The grounding pole line is an important equipment of the UHVDC transmission system. Due to the low steady state voltage of the grounding pole line during the normal operation, the grounding pole line adopts the external insulation of the 35kV AC line. However, 110kV voltage grade is used mostly for tower of the grounding pole. So the external insulation is lower. When the DC system runs out of balance, there will be overvoltage on the grounding pole line, which may cause the external insulation to be broken down. The "7.13" grounding line fault occurred in 2015 in +/- 800kV Bin-jin HVDC project. Due to the high voltage DC casing breaking down in Jinhua, the system turned directly into the unipolar operation. The overvoltage of the Yibin to Jinhua grounding line at the moment of the casing breakdown caused the insulation breakdown of the grounding electrode without causing action of the line arrester. Under the condition that the DC continuous flow can not be extinguished, the accident of grounding insulator, ground wire and conductor burning has occurred. The occurrence of "7.13" accident has exposed the overvoltage and insulation coordination design defects of the grounding electrode circuit of the HVDC system. In this paper, the "7.13" fault of +/- 800kV Yibin to Jinhua DC project was taken as the analysis object. The simulation model was established by using the power system analysis software, MATLAB Sim Power System. The internal overvoltage characteristics and distribution of the grounding circuit of the UHVDC system were analyzed. The grounding line internal overvoltage level was calculated. Analysis showed that the overvoltage inside the UHV grounding circuit was depended on grounding line length, system operation mode, outgoing arrester parameters. To inspect the grounding electrode line of the converter station, the closer the grounding position of the high voltage pole was to the inspected converter station, the higher the overvoltage of the line of the grounding electrode. For the ground electrode line overvoltage caused by the high voltage pole line grounding, it showed a gradual downward trend from the converter station to the grounding pole on the grounding electrode line. The maximum overvoltage on the grounding electrode line is often caused by grounding fault on the high voltage side when the DC system was full power. Such overvoltage is the control value for the external insulation operation of the grounding electrode line. Based on calculation results of overvoltage, the appropriate insulation configuration adjustment scheme for grounding electrode line was given. After the "7.13" fault, the SGCC made a unified adjustment for external insulation configuration of the UHVDC grounding electrode lines. Number of insulator strings on the grounding electrode line has been increased to 5 or 6 pieces from 3 or 4 pieces. Distance of the insulator parallel gap was increased to 68cm from 20cm. Thus the problem was avoided that the MOA on the grounding conductor outlet could not protect the external insulation. The conclusion is of great significance to guide the external insulation design, fault analysis and operation and maintenance of the DC grounding line.
