generator overspeed

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joncole

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Hello guys and gals.

I have a large generac generator that is backup power for a wastewater pumpstation that has started getting "overspeed" alarm on startup. It starts for a few secs and then shuts down. It starts just fine not under load. It started fine under load for 5 years until now. The load has not changed but we recently replaced a few powerflex ab drives with scheider altivar drives. There is a harmonic filter assembly across the input of each drive. The generac tech said it was a glitch in software or controller and reset it by unplugging controller and having it reboot. It does it every startup under load now.

Anyone have any input on the issue? Is it possible the new drives could cause this?

Thanks for the replies.
 

qcroanoke

Sometimes I don't know if I'm the boxer or the bag
Location
Roanoke, VA.
Occupation
Sorta retired........
Hello guys and gals.

I have a large generac generator that is backup power for a wastewater pumpstation that has started getting "overspeed" alarm on startup. It starts for a few secs and then shuts down. It starts just fine not under load. It started fine under load for 5 years until now. The load has not changed but we recently replaced a few powerflex ab drives with scheider altivar drives. There is a harmonic filter assembly across the input of each drive. The generac tech said it was a glitch in software or controller and reset it by unplugging controller and having it reboot. It does it every startup under load now.

Anyone have any input on the issue? Is it possible the new drives could cause this?

Thanks for the replies.
Does it actually overspeed? (high RPM'S)
or just shut down and flag overspeed?
I would look at the new drives, sounds fishy, generator may not like the drives.
 

grich

Senior Member
Location
MP89.5, Mason City Subdivision
Occupation
Broadcast Engineer
I had a 80kw Generac unit with (I think) a E controller which reset to factory after a battery explosion. I went through the system params menus and noticed items such as flywheel teeth did not match the commissioning documents. That made sense, assuming that figure would affect the RPM sensor. One I re-entered the settings to match the documentation, the genny ran normally again.
 

joncole

Inactive, Email Never Verified
Does it actually overspeed? (high RPM'S)
or just shut down and flag overspeed?
I would look at the new drives, sounds fishy, generator may not like the drives.

Im not sure really. What could be different about the drives? The control device in both drives are IGBTS. There is a passive harmonic filter on the input of new drives. What do you mean doesnt like the drive?
 

qcroanoke

Sometimes I don't know if I'm the boxer or the bag
Location
Roanoke, VA.
Occupation
Sorta retired........
Im not sure really. What could be different about the drives? The control device in both drives are IGBTS. There is a passive harmonic filter on the input of new drives. What do you mean doesnt like the drive?

By it doesn't like the drives I mean it may not like the sine wave or freq. it sees. The electronic governor of the genset may be hunting for what the load is. If it runs fine without any load. (just a fast test or exercise without load) I would think there is something about the load that the generator doesn't like.
Wait and see what others think in here. They are way beyond me in telling you what it could be.
 

coop3339

Senior Member
Location
NJ
I have seen smaller 20kw propane units do this when the regulators are not set right. They would run rough and go out on over frequency. Could there be engine or fuel trouble? Maybe its just a coincidence it started happening after the drive change. When the gen is under load the engine has to work harder and some problems may show up that aren't there at idle.
 

joncole

Inactive, Email Never Verified
Im the op. We found a bad current transformer on the output leads of the transformer and it fixed the overspeed problem.

Now another issue. I dont think its related but may be. We have six drives on this pump station. The power is fine from the generator until we put 4 or 5 pumps on line. When this happens the voltage from the generator starts fluctuating enough to start causing problems, lights flickering, drives faulting on input loss and overvoltage, etc.

This is a good sign of harmonics affecting the regulator on the generator. I understand that. But the 3 AB drives are an 18 pulse system and the 3 schneider drives have a harmonic passive filter that guarantees .5% thd or less. WTF!!

Anybody have experience with this? Any comments, suggestions, etc.:cry::rant::weeping:
 

willpoll

New User
Location
Everett WA, USA
joncole:

Can you clarify what you mean by this:-

"We found a bad current transformer on the output leads of the transformer and it fixed the overspeed problem."

I'm having the exact same issue and would love to know how you resolved it.

Cheers,

Will
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
...

This is a good sign of harmonics affecting the regulator on the generator. I understand that. But the 3 AB drives are an 18 pulse system and the 3 schneider drives have a harmonic passive filter that guarantees .5% thd or less. WTF!!

Classic case of salesmen engineering something for you...

An 18 pulse drive uses a complicated multi-phase transformer and 3 different diode front-ends on the drive that cases a phase shift of the various harmonics so that most of them cancel each other out to meet the IEEE 519 requirements, the net result is usually around 4% THD. A passive harmonic filter gets to the same 5% goal (sometimes), but does so differently. With a 6 pulse drive, MOST of the harmonic current is in the 5th and 7th harmonics. So the PHF unit is a tuned filter, SPECIFICALLY for the 5th and / or 7th harmonic (depending on how much they spent). It solves the problem by tuning out MOST of the offending harmonic frequency CONTENT, but not all of the harmonic FREQUENCIES. There is still the same amount of 11th, 17th, 19th, 23rd, etc. etc. If all you want to worry about is the THD number, i.e. the 5%, then it can work.

But the other harmonics still exist and can cause problems. In your case, it appears as though the AVR or the governor have something in them that is being affected by one of those higher order harmonics. When you had 18 pulse drives, those were being attenuated. When they changed to using PHFs, they were not.

How big are the drives? If small, you can try adding an EMI/RFI filter to them. But be forewarned, they might resonate with the PHFs too. Schneider should be helping you deal with this if they are the ones who convinced you that the PHF solution was a good fit.
 
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