Best Practices for Implementing High-Resistance Grounding in Mine Power Systems

Proper implementation of high-resistance grounding of mine power systems can reduce arc flash and shock hazards by limiting ground fault current while also permitting reliable detection and clearing of the fault. IEEE Standard 142 defines a high resistance grounded system as one with a purposely inserted resistance that limits ground-fault current to levels such that the fault current (usually thought of as less than 10 A) can flow for an extended time without exacerbating damage. However, the per-phase zero-sequence resistance of the system should not exceed the distributed per-phase capacitive reactance of the system; otherwise, the system will be prone to transient overvoltages and relay selectivity problems. Recent research has shown that in practice, the zero-sequence resistance may be considerably larger than the system capacitive reactance, thereby violating the definition of a high-resistance grounded system. This paper outlines procedures for proper sizing of the neutral grounding resistor considering the distributed system capacitance. The paper begins with a discussion of the problems caused by distributed capacitance in high-resistance-grounded mine power systems. Subsequently, procedures for determining system capacitance, sizing the neutral grounding resistor, and establishing relay pickup settings are given. These procedures are straightforward to apply and do not require computer modeling for implementation. Numerical examples of the procedure applied to a high-voltage longwall system and also an underground mine distribution system are provided.