Research Of Overvoltage Caused By Monopolar Grounding On UHVDC Transmission System

With the increase of electricity load levels, and the uneven distribution of the energy area and the load area, more and more long-distance, high-power, inter-regional transmission lines appear on our power system, and DC transmission system with economic good, reliable operating performance and the absence of stability of AC transmission system and other characteristics, it is ideal for long-distance and high-power transmission. Studing the overvoltage of DC transmission system to determine the insulation level of the DC transmission line is of great significance for our country’s energy security and economic development. Line-to-ground fault is the most common faults on the DC lines, and will produce the most severe overvoltage on the lines, so the calculatin and research of overvoltage caused by monopolar grounding on UHVDC transmission system is necessary.Firstly, this article analyze the principle of overvoltage caused by monopolar grounding on UHVDC transmission system to identify the factors that influence overvoltage. And then make the equivalent circuit diagram of UHVDC transmission system, set up the voltage and current status differential equations of every point on the line, and calculate the equations with improved Eular method. The results show that when the fault is located the middle of the line will produce a highest overvoltage on non-fault polar; shorter the line, the higher the level of overvoltage on the line; failure grounding resistance can reduce the transient component of the voltage wave caused by line to ground fault, the greater the failure grounding resistance, the smaller the magnitude of overvoltage.For accurate calculations and detailed analysis of the overvoltage caused by monopolar grounding on UHVDC transmission system, according to the actual parameters of YUN-GUANG UHVDC transmission system, combined with the existing theoretical models of transmission system equipment and transmission line, establish the simulation model of UHVDC transmission system by PSCAD, detailed calculate the influence of the DC filter, smoothing reactors, line length, failure grounding resistance, load level, the earth resistivity and fault location and other factors on the overvoltage caused by monopolar grounding, and the overvoltage on equipments of convertor station. The results show that the maximum overvoltage occurs when the fault location is the middle of the line, the DC filter, load levels and earth resistivity have some impact on overvoltage, the higher the load levels, the higher the overvoltage, the larger the earth resistivity, the smaller the overvoltage, but the impact is small; inductance values and the configuration of smoothing reactor have a greater influence to the overvoltage, the larger the inductance, the smaller the voltage, when smoothing reactor all install on the polar bus, the overvoltage is larger than smoothing reactor install on the polar bus and neutral bus separately; line length and fault grounding resistance impact on overvoltage is consistent with the previous conclusion by improved Euler method, when the line length is800km the line overvoltage reaches1.7pu, worthy of attention; overvoltage will appear on converter station equipments when a line to ground fault occurs, the DC filter is closed to the line side, it will be greater influenced when the fault is located near the station, the fault will cause a large overvoltage on its inductance, threat its insulation, so we should pay some attention to this. About measures to restrict the overvoltage, you can choose to install a lighting arrester at the middle of the line, which can effectively restrict the overvoltage.