New selective earth-fault protection scheme in mines using insulated-neutral system

The paper reports the development of a new selective earth-fault protection scheme in mines using an insulated-neutral system of power supply. The discrimination is based on the fact that in the event of a single phase to earth fault anywhere in the system of a number of parallel connected feeders, the core balance transformer (c.b.t.) associated with each healthy feeder would give a voltage output proportional to the vector sum of capacitive currents of its healthy 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 as compared to that of healthy feeders. Utilising this principle, phase-comparator circuitry has been developed to achieve selectivity between healthy and faulty feeders. In addition, by judicious design of the scheme, the sensitivity of protection has been made independent of the number of parallel circuits involved.