Generator Question

Yes. Or, no. It depends on a lot of information that isn't provided.

The Dg was not running Changeover contactor failed and ext voltage fed to dg.

Did the dg quit working after the contactor failed?

Post #3 said: "The Dg was not running"

"What we have here is failure to communicate". The warden in Cool Hand Luke.

I meant does the generator work now, after the incident? Did the incident cause the generator to fail?

I humbly agree. It appears, the OP is hesitant to crank his generator.

Scenario one: The generator was off. Normally the utility load is connected to common point of changeover switch & the other two ends, at opposite end are for grid & generator out put. Then I don't understand how can this happen?

Scenario two: Same logic, generator running with out put connected to load, when followed, does not help either.

Was grid supply directly fed on purpose, some bizarre experiment, or had I had one too many?

Basic tests.

1. With generator isolated, measure resistance from each line terminal to neutral. If you do not have an ohmmeter, a lamp and battery will do. If one phase is more than a few ohms (usually far more) or lamp does not light, you have a broken winding. Test field winding resistance also, if accessible.
2. Turn the engine by hand crank, if available. Decompress by lifting exhaust valve if possible. If the engine can be turned 1 revolution without jamming, sudden resistance or bad noises at least you do not have any seriously loose parts.
3. If you can't or won't do 2., then you can force the fuel pump to zero delivery and hold it there (many engines have a mechanical stop lever/pull which does this, or a solenoid operated one which can be energised for a few seconds). Operate the electric starter to turn the engine [obviously, this is more risky than 2. above]. If this all sounds good, take off temporary "fixes" and try a start with fuel.
   

Yes you can rewinding your generator just tell us about the generator .the type .brushless or brush exciter .single or three phase .the frequency 50Hz or 60Hz

It's all a matter of scale.

It depends on whether the reverse power relay and circuit breaker (if so equipped) operated successfully.

It really gets down to how much luck you have. I've seen generators back-fed to the point they were motoring and they were fine. I've also seen brand new generators get smoked even though the feeder opened on instantaneous. Certainly you should have the windings tested properly if this is a commercial unit. If it's a little home unit and you screwed up on the switching control then figure out what you did wrong and fire it up and see if it still works.

Your statement "The Dg was not running Changeover contactor failed and ext voltage fed to dg" doesn't make sense or at minimum, is missing a lot more information. If you are using contactors then you should have a proper interlock method, preferably mechanical.

I've lost count of how many times I've seen contractors get line and load cabling reversed.

Damage is not to the alternator but to the turbine bading, some more info = google

Regards JD.

Why did the usual circuit protection device(s) fail, then?