MMC直流输电网线路短路故障电流的近似计算方法

An Approximate Method for the Calculation of Transmission Line Fault Current in MMC-HVDC Grid

柔性直流电网可灵活消纳大规模可再生能源,但直流线路故障后严重的过电流问题是制约其发展的重要因素之一。短路故障电流的分析和计算对于系统参数的设计和选取具有重要意义,但是由于换流站是高度复杂的非线性系统,无法得到故障电流的精确解析表达式,因此短路故障电流往往通过仿真得到。对于故障电流近似表达式的研究也往往只能针对双端直流系统进行,对于直流电网下的线路短路故障电流则无能为力。该文根据模块化多电平换流器(modular multilevel converter,MMC)柔性直流电网系统参数的基本特点对其进行合理的简化,得到线路短路故障电流的近似解析解的求解方法,可以在故障后一段时间内较为准确的描述故障电流的整体趋势,可为直流线路保护定值和直流断路器相关参数的选取和整定提供一定的依据。

VSC-HVDC grid can flexibly absorb large-scale renewable energy, but the severe overcurrent problem after transmission line fault is one of the most important factors which restrict its development. Calculation and analysis of short circuit fault current is meaningful to the design and selection of system parameters. However, converter is a complex nonlinear system which makes it impossible to get exact analytical solution of fault current, so the fault current is always obtained from simulation. Research concerning approximate expression of fault current is always based on two-terminal HVDC systems, but incapable of HVDC grid. This paper reasonably simplified the MMC-HVDC grid based on the feature of its parameters, in order to get an approximate way to calculate fault current. The proposed method can describe the general trend of fault current in a relatively accurate way during a certain period after fault occurs. This method can provide a basis for the selection of the setting value of transmission line protection and the parameters relevant to DC circuit breakers.