The influence of low resistance grounding system in delta side of transformer on differential protection and its solutions

In the system with lead in delta side of transformer grounded by low resistance, differential current arises in the traditional differential protection when having grounding fault beyond protected zone in this side, which may cause differential protection mal-operation. The paper analyzes the two schemes by contrast. The first scheme is differential protection zero sequence elimination in delta side and the other is setting differential threshold value higher than the maximal ground fault zero sequence current. According to the study, the first scheme eliminates the unbalanced differential current effectively when having grounding fault beyond protected zone in delta side, and improves the reliability of differential protection greatly. For the transformer with the voltage of 35kV in delta side, the first scheme lowers the differential threshold value and improves the sensitivity of differential protection when having single-phase grounding fault in protected zone. For the transformer with the voltage of 10kV (or 20kV) in delta side, none of these two schemes can reflect the single-phase grounding fault in protected zone sensitively.