Method to improve digital distance relay impedance measurement when used in series compensated lines protected by a metal oxide varistor

A novel method is described that enhances the accuracy of digital distance relays applied on series-compensated lines where the series capacitors are protected against overvoltages by a metal oxide varistor (MOV). The technique is applicable to systems where the relaying voltage is taken from the busbar side of the series capacitor. The basis of the technique is a method known as voltage compensation. The voltage across the series capacitor and overvoltage protective device is calculated in the relay and hence the relaying signal is modified by the relay. The new compensation technique makes the impedance measurement immune to the MOV operation, thus enhancing the relay integrity and the accuracy of impedance measurement. The technique is simulated in a typical differential equation distance relay algorithm and is examined for ground and phase faults using the EMTP (ATP) simulation package