mbrooke
Batteries Included
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- United States
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When a generator is shutting down (or starting for the matter) does the AVR decrease voltage in proportion to the frequency?
I would think it would turn off excitation current completely when it's turned off.
I doubt very much that voltage-proportional-to-frequency would be useful at less than 80 or 90% of nominal speed, and it seems like an unnecessary (and unlikely) feature. Were I designing a voltage regulator, I think I'd incorporate a time-delay that leaves the excitation current off until the engine's up to speed, (and maybe even until it's warm enough to take a full load) then ramps it up over maybe 10 s or so, then tells the transfer relay that it's ready.
As for how it's actually done, I'm sure there are different approaches from different manufacturers, for different sizes of generators and for different applications. I'm sure the installation and service manuals will include the specifics.
Will you have an ATS? Will you have switch gear?
In my world, a generator will get a start command from an ATS. The generator will do its best to make rated voltage and hertz ASAP. When it gets around 80-90% then a breaker will close and energize a bus that feeds said ATS. When the ATS see voltage and hertz within tolerance it will transfer.
It is up to you to set parameters as to when that breaker closes and a given ATS transfers.
ATS- only. I know, but what happens when the gen shuts down with ATS still in emergency?
If normal power is available at the ATS it will transfer to the normal power. If normal power is not available at the ATS and the generator shuts down, so goes the voltage down.
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Please bear with me while I answer that with a question.Down with the frequency?
Please bear with me while I answer that with a question.
How is frequency produced in a generator?
What happens, even if the AVR continues like the old ones used to, when a generator shuts down?
Speed. There is no slip, so shaft speed directly controls frequency.
If the AVR continues to produce the same voltage as the frequency is falling; every motor, solenoid, transformer, and magnetic ballast will saturate possibly burning up. In fact the voltage doesn't even have to remain constant, if the voltage at any point is higher than the frequency saturation will take place.
How would an engine/generator shut down be any different than a POCO power outage? I really don't see any difference between the two.
As for the AVR I don't think it has any control over the frequency. It's my understanding generator shaft speed controls the frequency.
https://www.macallisterpowersystems...ering-toolbox/generator-set-engine-governors/
I have seen what happens when an electronic engine governor malfunctions. Changing loads on the generator caused wild swings in engine speed. Swings in generator voltage as well as frequency. High voltages caused lots of damage to sensitive electronic equipment loads that were connected to the generator. Was damage also due to the higher frequency as well? I don't think at that point frequency was an issue.
During shaft speed less than normal operating speed could the lower frequency cause damage to connected loads? Possibly. I think the length of time the frequency was less than normal would be a factor.
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POCO outages are typically caused by breakers or fuses opening due to a fault such as a down primary. However, a genenrator shutting down results in the rotor slowing down before coming to a complete stop. One stays are 60Hz the latter dips below that.
True, but the length, time, of the event happens very quickly. Just a matter of a few seconds.
JMHO, not enough time for the frequency to be a factor.
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