Advance of the Ground Relay of the Line Protection

This paper describes the advance of the ground relay of the line protection. First of all, we examined the ground fault in a resistively grounded power system. The protective relay detects the ground fault by the zero phase current in the system. There are two methods for the current input to the relay. One uses the residual circuit of CT secondary winding, and the other uses the CT tertiary winding. When the ground current is insufficient in the residual circuit, the CT tertiary winding is used. In order to omit the CT tertiary winding and to get more reliability, we reviewed the full-scale of relay and evaluating the error of CT. As result, almost all transmission lines could omit the CT tertiary winding. Next, we examined the ground fault with high fault resistance in a solidly grounded power system. When that fault occurs in the system, the PCM current differential relay acts by fault current. So, this relay possibly mal-operates when the CT secondary circuit is disconnected. Therefore, it uses a special CT to raise the reliability. Nowadays GIS has become small and the CT installation space has been decreased, it is needed to reduce these CT cores. In order to omit the special CT, we utilized the combination of the power relay and the rate of change of relay current. As a result, there is a prospect that the relay would not need the special CT. These improvements are effective to reduce cost too.