sector did not include the development of mechanisms for limiting the risks that arise from establishing capital groups. The situation was influenced by the lack of appropriate provisions to support energy companies in carrying out investments in power infrastructure, and provisions giving the possibility for effective regulation of energy markets by the President of Energy Regulatory Office. The Minister of Economy is expected to come to an agreement with relevant administrative authorities and the authorities of entities governing the electricity infrastructure in terms of developing the rules for response in situations of a threat to power system security. This should also concern the principles of monitoring and coordination of actions in crisis situations. It is also important to ensure the actual independence of system operators who work within the structures of these groups.

Artur Wilczyński / Wrocław University of Technology

Study [10] outlines three key directions of such changes: • initiation of regulation transfer from the end (consumer) to the beginning (investor), including a quick strengthening of locational signals, e.g. in the form of marginal costs; it is a way of establishing a segment of independent investors, who are open to competition • introduction of reference costs, taking into account the external environmental costs (in the production of electricity and heat), as well as potential stranded network costs and excessive employment; it is a way to at least partially block the cross-subsidies between energy/power technologies in consolidated groups and to avoid new stranded costs • modernizing regulation in the area of work of distribution operators (in the area of intensive development of renewable energy and creating new system services) and separating distribution operators from consolidated energy groups; it is a way to create a segment of a distributed and innovative renewable energy sector and to join the EU energy strategy.

Training of personnel Introduction of new technologies in the energy sector, including those using nuclear energy, requires the training of personnel, both at higher and medium level [10]. There is a need to conduct training on simulators, which affect: • the ability to make optimal decisions regarding personal responsibility for energy security • the ability to act actively and quickly in situations of disturbances in the power system operation. 3. IMPACT OF LEGISLATIVE RESOURCES ON SECURITY The analysis conducted as part of the research project indicated that without numerous and significant changes at various levels of the existing regulatory laws, it is impossible to carry out a full and effective implementation of measures to ensure energy security by the government and local authorities as well as energy companies engaged in licensed business activity. These changes relate to three areas of regulations [2, 9]. The first relates to the proposal for legislative changes in the higher order acts, especially in the Energy Law and its executive acts. The second area includes proposals for new procedures and regulations for operators of transmission and distribution systems. Proposed changes relate to: Instructions for Operation and Maintenance of Transmission Network (IRiESP) for the transmission system operator and Instructions for Operation and Maintenance of the Distribution Network (IRiESD) for the distribution system operators, as well as the regulations and procedures of system operators, relating to operation and maintenance of electrical equipment. The third area includes proposals for cooperation between TSO, DSO and the owners of distributed sources. In this area, the suggested solutions relate to issues such as: requirements for automation and mechanical systems, security for active distributed generation sources; cooperation of security systems for distributed generation sources and power networks; behaviour of distributed generation facilities in various conditions, and monitoring of distributed generation facilities. Ensuring energy security in the country requires cooperation and division of responsibilities between public authorities, energy companies and system users in terms of fuel and energy supply. Increased coherence and effectiveness of legal solutions in the area of security requires solutions based on development of adequate responsibilities of ministries. It should include the responsibilities of many ministries. It is necessary to create coherent and high-quality legal regulations regarding energy security based on two pillars: multi-scale electrical power and TSO, and distributed energy and DSO. Many proposals for changes to the responsibilities of various entities involved in emergency state and restoration of EPS [3, 4]. Most of the changes proposed in the Energy Law were included in the last amendment to the Act of 11 March 2010. The division of responsibilities was introduced together with the imposed responsibility for ensuring security of electricity supply to all major users of power system and public authorities. OSP was identified as the most important entity in the area of security of electricity supply. In addition, the amendment allows the system operators to take effective and efficient actions in the event of power shortage in the system. Although these changes in the Energy Law have significantly improved the legal solutions in the area of energy security, they have not introduced all the necessary changes.

The Inﬂuence of Economic, Organizational and Legal Factors on Energy Security in The Country

It is worth mentioning the presence of various risks associated with energy and climate policy of the European Union, and so [8]: • implementation of the energy and climate package in Poland according to the EC proposal of 23 January 2008 will result in many negative effects on the energy system, national economy and living conditions in households • effects of reducing CO2 emissions as a result of implementation of the new EU policy are very costly, to an degree signiﬁcantly higher than emission allowance prices after 2013.

4. SUMMARY The authors of the studies prepared under the research project PBZ-MEiN-1/2.2006, titled “National Energy Security” pointed out various threats to the security of electricity supply, as well as recommendations for their elimination. Some of these recommendations, e.g. regarding the proposal for effective development of tariffs, may be implemented immediately or after an amendment of relevant legislative acts. Another group consists of recommendations for organizational activities that regulate the relations between various entities in the energy market, procedures in case of system failures risks, associated with restoration after such failures, the concept of changes in market solutions, which require changes in legislative provisions in various acts – laws or lower order acts. Another group of measures concerns the elimination of risks arising from legislative acts of the European Union – they require the involvement of the highest ranking politicians, representatives of government administration and Members of the European Parliament. Conclusions and recommendations formulated by the authors of the studies are sometimes of a general character, but mostly include very detailed indications. They can certainly be considered in order to take actions to improve the security of electricity supply in the country; the threats outlined in the studies are quite numerous. The situation in the environment of the Polish power sector is undergoing permanent change, and this applies to both the macroenvironment and microenvironment. These changes often result in increased risk to the energy security in the country, and therefore, the studies conducted under the research project should not be treated as a completed process. They should be conducted in a continuous manner, and the conclusions and recommendations included in them should be used in practice.

Artur Wilczyński / Wrocław University of Technology

BIBLIOGRAPHY 1. Bućko P., Badanie struktury taryfy za energię z uwzględnieniem mocy biernej pod kątem stymulowania zachowania użytkowników energii elektrycznej, Gdańsk University of Technology, Gdańsk 2007, section 7.1.3D. 2. Dołęga W., Analiza propozycji legislacyjnych o różnych poziomach wymuszających odpowiedzialność za bezpieczeństwo elektroenergetyczne. Raport końcowy w zakresie propozycji legislacyjnych, Wrocław University of Technology, Wrocław 2010. 3. Grządzielski I., Kielak R., Długiewicz J., Eliasz S., Sroka K., Kuczyński R., Propozycje rozwiązań organizacyjnych zapewniających prawidłowy przebieg restytucji po awarii systemowej, Poznań University of Technology, Poznań 2009, section 7.2.4. 4. Grządzielski I., Andruszkiewicz M., Sroka K., Propozycje legislacyjne o różnych poziomach wymuszające odpowiedzialność za restytucję po totalnej awarii systemowej, Poznań University of Technology, Poznań 2009, section 7.3.4. 5. Kamrat W., Buriak J., Analiza obecnego stanu odpowiedzialności ekonomicznej za bezpieczeństwo, Gdańsk University of Technology, Gdańsk 2007, section 7.1.1.C. 6. Kądzielawa A., Kłos A., Propozycje rozwiązań organizacyjnych ustalających usprawnienia i wymuszających odpowiedzialność za bezpieczeństwo pracy SEE. Propozycje rozwiązań organizacyjnych w zakresie relacji międzyoperatorskich. Wnioski i propozycje działań w zakresie rozwiązań organizacyjnych dla zapewnienia bezpieczeństwa elektroenergetycznego, Warsaw University of Technology, Warsaw 2009, section 7.2.3.ABE. 7. Kłos A., Nowakowska E., Pawełkowicz Z., Analiza obecnego stanu wpływu środków organizacyjnych na bezpieczeństwo elektroenergetyczne, Warszaw University of Technology, Warsaw 2008, section 7.2.1. 8. Malko J., Identyﬁkacja czynników ryzyka dotrzymania bezpieczeństwa elektroenergetycznego wynikających z dyrektyw polityki energetycznej UE: analiza ryzyka przyjętej polityki regulacyjnej i ﬁskalnej (akcyzowej) w obszarze elektroenergetyki, analiza ekonomicznych skutków i ich ryzyk dostosowania krajowej generacji do wymogów ograniczeń emisji związków SO2, NOx, CO2, Wrocław University of Technology, Wrocław 2008, section 7.3.2.C. 9. Pawlęga A., Opracowanie zasad współpracy, obowiązków i uprawnień OSP, OSD i właściciela źródła rozproszonego, Warsaw University of Technology, Warsaw 2009, section 7.2.3.C. 10. Popczyk J., Bartodziej G., Tomaszewski M., Dzierżanowski Ł., Analiza możliwości zastosowania środków organizacyjnych do zapewnienia odpowiedniego bezpieczeństwa, Silesian University of Technology, Gliwice 2008, section 7.2.2. 11. Toczyłowski E., Kaleta M., Rogulski M., Żółtowska I., Kacprzak P. , Pałka P. , Smolira K., Propozycje nowych mechanizmów rynkowych wymuszających poprawę bezpieczeństwa – z uwzględnieniem rynku bilansującego. Propozycje wykorzystania mechanizmów obrotu wielotowarowego pod kątem poprawy bezpieczeństwa, Warsaw University of Technology, Warsaw 2008, section 7.1.5. 12. Toczyłowski E., Kaleta M., Pałka P., Smolira K., Żółtowska I., Kacprzak P. , Rynek Bilansujący: propozycje rozwiązań zasad funkcjonowania oraz propozycje rozwiązań legislacyjnych w tym zakresie, Warsaw University of Technology, Warsaw 2009, section 7.2.3.D. 13. Wilczyński A., Tymorek A., Analiza możliwości ujęcia taryfowego odpowiedzialności za bezpieczeństwo w różnych horyzontach czasowych. Wrocław University of Technology, Wrocław 2008, section 7.1.3.ABEF – w zakresie taryf za przesył i dystrybucję. 14. Wilczyński A., Ryś-Przeszlakiewicz M., Analiza możliwości ujęcia taryfowego odpowiedzialności za bezpieczeństwo w różnych horyzontach czasowych. Propozycje rozwiązań taryfowych wymuszających odpowiedzialność za bezpieczeństwo, Wrocław University of Technology, Wrocław 2008, sections 7.1.3.CDEF and 7.1.4.BD – w zakresie rozliczeń za energię czynną. 15. Wilczyński A., Borecki J., Badanie struktury taryfy za energię z uwzględnieniem mocy biernej pod kątem stymulowania zachowania użytkowników energii elektrycznej, Wrocław University of Technology, Wrocław 2008, section 7.1.3.DE – w zakresie rozliczeń za energię bierną. 16. Wilczyński A., Namysłowska-Wilczyńska B., Tymorek A., Propozycje rozwiązań taryfowych wymuszających odpowiedzialność za bezpieczeństwo, Wrocław University of Technology, Wrocław 2008, section 7.1.4.ACD – w zakresie taryf za przesył i dystrybucję. 17. Wilczyński A., Tymorek A., Analiza możliwości likwidowania ograniczeń w przesyle energii elektrycznej występujących w warunkach rozwijania mechanizmów rynkowych (ocena różnych metod likwidowania ograniczeń, rola taryf przesyłowych), Wrocław University of Technology , Wrocław 2009, section 7.1.6. 18. Wilczyński A., Wpływ czynników ekonomicznych, organizacyjnych i prawnych na bezpieczeństwo elektroenergetyczne. Raport końcowy z zadania 7, Wrocław University of Technology, Wrocław 2010.

Stanisław Wojtas / Gdańsk University of Technology

Authors / Biographies

Stanisław Wojtas Gdańsk / Poland Academic degree, specialization in electrical equipment, received at the Department of Electrical Engineering and Automation (of Electricity at the time) Gdańsk University of Technology (1970). After a few years of work in the energy sector, he was employed at the Department of Electrical Engineering and Automation on Gdańsk University of Technology (1974). Received PhD (1983) and still works there. He lectured at the University of Basrah in Iraq (1988-1990). He is interested in construction and diagnostics of high-voltage cables with polyethylene insulation and lightning protection, in particular the assessment of earthing using the systematically developed and implemented surge measurement method.

Earthing Measurements for Power Line Towers

EARTHING MEASUREMENTS FOR POWER LINE TOWERS Stanisław Wojtas / Gdańsk University of Technology

1. INTRODUCTION Earthing is an important and necessary element of any energy system. Properly designed and constructed earthing guarantees safety for both people and devices located in places where a flow of dangerous short circuit or surge current caused by a lightning discharge can occur. Therefore, the earthing resistance should be made as low as possible, and its value should meet the guidelines contained in the specified standards and regulations. During its construction and later operation, the earthing should undergo periodic inspection, mainly through measurements of its resistance. Control tests of the resistance carried out using the traditional method are often very time consuming, especially for earthing of power line towers. For example, a 100 km section of 110 kV line may consist of more than 300 towers; the earthing of each should be tested at least every four [2] or five [5] years. It is therefore important that the tests can be carried out without disconnecting the lines. The measurement time for one tower should be as short as possible, and the instruments should be light and easy to transport. The purpose of this paper is to describe the procedures for measuring and assessing the earthing for power transmission line poles equipped with lightning conductors. The subject of analysis is primarily the influence of the span length and front time of the used measurement impulses on the results of earthing impedance. The presented results of theoretical calculations and computer simulations have been supplemented with measurements on real objects.

2. CLASSIC METHODS FOR ASSESSMENT OF LINE TOWER EARTHING Static resistance of earthing of line towers is usually determined using meters operating at low frequency and implementing various types of technical methods. In the case of high voltage transmission line towers, their earthings are connected in parallel by lightning conductors, as shown in ﬁg. 1 Therefore, there are two main methods of measurement: disconnection of artiﬁcial earth electrode from the tower and using a meter equipped with a current clamp.

2.1. Disconnecting the earth electrode from tower structure When using a low frequency excitation, the control terminals should be disconnected for the time of measurement, and thus the connection between the artiﬁcial earth electrode and the tower structure should be removed. Such a procedure is quite cumbersome and requires removing four connections – one at each leg of Abstract The elements of artiﬁcial earth electrode of the line tower and its foundation participate in the discharge of short circuit or lightning current to the ground. Both earthing elements should be taken into consideration during the control measurements of resistance or impedance of such earthings. In addition, the measuring procedure must take into account the fact that the earthings of transmission poles are connected in parallel by lightning conductors. The article discusses the issue of measuring and assessing the features of power line pole earthing using slow- and fast-changing waveform. The measurements of earthing resistance of the poles using meters

with frequencies similar to those in the network are cumbersome and laborious. The inﬂuence of earthings of adjacent poles can be reduced by using wave impedance of lightning conductors at high frequency or impulse waveforms. As a result of comparing the two methods based on fast-changing waveforms, it turns out that impulse meters are much more resistant to interferences caused by electromagnetic ﬁelds of the lines. The study analysed the effect of impulse front time and the line span length on errors made during these meassurements using impulse meters.

Stanisław Wojtas / Gdańsk University of Technology

the tower (ﬁg. 1b). In addition, the resistance value obtained in this way is caused solely by the artiﬁcial earth electrode, whereas the natural foundation earth electrode does not affect the measurement result. It should also be noted that such measurements should be made when the line has been turned-off. a)

control terminals

earthing ring

foundations

Fig. 1. The connection of measured earth electrode, including the bypass effect of adjacent towers – a) and foundations of the tower with ring earth – b)

In real earthing systems, the foundation earth electrode can significantly affect the resultant value of earthing resistance and determine the final assessment of the measurement result. The measurement results for the tower in the ground with a resistivity of about 200 Ωm shown in fig. 2. show that such a situation may take place. When measuring the resistance of an artificial earth electrode separated from the tower, a result Rs equal to 18 Ω was obtained, which is too high a value in relation to the standard requirements [1]. The resistance value of the analysed tower foundation is 12 Ω, and the parallel connection of both earthing elements gives the value of 7,7, which means that the requirements of the aforementioned standards are met.

Fig. 2. The results of static resistance for earthing of 110 kV line tower with the lightning conductor disconnected from its structure: R – resistance of the parallel connection of the foundation and artiﬁcial earth electrode, Rs – artiﬁcial earth electrode resistance, Rf – tower foundation resistance

2.2. The use of a meter equipped with a current clamp A special type of technical method is implemented using a current clamp meter. When using such a meter, the test terminals are not disconnected, and the current generated in the meter flows to the ground in the system of connected earthings and is divided into two parts. One of them passes through the tested conductor and earth electrode, while the other (IS) through the rest of the earthing system. The above case is illustrated in fig. 3. The measurement result is determined on the basis of the part of the current that flows through the tested earthing. The voltage drop is determined in relation to the auxiliary probe placed in the zone of reference potential.

Earthing Measurements for Power Line Towers

Measurements of tower earthings using this method are possible only when the meter is equipped with current clamps with a very large diameter to cover a single leg of transmission line tower. In order to determine the tower earthing resistance, four separate partial measurements should be made, one for each leg of the tower. The final result is determined by calculation as a parallel connection of the measured partial resistances. Electricity generated in the meter enters the tower structure at the point of galvanic connection (P). From there, the current spreads in all directions through conductive structure of the tower. Part of the current flows upward as IS and flows to the other towers in the system through lightning protection wire. The rest of the current flows into the tested earthing and then to the ground through the four legs of the tower. Hence, the current flowing into the ground is the sum of currents from I1 to I4 in individual legs of the tower. Voltage U designated in relation to the zone of reference potential should have the same value for the measurements of each leg of the tower. Therefore, the differences in the results of those measurements can be caused only by the differences in currents discharged to the ground by each leg of the tower. So, it can be stated that the total earthing resistance of the tower is a result of the parallel connection of partial resistances obtained for each leg:

 1 1 1 1   R       R1 R2 R3 R4 

1

(1)

However, it should be noted that the above dependence is correct only if all the partial measurements have the same point P connecting the meter to the tower [10].

P CP

Fig. 3. Determination of static resistance of a high voltage transmission tower using current clamps, where: MR – resistance meter, CP – current clamps, P – galvanic connection point, PV, P – auxilliary voltage and current probes

3. IMPULSE METHOD 3.1. Measuring principle This method allows measuring the earthing of transmission line towers using a suitable measuring instrument without disconnecting the earthing from the tower construction. In most cases, the span length in lines exceeds 150 m, and the wave impedance Zfp in the conductor – ground system is equal to about 500 Ω [9]. During the measurements the tested earthing with impedance Zx is bypassed with wave impedances Zfp of lightning conductors running to the two adjacent towers and wave impedances Zfs of the towers, as shown in ﬁg. 4. The impedance value for earthing of each tower is marked as Zu and Zx.

Stanisław Wojtas / Gdańsk University of Technology

Zfp

Zfp Zfs

Zfs

control terminals

Zx 2

Zfs

ring earth

foundations

Fig. 4. Pole earthing with adjacent poles and selected values of wave impedances for each element of the system

In such a system the impedance value at the terminal of measured earthing Z can be calculated according to the following formula:

Zm 

[ Z fs  0,5 x( Z fp  Z fs  Z u ]xZ x

(2)

Z x  0,5 x( Z fp  Z fs  Z u )

Fig. 5. shows the effect of bypassing the adjacent towers during the earthing measurement Zx as a function of this earthing. The relative error of the measured value Zm as a result of bypassing is determined based on the formula (2) as (Zx – Zm)/Zx. Calculations were made using the following assumed values of impedances: Zfp = 500 Ω, Zfs = 100 Ω [9] and Zu = 10 Ω. The presented chart shows that for the most frequently used value Zx, that does not exceed 20 Ω, the relevant error made during the impulse measurement when the earthings of adjacent towers are connected is maintained at 5%. The presented procedure for measuring earthing of power lines with lightning conductors without disconnecting the earthing wires from the tower structure allows this type of inspection and measurement work without turning off the line. In addition, impulse measurements without disconnection of control terminals are affected by the tower foundation, which also participates in the discharge of actual lightning currents, and whose resistance is often comparable to the resistance of an additional artiﬁcial earth electrode; therefore, it should not be overlooked in assessing the earthing effectiveness.

��

Error[%]

��

Zx [Ω] Fig. 5. Measurement relative error as a function of the measured value Zx based on the expression (2)

Earthing Measurements for Power Line Towers

3.2. Comparison of high-frequency and impulse meters Basic recommendations for the measurement of earthing in towers are contained in Annex N to the standard PN-E 05115: 2001, designated as information annex [3]. According to the above standard “Various methods can be used for measuring earthing resistance and impedance. Selection of the right method depends on the size of earthing system and level of disturbance.” It is recommended to measure earthing resistance with an earthing tester with a frequency of measuring voltage not exceeding 150 Hz using a current and voltage probe. In the case of connection to the system of lightning conductors of the power line, all earthings of the line towers have an influence on the obtained result. For such large systems the discussed standard allows any measuring method that is useful under the circumstances. The given examples use a high-frequency earthing tester to avoid turning off the line and disconnecting earthings from tower structures. Test frequency should be high, so that impedance of lightning conductors to adjacent towers is high enough to avoid this route of the measured current flow. In this case, a meter that generates impulses with a proper front time can be used instead of a high-frequency meter. Fig. 6 shows the results of comparative measurements of the impedance of horizontal earthing with a length of 70 m, made using the impulse and high frequency methods. Impulses with a front time of 4 µs were used in the measurements. The results obtained using both methods are comparable and show an increase in earthing impedance in relation to the resistance obtained using the static method [6]. In Poland the resistance of line tower earthing is commonly measured using the impulse method, without disconnecting the control terminals [8, 11]. Amplitude of the measuring current impulse is about 1 A. In the case of high frequency testers, the measuring current is at the level of miliampers, which can make such measurements unresistant to interference from stray currents and currents induced by electromagnetic fields of the lines. 80�

Z [ �] �

70� 60� � 50�

�

40� 30� 20� � 10� �� 0� 100�

�

� s� � 4�

�

� �

1 k�

41k� 100 k� 10 k� f [Hz] �

1000 k�

Fig. 6. The measurement results for horizontal earth electrode impedance with a length of 70 m as a function of frequency, made using a high frequency tester; the measuring point obtained using the impulse method for a front time of 4 µs [8] is marked on the curve

�

The results of measuring a 400 kV line tower obtained using both methods confirm the above problem. The tower was placed in the ground of low resistivity and the value of 2.5 Ω obtained using the impulse method is justified. The curve Z = f (f) obtained using a high frequency tester presented in fig. 7 shows a clear influence of external interferences (field, earthing currents), which overstate the impedance results. Values similar to the impulse results were obtained for very low frequencies – about 150 Hz. Such a frequency range is to be carried out using the static method and then the earthing resistance refers to the parallel connection of all towers, so it should reach the value of 1Ω. No influence of adjacent earthing should be present for a frequency of several kilohertz – the meter shows the earthing impedance value of 20 � for such a frequency range, which is definitely too high a result. The observed differences were caused by external interferences – their source in the working high voltage line. Due to a significantly higher test current amplitude, the impulse meters are much more resistant to such interferences.

Stanisław Wojtas / Gdańsk University of Technology

70 ��? �

��

� � �

��

Fig. 7. The results of impedance measurements for earthing of a 400 kV line tower as a function of frequency; the dotted line indicates the value of 2.5 Ω obtained using the impulse method with an impulse ftront time of 4 μs

4. TEST RESULTS The subject of the tests was the influence of the impulse front time and span length on the obtained values of the impedance of earthing for power transmission line towers equipped with lightning conductors. The tests were carried out using both computer simulations and measurements on real towers earthings. 4.1. Computer simulations Calculations based on computer simulations were made using Matlab software with the Simulink package. The tower earth electrode consists of parallel artificial earthing ring and foundation earth electrode. The earthing ring is modelled using the elements R, L and C, which were determined according to the methodology developed by R. Verm [12]. The foundation is modelled by the resistance Rf calculated based on the dimensions of the foundation footing. The whole replacement model of earth electrode is presented in fig. 8.

Fig. 8. Replacement model of earthing ring (R, L, C) with foundations (Rf )

The parameters of a square earthing ring with a side of 12 m and foundation of 0.9 m3 were designated for the assumed ground resistivity – 200 Ωm. The impedance of such a modelled earth electrode was determined at the impulse current with an amplitude of 1 A and front times equal to 0.5; 1.0; 4.0 and 8.0 µs and at alternating current with network frequency. The simulation results are shown in ﬁg. 9. With the increase of front time, the impedance value of earthing decreases, but at the time of 4 µs its value reaches the state close to the ﬁxed state obtained for network frequency, which is primarily due to the presence of resistive elements.

Earthing Measurements for Power Line Towers

In the next stage of calculations in Matlab, the analysed tower earthing was bypassed by two corresponding earthings, connected by a lightning conductor with the configuration shown in fig. 4. Wave impedance in the lightning conductor – earth system was modelled as a long line using fixed parameters. The assumed span length was equal to 200, 300 and 400 m. Wave impedance of towers was omitted, since the used impulse front lengths cause multiple wave reflections at the ends of towers, which reduces their influence on the resultant waveforms in the analysed connection system. The simulation measurement results for impulse impedance between terminals 1 and 2 in fig. 4 as a function of the impulse front time for the assumed span lengths are shown in fig. 10. The curve marked with a description “without lightning conductors” corresponds to the results shown in fig. 10 and shows how the impedance of the modelled earthing decreases with the increasing front time of the measuring impulse. Subsequent curves show the influence of parallel connection of earthings of adjacent towers on the obtained results, and their deviation from the initial curve (without lightning conductors) is a measure of error made when measuring without isolating the lightning conductors on top of the tower. Errors caused by bypassing earthings increase with the decrease in span length and increase in front of the measuring impulse, as can be seen in fig. 10.

. . .

static

Fig. 9. Results of simulation calculations for the impedance of tower earthing at the impulse current with an amplitude of 1 A, given front times and the frequency of 50 Hz

The current Polish practice for measuring power line pole earthings uses impulse front times of 1 and 4 µs as the values provided in the standard PN 04060:1992 [4]. In the case of impulses with the front time of 1 µs the decrease of the measuring impedance value for earthing caused by the bypassed influence of adjacent earthings is at the lowest level and does not exceed 2-3%. However, it should be noted that the same value of impulse impedance at such a short time of front significantly exceeds the earthing resistance measured in static conditions, which usually is a reference point in assessing earthings. The impulse coefficient for the tower earthing is defined as the ratio of the impulse impedance to the static resistance; in the case of impulse with a front time of 1 µs it can achieve high values, usually in the range of 1.2-2.5. Higher values refer to earthings in towers located in grounds of high resistivity where it is necessary to use extended artificial earth electrodes [7, 10]. In such cases, the impedance of tower earthing measured with the impulse with a front time of 1 µs can too often exceed the normative values reported for static conditions.

Stanisław Wojtas / Gdańsk University of Technology

impulse resistance [Ω]

without lightning conductors 400 metres 300 metres 200 metres

impulse front time [µs]

Fig. 10. Inﬂuence of the current front time of measuring impulse on the impedance of the tower bypassed with earthings of two adjacent towers and lightning conductors of various span lengths.

Measurements of tower earthings using impulses of front time equal to 4 µs may include errors at short spans due to bypassing with adjacent earthings. This error does not exceed 10%, even in extreme cases. Impulse coefﬁcient of tower earthings measured at impulse of 4 µs is not very high and usually does not exceed the value of 1.5. Compared to the static resistance, the higher value of measured impedance is partially offset by an error introduced by the bypassing with adjacent earthings, so the results obtained in impulse measurements without isolating earthings from the lightning conductors may be related to the requirements for earthing static resistance with a good approximation.

4.2. Measurements of the actual line tower earthing The simulation calculations for the bypassing influence of the adjacent towers on the measurement results described in the previous section were verified with tests conducted on the actual power line. The tests were conducted on seven towers belonging to two lines with a voltage of 110 k V, and the test program included the measurement of impulse impedance with the impulse front times of 1 and 4 µs and static resistance. Measurements were made with closed control terminals connecting the artificial earth electrode to the tower structure in two connection configurations: with no lightning conductors and with conductors mounted at the top of the tested tower. Average values of the obtained impedances and resistances are shown in fig.11. Errors resulting from bypassing the measured earthings with the earthings of adjacent towers are shown in the bottom part of the figure. The biggest error, exceeding 40%, was observed in static measurements; this confirms that the static method can not be used to measure the tower earthing without disconnecting the control terminals or isolating the lightning conductors from the tower structure. Much smaller errors occurred during measurements using the impulse method: at impulse front time of 1 μs the average error was 3.5%, and 4.3% at the front time of 4 μs. Lowering of the obtained earthing values due to bypassing with earthings of adjacent towers does not exceed the error values obtained from computer simulations and shown in fig. 5 and 10. .

. without lightning conductor with lightning conductor

. .

static

Fig. 11. Inﬂuence of lightning conductors on the impedance of actual earthings measured at given impulse front times and on their static resistance

Earthing Measurements for Power Line Towers

5. CONCLUSIONS The discharge of the line tower current to the ground is done by an artificial earth electrode and the foundations of that tower. Therefore, assessment of earthing resistance for the power line tower should be made when both earthing elements are parallel. Measurement using low frequency meters with disconnection of control terminals from the tower structure does not meet the aforementioned condition. Moreover, it requires that the line is turned-off during measurement. Although there are methods of measurement at low frequency using current clamp meters, which allow testing the complete tower earthing without disconnecting the control terminals, they are quite cumbersome as they require analysis of the current flowing into the ground through each of the four legs of the tower and due to the relatively low accuracy of such measurements. The use of fast-changing waveforms (impulse or high frequency meters) allows the measurement of earthing without disconnecting the control terminals from the tower structure, because the earthings of adjacent towers are connected in parallel to the tested earthing through lightning conductors, whose impedance increases to the value of wave impedance in the conductor – ground system in the case of fast-changing waveforms. In practice, impulse meters are used for the earthing measurements in the case of high-voltage line towers, due to a very high susceptibility to interference of the meters operating at high frequency. Measuring current of high-frequency meters is at the level of milliamperes and their work is interfered by voltages induced in the measuring circuits by electromagnetic field under the line, as well as by stray currents. Impulse meters operate at currents at the ampere level, which makes them much more resistant to this type of interference. Parallel connection of earthings in individual line towers slightly lowers the impedance value measured using the impulse method. The difference between the actual and measured value of the earthing impedance increases with increasing impulse front time, and decreases with increasing length of the line spans. The proposed 4 µs impulse front time is a compromise between the required accuracy of measurement and the obtained impedance values referred to the earthing resistance specified in standardization rules. Even under the most adverse conditions, the theoretical error made in applying the proposed impulse method of measurement does not exceed 10%, which is acceptable in earthing tests. The calculations and computer simulations were confirmed by the results of measurements carried out on the actual earthings of 110 kV towers. These tests show that the error caused by the bypassing influence of earthings of other towers under real conditions is smaller than the one resulting from theoretical calculations and does not exceed 5%.

BIBLIOGRAPHY 1. PN-EN 62305-1:2008 – Protection against lightning, Part 1, General principles. 2. BS EN 62305-3:2009 – Protection against lightning, Part 3, Physical damage to structures and life hazard. 3. PN-E 05115:2002 – Power installations exceeding 1 kV a.c. 4. PN-E 04060:1992 – High voltage test technique. General principles and test requirements. 5. Construction Law, 1994, consolidated text: Journal of Laws of 2006, No. 156, item 1118. 6. Wojtas S., Ocena uziemień odgromowych metodami: udarową i wysokoczęstotliwościową, Pomiary, Automatyka, Kontrola, vol. 53, nr 4, 2007. 7. Wojtas S., Wołoszyk M., Galewski M., Rezystancja udarowa uziemień obiektów budowlanych, Elektrosystemy, no. 4, 2004. 8. Wołoszyk M., Pomiary impedancji (rezystancji) udarowej uziemień odgromowych, [in:] Gryżewski Z., Prace pomiarowo-kontrolne przy urządzeniach elektroenergetycznych o napięciu do 1 k V, COSiW SEP, Warszawa 2002. 9. Szpor S., Samuła J., Ochrona Odgromowa, WN-T, Warszawa 1983. 10. Wołoszyk M., Wojtas S., Galewski M., Badania udarowe uziemień słupów linii elektroenergetycznych, Elektrosystemy, no. 11, 2006. 11. Wojtas S., Impulse measurement accuracy of transmission line earthings, [w:] 29th International Conference on Lightning Protection ICLP2008, 23rd–26th June 2008, Uppsala 2008. 12. Verma R., Mukhedar D., Fundamental considerations and impulse impedance of grounding grids, IEEE Transaction on Power Apparatus and Systems, vol. PAS-100, no. 3, 1981.

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