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Proc. of Int. Conf. on Control, Communication and Power Engineering 2010

Direct and Indirect Control Strategies of Dynamic Voltage Restorer K.Sandhya1, Dr.A.Jaya Laxmi2, Dr. M.P.Soni 3

1&3

Department of Electrical and Electronics Engineering, VNRVJIET, Bachupally, Hyderabad, INDIA Email: sandhyakp_msr @yahoo.co.in 2 Department of Electrical and Electronics Engineering, College of Engineering, JNTU Hyderabad, Hyderabad, INDIA

Abstract— This paper presents direct and indirect control strategies of Dynamic Voltage Restorer (DVR). With the development of information and automation techniques, dynamic voltage problems are once again in spot light. As an important custom power device, DVR is powerful in solving these power quality problems. The program investigated was the MATLAB/SIMULINK. Simulation results were presented to illustrate and understand the performances of DVR with direct control and with indirect control strategy.

The reliability and robustness of these control schemes in the system response to the voltage disturbances due to system faults or load variations are proved in the simulation results.

Index Terms—DVR, Custom Power Devices, Dynamic Power Quality, Control Strategy.

I.

INTRODUCTION

Quality power supply is essential for proper operation of industrial processes which contain critical and sensitive loads. For Power Quality improvement, the developments of power electronics devices such as FACTS and Custom Power Devices have introduced an emerging branch of technology providing the power system with versatile new control capabilities. Voltage sags and swells in the medium and low voltage grid are considered to be the most frequent type of Power Quality problems. Their impact on sensitive loads is severe. Different solutions have been developed to protect sensitive loads against such disturbances. Among these DVR is most effective device. A DVR injects a voltage in series with the system voltage to correct the voltage fluctuations.

II. CUSTOM POWER TECHNOLOGY The concept of custom power was introduces in 1995

by N.G. Hingorani. Like Flexible AC Transmission Systems (FACTS) for transmission systems, the new technology known as Custom Power pertains to the use of power electronics controllers in a distribution systems [1], [2]. Just as FACTS improves the power transfer capability and stability margins, custom power makes sure consumers get pre-specified quality and reliability of supply.

Some of these Custom Power Devices are: Seriesconnected compensator like DVR (Dynamic Voltage Restorer), Shunt-connected compensator like DSTATCOM (Distribution Static Compensator), Series and shunt compensator like UPQC (Unified Power Quality Conditioner) and SSTS (Solid State Transfer Switch). Among these, the DVR is an effective custom

power solution which is based on the VSC principle [9] can deal with voltage sags and swells.

III.

A DVR is a device that injects a dynamically controlled voltage Vinj(t) in series to the bus voltage by means of a booster transformer. The Dynamic Voltage Restorer employs series boost technology using solid state switches to correct the load voltage amplitude as needed [7],[8]. A DVR consists of a voltage source converter and is shown in fig.1. There are three single phase transformers connected to a three phase converter with energy storage system and control circuit [11]. The objective is to avoid voltage sags/swells in the network.

Moreover, it is consider that the DVR acts only during fault period. On the contrary, it is considered by-passed.

Figure1. Structure of DVR

An equivalent circuit diagram of the DVR and the principle of series injection for sag/swell compensation is depicted in fig.2. The load voltage is given by VL = VS + Vinj

(1)

Where VS is the supply voltage and Vinj is the voltage injected by the mitigation device.

Under nominal voltage conditions, the load power on each phase is given by SL =VL IL* = PL-jQL

(2)

Where IL is the load current, PL and QL are the real and reactive power taken by the load respectively during sag/swell.

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DYNAMIC VOLTAGE RESTORER(DVR)

Proc. of Int. Conf. on Control, Communication and Power Engineering 2010

Figure 2. Equivalent circuit of DVR

Figure3. Direct control DVR

When the compensating device is active and restores the voltages back to normal, then

The direct control DVR model is shown in fig. 3 and the direct control analyzed in this paper is exhibited in fig.4, which employs the dqo rotating reference frame, a PLL and four proportional-integral (PI) regulators. The first one is responsible for controlling the terminal voltage through the reactive power exchange with ac network.

This PI regulator provides the reactive current reference Iq*, which is limited+1p.u. capacitive and -1p.u inductive.

Another PI regulator is responsible for keeping the dc voltage constant through a small active power loss in the transformer and inverter. This PI regulator provides the active current reference Id*. The other two PI regulators determine voltage reference Vd* and Vq*, which are sent to the PWM signal generator of the converter, after a dq0-toabc transformation. Finally VABC are the three-phase terminal voltages, IABC are the three-phase current injected by the custom power device into the network, VRMS is the Root Mean Square (RMS) terminal voltage, Vdc is the DC voltage measured in capacitor. Finally V*abc re the threephase voltages desired at the converter output.

SL=PL-jQL=(PSag-jQSag) + (Pinj – jQinj)

(3)

Where the sag subscript refers to the sagged supply quantities, the inject subscript refers to quantities injected by the compensator device (DVR).

IV.

CONTROL SCHEME

The aim of control scheme is to maintain constant voltage magnitude at the point where a sensitive load is connected, under system disturbances. The control system of a DVR plays an important role, with the requirements of fast response in the face of voltage sags and variations in the connected load. In this paper the proposed DVR control strategies are direct and indirect methods.

Converters presently employed in FACTS controllers are the Voltage Sourced Converters (VSC) type rather than Current Sourced type converters. The most dominant converters needed in FACTS controllers are the voltage sourced converters. Such Converters are based on devices with gate turn-off capability. A voltage-source converter is a power electronic device, which can generate a sinusoidal voltage with any magnitude, frequency and phase angle. In distribution voltage level, usually, the employed switching element is the Integrated Gate Bipolar Transistor (IGBT), due to its lower switching losses and reduced size. As the converter rating employed in these devices is relatively low, hence the output voltage control can be executed through Pulse Width Modulation (PWM) switching pattern. The VSC converts the dc voltage across the storage device into a set of three-phase ac output voltages. These voltages are in phase and coupled with the ac system through the reactance of coupling transformer.

A. Direct Control Scheme In this scheme, the switching elementsIGBTs/diodes, the PWM signal generator and dc capacitor are explicitly represented. Here, a DVR model is implemented using MATLAB Sim Power Systems. It consists of six pulse voltage source converter using IGBTs/diodes, a dc capacitor, PWM signal generator, a passive filter to eliminate harmonic components and a voltage controller. Here converter is directly controlled i.e., both the angular position and the magnitude of the output voltage are controllable by appropriate on/off signals [5].

Figure 4. Direct Control

B. Indirect Control Scheme The indirect control DVR model is shown in fig.5 and the indirect control analyzed in this paper is exhibited in fig.6, which employs proportional-integral (PI) egulator. In this, VABC are the three-phase terminal voltages, VRMS is the Root Mean Square (rms) terminal voltage. Finally V*abc are the three-phase voltages desired at the converter output. The controller input is an error signal obtained from the reference voltage and the rms value of the terminal voltage measured. Such error is processed by a PI controller and the output is the angle, which is provided to the PWM signal generator. The PWM generator then generates the pulse signals to the IGBT gates of Voltage Source Converter (VSC) [10]. It is

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Proc. of Int. Conf. on Control, Communication and Power Engineering 2010

important to note that in this case, indirectly controlled converter, there is active and reactive power exchange with the network simultaneously.

Figure 8(a). Voltage at bus4 without DVR. (20% Voltage Sag)

Figure5. Indirect control DVR

Figure 8(b). RMS value of voltage at bus4 without DVR. (20% Voltage Sag) Figure 6. Indirect Control

V.

TEST SYSTEM

A.

Test System The test system employed to carry out the simulations is shown in fig.7. Such a system is composed by a 13KV, 50Hz generation system, feeding two lines through Y/∆/∆, 13/115/115KV three winding transformer.

The two such lines are feeding two distribution networks via two ∆/Y, 115/11KV transformers. The sequence of events simulated is explained as follows. Initially there is no load at bus 4, load L2 (inductive load) is connected during t = 100 – 300 ms. In next event of simulation load L3 (capacitive load) is connected to bus 4, during the time t = 100 – 300 ms. For the events described above, in the absence of the DVR the terminal voltage varies considerably. Initially there is no load at bus 4, so voltage was 1.0 p.u. until the load 2 (L2) is connected, then it is decreased to 0.8 p.u. from t = 100 – 300 ms due to introduction of inductive load L2. The simulation results are shown in fig.8 (a) & fig.8 (b). And in next event due to the introduction of capacitive load L3 from t = 200-300 ms, the voltage is increased to 1.2 p.u.. The simulation results are shown in fig.9 (a) & fig.9 (b).

TABLE. 1 LOAD DESCRIPTION Loads P(MW) Q(MVAR) L1 10 9 L2 10 22 L3 10 -18

Figure 9(a). Voltage at bus4 without DVR. (20% Voltage Swell)

Figure 9(b). RMS value of voltage at bus4 without DVR. (20% Voltage Swell)

B. Test System with DVR The test system connected with DVR through series transformer is shown in fig.10.

Figure 10. Test system without DVR Test System with Direct Control DVR:

For the events described previously, now a DVR with direct control is connected as shown in fig.10.

The voltage is improved to its nominal value (1.0 p.u.) in the presence of DVR. The voltage is improved from 0.8 p.u. to 1.0 p.u. during the period t = 100 – 300ms in presence of load L2. It is improved from 1.2 p.u. to 1.0p.u. in the presence of load. The simulation results are shown in fig.11 (a) and fig.11 (b), where the very effective voltage regulation provided by the DVR with direct control can be clearly appreciated.

Figure 7. Test system without DVR

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Proc. of Int. Conf. on Control, Communication and Power Engineering 2010

Figure 11(a). Improved voltage at bus4 with direct control DVR. (mitigation of20% sag)

Figure 14(a). Voltage at bus4 without DVR during the fault

Figure 11(b). RMS value of improved voltage at bus4 with direct control DVR. (mitigation of 20% sag)

Figure 14(b). RMS value of voltage at bus4 without DVR during the fault

Indirect Controller DVR:

For the events described previously, now a DVR with indirect control is connected as shown in the fig.10. The simulation results are shown in fig.12 & 13.

Figure 15(a). Improved voltage at bus4 with direct control DVR during fault

Figure 12. RMS value of improved voltage at bus4 with indirect control DVR. (mitigation of 20% sag)

Figure 15(b). RMS value of improved voltage at bus4with direct control DVR during fault

Figure 13. RMS value of improved voltage at bus4 with indirect control DVR. (mitigation 20% swell)

C. Results with single line to ground fault A single line to ground fault is applied at bus 4 without load, via a fault resistance of 0.1Ω, during the period 100-300 ms. The voltage sag at bus4 is 50%. The voltage without DVR during the single line to ground fault is as shown in Fig.14 (a) and the RMS value of voltage with fault is shown in Fig.14 (b).

Similarly a new set of simulations was carried out with single line to ground fault with the fault resistance 0.1Ω, without load, but now with the DVR with direct control and then with indirect control. The voltage at bus4 and the RMS value of voltage with direct control are shown in Fig.15 (a) and Fig.15 (b). The simulation results with indirect control DVR are shown in fig.16. Both of them gave effective voltage regulation against fault conditions.

Figure 16. RMS value of improved voltage at bus4 with in direct control DVR during fault

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TABLE .2.

COMPARISON OF TERMINAL VOLTAGE (P.U.) Time in ms

t=100-300

Without DVR

With Direct Control DVR

With Indirect Control DVR

1.0 p.u.

1.0 p.u.

1.2 p.u.

1.0 p.u.

1.0 p.u.

0.5 p.u.

1.0 p.u.

0.9 p.u

0.8 .u.

Voltage sag t=100-300 Voltage swell t=100-300 with fault

Proc. of Int. Conf. on Control, Communication and Power Engineering 2010

CONCLUSIONS

About the authors:

This paper has presented the power quality problems such as voltage dips, swells and mitigation techniques by DVR. A PWM based direct control scheme has been implemented to control the VSC. Here both the angular position and the magnitude of the output voltage are controllable by appropriate on/off signals with direct control. An Indirect control scheme is also presented. The simulation results showed clearly the performance DVR with both of the control schemes in mitigating voltage sags and swells. Table 2 gives comparison of terminal voltage (p.u.). The post disturbance voltages are slightly higher with the indirect controller DVR. The simulation results showed that the DVR with direct controller provides better voltage regulation. Although a direct controller is more difficult and expensive, it gives superior dynamic performance.

REFERENCES [1]

N.G. Hingorani, “Introducing Custom Power”, EEE Spectrum. Vol.31, pp.41-48, 1995.

[2] E Acha, V.G.Agelidis, O.Anaya-Lara, T.J.Miller, “Power Electronic Control in Electrical Systems”, 1st edition, Newnes, 2002.

[3] Anaya-Lara o, Acha E., “Modeling and analysis of custom power systems by PSCAD/EMTDC”, IEEE Transactions on Power Delivery, Vol.17. Issue: 1, Jan.2002, Pages:266272.

[4] Bollen, M.H.J., “Voltage sags in three-phase systems” Power Engineering Review, IEEE, Vol.21, Issue: 9, Sept.2001, pp: 8-11, 15.

[5] N.G.Hingorani and L.Gyuyi, Understanding Facts – Concepts and Technology of Flexible AC Transmission Systems, 1st Ed: Inst.Elect. Electronic Eng.Press, 1999.

[6] Gareth A. Taylor, “Power quality hardware solutions for distribution systems: Custom power”, IEE North Eastern Centre Power Section Symposium, Durham. UK, 1995, pp.11/1-11/9.

[7] Ran Cao, Jianfeng Zhao, Weiwei Shi, Ping Ziang and Gouquing Tang, “Series Power Quality Compensator for Voltage Sags, Swells, Harmonics and Unbalance”, IEEE/PES Transmission and Distribution Conference and Exposion, Vol.1,28 Oct.-2 Nov., pp.543-547.

[8] Chris Fitzer, Mike Barnes and Peter Green, “Voltage Sag Detection Technique for a Dynamic Voltage Restorer”, IEEE Trans. Power Electronics, Vol.22, No.2, Mar.2007, pp.626-635.

[9] Stump, M.D., G.J. Kaene and F.K.S Leong, 1998. “Role of custom power products in enhancing power quality at industrial facilities.” In: Conf. Rec. IEEE/EMPD, pp: 507517.

[10] o.Anaya-Lara, E. Acha, “Modeling and Analysis of Custom Power Systems by PSCAD/EMTDC”, IEEE Trans., Power Delivery, PWDR Vol-17(1), pp.266-272, 2002.

[11]R.Buxton, “Protection from Voltage Dips with Dynamic Voltage Restorer”, in IEE Half day colloquium,(1998).

[12] A. Ghosh and G.Ledwich, Power Quality Enhancement Using Custom Power Devices, Kluwer Academic Publishers, 2002.

K.Sandhya, born on 30th Aug 1980, in Kadapa District, A.P., India. Obtained B.Tech degree in 2001 and M.Tech in 2007 with specialization in Electrical Power Systems from Jawaharlal Nehru Technological University, and pursuing Ph.D (Power Quality) from Jawaharlal Nehru Technological University, India. Presently working as Assistant Professor, Electrical & Electronics Engineering in the Department of EEE, VNR Vignana Jyothi Institute of Engineering and Technology, Bachupalli, Kukatpally, Hyderabad. She had 8 years of teaching experience. During her teaching career she taught various subjects like Network Theory, Electrical Mechanics, and Power Systems etc. Her research interests are Power Systems & Power Quality, FACTS, and Custom Power Devices. She is a Member of Indian Society of Technical Education (M.I.S.T.E) DR. A. Jaya laxmi was born in Mahaboob Nagar District, Andhra Pradesh, on 07-11-1969. She completed her B.Tech. (EEE) from Osmania University College of Engineering, Hyderabad in 1991, M. Tech.(Power Systems) from REC Warangal, Andhra Pradesh in 1996 and completed Ph.D.(Power Quality) from Jawaharlal Nehru Technological University College of Engineering, Hyderabad in 2007. She has five years of Industrial experience and 8 years of teaching experience. She has worked as Visiting Faculty at Osmania University College of Engineering, Hyderabad and is presently working as Associate Professor, JNTU College of Engineering, Hyderabad. She has 18 International and 5 National papers published in various conferences held at India and also abroad. She has 5 international journal papers to her credit. Her research interests are Neural Networks, Power Systems & Power Quality. She was awarded “Best Technical Paper Award” for Electrical Engineering in Institution of Electrical Engineers in the year 2006. Dr. A. Jaya laxmi is a Member of Institution of Electrical Engineers Calcutta (M.I.E) and also Member of Indian Society of Technical Education (M.I.S.T.E).

Dr. M. P. Soni, Worked as Addl. General Manager in BHEL R & D in Transmission and power System Protection. Worked as Senior Research Fellow at I.I.T. Bombay for BARC Sponsored Project titled, ‘Nuclear Power Plant Control’ during the year 1974 - 1977. Presently Working as Professor and Head, Research and Consultancy Centre, VNRVJIET, Bachupally, Hyderabad. India. He has undertaken the following projects like “Dynamic Simulation Studies on Power System and Power Plant Equipments”, “Initiated developments in the area of Numerical Relays for Substation Protection”, “Developed Microprocessor based Filter bank protection for National HVDC Project and commissioned at 220 kV Substation s ,MPEB Barsoor and APTRANSCO Lower Sileru, Terminal Stations of the HVDC Project. “Commissioned Numerical Relays and Low cost SCADA System at 132kV, GPX Main Distribution Substation ,BHEL Bhopal”.

He has 12 international and national conference papers to his credit. His research interests include power System protection and advanced control systems.

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