Along with China’s rapid economic growth over the past three decades has come a major revamping of that country’s energy and load planning strategies. In particular, DC transmission has begun to take on an increasingly important role since 1990 due to its known advantages for HV & UHV lines running great distances (see article on page 46).
Starting with the ±500 kV of Ge-Nan project, first commissioned in 1989, and the ±800 kV Xiang-Shang and Chu-Sui lines completed in 2010, the framework of a huge DC backbone has now taken shape across the country. Indeed, by the end of 2012 China was already operating three ±800 kV lines, one ±660 kV line, ten ±500 kV lines, one ±400 kV line as well as three back-toback DC connection projects. More DC lines are now in planning, under construction or recently commissioned and some will operate as high as ±1100 kV.
At the same time, the source of DC transmission technology used in China has undergone a complete changeover – from only foreign turnkey projects at the start, to foreign-dominated projects, to foreign cooperation on projects, to Chinese-dominated projects and finally to total Chinese self-reliance. For example, at ±500 kV DC, China has achieved almost full local production capability.
During development of these various DC projects, there has been considerable experience with failures of external insulation on certain apparatus. At the beginning, there were a relatively large number of such faults. But after applying suitable countermeasures, these have progressively been reduced to the current situation of mostly trouble-free operation.
This INMR article from 2014, contributed by experts Wie Jie and Su Zhiyi of the Electric Power Research Institute in Beijing, reviewed past research conducted by the State Grid and the China Southern Grid on external insulation faults at ±500 kV converter stations, using voltage dividers as the reference for experience with a range of equipment
Overview of ±500 kV Point-to-Point DC Transmission in China
At the start of 2012, China already had a total of 10 operating ±500 kV point-to-point DC systems with a combined transmission capacity of some 28,800 MW (see Table 1).
Ge-Nan, China’s first long-distance DC transmission line, ran from the Gezhouba Station in Hubei to Nanqiao in Shanghai and its equipment and design came entirely from suppliers based in Europe. About 2009 to 2010, this initial system was retired as part of a comprehensive retrofit lasting some 6 months and during which capacity was increased to 5.786 billion kWh.
The Jiang-Cheng, Long-Zheng and Yi-Hua DC lines, transporting up to 9000 MW from the Three Gorges to Guangdong and Huadong, were designed jointly by Chinese and western-based suppliers. During 2010, the availability of these systems was high (respectively 17.23, 13.89 and 14.91 billion kWh) and together they accounted for 55% of the total capacity generated at that time at the Three Gorges.
Similarly, China Southern Power Grid’s Tian-Guang, Gao-Zhao and Xing-An DC transmission systems were designed jointly by Chinese and German suppliers and had a combined capacity of close to 7800 MW. By early 2010, the Tian-Guang system’s transmission capacity was up to 5 billion kWh while those of the Gao-Zhao and Xing-An lines were 12.25 and 13.38 billion kWh respectively.
The ±500 kV De-Bao, Yi-Mu and Lin-Feng DC systems, commissioned after 2010, were entirely designed in China and, aside from certain key technologies and equipment, everything was locally produced. Up to now, these lines have had mostly stable operation, apart from individual discharge faults on the line side flat wave reactor, found to be due to a design defect.
External Insulation Faults Affecting ±500 kV DC Voltage Dividers
Faults at JiangLing, LongQuan & YiDu Converter Stations
Flashovers of external insulation on ±500 kV DC voltage dividers at Jiangling, Yidu and Longquan Converter Stations were reported late in 2004, early 2009 and early 2010. These flashovers occurred under similar conditions of more than 2 weeks of persistent rain, including 2 to 3 days of heavy fog during which visibility was reduced to only about 5 m.
Suppliers of the affected equipment were contacted and confirmed that the same silicone rubber material was used on all bushings of the affected DC voltage dividers. Table 2 lists the technical parameters of these units, which are shown in Fig. 1.