It is readily apparent that a loss of excitation can be damaging
to the generator as well as detrimental to the overall operation
of the system. Therefore, loss of excitation protection should
be provided on all types of generators.
To detect a loss of excitation with any machine loading, the relay
characteristic should be set with a circle diameter equal to
direct axis synchronous reactance (xd) of the generator.
The offset mho loss of excitation relay can detect a generator
loss of synchronism for some system conditions. However, the
relay will not detect a loss of synchronism under all system conditions
and therefore separate loss of synchronism relaying
should be provided to protect the generator.
Consideration should be given to the effect of stable swings on
relay performance.