UPS Question

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ceb58

Senior Member
Location
Raeford, NC
We have a situation where someone thought a generator had taken a hit from lightning. The Gen. teck. and one of our guys found it had shutdown on over rev. The teck check the draw on the building and it was 24amps. every thing running. He found the UPS system tripped out. With the gen. running 100% of the load when they reset the UPS it would bog the gen. down and make it over rev trying to keep up. The gen set is a 45kw 240 1ph so it should be good for 180 amps. The gen. teck said that the UPS is pulling to much load. I do not know the kw rating or the power factor. The UPS has 6 modules in it and they said each module is pulling 5 amps ea. The UPS powers a panel that runs communication equipment so if you remove the a/c and lighting load which is not on the UPS panel it should be around 13-15 amps. The UPS systems we use will supply the load for 30-45 min. however they never go that long unless there is something is wrong with the gen. The gen. teck kept saying that the UPS was a restive load and two of the modules should be removed to keep from overloading the gen. Help me understand the reasoning as I was not there to see this for my self.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Sounds to me as tough your Generator Tech is one of those kind of guys who doesn't want to bother with details and investigation, he just wants to point to the box that he didn't supply and blame that for everything.

That's not to say it ISN'T the UPS, but even assuming the batteries were completely discharged and the connected load was completely at maximum, the UPS input current would most likely be limited by the electronics in the UPS, what they call "current limit mode", which is supposed to prevent this exact sort of thing. That doesn't rule out that there is something wrong with the UPS of course, but from your descriptions, that UPS doesn't sound as though it's big enough to bury that generator.

One thing I DO know however is that it used to be really common for low cost generators to have low cost analog AVRs (Automatic Voltage Regulators) that are tied into the engine Governor. Non-linear loads on the alternator, i.e. those with high harmonic content such as VFDs, Soft Starters and UPSs, can affect those cheap AVRs which in turn give false signals to the Governors, which respond by... guess what?... over speeding! It used to be a very common problem. Most genset mfrs solved it when they upgraded to digital AVRs a few years ago by putting better filters on the sensing circuits. I would be suspicious that yours does not have that. I would also be suspicious that your tech knows this because he has run into it before, but has been instructed to side step the issue and not bring it up because nobody wants to spend the money to correct it.

If your UPS has a manual bypass, engage it and load the generator as much as you can. If it doesn't bog down, disengage the bypass so the UPS is back in the circuit again an see if the generator goes nuts. If it does, it's probably the AVR problem.

If they continue to refuse to address the problem, some people have had success putting the filtering on the UPS side. The problem is, that filtering needs to be able to handle the entire UPS input power and is therefore expensive, compared to a small cheap little filter on the sensing circuit of the AVR. Unfortunately I don't know of anyone who sells a ready-made retrofit filter kit for older AVRs, mostly because genset mfrs solved the problem for themselves and eliminated the market for them.
 

Besoeker

Senior Member
Location
UK
One thing I DO know however is that it used to be really common for low cost generators to have low cost analog AVRs (Automatic Voltage Regulators) that are tied into the engine Governor. Non-linear loads on the alternator, i.e. those with high harmonic content such as VFDs, Soft Starters and UPSs, can affect those cheap AVRs which in turn give false signals to the Governors, which respond by... guess what?... over speeding! It used to be a very common problem. Most genset mfrs solved it when they upgraded to digital AVRs a few years ago by putting better filters on the sensing circuits.
I think you may be correct in attributing the problem to the AVR.
Not so sure about the analog(ue) and digital comparison though.
Although not our main line of business, we make AVRs. We use analogue firing circuits because of their immunity to supply voltage distortion.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
I think you may be correct in attributing the problem to the AVR.
Not so sure about the analog(ue) and digital comparison though.
Although not our main line of business, we make AVRs. We use analogue firing circuits because of their immunity to supply voltage distortion.
I only meant it in terms of the fact that most generator mfrs went to digital AVRs a few years ago and used that as the impetus to fix the harmonics issue at the same time. It allowed them to avoid having to admit there was a problem.
 

dbuckley

Senior Member
Most UPS units have a lot of capacitance on the input to try and clean up the mess they make of the input waveform. The problem is when the UPS is lightly loaded the caps are way over-compensating, and make the UPS unit have a very poor power factor, and that annoys many an AVR.

A five module UPS drawing just 24A sounds like a very lightly loaded UPS system to me.
 

jrannis

Senior Member
When you mentioned "lightning strike" one would have to assume that the system was in operation and working properly.
My question would then be, what changed?

Maybe not the best troubleshooting method but, pay the generator tech to change the voltage regulator, then take it from there.
 

ceb58

Senior Member
Location
Raeford, NC
When you mentioned "lightning strike" one would have to assume that the system was in operation and working properly.
My question would then be, what changed?

Maybe not the best troubleshooting method but, pay the generator tech to change the voltage regulator, then take it from there.

The mention of lightning strike was because there was a bad lightning storm which knocked out the utility power. The gen. cranked and switched over but shut down on over rev. When the ups discharged it shut down communication equipment which set off alarms.
I have gained some more info on the UPS. Each module is 3.5kw so with 6 it would be 21 kw. The gen set is not a 45kw but a 35kw. From what I have learned from you guys and links posted. What the gen. teck said now makes sense The gen. set and the UPS are too close in size and the sine wave is causing the gen. set to go stupid. I will be going to the site in a week or so. Some one has put the A/C circ. on the UPS panel along with some other circ. that do not need to be there also. Then it will give me a chance to see what load is on the panel and then we can reduce the kw on the UPS and see if this corrects the problem.
 
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