Field Failure Relay with and w/o Under Voltage

Do you mean field failure protection?...To protect from the thermal damage due to sudden loss of excitation.

This prevents the generator documentation from being incorrectly filed. Unfortunately most of these relays work very poorly. Anyone who has ever looked for generator documentation knows this.

I think you mean to say you have a generator with two rotor field failure relays, one of which is only armed (active) when the generator terminal voltage is below normal, the other is active always. You do not mean you have different field failure system for a generator with output under-voltage protection compared to a generator without output under-voltage protection!! The reason, I think, would be that any good excitation scheme will increase excitation to several times rated for as long as possible after a short circuit. This ensures the short circuit current due to a load fault is kept high so that the fuse or circuit breaker feeding the faulty part of the load opens quickly. You will realise that a fuse breaks much faster if current is many times its rating than if it is only say 1.5 times. The field-fail relay operating ONLY at low generator terminal (output) voltage would have a higher current setting because a healthy excitation system ought to give higher [above rated-current] field current under these conditions. A short-circuit or severe overload always pulls generator output voltage below normal.

Two different relay for field failure one is with under voltage second is without under voltage . what is field failure without under voltage ?

"No field voltage" or "no field current" is easy to detect (when the field is connected by slip-rings or if an exciter field is monitored), but a better field failure relay will identify that field current is not as high as it should be. Field current is more likely to be too low or high because the voltage regulator/excitation system has a problem than because the field windings have failed. If the field strength is too low, a generator in parallel with other generators will work at a leading power factor and can lose synchronism and "pole slip". If you mean undervoltage at the main generator output terminals, this may not happen with weak field because parallel generators or the Grid keep the voltage up to normal at these terminals.

i don't think you guys grasp the original question. i think the question originates from the setting of a field failure relay on some multifunction generator protection relays that have an added feature which activates the field failure only if the voltage drops below a selected threshold. Or one can elect to enable the field failure relay solely on the generator mho characteristic i.e. the generator impedance as seen by the system.

You might be right. I'm sure the documentation that was filed away would explain how to use these relays.