A Review of Phase-Ground Fault Current Relay Type and Settings in an HRG Medium-Voltage Mining Power System

This paper cites current mandatory requirements in the “Title 30-Mineral Resources, Code of Federal Regulations, “30 CFR 75.814, and MSHA” for the ground fault current protection relay settings and neutral resistor rating for a high-resistance grounded (HRG) underground mining power supply system. Very high system charging current of a mining power system poses design concern of meeting the MSHA requirements, “ground-fault current shall be limited by a neutral grounding resistor (NGR) to 3.75A and associated ground fault protection relay shall be set at 40% of the NGR rating when power system nominal voltage exceeds 2.4 kV”. This paper is based upon a published paper with very high system charging current of 68.2A. The previous authors have correctly identified the problems of meeting MSHA requirements; NGR rating and ground fault relay settings. This paper recommends a voltage polarized ground fault current relay that can be set to meet MSHA requirements even if the NGR rating does not meet MSHA requirements. Such a relay identifies forward direction and reverse direction of ground fault current during phase-ground fault condition. The ground fault current flow in the three-line diagram indicates that the faulted feeder relay senses higher ground fault current whereas the parallel feeders and the neutral relay senses less current to cause delayed tripping of NGR relay and parallel feeders relays to improve power system reliability. Per NEC (NFPA 70) code requirement, a movable equipment enclosure grounding to keep voltage less than 100V across the grounding conductor is included. For the underground mine medium-voltage (MV) feeders; the practice of using an integrated continuous monitored ground check wire (with the phase conductors) is explained.