The application of 2400 V to longwall face equipment

Operational and safety aspects of 2400 V longwall mining systems, as compared with 995 V systems, are presented. A 900 ft face width was chosen for the comparison, and typical longwall equipment, power requirements, and cable lengths were selected. The kilovoltampere rating of the power-center transformer was specified as 2500 kVA, with a 0.05 pu impedance and a resistance-to-reactance ratio of 4.0. Maximum fault currents are calculated for each system voltage, assuming a bolted three-phase fault and an infinite bus at the primary of the power-center transformer. Maximum fault currents and interrupting capabilities of circuit breakers at both utilization voltages are compared. Typical cable sizes are specified for each system voltage, and their respective voltage regulations are calculated. Voltage drops and motor-torque reductions are also calculated and compared for conveyor-motor starting conditions. It is shown that the improvements anticipated with higher utilization voltage (better voltage regulation, improved motor torque and decreased cable sizes) are achieved in this longwall application. Voltage studies with an infinite bus at the power-center primary uncovered marginal performance of the 995 V system with the selected face equipment and width. Inclusion of the upstream power system illustrated the likelihood of unsatisfactory operation of 995 V, whereas the 2400 V installation would likely succeed