GENERATOR EXCITATION SYSTEM

[duki]

Does anyone know why it is a good practice to turn off a generator's excitation system as soon as the generator breaker is opened and the driver is still running? It seems like it would make no difference if excitation remains on since there is no load.

 

[waross]

It is not as important as it once was. Most newer AVRs (automatic voltage regulators) have UFRO (under frequency roll off). These reduce the output (or the voltage set point) when the frequency falls. This gives some load shedding and gives the engine a chance to get back up to speed and frequency. This is useful for motor starting and helps sets handle motor starting surges that and other loads with an initial surge on energization. Also, UFRO prevents AVR failure when running at reduced speeds.
For example, if the prime mover is operated at 25% speed, it may take 4 times the field current to develop rated voltage. (The exact amount depends on the percentage load on the set and the characteristics of he load at reduced frequencies.)
Also at low speeds/frequencies the inductive reactance of the coils and transformers in the AVR is much reduced and saturation is to be expected.
Bottom line: Older AVRs often failed if an operator forgot to turn off the AVR before slowing the set down for whatever reason.

Bill
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"Why not the best?"
Jimmy Carter

 

[ScottyUK]

There's no reason to cut the excitation when the main breaker opens providing that the prime mover is still running. It helps to prevent voltage excursions if the excitation is backed off to place the generator somewhere near a neutral VAr condition prior to opening the main breaker. A modern AVR will quickly correct the situtation regardless of the excitation level at breaker opening, but it is a good practice none the less.

It is common for a generator to remain at a synch idle condition with the unit at nominal speed and with excitation raised under a load dump situation, which leaves the machine in a state where it is capable of reclosing the main breaker. On a large generating unit this can be valuable as it takes a fair time to spin up a turbine from a trip.

Bill has already mentioned UFRO, something I know as the volt-Hz limiter, which is a supervisory controller intended to prevent saturation of the stator core. At the 25% speed Bill quoted the stator core will saturate unless the designer has been ridiculously generous with the stator iron. Normally the AVR has two stages of protection: the first stage limits the machine conditions within safe operating limits, then it trips excitation if the limiters can not stay within the safe operating limits. A prime mover trip should always trip the excitation.


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If we learn from our mistakes I'm getting a great education!

 

[Skogsgurra]

There is a secondary and probably very minor (in most cases) effect. And that is that rotor cooling is reduced when the machine stands still. Leaving excitation on can lead to increased rotor temperature, sometimes too high.

This is, of course, not possible if you have a rotating exciter. But in a machine with slip rings, it is a risk. Also, why consume energy when not needed?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[edison123]

It all depends on type of generator (hydro/Steam/Gas) and conditions of trip (fault/normal) and amount of load at the time of trip.

In hydro machines, fault trip during load will result in high overspeeds due to water flow and it is imperative to trip the field immediately in order to avoid dangerous high stator volts.

In steam & gas turbines, fault trip does not result in high overspeeds because of lower steam mass and easier control and it may be possible for avr to control the generator voltage. In many cases, the machine is resynchronized soon if no machine fault was present.

In controlled manual trips, the load is invariably brought to zero before tripping the generator and hence no problem to keep the field on unless the machine is run down to shutdown.

But then, as Gunnar says, why waste energy when not needed ?

 

[waross]

As the regulars here know, most of my experience has been with smaller diesel machines, up to around 1MW.
Diesel plant operators sometimes like to run the engines at slow speed on start up to let the engine warm up before running up to operating speed. On shutdown they would also cut the speed down to 25% to 50% of operating speed to allow the engine (and the turbo, if fitted) to cool down before stopping.
At times a mechanic would run the engine slow while trouble shooting and/or adjusting.
Slow running would usually destroy an old style AVR (without UFRO) before rotor heating became an issue. One or two minutes or less.
The operating procedure on shutting down was:
Shed load if possible.
Open the main breaker.
Turn off the AVR.
Slow down the engine.
The issue was not opening the breaker, the issue was slowing the engine. (I am using the word engine rather than prime mover to restrict my remarks to engine driven sets.)

Bill
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"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Thanks Bill - that's interesting.


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If we learn from our mistakes I'm getting a great education!

 

[Runsor]

Question:
If there is a short circuit fault inside the generator or a fault between the generator terminals and the circuit breaker, does it help to immediately switch off the excitation with the CB trip signal? Or is it enough just to start the shutdown procedure after some delay?

 

[waross]

Yes.
Shutting down the excitation will limit internal damage. The better AVRs fitted to some diesel sets will shut down automatically on high exciting current, after a few seconds.. (Over excitation) There is an intentional delay to allow for picking up block loads and to supply adequate fault current to trip external breakers.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Larger machines use a generator differential relay to (typically) provide a high speed trip of the generator breaker, field breaker and prime mover.



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If we learn from our mistakes I'm getting a great education!

 

[slavag]

Agree with Scotty and would like add, small machines too
( don't say about hydro, don't know). And not only diff protection.
Regards.
Slava

 

[turbinetony]

Is your excitation feed from a PPT or seperately fed?

On our units the exciter stops at 95% speed to prevent V/HZ limiter from activation. I know that the excitation can assist with providing a drag for steam turbine/generators to allow coast to gear sooner.

regards,
Steve

http://www.powerplantforum.com

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