Study On The Voltage Distribution Of Insulator Strings For 750kV Transmission Lines

The insulators is one of the key components for overhead transmission, and the performance of insulators will directly affect the safe operation of the entire line. The stray capacitance which between the insulators and tower and conductors make the voltage distribution along the insulator string uneven. The 750kV power grid will become the main grid in Northwest China, with the construction of 750kV power grid in anywhere of Northwest of China. The insulator strings is longer and the type of insulator is various, which make the voltage distribution of insulator strings more uneven and Implementing the measures to improving the voltage distribution is difficulty. Therefore, to determine the potential distribution of insulator string is very important to detect the poor insulator and safety and stable operation of transmission line. There is on standard voltage distribution of insulator string for 750kV transmission lines, which is not conductive to detect the poor insulators and affect the safe operation of the transmission lines.According to the actual situation of the insulators, insulator three-dimensional electric field computation models considering the effects of tower, bundle conductors, grading ring, shielding line and so on were set up based on the finite element method. That used to determine the voltage distribution of insulator strings for 750kV transmission lines. The impact of some factors, such as bundled conductors, tower, shield line, grading ring, the type of insulators, material of insulators, insulator suspension mode, conductor arrangement and so on, on the potential distribution of insulator strings located in different position were analyzed and the structure of grading ring was optimized, and then the curve of potential distribution of the insulator strings for different tower, different number of insulators, different types of insulators and different materials of insulator of 750kV transmission lines was determined. The results showed as follows.①The bundle conductor make obvious effect for potential distribution of insulator strings. It can make the potential distribution of insulator string more uniform. The results show that if the length of conductor is more than 8 times of the length of insulator strings, the effect of conductor is equivalent with the actual wire. The other phase of conductors on the same towers impact the potential distribution of insulator strings more uneven. The tower make obvious effect for potential distribution of insulator strings too. Ignored the impact of tower, the potential distribution of insulator string will be more uneven, especially take significant for the ground side of the insulator withstand voltage. Other factors such as shield line, the type of insulators, insulator suspension mode, the arrangement of conductors, material of insulators and so on, have influence on the potential distribution of insulator strings.②The best parameter of grading ring to control the electric field distribution of insulators for porcelain or glass insulators is that the position of ring is located between the second and third insulator,and the diameter of grading ring is taken as 900mm~1000mm, and the caliber of grading ring taken to be 100mm~120mm. And the best for the composite insulators is that the value range of h is 150mm~300mm, 1100mm~1300mm for R andΦis taken as 140mm~180mm.③Increasing the number of insulator could reduce the withstand voltage of single insulator. But that only make obvious effect for the insulators which near by the tower and not effect for the side of conductor. The potential distribution of insulator strings is more and more uneven when increase the number of insulator.④For the double-circuit transmission lines on the same tower, the potential distribution of the upper phase insulator strings is the most uniform and the middle phase is the most uneven. Potential distribution of insulator string for the corner tower is more uniform than the straight tower. The maximal withstand voltage of single insulator for the middle string of single circuit tower is more than the side string. The voltage distribution for V string is more uneven than I string. When the number of insulators increase, theβof the insulator strings for single circuit tower increases more obvious than the double circuit towers.