Zero sequence circuit of three-legged core type transformers

For three-legged Y-YG core type transformers, due to the high reluctance of zero-sequence magnetic return path, the zero-sequence magnetizing impedance is low. The low zero-sequence magnetizing impedance will allow fault current to flow for a line-ground fault in the YG side of a three-legged core type transformer. When calculating line-ground fault current of a three-legged core type transformer, the zero sequence equivalent circuit needs to include the zero-sequence magnetizing impedance. The zero-sequence equivalent circuit from most classic reference books does not take magnetizing impedance into account and needs to be modified. This paper gives the modified zero-sequence equivalent circuit for a three-legged core-type transformer. Theoretical calculation, computer simulation, and actual staged tests verify that fault current based on the modified zero sequence circuit matches the result from the simulation and actual test.