The effects of very-high-resistance grounding on the selectivity of ground-fault relaying in high-voltage longwall power systems

With the advent of high-voltage (greater than 1 kV) utilization circuits on longwall mining equipment in the late 1980s, the Mine Safety and Health Administration (MSHA) initially required maximum ground-fault current limits of 3.75 A for 4160 V systems and 6.5 A for 2400 V systems. (It should be noted that the Code of Federal Regulations defines low voltage, medium voltage, and high voltage for mine power systems as 0-660 V, 661-1000 V, and greater than 1000 V, respectively.) Ground-fault relay pickup settings were not permitted to exceed 40% of the maximum ground-fault current. Shortly thereafter, the MSHA began, and presently continues, requiring a much lower maximum ground-fault current limit of 1.0 A, or even 0.5 A, with ground-trip settings of 100 mA. Shielded cables, which have significantly more capacitance than their unshielded counterparts, are required for high-voltage applications in the mining industry. In an earlier paper, the author showed that with the long cable runs of a high-voltage longwall system, capacitive charging currents could easily exceed grounding-resistor currents under ground-fault conditions. As a result, overvoltages from inductive-capacitive resonance effects can occur. Because of the large system capacitance and low ground-trip setting, the relay selectivity of the ground-fault protection system may also be compromised. Therefore, an analysis of a typical 4160-V longwall power system that utilizes very-high-resistance grounding (ground-resistor-current limit of 0.5 A) is performed to determine if potential problems exist with the selectivity of ground-fault relaying.