Excitation Fail Condition in Power Generation Unit

It is difficult to answer, since you wrote"breaker out" after "excitation fails"and "its protection" does not make clear if generator (electrical?) protection or turbine (mechanical?) protection also fails (or both).

If "breaker 0ut" meant main "generator to Grid" breaker, then little will happen in short term, because the unit is off-load. If you mean an excitation breaker opens (no excitation), it depends on the load - specifically if the load is enough to cause loss of synchronism.

A turbo generator can typically stay in sync up to about 1/3 rated output without excitation. Beyond that, it will lose sync and "pole slip". This means the generator torque will cycle from positive to negative as the phase slips. If the machine is fully loaded it will accelerate to overspeed, with the torque oscillations getting faster (if there is a governor, it may act to limit the overspeed).

The oscillations will be no good for either generator or turbine, but survival depends on how long this goes on and how tough the unit is - small machines are tougher.

Likely, survival will depend on the operator noticing the current/megawatt swinging and emergency stopping the unit.

Possibly, vibration protection will stop the unit automatically. Also, it is usual for there to be both excitation failure tripping by the AVR system and a relay driven by only generator current/voltage which detects pole slipping or out of sync conditions.

My question was about excitation breaker open condition and electrical/excitation protection failure.

thanks for the answer.

The machine will continue to deliver active power but would draw reactive power from the bus.

Stator current would increase to about 200% rated.

Machine will overspeed.

Rotor would heat up abnormally.

yes your correct my friend and the generator have the protection in the module control but in the exciter only the protection is the diod and the varistor if the genset oveloaded automatic burnout the exciter diod or the varistor !!! and the AVR have a small fuse on it you can reset that manually, or some of the AVR dont have so you need to replace new one !!

Hi..

I still did not understand this case, if exciter breaker open, there will be no excitation and no voltage built and ACB will tripped, done.

Dear Loadshare,

If you read the original post and all answers, I think you will realise that the electrical protection was specified as inoperative, so there would not be a trip of the ACB and shutdown of prime mover to avoid damage. If the generator remains connected to Grid it will be energised as an induction generator, just like an induction motor. The difference is that the damping bars (similar to "squirrel cage") built into a synchronous generator are not rated to carry induced rotor current at rated output for more than a short time.

I assume here that ACB means Alternator Circuit Breaker, not Air Circuit Breaker. I tried to be clear by writing "generator to Grid" breaker, for there are several common descriptions used, like "Main Breaker" but they are not self-explanatory.

67model

Dear 67model,

oh, its clear now.

Thank you

LS