what happens when d cexcitation given to alternator is
removed while it is running or what happens when dc
excitation is failed in alternator while running?

Answer Posted / nishad

Generator will not produce any output voltage and it will trip the generator output breaker by the loss of excitation monitor protection relay or under voltage relay. If the breaker not trips by these relay, it will trip by reverse power relay. Due to the presents of Reverse Power relay, it will not allow the Generator to run as an induction generator and will trip the Generator breaker, thus it protect the generator from further severe fault. Again, if it not trips by these all protections, alternator will work as induction generator due to the residual magnetism in the field core when it is running in parallel and it will give the over load to another generator as a result, it will trip entire system. Also, as an alternator typically isn't designed to operate as an induction machine, the windings on these machines will heat up and damage. Therefore, need to make sure the reliability of protection relays to avoid such faults.

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