Selective High-Resistance Grounding System for a Cogeneration Facility

This paper describes a selective high-resistance grounding (HRG) system for a large industrial facility with two in-plant generators in cogeneration mode and two utility transformers. In a five-step coordination using sensitive ground fault nondirectional and directional relays, ground faults throughout the distribution system are cleared with a maximum pickup setting of 6 A, and time delay of 0.25 s. A major consideration for HRG systems is that the stray capacitance charging current of the system should be small (10-15 A) to prevent ground fault damage. If the fault is left in place for some duration for a single-line-to-ground fault, insulation stresses occur due to higher line-to-ground voltages on two unfaulted phases. This paper demonstrates that with selective HRG, both these concerns are mitigated. A selective HRG is a bridge between the classical concepts of HRG and low-resistance grounding. The modern advancements in ground sensors and protective relaying can selectively clear low levels of ground fault currents, which make selective HRG a reality.