Page 212 - Practical Power System and Protective Relays Commissioning

212 Practical Power System and Protective Relays Commissioning

The relay will also operate if Z F , Z, where Z 5 the setting impedance set
on the relay.
Increasing V R has a restraining effect, so this voltage is called the
restraining voltage and increasing I R has an operating effect, and this current
is called the operating current.

18.6.3 DISTANCE RELAY OPERATING CHARACTERISTIC
The relay operating boundaries are plotted, on an R/X diagram, its impedance
characteristic is a circle with its center at the origin of the R-X coordinates
and its radius will be the setting of the relay (relay reach) in ohms as shown
in Fig. 18.6.4.
As shown in Fig. 18.6.5, the distance relay with plan characteristic is a
nondirectional relay. This means the relay can trip for faults in a forward
direction of the transmission line, and also for faults behind the relay in the
reverse direction.
Another characteristic is used to give the relay the directional feature to
see the faults on the forward direction of the line only. This is called Mho
characteristic, as shown in Fig. 18.6.6.
The main advantages of this Mho characteristic are as follows:
1. It is a very popular characteristic;
2. It is very simple;
3. It has more immunity to power swings.

FIGURE 18.6.4 Distance relay simple plan characteristic.