Among the five provinces served by the China Southern Power Grid (CSG), the western provinces, including Yunnan and Guizhou, are rich in water resources and have a low demand for electricity. On the contrary, the eastern province of Guangdong lacks water resources yet has a huge power demand because of its well-developed economy.
CSG began to develop a transmission plan for an additional 11,500 MW to 13,500 MW to be transferred from west to east, to Guangdong. In 2003, the utility launched a series of feasibility demonstrations and studies on power transmission levels and capacities. Included were the practicability and cost-effectiveness of an ±800-kV dc transmission line linking the hydropower plants in Yunnan to load centers in Guangdong. The studies confirmed the growing power demand in eastern China required construction of the ±800-kV power transmission project. This west-to-east transmission project offers a large load-transfer capacity over a distance of more than 1,400 km (870 miles) with high efficiency.
Founded in December 2002, CSG is responsible for the investment, construction, operation and management of the southern power grid across five provinces: Guangdong, Guangxi, Yunnan, Guizhou and Hainan. This is a service area of 1million sq km (386,102 sq miles) supplying energy to a population of 230 million through a transmission system characterized by long distances and a large capacity that includes high-voltage alternating-current (HVAC) and high-voltage direct-current (HVDC) circuits operating in parallel.
The Yunnan-Guangdong project crosses Yunnan, Guangxi and Guangdong, starting at Lufeng, Chuxiong, Yunnan, in the west and ending at Zengcheng, Guangzhou, Guangdong, in the east. This project has a rated voltage of an ±800-kV dc, a capacity of 5,000 MW and a transmission distance of 1,418 km (881 miles). The main parts of the system are the Chuxiong converter station, the Suidong converter station, the dc transmission lines and electrodes at both ends.
This major HVDC project transmits clean energy from Yunnan, Xiaowan, and other hydropower stations to the load center of the Pearl River Delta region. CSG decided to adopt an ±800-kV HVDC line because it increases the transmission efficiency by reducing line losses by 40% and reducing the land required for the right-of-way compared with a ±500-kV dc option. Furthermore, it will solve the problem of excessive short-circuit current at the receiving end load center, hence improving the safety and stability of the power grid.
Converter Station Challenges
At the sending end of the Yunnan-Guangdong project, the Chuxiong converter station is at an altitude of about 1,850 m (6,070 ft), so the external insulation (including air clearance and creepage distance), electromagnetic environment and test voltage parameters had to be corrected accordingly. The external insulation of equipment was determined to take into account aging problems caused by intense ultraviolet radiation. For the main equipment, especially the 800-kV converter transformers, the width limit of 3.5 m (11.5 ft) for the railway transportation of equipment to Chuxiong imposed a strict design condition.
The Yunnan-Guangdong dc system at the sending end has two operational conditions: parallel operation with the ac system and isolated operation. The dc system had to be designed to satisfy the two operating conditions. The Chuxiong converter station is linked to the Xiaowan and Jinanqiao substations by two 500-kV ac transmission lines. In addition, it also is linked to the 500-kV Heping substation (Kunxibei substation) in the main grid of Yunnan by two 500-kV ac lines. A situation in which only the Xiaowan or Jiananqiao substation is connected to the Yunnan-Guangdong dc transmission lines would be deemed an isolated operational condition.
The converter station at the receiving end is located in a heavily polluted area. With an equivalent salt deposit density up to 0.066 mg/sq cm (4.26 mg/sq inch), the design of the external insulation of equipment and the installation height of dc switchyard equipment proved challenging. The Yunnan-Guangdong ±800-kV dc transmission system shares the grounding electrodes on the Guangdong side with the ±500-kV dc transmission system. Therefore, the problems related to the design of the electrodes and the operation of these two dc systems, arising from the shared grounding electrodes, had to be overcome.