DISTANCE RELAY – PROTECTION FOR LONG DISTANCE LINES

Distance relay is one of the most important protection schemes in the power system. Distance relaying is relatively complex in nature. This article however will try to give the readers a basic understanding of the concept and process.

There are basically three types of distance relays:

(1) an impedance type of relay measures the voltage-to-current ratio on a faulty line, which is the impedance to fault;

(2) a reactance type of relay measures a signal proportional to the imaginary component of the voltage-to-current ratio; and

(3) an admittance type of relay, also called a mho relay, measures current to voltage and is inherently directional.

The relays thus respond to impedance, reactance, or mho, rather than the fault current. Static distance relays may operate within 0.5 cycle of fault inception. External timers and the distance settings are used to provide selectively.

The characteristics of a distance relay are plotted on an R–X diagram. Thus, the characteristic of an impedance relay is a circle, and the relay will operate in either direction for an impedance setting within the circle.

The characteristic of a reactance relay is a straight line parallel to the R-axis and is nondirectional. A mho relay has a circular characteristic, which passes through the origin and hence is directional.

Variations in these basic characteristics are conventional. Consider that a distance relay is applied to a short-line, the arc fault resistance can alter the reach of the relay and result in a no-trip or nuisance trip.

Conversely, a reactance relay will remain unaffected by the arc fault resistance but may operate on load currents. It should be used in conjunction with other relays to restrict its reach along the R-axis and in a negative reactance direction.

For specific applications, the characteristics of a mho relay can be offset in the forward or reverse direction. Blinder relays have an angle impedance characteristic that can be set parallel to the impedance characteristics of a line.

Figure 1(a) shows characteristics of distance relays. The reverse offset mho characteristic is used for loss of excitation protection of a generator.

The concentric circles and blinder characteristics, shown in Fig. 1(b) are used for out-of-step protection.