不受直流换相失败影响的交流故障分量差动保护

AC Superimposed-current Differential Protection Immune to DC Commutation Failures

针对交直流混联系统中传统故障分量差流保护因直流换相失败及过渡电阻影响可能拒动的问题,提出一种新的故障分量电流差动保护方案。首先,分析了传统故障分量差流保护在不对称接地短路下拒动的原因。据此,利用线路两端零序电流相位差的余弦值对传统故障分量差流保护判据中的差动电流幅值进行自适应补偿,实现区内外故障的准确识别;并结合故障相和非故障相的差动电流幅值差异构建选相函数。其中,采用线路两端零序电流的半周采样值之和构造故障类型识别判据,确保所提保护方案只在不对称接地短路时投入。最后,在PSCAD/EMTDC中搭建±800 kV直流/500 kV交流混联系统仿真模型,验证了所提保护方案的可行性。结果表明所提保护方案不受直流换相失败及过渡电阻影响,能够可靠保护线路全长。

To solve the problem that the the traditional failure differential protection based on fault-component currents may block due to commutation failure and fault resistance in AC/DC hybrid system,a new differential protection scheme based on fault-component currents is proposed.First,the reasons that the traditional differential protection based on fault-component currents failing to operate under the unsymmetrical earth fault are analyzed.Based on this,the cosine value of zero-sequence-current phase difference at both ends of the transmission line is used to compensate the differential current amplitude in traditional differential protection,so that the internal and external faults can be identified accurately.The phase selection function considering the differential current amplitude difference between fault and healthy phases is constructed to discriminate the fault phase.Therein,the fault type identification criterion using the total sum of half-cycle sampling values of zero-sequence current at both ends of a line is constructed to ensure that the proposed protection scheme only works during unsymmetrical earth faults.Finally,the simulation model of a±800 kV DC/500 kV AC hybrid system is built in PSCAD/EMTDC to verify the proposed scheme.The results show that the protection scheme is not affected by DC commutation failure and fault resistance,and can protect the entire line reliably.