The effects of cable capacitance on longwall power systems

Utilization voltages used on longwall equipment operating in coal mines in the United States have increased steadily over the past 15 years from 1000 V up to 4160 V. This voltage increase has directly influenced the 200% gain in overall longwall productivity from 1987 to present. The transition from medium voltage (661 V - 1000 V) to high voltage (greater than 1000 V) has permitted significant increases in face widths and equipment sizes. Longwall systems with total connected loads of over 5000 hp are now common, and all longwalls in the United States now use high-voltage equipment, with the majority utilizing 4160 V. Shielded cables, which have significantly more capacitance than unshielded cables, are required for high-voltage applications in the mining industry. This capacitance can have detrimental influences on system overvoltages and relay selectivity during ground-fault conditions if the values for the grounding resistor and the ground-fault-relay pickup settings are improperly chosen. These issues are addressed in this paper.