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 dist ance relays applied on series-compensated lines where the series capac itors are protected against overvoltages by a metal oxide varistor (MO V). The technique is applicable to systems where the relaying voltage is taken from the busbar side of the series capacitor. The basis of th e technique is a method known as voltage compensation. The voltage acr oss the series capacitor and overvoltage protective device is calculat ed in the relay and hence the relaying signal is modified by the relay . The new compensation technique makes the impedance measurement immun e to the MOV operation, thus enhancing the relay integrity and the acc uracy of impedance measurement. The technique is simulated in a typica l differential equation distance relay algorithm and is examined for g round and phase faults using the EMTP (ATP) simulation package.