Solution for Effect of Zero Sequence Currents on Y-Y Transformer Differential Protection

In this paper, Transformer Differential protection mal-operations during external faults due to zero sequence current circulation on one side of the transformer is described with an example 132/33kV 60MVA Y-Y transformer. A simplest and modern solution is recommended with the comparison of conventional technology. Differential protection is a unit protection, which should not issue trip for external faults. When a single phase fault occurred in a transmission line, a small part of zero sequence current feeding from remote end will take a path from one side of the transformer connected to this line. Generally this current is higher than minimum pick-up of Transformer Differential protection. Since there will be no reflected current on other side of the transformer corresponding to this fault current, transformer differential protection can issue trip when this zero sequence current is higher than pick-up value. This scenario is simulated in MiPower software by considering HV side of the transformer connected to the grid via a transmission line and LV side is connected to strong, weak and no source cases to observe zero sequence current flow in transformer. These results are verified with manual calculations also. Generally zero sequence currents during external faults will cause mal operations of transformer differential protection for Yd and Dy type transformers. But this paper explains a different scenario with Y-Y transformers differential protection mal operation for eternal transmission line faults. Recently this problem is facing by many of the solar plant substations even with YN-yn transformers and generating plants. Hence a solution is prepared with numerical relay of ABB make RET670 and prepared solution is tested in the laboratory for the disturbance took place in a 132kV substation using Omicron injection unit. Conventional solution during electromechanical and static relays with interposing delta connected CT to filter zero sequence currents for external faults is also described in this paper.