基于电力电子变压器故障穿越策略的低压直流配电网保护

Protection for Low-voltage DC Distribution Network Based on Fault Ride-through Strategy of Power Electronic Transformer

以级联型电力电子变压器(CHB-PET)作为主站的多分支柔性直流配电网在低压直流侧发生双极短路故障后,故障电流上升迅速且峰值较高,而电力电子器件在故障后会很快闭锁,造成故障信息的丢失,给直流配电网线路保护的可靠动作带来了挑战。针对此问题,提出了一种基于CHB-PET输入级与隔离级协同配合的无闭锁故障穿越方案,通过设置隔离级中双有源桥高压侧开关管的内移相比来控制输入级直流侧电容放电,使其不会过充或过放,消除了CHB-PET内部电力电子器件的过流,从而保证故障后CHB-PET可持续输出稳定电流。在此基础上,采用傅里叶变换提取故障穿越策略下CHB-PET低压直流侧电流的高频分量,以高频电流幅值积分值与全电流积分值之比构造保护判据,进而设计一种单端量保护方案。在PSCAD/EMTDC仿真平台搭建含CHB-PET的多分支直流配电网模型,仿真分析表明所提方法能快速、可靠识别并切除故障线路。

When the bipolar short-circuit fault occurs at the low-voltage DC side of the multi-branch flexible DC distribution network which takes the power electronic transformer with cascaded H-bridge(CHB-PET) as the main station,the fault current rises rapidly and the peak value is high.However,power electronic devices lock up soon after the fault,resulting in the loss of fault information,which brings challenges to the reliable operation of DC distribution network line protection.To solve this problem,a block-free fault ride-through scheme is proposed based on the cooperation between the input stage and the isolation stage of CHBPET.By setting the inward shift ratio of the dual active bridge high-voltage switching tube in the isolation stage,the DC capacitor discharge of the input stage is controlled,so that it does not overcharge or overdischarge.And the overcurrent of the power electronic devices in the CHB-PET is eliminated,so as to ensure the sustainable stable current output of the CHB-PET after the fault.On this basis,the Fourier transform is used to extract the high frequency component of the low-voltage DC-side current of CHB-PET with the fault ride-through strategy,and the protection criterion is constructed by the ratio of the integral value of the high-frequency current amplitude to the integral value of the full current.Then a single terminal protection scheme is designed.A multi-branch DC distribution network model including CHB-PET is built on the PSCAD/EMTDC simulation platform.The simulation analysis shows that the proposed method can identify and remove faulty lines quickly and reliably.