High-resistance grounding protection scheme of 11 kV generating plant

This paper describes design application of 11 kV high-resistances grounding (HRG) protection schemes for the power plant generators to minimize phase-ground fault damage. Generated power is stepped-up to connect to utility grid at 66 kV by transmission lines, and it is stepped-down to 6 kV to supply power to the plant auxiliaries. One HRG protection scheme consists of a generator neutral resistor sized per the HRG criteria and a separate HRG protection scheme designed for the ungrounded 11 kV power system when the generator breaker opens and the utility back-feeds power to the generator auxiliaries. How two separate HRG schemes operate to provide minimum phase-ground fault current is included in this paper. Three-line diagrams are presented to show bolted phase-ground fault conditions' fault current flow directions, equivalent circuit, and phasor diagrams. Discussion on sensitivity of current relays to detect low phase-ground fault and to clear the fault by over voltage relay is included. This paper clarifies that the system charging current under normal operation and during phase-ground fault condition is provided by the power source.