Generator During Shutdown

[mbrooke]

When a generator is shutting down (or starting for the matter) does the AVR decrease voltage in proportion to the frequency?

 

[drcampbell]

I would think it would turn off excitation current completely when it's turned off.

 

[mbrooke]

I would think it would turn off excitation current completely when it's turned off.

What about start up?

 

[drcampbell]

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.

 

[mbrooke]

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.

Agree- but in theory its possible that if an engine shuts down with an ATS in emergency, the load could get destroyed if the AVR keeps the voltage up.

 

[rambojoe]

Depends on the generator... and how the transfer is set up...if a grid is solely on a gen (no power company)...run it out of gas or hit the e-stop, all lights will slowly diminish along with rpm, which is directly related to herzt...

 

[rambojoe]

cant figure out how to edit w/the new forum, but the dynamo will be fine, the danger lies in the loads downstream- again all how the backup is configured.

 

[Todd0x1]

I would say it depends on the generator. My only experience is with portable units, the higher end ones have a vacuum contactor which drops the load before idling down and eventual shutdown, or drops the load instantly (well as fast as the contactor will move) if the output falls outside predefined parameters. The lower end ones say to turn the breaker off before shutdown.

I had a lower end one (without the fancy controls) on rent powering a new building prior to utility connection. The customer forgot to check fuel and call for refuel and ran it dry. When it ran out of fuel the engine surged, voltage and frequency went haywire and blew up all their occupancy sensors.

So I would say you want a generator that can drop the load if the output is abnormal.

 

[mbrooke]

Thanks

Any ideas of standby units like Kohler, Generac, Caterpillar, and Onan?

 

[Todd0x1]

I don't have any experience with Kohler (well I do with their toilets), generac only an old diesel unit, no onan, and Cat only towable rental units. Not sure what those had for dropping load. I know some will have a shunt trip breaker instead of the vacuum contactor, which is fine for disconnecting but has to be manually reset.

 

[Russs57]

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.

 

[mbrooke]

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?

 

[Jamesco]

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.

.

 

[mbrooke]

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.

.

Down with the frequency?

 

[Hv&Lv]

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?

 

[mbrooke]

Please bear with me while I answer that with a question.
How is frequency produced in a generator?

Speed. There is no slip, so shaft speed directly controls frequency.

What happens, even if the AVR continues like the old ones used to, when a generator shuts down?

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.

 

[Jamesco]

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.

.

 

[mbrooke]

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.

.

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.

 

[Jamesco]

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.
.

 

[mbrooke]

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.
.

Unless the voltage remains high.