Analysis of very-high resistance grounding in high-voltage longwall power systems

The application of very sensitive ground-fault protection in underground coal mines was demonstrated in the early 1980's for low- and medium-voltage utilization circuits (less than 1 kV), but its commercial application did not occur until the advent of high-voltage utilization circuits on longwalls in the late 1980's. With these high-voltage (greater than 1 kV) systems, MSHA initially required a maximum ground-fault resistor current limit of 3.75 A for 4160-V systems and 6.5 A for 2400-V systems in 101-c Petitions for Modification. However, more recent Petitions for Modification have been required to limit maximum ground-fault resistor currents to 1.0 A, or even 0.5 A. Standard practice in other industries generally requires high-resistance grounding to be designed so that the capacitive charging current of the system is less than or equal to the resistor current under a ground-fault condition. The intent of this practice is to prevent the system from developing some of the undesirable characteristics of an ungrounded system, such as overvoltages from inductive-capacitive resonance effects and intermittent ground-faults. Shielded cables, which have significantly more capacitance than their unshielded counterparts, are required for high-voltage applications in the mining industry. Thus, with the long cable runs of a high-voltage longwall system, capacitive charging currents may exceed grounding-resistor currents under ground-fault conditions. An analysis of a typical 4160-V longwall power system that utilizes very-high resistance grounding (grounding-resistor-current limit of 0.5 A) is performed to determine whether or not potential problems exist.