大规模直驱风电场经柔直送出系统交直流线路故障特征分析

Analysis on Fault Characteristics of Large⁃Scale Direct⁃Drive Wind Farm PassingThrough AC/DC Line of VSC⁃MTDC Transmission System

针对具有故障穿越策略的永磁风电场经多端柔性直流(Voltage Source Converter Based Multi-terminal Direct Current,VSC⁃MTDC)输电系统外送的交直流混联系统,分析故障穿越控制目标下受端交流电网故障和考虑输电线路频变特性的直流系统输电线路故障发生后的故障特征,以及大时间尺度下,直流系统对交流故障的响应。研究结果表明:基于实时数字仿真(Real Time Digital Simulation,RTDS)搭建的交直流混联系统模型在有效控制故障发展的前提下,不同故障条件、控制策略下具有相对应的故障特征,如:交流并网线路故障时,负序电流将被抑制,换流站仅向故障点提供有限的正序电流,且优先输出无功功率,直流线路累积不平衡功率出现直流过电压现象,计及直流电压波动引起受端换流站的dq轴电流发生变化,造成交流线路故障电流短时间内增大约20%。直流线路发生单极接地故障时,故障极电压以最大700 kV/ms的速率迅速下降到0,非故障极电压上升为原来的两倍,且故障电流会随着接地电阻的增大而减小;当发生双极故障时,故障电流能够在数毫秒内达到额定电流的几十倍,最大速率可达8.25 kA/ms,严重威胁电力电子器件的安全,并且电压迅速降至0,此时通过混合直流断路器快速动作,在故障电流上升至额定电流2倍前切除故障线路,使系统满足N-1运行原则。仿真结果与理论分析结论相符。

Aiming at the AC/DC hybrid system with permanent magnet wind field with fault ride⁃through strategy and multiterminal flexible DC transmission,the fault characteristics of the AC grid fault at the receiving end under the fault ridethrough control target and the fault of the DC system transmission line considering the frequency variation characteristics of the transmission line are analyzed,as well as the response of DC system to AC fault at a large time scale.The research results show that the AC/DC hybrid system model based on RTDS has corresponding fault characteristics under different fault conditions and control strategies,while effectively controlling the development of faults.For example,When the AC grid⁃connected line fails,the negative sequence current will be suppressed,and the converter station only provides limited positive sequence current to the fault point,and preferentially outputs reactive power.The accumulated unbalanced power of the DC line will cause DC overvoltage phenomenon.Considering the fluctuation of DC voltage,the dq axis current of the receiving end converter station will change,resulting in a short⁃time increase of about 20%in the fault current of the AC line. When a single⁃pole grounding fault occurs in the DC line, the fault pole voltage drops rapidly to 0 at a maximumrate of 700 kV/ms, while the non⁃fault pole voltage rises by two times, and the fault current decreases with the increase ofthe grounding resistance. When a bipolar fault occurs, the fault current can reach dozens of times of the rated currentwithin milliseconds, with a maximum rate of 8.25 kA/ms, which seriously threatens the safety of power electronic devices,and the voltage rapidly drops to 0. At this time, through the rapid action of the hybrid DC circuit breaker, the fault lineis removed before the fault current rises to two times the rated current, so that the system meets the N ⁃1 operationprinciple. The simulation results are consistent with the theoretical analysis conclusions.