Study On The Causes And Countermeasures Of Differential Protection Misoperation Caused By Inrush Current

Reliable and fast operation of differential protection, which is used as the main protection of transformers, is very important to transformers and the entire power system. To avoid mal-operation caused by magnetizing inrush, magnetizing inrush restraint is designed for differential protection. However sympathetic inrush could fail the restraint and make differential protection mal-operate. Besides, with increasing requirement of development of power grid, more and more AC/DC compound systems are promoted. And electronic CT(ECT) will take the place of traditional electromagnetic CT to become the hub of relay protection.Firstly, because of the field operation recorded datas are not easy to collect, sympathetic inrush’s generation modes, location, voltage level, CT configuration, inrush restraint, load and other conditions in traditional transformer and converter transformer are summarized and discussed according to the cases in which sympathetic inrush causes differential protection mal-operate cases. And effect caused by DC magnetic bias is analyzed especially suitable to the features of converter transformers. Secondly, based on the statistical results, some common kinds of electromagnetic CTs’ saturation is analyzed, and how CT’s saturation and load current affects second harmonic ratio and dead angle in sympathetic inrush and differential current is studied, which leads to deeper reasons why sympathetic inrush causes differential protection mal-operation. Thirdly, for CT saturation is the deeper reason, Rogowski ECT’s transform features and its impact to differential protection are discussed and improved according to the defect. The research shows that improved Rogowski ECT can transferre sympathetic inrush without distortion, and its second harmonic ratio and dead-angle characteristics are unchanged, resulting in the uninfluenced dead-angle and second harmonic restraint criteria. Finally, precaution methods promoted in other articles are summarized in five types, and each type precaution is reviewed in practicality, advantages, disadvantages and applicability aspects. Research’s results are verified by simulation using PSCAD/EMTDC and MATLAB/Simulink.