Selective High-Resistance Grounding System for a Cogeneration Facility

Abstract

The paper describes a selective high resistance grounding (HRG) system for large industrial facility with two in-plant generators in cogeneration mode and two utility transformers. In a 5-step coordination using sensitive ground fault non-directional and directional relays, ground fault throughout the distribution system are cleared with a maximum pickup setting of 6A, time delay of 0.25s. A major consideration for HRG systems is that the stray capacitance charging current of the system should be small (10–15A) 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 un-faulted phases. The paper demonstrates that with selective high resistance grounding, both these concerns are mitigated. A selective HRG is a bridge between the classical concepts of HRG and LRG (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.