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New insulators and apparatus for 800kVdc transmission Ralf Hartings, ABB, Ludvika, Sweden Roger Sundqvist, ABB, PiteĂĽ, Sweden
1. Introduction As is generally known, HVDC transmission systems are to be preferred over HVAC for long distance transmission. This is due to a combination of several aspects, such as fewer lines are needed per MW, and as a consequence, a smaller right-of-way for the power lines, simpler and less expensive tower constructions (only 2 conductor bundle systems for DC instead of 3 for AC) and of course significantly less power losses in DC. For a certain transmission power, increasing the voltage will require a lower current and therefore give less total power losses. With the increasing need for more power to be transported along high voltage power lines, it has become economically interesting to increase the system voltage of transmission power lines. In 1984, the first 600kVdc system was put into commercial operation in Itaipu, Brazil. Since then, a number of 500kVdc systems have been taken into operation world-wide. In recent years, power grid companies in for instance Brazil, China, India, and Africa, have become aware of the economical advantage of increasing the transmission voltage from 500-600 to 800kVdc. One of the contributing factors for this change is the application of composite insulators for apparatus. As the voltage is increased from 500/600 to 800kVdc, the required insulation length passes the upper feasible limit for porcelain. However, composite insulators can be produced at lengths exceeding the limit for porcelain. Until relatively recently, the maximum length of composite insulators available on the market was about 6m. This limitation has now been set aside by ABB in Piteü, Sweden, by pushing it up to 11m. Another important demand for UHVDC power transmission at 800kVdc is reliability and availability. With so much power from only one transmission line, it is of vital importance that continuous operation can be guaranteed in all environmental conditions. Composite insulators have shown an excellent performance on HVDC power lines in for instance China [1] and apparatus with composite insulators have also successfully been installed in many places around the world [2]. Maintenance free composite insulators for apparatus, with their excellent dielectric and mechanical performance, are therefore required for 800kVdc power transmission. A complete set of 800kVdc substation apparatus, equipped with composite insulators, has recently been manufactured and installed in the world’s first 800kVdc test site in Ludvika, Sweden to demonstrate the feasibility of 800kVdc transmission. This paper describes an application and testing results of a transformer- and wall bushing equipped with composite insulators for 800kVdc.
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2. Manufacturing of composite insulators for 800kVdc ABB has been manufacturing composite insulators since 1998. Today a total number of more than 40 000 insulators are installed in different apparatuses, environments and countries around the world. The service record of these insulators is excellent with no known failure in service. Thus, the composite insulator gives a good reliability together with well known advantages, such as. -
High level of safety Superior seismic performance Improved electrical performance
An outdoor electrical insulator need to be a self-contained component that withstand mechanical and electrical loads under harsh environments and long times. The different materials must therefore be engineered to form a robust unit with a sufficient strength for all joints and interfaces. There are three important material boundaries, excluding the microlevel, between different materials in the composite insulator i.e. • Tube – outer housing • Tube – flange • Flange – outer housing (overlap) These three materials and boundaries need thorough design considerations. First, all service loads must be examined and converted to design loads including environmental conditions and service time. Given the design loads we can easily test and check the materials and interfaces according to standards and customer approval requirements. This gives an approved design. However, to fully secure a design, complementary analysis of interfaces as well material properties is needed. The outer housing, in this case an HTV silicone rubber, needs to have a secured joint to both the tube and the flange. These joints can affect the long term insulator functionality if it is not properly designed. The ABB design is in this respect verified according to valid standards and further several in-house tests. The silicone-joint is chemically adhered to both the surfaces creating strong bonds. This is obtained by a surface conditioning followed by a primer treatment of the adhesive surfaces. This process of surface preparation is carefully controlled and is built in as a part of the production line. The HTV silicone rubber has been in service for many years in different environments with excellent performance [3]. ABB has developed a composite insulator design that can be efficiently adapted to new types. The design criteria’s are founded on an extensive mapping of materials properties for the used materials as well as the joints and interfaces between the separate materials. The production unit at ABB is capable of producing composite insulators of above 10 meters in length. This means one continuous 10 meter tube and outer housing! Several insulator types for 800 kVdc have been developed lately. These are 800 kVdc insulators for bushings, busbar supports, DCVDs, reactor support and disconnectors.
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The load bearing part of an insulator is the fiber composite tube. ABB is using filament winding technology for manufacturing of the tubes. This makes it possible to design the tube material for optimization of the mechanical properties. Filament winding is an efficient process for series production of closed structures such as tubes. The complete process cycle is fully automated giving low process variations.
All the insulators installed in the 800kVdc test site in Ludvika (see fig. 1. ), have been approved in type tests according to IEC 61462. An example of the load capability of one the newly developed 800 kVdc insulators is an SML level well above 500 kNm.
fig. 1.
800kVdc test site in Ludvika, Sweden for all key apparatus in an 800kVdc converter station.
A unique process is used for manufacturing the silicone outer housing directly onto the tubeflange unit. This process, named Helical Extrusion, is developed by ABB, see fig. 2. A helical shed pattern is obtained and at the same time a strong interface is formed between siliconetube and silicone-flange. The tooling or die is one key part of the process. This tool concept has inherent a high flexibility for different shed as well as tube geometry’s.
INMR, May 2007 Rio de Janeiro, Brazil
fig. 2.
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Helical Extrusion
All insulators are routine tested before delivery. These tests are done with test equipment that is designed for high quality standards and meet the highest requirements for safety and reliability. fig. 3. shows the equipment for routine pressure test and tightness test. Typically, an insulator is pressurized to twice the service pressure in the routine pressure test.
fig. 3.
Routine pressure and tightness test of Composite Insulator.
3. Development of 800kVdc bushings Making 800kVdc transmission possible, required the development of an 800kVdc converter station, which in turn, implied the development of a long list of equipment installed in such a station. Of all equipment, the converter transformer, including the converter transformer bushing and the wall bushing were identified as the most critical components. When the 800kVdc bushing development project was initiated at ABB in Ludvika, all available information was compiled, This includes experience with the 600kVdc bushings in Itaipu, Brazil, and in particular, the more recently and successfully developed bushings for the 500kVdc Three Gorges projects in China. These SF6 enhanced bushings have been in service since 2003 for almost 3.000.000 apparatus-service-hours without any dielectric problems. This design was therefore used as a basis for the development of the 800kVdc bushing. A limit test, combined with DC electric field measurements, was performed on the existing
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design for 500kVdc. The aim was to verify the design limits and to see if any non-linear phenomena could arise, when increasing the voltage and stresses. In DC applications, the voltage is constant over time and not varying as in AC: 10ms positive, 10ms negative and so on. The impact of this constant electric field strength is a continuous driving force on charges and charged particles. These are drawn towards and accumulated on the bushing and these charges will have an impact on the total field distribution, not only on the outside of the bushing, but also on the internal parts of the bushing. It is the electric field distribution, and the lack of “electric field hotspots�, which determines the dielectric performance of the bushing. It is therefore of vital importance to know where those charges will and should be accumulated when designing the internal parts of the bushing.
Probe fig. 4.
Test set-up for DC electric field measurements along transformer bushing. The field probe is in the middle of the red circle.
The test set-up for the electric field measurements is shown in fig. 4. The rotating field probe measures the axial and radial field along the bushing, by moving the white horizontal rod, holding the probe, along the vertical (brownish) robot pillar. A typical field plot is shown in fig. 5. , measured after the bushing was grounded and thus indirectly showing the amount of charges left in and on the bushing. It is clear that charges are present and that they will influence the field distribution during normal service conditions. To integrate this phenomenon into the design of the bushing is a requirement for any HVDC bushing in order to guarantee a life-long problem free operation in service.
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Toroid position one. Remaining field 9 minutes after 500 kV. 4 sweeps. 250,0
Electric field strength (no units)
Radial
200,0 Axial
Electrical field strength (kV/m)
150,0
100,0
50,0
0,0 0,00
0,50
1,00
1,50
2,00
2,50
3,00
3,50
4,00
4,50
5,00
-50,0
-100,0
-150,0 Height (m)
fig. 5.
Axial and radial electric field along 500kVdc transformer bushing after grounding (Uapplied=0). The different curves represent different measurements in time. Time step is about 1 minute. Y-axis: Axial and radial field strength in arbitrary units; X-axis: length along bushing in meter
The field distribution inside and outside the bushing is controlled not only by the deposited charges on the outside, but of course also by the inner structure of the bushing and the external flanges and corona rings. The charges deposited on the outside of the bushing will decay rather slow, due to the excellent insulation properties of the silicone rubber used for the weather sheds of the insulator, depending on the humidity of the surroundings. Whereas in AC the field distribution is determined by the dielectric permittivity (ε, epsilon) of the insulation materials, in DC the steady state field distribution is controlled by the resistivity (ρ, rho) of the insulation materials. In real service operation, the bushing will be exposed to both DC and transients of varying frequency. The design of the bushing needs therefore to be such that the field distribution based on the permittivity (“AC” like capacitive field distribution) is very similar to the field distribution due to the resistivity of the insulation materials. When switching on the voltage, an AC-like voltage distribution will occur, and after a certain time (corresponding to the “overall time constant” of the insulation system, τ = ρ ∗ ε), the DC-like field distribution will obtained. Due to the complex nature of the insulation system, the geometry and position of the different parts, the transition from “AC to DC” may occur very differently in different locations inside and outside the bushing. The resistivity of the insulation materials of a bushing is of course very high, as the material is designed to be an insulator. As a result, the typical overall time constant will vary orders of magnitude and can be very long. In fact, days, weeks or months are quite typical. Furthermore, the resistivity is normally a function of the temperature, which means that the resistivity may vary several orders of magnitude, with varying temperatures.
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In the bushing development project, extensive studies and measurements have been done at ABB Corporate Research to clarify the basic insulation characteristics of the insulation materials used and their impact on the performance of the bushings. The material properties, such the permittivity, resistivity, including its temperature dependency and its dielectric withstand characteristics, were essential for the development of the 800kVdc bushings. The final bushings, equipped with composite insulators, are shown in fig. 6. (transformer bushing) and fig. 7. - fig. 9. (wall bushing)
fig. 6.
800kVdc transformer bushing during type testing at STRI. Insulation length of composite insulator 7,6m.
In addition to the mentioned measurements, a study is initiated at High Voltage Valley in Ludvika, Sweden [4], which is a collaboration between academy and industry in high voltage power transmission, facilitating competence development and research innovations with focus on implementation. Work is going on to dynamically simulate the creation and movement of charges inside and outside the bushing, based on and resulting in a total electric field distribution inside and outside the bushing. The measured electric field distribution outside the bushing are used together with the simulations to get a complete picture and
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understanding of the dielectric behaviour of the bushing and this knowledge is condensed in the design rules for bushings and other high voltage equipment. The transformer bushing, with a total length of 10m, passed all dielectric routine- and type tests, among which a 2 hours dry withstand tests at +1456kVdc (World Record). The high testing voltage is due to the requirement to test the transformer bushing at 115% of the maximum DC transformer voltage (+1266kV), based on a maximum system voltage of 102% of the system voltage of 800kVdc. The wall bushing, with a total length of 20 meter including standard corona rings, passed all dielectric routine- and type tests, among which a 2 hours dry withstand test at +1238kVdc and a 1 hour wet test at +1030kVdc. The wall bushing was mounted in the test hall having the outdoor part inside the hall, to be able to perform a well controlled wet test inside the test hall, see fig. 9.
Insulation length indoor 6,7m Insulation length outdoor 9m
fig. 7.
800kVdc wall bushing during mounting at STRI before type testing.
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fig. 8.
Overview of +1238 kVDC withstand test set-up at STRI in Ludvika. The white structure mounted at the base of the wall bushing is the robot used for the electric field measurements.
fig. 9.
Outdoor part of wall bushing during DC rain test indoor at +1030kVdc
All tests were performed at STRI in Ludvika, Sweden, under the supervision of SATS to obtain a complete type test report from an independent testing organisation.
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Summary of test results (all tests were successful): Wall bushing: ; Lightning impulse (LI). 15 positive and 15 negative impulses at 1900kV, 5 chopped impulses at -2185kV. ; Switching impulse (SI), wet. 15 positive and 15 negative impulses at 1700kV. ; AC: Radio Interference Voltage (RIV) test up to 872 kV. Capacitance, tan delta and pd measurements Partial discharge test during 60 min at 874kV Withstand test at 905 kV. ; DC: Polarity reversal test at +/- 979kVdc. 1 hour wet test at +1030kVdc. 2 hours dry withstand test at +1238kVdc. Transformer bushing: ; Lightning impulse (LI). 15 positive and 15 negative impulses at 1900kV, 5 chopped impulses at -2185kV. ; Switching impulse (SI), wet. 15 positive and 15 negative impulses at 1700kV. ; AC: Capacitance, tan delta and pd measurements Withstand and partial discharge test 1 hour at 1032kV ; DC: Polarity reversal tests at -/+/- 979kVdc and -/+/- 1126kVdc 2 hours dry withstand tests at +1266kVdc and +1456kVdc
4. Long duration test at 850kVdc in Sweden Introducing a new system voltage for dc transmission, demands a thorough verification of the long term behaviour of the equipment in a converter station as the availability requirements are high for an 800kVdc transmission system. From the beginning, it was clear that a long term test would be required to confirm the feasibility of an 800kVdc converter station. In early 2006 the work was started to build a dedicated 800kVdc test site on the grounds at STRI in Ludvika and in November 2006, the site was ready and energized, see fig. 1. All key components are included, such as a voltage and current transformer, voltage divider, RI capacitor, circuit breaker, arrester, disconnector, smoothing reactor and of course the composite support insulators, see fig. 10. All equipment installed in the 800kVdc test site has passed the required type tests. The site is operated at +850kVdc to confirm the rigidness of the equipment installed, even at higher stresses than can be obtained during normal service conditions.
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fig. 10. 800kVdc apparatus in outdoor test yard, incl. wall bushing. The transformer bushing is inside the gray test hall. Applied voltage is +850kVdc.
To simulate the service conditions as good as possible, it was decided to build a separate testing hall for the bushings. The test hall is to simulate the conditions in the valve hall for an 800kVdc converter station. The valves and their cooling system create indoor conditions at roughly 40-50 degrees C. At these elevated temperatures the transition in electric field, from capacitively to resistively controlled, is also faster thus making the long duration test more effective. It was therefore decided to have the same temperature of 40-50 degrees C in the test hall for the bushings, see fig. 11.
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fig. 11. Indoor part of wall bushing (left) and transformer bushing (right) inside the dedicated test hall at 50 degrees C to simulate valve hall conditions at a 800kVdc site. The robot for the field measurements is mounted at the base of the transformer bushing. Applied voltage is +850kVdc.
During the long duration tests, the electric field is measured along the transformer bushing to monitor the transition from purely capacitive AC-field distribution to a steady state resistive DC field distribution, including the impact of accumulated charges, see fig. 12. The measured electric field distributions also confirm the calculation models based on the resistivity of the insulation materials used. To maximize the outcome from the long duration test, several other types of measurements are done continuously, such as measurement of applied voltage, leakage current on specific equipment and weather conditions. Other measurements are done depending on specific weather conditions, such as Infra Red and Ultra Violet light, and normal video recordings.
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Probe
fig. 12. Measurement of 2-dimensional DC electric field along transformer the bushing, during the long duration test. The wooden structure is used to put the robot in parallel with the bushing. The position of the probe is indicated by the red circle and it is remotely moved along the bushing.
5. Summary and conclusions In order to effectively increase the power transmission capacity of HVDC transmission systems, all necessary steps have been taken to verify the rigidness of the equipment needed in an 800kVdc converter station. The activities performed include development, type testing and long term testing of all involved 800kVdc equipment and support insulators, as well as a confirmation of the fundamental material properties and subsequent short- and long term electric field distributions and withstand capabilities. The basic insulation material properties have been obtained at ABB Corporate Research in close collaboration with the production units for composite insulators and bushings in Sweden. The most critical components of an 800kVdc converter station, the transformer and the wall bushing have successfully been developed at ABB in Ludvika, Sweden. This development was possible thanks to newly developed composite insulators at the recently expanded composite insulator manufacturing plant of ABB in Pite책, Sweden. Although a major milestone has been taken by showing the feasibility of 800kVdc converter stations, the need for an even higher transmission capacity has already initiated plans for uprating the equipment (higher current) as well as to develop equipment for 1000kVdc.
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6. References [1]
[2]
[3]
[4]
“Experience with Composite HVDC and HVAC Insulators in China: From Design to Operation”, Prof. Liang Xidong, INMR, PROCEEDINGS OF THE 2003 WORLD CONFERENCE ON INSULATORS, ARRESTERS & BUSHINGS, 2003 “Optimized use of HV composite apparatus insulators: field experience from coastal and inland test stations”, I. Gutman, L. Stenström, D. Gustavsson, D. Windmar, W.L. Vosloo, Cigré paper A3-104, 2004 ”Today’s operating performance and manufacturing processing requirements for silicone rubber in electrical components”, Dan Windmar, Henrik Hillborg, INMR, PROCEEDINGS OF THE 2005 WORLD CONGRESS ON INSULATORS, ARRESTERS & BUSHINGS, 2005 High Voltage Valley, http://highvoltagevalley.org