Differential Relay In Generator

please see:

www.basler.com/downloads/genprotguidC.pdf

Differential Relay of a Generator is one of the most important protective device that protects the generator from internal faults. It is also known as Unit protection and very selective in fault discrimination. There are two CTs at the both end of stator winding ( each phase - before Neutral formation ) connected in opposition and output is connected to the relay element. Under normal operating conditions or fault outside the zone of protection ( both CTs) , the net current output is zero and hence no operation of the relay. But for a fault in the zone , the current balance is disturbed and a differential current flows through the relay that trips the generator.

The generator has 3 windings and these are all separate from one another. When there is a load on the generator each winding has a current flowing through it. Simply put, the current entering the generator at one end of a winding equals the current leaving the generator at the other end of the same winding.

If a fault develops in a winding, for instance the insulation breaks down and there is a leakage to earth or to another winding, then because of the leakage inside the generator, the current for the winding entering the generator will not equal the current leaving at the other end.

The differential differential relay senses the difference and calls the turbine controls to shut down the unit. Unlike an overcurrent situation or the like which is an external fault threatening the generator and can be handled by opening the generator breaker, with a differntial fault, the fault is inside the generator and to protect it, the generator must be shut down.

The differntial relay senses the currents with two CT's placed on each end of the winding.

As for the workings of the relay, I leave this to others..........

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