Compared with the ±600 kV class and the UHV DC below 600 kV, the main technical and economic advantages of UHV DC transmission can be summarized in the following six aspects:
1. Large transmission capacity. Using 4000 ampere thyristor valves, the ±800 kV DC UHV transmission capacity can reach 6.4 million kilowatts, which is 2.1 times that of the ±500 kV, 3 million kilowatts high-voltage DC method, and 1.7 times that of the ±600 kV level, 3.8 million kilowatts high-voltage DC method, enabling full scale transmission advantages.
2. Long transmission distance. The use of ±800 kV DC transmission technology makes it possible to transmit power over long distances, and the economic transmission distance can reach 2,500 km or even further, providing a transmission guarantee for the development of a large hydropower base in southwest China.
3. Low line losses. In the case of total wire cross-section, transmission capacity are the same, ± 800 kV DC line resistance loss is ± 500 kV DC line of 39%, is ± 600 kV level DC line of 60%, improve transmission efficiency, save operating costs.
4, engineering investment savings. According to the relevant design department’s calculations, for ultra-long distance, very large capacity transmission needs, ± 800 kV DC transmission program unit transmission capacity integrated cost of about ± 500 kV DC transmission program of 72%, saving project investment benefits significantly.
5. High corridor utilisation rate. ± 800 kV, 6.4 million kilowatts DC transmission scheme line corridor for 76 meters, unit corridor width transmission capacity of 84,000 kilowatts / m, is ± 500 kV, 3 million kilowatts scheme and ± 620 kV, 3.8 million kilowatts scheme of about 1.3 times, improve the efficiency of transmission corridor utilization, save valuable land resources; due to the single line transmission capacity, significant savings in the valley, the River crossing points of the limited resources.
6. Flexible operation mode. The proposed 400+400 kV double twelve pulsating converter series connection scheme of the State Grid Corporation’s UHV DC transmission has a flexible mode of operation and greatly improves system reliability. If any of the converter modules fail, the system can still guarantee 75% of the rated power delivery.
