Overvoltage and Insulation Coordination
Installing arresters at appropriate locations is a key measure taken to reduce transient overvoltages, but the configuration of arresters in ±800-kV dc systems is complex. To minimize the transient overvoltage arising from switching or faults, the configuration scheme for this project was determined in conjunction with the voltage/current characteristics of the arresters. Additionally, smoothing reactors are installed separately at the pole line and neutral line to reduce the overvoltage level on the high-voltage pole line.
The corresponding withstand voltage of switching impulse and lightning impulse in the previous ±500-kV dc systems in China is 1,300 kV and 1,550 kV, respectively. Compared with the linear extrapolation from the ±500-kV dc insulation level, both the overvoltage and the insulation levels of the equipment used in the ±800-kV dc project are substantially reduced through reasonable arrester protection and insulation coordination. The results from the research CSG had performed earlier made it possible to reduce the research and development (R&D) complexity and manufacturing cost of the equipment on the premise that sufficient safety margins would be maintained.
The isolated operational mode is of significant importance to improve the transmission capacity and troubleshooting capability of the power grid. It imposes strict requirements on the overvoltage suppression and control sequence of the Yunnan-Guangdong project. In case of isolated operation of the dc system on the Chuxiong side, the temporary overvoltage (double dc poles blocked successively) of up to 1.57 p.u. may occur, so the following necessary technical measures were taken:
- Four interrupter circuit breakers were used at the incoming lines of four ac filter banks, and double-interrupter circuit breakers were used as the filter sub-banks. The circuit breakers of ac filter sub-banks can be tripped only when the ac overvoltage of the Chuxiong converter station is lower than 1.3 p.u.
- In isolated operation, the circuit breakers of ac filter banks at the Chuxiong converter station were set to be disconnected 100 msec after bipolar blocking of the Yunnan-Guangdong dc system.
- The dc control and protection system was designed so that all circuit breakers of converter transformers would not trip before ac filters in the event of dc bipolar load rejection.
- Two-column arresters were used for ac bus bar at converter stations instead of single-column arresters to improve the ability of energy dissipation.
At the sending end, because the Chuxiong converter station is at high altitude, external insulation and test voltage parameters had to be designed accordingly. Meanwhile, as heavy pollution prevailed at the receiving end, the external insulation of the ±800-kV dc system involved several considerations:
- Air clearance and altitude correction
- Pollution in the Yunnan-Guangdong dc line corridor and station sites as well as operational experience
- Pollution flashover characteristics of insulators under the conditions of high altitude, pollution and ice coating
- Characteristics of dc composite insulators and application of external insulation at converter stations under polluted conditions
- Development of dc composite insulators as well as a full-voltage and full-size pollution characteristic test
- Lightning protection and the grounding of dc lines.
Composite insulation material is an effective way to solve the excessive structural height of porcelain post insulators for the ±800-kV dc converter station in a polluted ambient environment. The research confirmed the specific creepage distance of composite insulators is 75% that of porcelain insulators, thus the structural height of composite post insulators was designed with a length of 10 m (33 ft).