Influence Factors Study On The Lightning Withstand Level Of500kV EHVAC Transmission Lines

With the rapid development of power system, extra high-voltage(EHV) system of a considerable scale has been built in China. Therefore, transmitting capacity of the Grid is enhanced due to the vast improvement of its structure. As a significant composition of the EHV system,500kV transmission lines take an important role in the delivery of electrical energy. The operational experience of the high-voltage(HV) and EHV transmission lines at home and abroad shows that the lighting fault is the key factor affects the safe operation of the transmission lines. Lightning withstand level is an important performance index to measure the lightning tolerance of transmission lines, therefore it is influential in the safety and reliability of power system.Basically, there are three causes for the lightning flashover of a transmission line:top of the tower and the overhead ground wire(OHGW) nearby are stroked, OHGW near the middle of a span is stroked, lightning strokes the conductor round the ground wire. In this paper, lightning withstand level of the500kV EHV transmission lines is analyzed under the above-mentioned three situations. Lightning current model, transmission line model, tower model, insulator string flashover model are built based on the electromagnetic transient simulation program PSCAD/EMTDC.Lightning withstand level of transmission lines is related to the following parameters: impulse grounding resistance of tower, number of insulator piece, line span, erect way of ground wire, etc. In this paper, these parameters are changed and the corresponding PSCAD models are established. And the influence on the lightning withstand level of500kV EHV transmission lines caused by above-mentioned factors is analyzed by simulation. Reasonable lightning protection methods are proposed against the three situations:reducing the grounding resistance of tower, increasing the number of insulator, fixing double-OHGWs can improve the lightning withstand level of line when lightning strikes to the top of tower; increasing the number of insulator, fixing double-OHGWs may improve the lightning withstand level when lightning strikes the conductor round OHGW; reducing the grounding resistance, enlarging the span of line, fixing double-OHGWs will improve the lightning withstand level when lightning strikes to the mid span of OHGW. Finally, spark discharge process is simulated experimentally by home-made spark discharge apparatus. Substep progress process and experimental phenomenon of spark discharge are analyzed. Fitting formula of striking distance coefficient is obtained by statistical analysis on experimental data. As a result of the limitation of experimental device, further improvement is needed in the following experiment.