How do AVRs handle unbalanced systems?

[I_am_learning]

(Referring to functioning of AVR on a system where an Isolated Synchronous Generator is supplying load to consumer)
What voltage do AVRs normally sense? I guess they must sense Line-to-phase voltage of all three phases. (and not just a single phase)
And when it detects that one phase is loaded and other phase is unloaded,
Say, the line to phase voltage reading of three phases comes out to be
A: 1 pu
B: 1 pu
C: 0.5 pu

What should AVR do now? Will it increase excitation slightly or what?

 

[jim hardy]

A large central station machine would be equipped with unbalance detection which would place voltage regulator to manual control, on presumption the unbalance was not real but caused by a failure in the metering circuitry.
a real unbalance that large would indicate something drastically wrong inside the generator and trip its protective relays, shutting it down immediately. ( i think on negative sequence, need a genuine power engineer to check me on that one.. Something that bad wrong would probably give differential too.)

a very small machine might only sense two phases.
it's common to sense phase AC for voltage control and phase B current for var control because they're 90 degrees out of phase, draw your triangle...
in which case your supposition could be right, regulator might try to raise voltage.

 

[I_am_learning]

Thanks jim.
I agree that such a large unbalance would already trip some other protection circuit. So, let's talk about little less unbalance, say, 1.0, 1.0 and 0.8 pu.
For isolated system (used in places not fetched by the national grid), what type of AVR should be suitable?
I didn't quite get what you meant when you said -
" large central station machine would be equipped with unbalance detection which would place voltage regulator to manual control, [BOLD]on presumption the unbalance was not real but caused by a failure in the metering circuitry [/BOLD]."

In other words, ideally, what should the AVR be doing in such condition ?

 

[jim hardy]

the voltage regulator would see voltage decrease and try to raise it back up.

Some regulators sense all three phases , smaller ones only one.
So if regulator didnt look at affected phase it would remain oblivious.

it's a simple closed loop controller and you understanding is correct.

The bigger the machine the more elaborate the schemes to handle "What If's "


Most large regulators have a "manual" section called "BASE"
and an Auto section that adds or subtracts a small amount to BASE
and a null-meter that shows how much auto corection is applied.
That way if the auto quits or goes haywire you simply turn it off.
A huge imbalance would be interpreted as a measurement error and the automatic section would be automatically switched off.
Operators tweak the Base adjustment to keep auto-correction near zero so that should an unexpected transfer out of AUTO occur it will be bumpless.