A distance relay is one whose operation is based on measurement of impedance, reactance or admittance between the location of the relay and fault point.
The rapid growth in power system requires:
1. Continuity of supply
2. Good voltage regulation
3. Fast fault clearance,
To meet the above: High speed protection system along with automatic reclosing of circuit breakers has been developed and are widely used for medium, high and extra high voltage transmission lines.
Principle of Distance Relay
The impedance/reactance of transmission line is proportional to its length. A relay capable of measuring the impudence/reactance up to a given point can be used to protect a transmission line.
This designed to operate only for faults occurring between the relay location and selected point.
The relay compares the current flowing and voltage at the relay point.
In case of fault on transmission line, the current flowing to fault point increases and the voltage at relay point decreases.
The relay sees this as fall in impedance and this is below a pre-fixed setting, the relay operates.
Distance relays are:
Double actuating quantity relays that is, one coil energized by voltage and the other coil energized by current.
The torque produced is such that when V/I reduces below a set value, the relay operates.
