TABLE OF CONTENTS It is essential that any faults on a power system circuit are cleared quickly; otherwise they could result in the disconnection of customers, loss of stability in the system and damage to equipment. Distance protection meets the requirements of reliability and speed needed to protect these circuits, and for these reasons is extensively used on power system networks.
Distance protection is a nonunit type of protection and has the ability to discriminate between faults occurring in different parts of the system, depending on the impedance measured. Essentially, this involves comparing the fault current, as seen by the relay, against the voltage at the relay location to determine the impedance down the line to the fault. For the system shown in Figure 9.1, a relay located at A uses the line current and the line voltage to evaluate Z = V ? I. The value of the impedance Z for a fault at F 1 would be Z AF1, and (Z AB + Z BF2) for a fault at F 2.
The main advantage of using a distance relay is that its zone of protection depends on the impedance of the protected line that is a constant virtually independent of the magnitudes of the voltage and current. Thus, the distance relay has a fixed reach, in contrast to overcurrent units where the reach varies depending on system conditions.
