Zero Sequence Network for Two Winding Transformers

From above discussion we can also note that zero sequence current can flow only if there is return path. Hence for Y-y connected transformer without neutral earthed (either of HV side or LV side) there will not be zero sequence current through transformer. Similarly for D-y connections with LV side neutral earthed zero sequence current will flow through delta side only and will not reflect into power system connected to HV side of the transformer. This fact can be represented as shown in figure. In this case zero sequence impedance of transformer is same as that of its’ +Ve sequence; only connections in zero sequence network differs depending upon transformer winding connection type. While representing zero sequence network of the power system; if transformer HV or LV side winding is Y connected and earthed that side of the transformer get connected to power system. Whereas Delta connected transformer winding get connected to zero sequence reference bus. Zero sequence equivalent circuits for different types of transformer are shown in the figure. Fortunately in transmission system most of the transformers are Y-y connected with both side neutral grounded. In such case zero sequence network of the power system is same as positive sequence network (except change in zero sequence impedance values for transmission lines).