Common Distance Relay Types

Distance relay applications

Distance relays respond to the voltage and current at the relay location, i.e. the impedance. The impedance is fairly constant per mile, so these relays respond to the distance between the relay location and the fault location.

As power systems become more complex and fault currents vary with generation and system configuration, directional overcurrent relays become difficult to apply and set for all contingencies, whereas the settings of distance relays are constant for various variations outside the protected line.

There are three common types of distance relays, as shown in Figure 1. Each differs in its application and operating characteristics.

Remote relay

Impedance relay

The impedance relay has a circular characteristic centered at the origin of the RX diagram. It is non-directional and is primarily used as a fault detector.

Admittance relay

The admittance relay is the most commonly used distance relay. In the test scheme it is a trip relay and in the step scheme it is a backup relay. Its characteristic passes through the origin of the RX diagram and is therefore directional. In electromechanical design it is circular, while in solid-state design its shape can correspond to the transmission line impedance.

Reactance relay

The reactance relay is a straight line characteristic that responds only to the reactance of the protected line. It is non-directional and is used to supplement the admittance relay as a trip relay, making the overall protection independent of resistance. It is particularly useful on short lines where the short circuit fault arc resistance is of the same order of magnitude as the line length.

Figure 1 shows a three-zone stepped distance relaying scheme that provides instantaneous protection for 80–90% of the protected line section (Zone 1), delayed protection for the remainder of the line (Zone 2), and backup protection for adjacent line sections. Zone 3 also provides backup protection for adjacent line sections.

In a three-phase power system, there are 10 possible faults:

Three single-phase grounding,

Three phase to phase,

Three dual phase to ground, and

A three-phase fault.

The relays provided must have the same settings regardless of the fault type. This is possible if the relays are wired to respond to incremental voltages and currents. The incremental quantity is defined as the difference between any two phase quantities, for example, ea – Eb is the incremental quantity between phase a and phase b. Typically, for a multiphase fault between phase x and phase y,

Three-zone step-distance relay protection is used to protect 100% of the line and backup adjacent lines.

Three-zone step-distance relay protection for 100% protection of the line and backup of adjacent lines. (Source: Horowitz, S. H. and Phadke, A. G., Power System Protection, 2nd ed., 1995. Reproduced with permission from Research Press, UK.)

Where x and y can be a, b or c and Z1 is the positive sequence impedance between the relay location and the fault. For ground distance relays, the fault phase voltage and the compensating fault phase current must be used.

Where m is a constant that depends on the line impedance and I0 is the zero sequence current in the transmission line. The full set of relays includes three phase distance relays and three ground distance relays. This is the preferred protection scheme for high and extra-high voltage systems.