What are the working principles of generator main protections?

What are the working principles of generator main protections?
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1 Answer

Answer :

GENERATOR START UP PROTECTIONS


SUPPLEMENTARY PROTECTION OF GENERATOR

The generator is normally expected to run rated speed before excitation power is applied by closing the field breaker. However the residual magnetism in the field circuit may provide small voltage build up even when the machine is run upto its rated speed without excitation. At this stage fault if any in the generator stator circuit may not be sensed by the regular protection, as must of the relays are having higher current ranges. Hence separate protection (Phase & Ground) are provided with low current ranges.


a) PHASE OVER CURRENT PROTECTION 

The CT current is stepped down by an internal CT and converted to voltage signal. The signal is compared with the internal reference. The protection is interlocked with the auxiliary relay for the generator transformer breaker closed position to ensure that the protection is inoperable when the machine is synchronized to grid. 


b) GROUND FAULT PROTECTION DURING START UP

The generator neutral current as measure in series with the resistance of the secondary of the earthing transformer is fed to the relay through CT. CT current is converted to a voltage. This is compared with the internal resistance references. This protection also interlocked with generator breaker position to ensure that the protection is inoperable when the machine is connected to grid.  

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