AORC Technical meeting 2014

Transcription

1 AORC Technical meeting 2014 http : // B Development of ±160 kv XLPE Cable and its Application to the World s First Three-terminal VSC HVDC System in China Lin-jie Zhao, Hong Rao, Xiao-lin Li, Wei Zhang, Yang Xiang, Ming-li Fu Electric Power Research Institute, China Southern Power Grid Co., Ltd P. R. China SUMMARY This paper reports the experience of first ±160 kv Cross-Linked Polyethylene (XLPE) insulated HVDC power cable development and its application to the world s first threeterminal modular multilevel (MMC) voltage sourced converters (VSC) high voltage direct current (HVDC) power transmission pilot project in China. Special focus is on the solution of three more difficulties challenges which the development of ±160 kv XLPE insulated HVDC power Cable and its accessories faced. The first one is the different insulation co-ordination principle considering the connection of HVDC power cables and overhead lines. The second one is the DC voltage withstand factory test method in the absence of proper testing terminations. The third one is how to evaluate the effect of AC harmonic voltage to the electrical performance of cable system under certain operation conditions. The pilot project has been commissioned and was successfully put into operation on December 25th Now, the experience shared in this paper are applied to the development of ±320 kv XLPE Cable and its accessories in China. KEYWORDS High voltage direct current (HVDC) power transmission Voltage sourced converter (VSC) Modular multilevel converter(mmc) HVDC power cable Crosslinked polyethylene (XLPE) Insulation coordination DC Voltage withstand test AC Voltage withstand test zhaolj@csg.cn

2 1. Introduction With steadily growing demand for electricity, China is facing the challenge of expanding its grid to keep up with requirements, whilst addressing environmental issues. The voltage sourced converter (VSC) high voltage direct current (HVDC) power transmission technology has been well developed over last a few years both in China and elsewhere in the world. A pilot project with the rating of ±160kV/200MW/100MW/50MW has been built and comissioned to bring dispersed and intermittent wind power generated on Nan- Ao island into onshore grid of China Southern Power Grid (CSG), of which 32km connection consists of XLPE insulated HVDC land and submarine cables and overhead lines. It is the world s first three-terminal modular multilevel converter(mmc) HVDC system developed under the support of National High Technology Research and Development Program of China (863 Program) (2011AA05A102). It is also the first ever in China that a XLPE insulated HVDC power cable was developed and used in an engineering application. Several equipment suppliers are involved in the project: they are three VSC HVDC valve suppliers, three control & protection system/equipment suppliers and three XLPE land/sea cable suppliers. All of them are Chinese domestic manufacturers. SEPRI (Electric Power Research Institute, China Southern Power Grid) is technically responsible for the entire project. Our study fields focus on designing main wiring and control & protection strategy of MMC HVDC system, analysing over-voltage and putting forward insulation co-ordination principles, drawing up main equipment technical specifications, developing test method and evaluating main equipment performance, system commissioning, and so on. This paper briefly introduces the design of the pilot project and its requirements to HVDC power cable, and shares the solution of three more difficulties challenges which the development of ±160 kv XLPE insulated HVDC power Cable and its accessories faced. 2. The design of the pilot project and its requirements to HVDC power cable The aim of building the pilot project is connecting dispersed wind-farms on Nan-Ao island of Guangdong province, China, into onshore AC grid of China Southern Power Grid (CSG) and ensuring the power transmission stability. There are three converter stations (named as A, B and C) designed, as shown in Fig.1. Station A and B separately connect the dispersed wind-farms by AC transmission lines on Nan-Ao island and Station C connects the onshore AC grid. Station A and B are inter-connected via a HVDC transmission line, and Station B and C are also inter-connected via another HVDC transmission line. MMC technology as show in Fig.2 is used [1]. Thus, a three-terminal MMC HVDC power transmission system is implemented. According to the wind power capability of the present and planning wind farms, rated DC transmission voltage is designed as ± 160kV and the power ratings of Station A/B/C are as 50MW/ 100MW/200MW. The three flexible operation modes of the pilot project are designed and as follows. Parallel operation mode of MMC HVDC transmission system and the present AC submarine cable transmission line Mode of wind power transmission only via MMC HVDC system Pure reactive power compensation operation mode (MMC converters act as Static Synchronous Compensators (STATCOM)) Due to the limitation of the geographical conditions, as shown in Fig.1, Station A and B shall to be connected by an overhead line. The interconnection between Station B and C shall to be of 2 sections of submarine cable with length of about 10km and 1.2km, 3 sections of land cable and 1 sections of overhead line with length of about 8.5km. The maximum water depth of the submarine cable laying patch is about 15m. It is the most complex VSC HVDC transmission line system up to now. 1

3 1-2, 3-4, 6-7: Land cable 2-3, 5-6: Submarine cable 4-5: Overhead line 1, 4, 5, 7: Cable termination connected to overhead line or station bus bar 2, 3, 6: Prefabricated joints between land cable and submarine cable Fig.1. Diagram of the pilot project and the requirements to HVDC power cable. Fig.2. The Wiring Diagram of Single Converter with Bipolar Symmetric Mode. 3. Development of ±160 kv XLPE Cable and its insulation coordination In this pilot project, single-core XLPE land/submarine cable and its accessories with ratings of ± 160 kv/625a are designed and developed by three Chinese domestic manufacturers. The conductor cross-sectional area is selected as 500mm 2 in order to make that the conductor operation temperature does not exceed 70 Celsius degree under different environmental conditions. Cable insulation is made of XLPE, and accessories insulation is made of EPDM. However, determination of the insulation thickness is difficult for cable manufacturers because of the different insulation coordination principles under the connection status of HVDC power cables and overhead lines. XLPE cables not only experience the DC voltage from the HVDC system, but also the potential lightning impulse from the overhead lines under operation conditions, which makes the cable insulation thicker than that decided by system DC voltage. The detailed simulation calculation using PSCAD/EMTDC software by SEPRI shows that the maximum potential lightning impulse voltage from which the HVDC power 2

4 cable insulation suffers could reach 337kV. Arrester protection for HVDC power cable is necessary. According to the insulation coordination procedures recommended by IEC/TS and based on the lightning/switching over-voltage analysis results of the whole MMC HVDC system, the rated lightning/switching impulse withstand voltages of the cable system should be 550kV/450kV based on the protection of arrester with the lightning lightning/switching impulse protection level 302kV/271kV. After that, the insulation thickness of HVDC power cable system is selected by manufacturer as 16mm, properly higher than the valve 12mm of which cable system can not experience the lightning impulse voltage. The final specified technical parameters of the developed cable system are described in Table 1. Table.1 The final specified technical parameters of the developed cable system No. Parameters unit value note 1 Rated DC voltage(u0 ) kv ±160 2 Max. continues operation DC voltage (Um ) kv ±168 3 Conductor cross-sectional area(s ) mm Max. allowed conductor temperature( T ) C 70 5 Rated current-carrying capacity(i ) A DC test voltage (UT ) kv 1.85U0 Type test and routine test 7 DC test voltage (UTP1) kv 1.45U0 prequalification test and test after installation Superimposed lightning impulse test voltage(type test) -opposite polarity Superimposed switching impulse test voltage(type test) -same polarity Superimposed switching impulse test voltage(type test) - opposite polarity short current duration of metal sheath kv kv kv ka LI(+550)+DC(-160); LI(-550)+DC(+160) SI(+290)+DC(+160); SI(-290)+DC(-160) SI(+450)+DC(-160); SI(-450)+DC(+160) 8kA,1s 2.1U0 for prequalification test according to CIGRE TB U0 for prequalification test according to CIGRE TB DC voltage withstand factory test method The type test was successfully applied to the developed cable samples on witch formal joints and terminations assembled according to the electrical performance test method recommended by CIGRE TB [2] and mechanical performance test method by ELECTRA [3], as shown in Fig.3. However, how to properly evaluate the DC withstand performance of many delivery length of cable in the factory is a challenge. In this pilot project, as shown in Fig.4, it is experimentally demonstrated that the 1.85U0 DC test voltage can be successfully applied to the insulation of the cable within the specified testing time, after about 2m-length metallic sheath and outer semi-con layer 3

5 are carefully stripped for cable termination. It makes the DC voltage withstand test easier and cheaper in case there is no proper expensive testing terminations. Fig.3. The electrical performance type test field. Fig.4 The DC voltage withstand factory test field. 5. Test evaluation of the effect of AC harmonic voltage on the electrical performance of HVDC power cable HVDC cable could experience the combination of system DC voltage and AC harmonic voltage under certain operation conditions. The occurance of AC component will increase the dielectirc loss and decrease the partial discharge incept voltage, which may lead to cable insulation failure due to accumulative effect. In this pilot project, in order to evaluate the effect of potential AC harmonic voltage, an explorative sampling test was carried out by applying 0.8U0 for AC voltage withstand test with partial discharge measurement in 30 minutes, then a 1.85U0 DC voltage withstand test in 60minutes and finally being followed by a 0.8U0 AC voltage withstand test with partial discharge measurement in 30minutes. This method combination is also used for defect inspection of the HVDC XLPE cable. All of the samples from formal cable and accessory products successfully passed the combined test. Fig.5 shows the submarine cable laying field. After the developed ±160 kv XLPE cable system was installed in the field to connect the stations, MMC HVDC system was commissioned. Fig.6 indicates cable insulation shall bear the mixed voltage of DC 120kV plus AC 17kVrms after MMC converters energized but before de-blocked. The frequency of AC voltage is about 150 Hz. This status keeps from several minutes to no more than 30 minutes. Once MMC converter is de-blocked, AC harmonic voltage shall disappear, as shown in Fig.7. Under the premise of that XLPE DC breakdown field strength is 2.3 4

6 times of its AC breakdown field strength, DC test voltage value (1.85U0) is 1.86 times of the equivalent DC voltage value of the mixed operation voltage, AC test voltage value (0.8U0) is times of the equivalent AC voltage value of the mixed operation voltage. This means that there is no effect of AC harmonic voltage on the electrical performance of XLPE HVDC power cable during the MMC converter energizing process. Fig. 5. The developed submarine HVDC cable laying field. Fig. 6. Oscillograph of DC operation voltage during MMC converter energizing process. Fig. 7. Oscillograph of DC operation voltage after the MMC converter deblocked. The pilot project has been commissioned and was successfully put into operation on December 25th

7 7. Conclusion China has well developed ± 160 kv XLPE HVDC land and submarine cable and successfully apply it to the world s first commercial three-terminal MMC HVDC system. It is regarded as a very important milestone of this technology, not only for China, but also for the global power industry as a whole. Now, the development of ±320 kv XLPE HVDC cable is undergoing under the support of National High Technology Research and Development Program of China (863 Program) (2013AA050102). References [1] CHEN Ming, RAO Hong, LI Licheng, et al. Analysis on the Main Wiring of Nan ao VSC-HVDC Tranmission System [J]. SOUTHERN POWER SYSTEM TECHNOLOGY, 6(6), 2012: 1-5. [2] CIGRE TB 496, Recommendations for Testing DC Extruded Cable Systems for Power Transmission at a Rated Voltage up to 500 kv [R] [3] ELECTRA 171, Recommendations for mechanical tests on submarine cables[r]