Distance Relay

The basic principle of distance relay is that the apparent impedance seen by the relay, which is defined as the ratio of phase voltage to the line current of a transmission line and in the present of fault it is reduced intensely.
Now, a distance relay compares this ratio with the positive sequence impedance of the transmision line.

suppose
Z1 = the ratio of phase voltage to line current of transmission line
Z2 = positive sequence impedance

if Z1/Z2 is less than unity, it indiactes a fault. This ratio also indicates the distance of the fault from the relay.

Becuase, impedance is a complex number, the distance protection is directional.

There are two types.

1- impedance relay
2- Mho relay

1- impedance relay: the impedance relay trips if the magnitude of the impedance is within the circular region. Since the circle spans all the quadrants, it leads to non directional protection scheme.

2-Mho relay: only covers primary the first quadrant, that is why it is called directional in nature

Here is an example: