Protection against Loss of Excitation

Generator Protection Part 11
Protection against Loss of Excitation
       The loss of excitation of the generator may result in the loss of synchronism and slightly increase in the generator speed. The machine starts behaving as an induction generator. It drawn reactive power from the system which is undesirable. The loss of excitation may lead to the pole slipping condition. Hence protection against loss of excitation must be provided.
        The protection is provided using directional distance type relay with the generator terminals.
       When there is loss of excitation, the equivalent generator impedance varies and traces a curve as shown in the Fig. 1. This Fig. 1 shows the loss of excitation characteristics alongwith the relay operation characteristic, on R-X diagram.
Fig.1  Loss of excitation characteristic

        The equivalent generator impedance locus traces a path from first quadrant of R-X diagram to the fourth quadrant. The distance relay is used which covers the portion of the fourth quadrant where impedance locus path exists. Thus when the impedance takes value in the region covered by the relay characteristics, the relay operates. The relay operates when generator first starts to slip poles. Then relay trips the field circuit breaker. And it disconnects the generator from the system, too. When the excitation is regained and becomes normal, the generator can then be returned to service instantly. 

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