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Aiming at the erection structure of 750kV four-circuit transmission lines on the same tower and the overvoltage characteristics of the system, the long wave-front discharge characteristics of phase-to-phase air-gaps are studied, and the discharge characteristics curves under different gap distances are obtained. Based on the overvoltage characteristics of 750kV power transmission system, the phase-to-phase insulation configuration of 750 kV four-circuit transmission lines is proposed. The results show that the distance between two phase conductors is longer than that of conventional double-circuit 750kV transmission line.

Composite cross-arm of ultra-high voltage (UHV) tower bears both electric and mechanical loads. Owing to its high operation voltage and very complex insulation structure, potential and electric field distributions of composite cross-arm are extremely uneven. It may affect long-term stable operation of transmission line. Using three dimension finite element method (FEM), this paper presented research on voltage equalizing characteristics of composite cross-arms in 1000 kV tower. Computation model was established. The potential and electric field distributions of middle phase and side phase under several design schemes were computed. The program for grading and shielding devices was designed and then optimized. The electric field computation results showed that the electric field strengths on key positions were within permitted range. Corona tests showed that no visible corona discharge was found from insulators or fittings when the applied voltage was 1.2 times of test voltage. The design of the grading and shielding devices can be applied in actual projects.

Due to unbalanced energy resources and allocation of productive force in China, our country has to develop and build ultra-high voltage grid. As most of western regions in China are located in high attitude area, the construction of power transmission and transformation project would inevitably go though these different altitude regions with average elevation of over 1000 meters. The atmospheric condition in different altitude regions has distinctive impact on the discharge voltage of outdoor insulators. The experimental results from low altitude regions cannot be used for engineering design of high altitude applications. The characteristic of air gap discharge is the basis of UHV transmission line external insulation design. Studying the discharge characteristics of side-phase discharge and its altitude correction can not only provide reference for choosing air gap distance in power projects at high altitude, but also be used for correcting air gap discharge voltage at high altitude. For the purpose of obtaining air gap discharge voltage and its correction in different altitudes, typical phase-to-phase air gap was selected, and comparative experiments were conducted under the same configuration condition at the different altitude areas such as Wuhan (35m), Chengdu (500m), Guiyang (1000m), Kunming (2100m). The experiments were carried out under extreme weather conditions which considered air pressure, temperature and humidity, including direct alternating current (AC) voltage discharge tests with gap length of 1.3m, lightning impulse (LI) voltage discharge tests with gap length of m, and switching impulse (SI) voltage discharge tests with gap length of 3.3m and 4.2m. According to the test results, the influence of atmospheric conditions and altitude correction factors of AC, LI, and SI discharge voltage were calculated and analyzed systemically in this paper. Finally, altitude correction parameters of typical air gap discharge voltage at high altitude of 2100m in voltages of AC, LI and SI were proposed, and the impact of proposed changes in the voltage range under the influence of factors such as temperature, humidity, and barometric pressure, etc., obtained modified formula to meet the needs of practical engineering.

In this paper, experiments on artificial polluted insulator are conducted to study the 50% withstand voltage U50 of artificial polluted suspension insulators under different levels of contamination and asymmetric contamination on the top/bottom surface. The electrical fields along the insulator surfaces under different scenarios, such as asymmetric contamination on top/bottom surface, and uneven circumferential distribution of surface contamination, have been calculated by the use of finite element analysis for field simulation. The simulated result shows that asymmetric top/bottom surface contamination has a greater impact on the insulator electrical field distribution, which has practical meanings to the anti-fouling design of the transmission line under different pollution level across the country.