Generator and UPS don't play well together.

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qcroanoke

qcroanoke

Sometimes I don't know if I'm the boxer or the bag
Location
Roanoke, VA.
Occupation
Sorta retired........
Have a set up with a 25 KW generator that serves a 15 kva UPS.
The problem is that everything is hunky dorry with the generator when the UPS is not on.
But when the UPS is added to the load within a minute or two the generator will shutdown on overspeed. But,there is NO change in engine RPM's. I've heard of UPS's rejecting generators but I've never heard of this. Site guy says he read frequency of the generator and it was all over the place.
Any ideas?
 
If the frequency was really "all over the place", then the motor is not spinning properly/right speed.
YOU need to check the frequency
 
qcroanoke said:
I've heard of UPS's rejecting generators but I've never heard of this. Site guy says he read frequency of the generator and it was all over the place.
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I've had UPS's reject the generator power, and the frequency was spot-on and steady. The generator people blamed the UPS's and the UPS people blamed the genny people. :roll:

"Waddya gonna do?"


Oh, Happy Square Day! :smile:
 
I've found the smaller gensets tend to run slightly higher than 60 HZ, which usually give UPS units fits. Could be a grounding issue, is it a three pole or is it a four pole switch? (assuming it's three phase) The output of the UPS may be grounded, but the genny's not or vice versa. Also the genny may be too small, I think the rule of thumb is 2-3 times the size of the UPS is normal. Non linear loads seem to make generators do goofy things.
 
Sounds like the voltage regulator on the generator isn't working, which can happen when you have a generator feeding a UPS. I'm not sure, but it sounds possible that haveing the output frequency all over the place could be seen as a overspeed by the generator.

Can you have the site guy read the freq. of the generator without the UPS connected, and verify it works OK without the UPS. (It might just be a bad regulator, and for some reason the problem just doesn't show up without the UPS connected).

Do you have a load on the UPS? How much? Is there other loads on the generator? Again, how much? And what sizes are the generator & UPS.

I think keeping the generator sized larger than the UPS size, and having another constant load on the generator (cooling units or motor loads) helps prevent this.
 
Some small generators do not have good voltage regulators. What was the voltage out from the gen just prior to the frequency changing? Many times the UPS causes enough harmonics (voltage distortion), that the gen cannot compensate or figure out how to control the voltage and inevitably the frequency properly.
Try the gen with other loads. Make sure it works well on its own then start checking the UPS on utility power.
 
I would bet if you checked the frequency with the UPS running it would be all over the place. In this case it is Harmonics.

Whose generator whose UPS, a rule of thumb is 2 KW generator to 1 kw UPS, BUT I would always check with the manufacture.

An input filter on the voltage regulator may work.
 
steve66 said:
Sounds like the voltage regulator on the generator isn't working, which can happen when you have a generator feeding a UPS. I'm not sure, but it sounds possible that haveing the output frequency all over the place could be seen as a overspeed by the generator.

Can you have the site guy read the freq. of the generator without the UPS connected, and verify it works OK without the UPS. (It might just be a bad regulator, and for some reason the problem just doesn't show up without the UPS connected).

Do you have a load on the UPS? How much? Is there other loads on the generator? Again, how much? And what sizes are the generator & UPS.

I think keeping the generator sized larger than the UPS size, and having another constant load on the generator (cooling units or motor loads) helps prevent this.
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The problem does not appear until the UPS is connected and right now there is very little load on the UPS. The rest of the building is also on the Genset under power outages so it does have some cooling loads and heat loads with winter coming. (the building is in New Jersey)
Generator is 25 KW UPS is 15 KVA I've already been told this is very close to unacceptable size wise for the generator.
As I said earlier the Gensets RPM's never change 1800 all the way.
But the site tech took readings and said his meter showed the frequency all over the place. Most gensets will shut-down on overspeed at about 65-67 hertz I believe.
I'm leaning toward harmonics but I just really am not sure. I have never seen a generator shut down on overspeed when there is no reason for it to do so.
 
brian john said:
I would bet if you checked the frequency with the UPS running it would be all over the place. In this case it is Harmonics.

Whose generator whose UPS, a rule of thumb is 2 KW generator to 1 kw UPS, BUT I would always check with the manufacture.

An input filter on the voltage regulator may work.
Click to expand...

I will have the site tech check frequency on the output of the UPS monday.
Genset is Onan
UPS is 9350 Powerware
Building is 3 phase 120/208
We have checked with both manufacturers and they will be on site next week to check it over and run them through their paces.
I just really want to know what is causing this.
 
You need to add some more resistive load to the generator if there is a light load on the UPS.
The harmonics of the UPS creates havoc on the voltage regulator of the generator.
 
If I recall my motor theory, the RPM is directly related to output frequency of the alternator. I don't think it is possible to have 1800RPM all the time and the output frequency to change.
 
Definitely harmonics. If you have a scope or power analyser, look at the UPS input when it's on street power, then look when it trys to run on the UPS. I suspect the word ugly will apply to the waveforms.

More load on the generator will help, but there's also the rule of thumb that a genny should be rated at twice (or more) capacity as the UPS it's feeding. Also, as noted, UPSs are much harder on the unput line when running at low capacity.
 
qcroanoke said:
Have a set up with a 25 KW generator that serves a 15 kva UPS.
The problem is that everything is hunky dorry with the generator when the UPS is not on.
But when the UPS is added to the load within a minute or two the generator will shutdown on overspeed. But,there is NO change in engine RPM's. I've heard of UPS's rejecting generators but I've never heard of this. Site guy says he read frequency of the generator and it was all over the place.
Any ideas?
Click to expand...

I wish I had a $ for every occasion I have heard this complaint.

Unless the generator is specifically built to be used with UPS's AND had been used with the model of that specific UPS then all bets are off. Same is for ASD's or large, non-linear loads connected to generators.

It should have been an engineered installation and if it was you would have a rightfull complaint to the Licensing Office. Back to the drawing board.
 
15 KVA UPS on a 25 kw generator is on high side.

You have to upsize the generator size in KVA. Variable speed drives and UPS's distort the supply waveform and this confuses the AVR and speed control electronics of the generator.

UPS harmonics can cause voltage waveform distortion at the generator. But permanent magnet generators that derive their own excitation can help to mitigate this problem.

Please read sheet 7 of 12 on this link.

http://www.abco.co.za/C - Electrical Load Impact t-030_p23-34.pdf

So i would suggest a generator with Permanent magnet excitation.

You may contact Cummins rep and inform your load characterstics and let them suggest the correct rating/specs.

Cheers.
 
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