NEW SELECTIVE EARTH-FAULT PROTECTION SCHEME IN MINES USING INSULATED-NEUTRAL SYSTEM

The development of a new selective ground-fault protection scheme in mines using an insulated-neutral system of power supply is reported. The discrimination is based on the fact that in the event of a single phase to ground fault anywhere in the system of a number of parallel connected feeders, the core balance transformer (c. b. t. ) associated with each fault-free feeder would give a voltage output proportional to the vector sum of capacitive currents of its fault-free phases, whereas the faulty feeder c. b. t. would give a voltage proportional to the vector sum of capacitive currents of all sound phases less the capacitive currents of the feeder itself. This vector sum of currents is of reverse polarity compared to that of fault-free feeders. Utilizing this principle, phase-comparator circuitry has been developed to achieve selectivity between fault-free and faulty feeders. By judicious design of the scheme, the sensitivity of protection has been made independent of the number of parallel circuits involved.