A differential relay is defined as the relay that operates when the phase difference of two or more identical electrical quantities exceeds a predetermined amount. The differential relay works on the principle of comparison between the phase angle and magnitude of two or more similar electrical quantities. Comparing two electrical quantities in a circuit using differential relays is simple in application and positive in action.
For example, consider the comparison of the current entering a protected line and the current leaving it. If the current enters the protected line is more than the current leaves it, then the extra current must flow in the fault. The difference between the two electrical quantities can operate a relay to isolate the circuit.
For the operation of the differential relay, it should have two or more electrical quantities, and these quantities should have a phase displacement (normally approximately 180). Any types of the relay can operate as a differential relay depends on upon the way it is connected in a circuit. In other words, it doesn’t depend on the construction of the relay it depends on the way it is connected to the circuit.
Differential protection provides unit protection. The protected zone is exactly known by the location of current and potential transformers. The phase difference is achieved by suitable connections of secondaries of CTs and PTs.
The differential protection principle is employed for the protection of generator, generator-transformer units, transformers, feeders, large motors, and bus-bars. The differential protection relay is mainly classified into four categories. These are
1.Current Differential Relay
2.Voltage Differential Relay
3.Biased or Percentage Differential Relay
4.Voltage Balance Differential Relay