含交流影响的MMC-HVDC直流故障电流递推计算方法

Recursive Calculation Method of MMC-HVDC DC Fault Current With AC Effect

基于模块化多电平换流器的双极柔性直流输电系统发生直流侧单极接地故障后,其等值电路为RLC放电电路,该等值电路无法将交流量及控制环节纳入计算,结果误差较大。该文从换流器能量角度出发,采用递推公式的方法对短路电流进行计算,首先推导直流侧故障前后的放电递推公式,依据本时刻的瞬时直流电压及控制器输出的参考波求出上下桥臂子模块拟合的交流电压瞬时值,进而求出交流电流瞬时值。根据直流侧和交流侧向换流器注入或泄放的能量计算出换流器的剩余能量,利用剩余能量反求直流电压及直流电流实际瞬时值,并将所得数据作为下一计算周期的初值。使用Matlab将上述递推算法编程,并与PSCAD详细仿真模型进行对比,得出,所提出的递推方法较为精确,弥补了纯直流放电算法的不足,误差极小,可以等效代替故障前后的直流系统。

The equivalent circuit of the bipolar flexible HVDC system based on modular multilevel converter(MMC)is RLC discharge circuit after the DC-side single-pole grounding fault occurs.The equivalent circuit do not include AC system and controller in the calculation,and the result error is large.From the angle of converter energy,this paper calculated short-circuit current by recursive formula.Firstly,the recursive formula of discharge before and after DC side fault was deduced.According to the instantaneous DC voltage at this moment and the reference wave output by the controller,the instantaneous value of AC voltage fitted by upper and lower bridge arm sub-module was calculated,and then the instantaneous value of AC current was obtained.The residual energy of the converter was calculated according to the energy injected or released by the DC side and AC side.The actual instantaneous values of DC voltage and DC current were calculated by using the residual energy,and the data obtained were taken as the initial values of the next calculation cycle.The recursive algorithm was programmed with Matlab and compared with the detailed simulation model of PSCAD.It is concluded that the recursive method proposed in this paper is more accurate and has minimal error than the method,which makes up for the shortcomings of the pure DC discharge algorithm,and can replace the DC system before and after the fault equivalently.