OK. I am running out of ideas and looking for help. We installed a 300kva genset to back up our 300kva, 240 kw ups. After 2 test, still unable to get the ups and genset to work together. UPS tech. in private has stated that the genset is undersized for this app. Our load on the UPS is about 40 kw, due to decrease in size of servers over the years. Where can I find info to back up UPS tech. claim or disprove it. Any help would be great. email me here
keith.white@ncr.com
thanks
Jbfan,
The tech is probably right. If the UPS does not have an input filter that is interlocked with the ATS then it is very possible your problem is an undersized generator. What happens is the UPS throws lots of harmonic current back on the system and this causes havoc with the generators regulator. The fix in the past has been to double the size of the generator to compensate. The newer large UPS’ have an input filter that allows you to size the generator closer to the load. I don’t have any back up literature handy but if I find it I’ll post back.
Nick
Thanks NIck. What the generator people are telling us is the genset only see the UPS as the load and not what the UPS sees as a load. Therefore it is mainly a capatiance load only, which drives the voltage higher. What they have come up with is to remove the filter caps, and hope the genset will not overpower the UPS. We shall see in a couple of weeks when we retest. Also, I found something, that supported the tech's statement. We have a confrence call tomorrow , and we will see what the engineers that recommended this genset have to say about it. Thanks for the input.
Also. the UPS is about 12 years old
Oh Boy! Another {electrical} pissing contest!!
The problem has bee around for at least 20 years, but everyone hasn’t had to deal with it at the same time. The simplified problem is that the genset exciter wants to see a nice clean sinusoidal load, and the UPS wants to see an very stiff source, and so they end up fighting each other. It just continues as finger pointing until someone caves, and the end user {and/or electrical contractor, depending on contract verbiage} often get the very short end of the stick.
A delta-wye drive-isolation transformer or 3ø reactor between the two may help—like those used on AF drives, but don’t bet on it. The UPS and genset vendor susually insit that it’s not their problel, and it never happened before, but they LIE! Ususally the CSI-type specidfications don’t handle this very well, and reapozilbiblues need to be worked out before the problem occurs.
Good luck. Keep us posted.
Here is an update. We are going to retest again Sat. night. We will disconnect our load and the generator people are bringing in a load bank which will be used to adjust everything. The UPS people are going to remove the ac line filters. We asked point blank if the genset was to small, and they assured us it would work. I will let you know how it goes. Thanks for the replies so far.
Are you going to attach the load bank to the UPS or the genset. I think it should be attached to the UPS. Let us know what happens. Load banks we use on our genset are pure resitive heat. I don't know if that will throw your test off as real loads are computers etc and throw harmonics onto the lines that could be having some effects.
[This message has been edited by jdevlin (edited 03-14-2003).]
Thanks for the update, jbfan. It’s not clear who is saying that the genset size is adequate. That is a critical bit of information that, if it hasn’t already, needs to be put in writing. The verbal finger pointing will continue until someone’s feet are held to the fire. It sounds like the genset is new, versus a decade-old+ UPS. It also sounds like the genset voltage regulator doesn’t know what to do with a nonlinear/nonsinusoidal load. Permanent-magnet excitation helps a little, but a 300kVA genset can have trouble with even a purely resistive load near its nameplate rating. I’m not up on the latest genset capabilities, but it may be difficult for some gensets to simply handle a resistive {1.0PF} 100-to-200kW-load step change. That can be a hard thing for a genset {and its AVR} to negotiate—much less if the load is nonsinusoidal.
For your testing session you might want to look at phase voltages and currents with something like a Fluke 41 or Tektronix 700-series, to get a clearer idea of how the two major components are interacting.
Ideally, for AC circuits, distorted voltage points to the source, and distorted current points to the load. In this case, though, it’s more complicated, for the source does not present a very low impedance to the connected load, and UPSs are {in the fine print} promised to work acceptably with a low-impedance source, like a nice utility-owned 5%Z padmount transformer. Aside from that simple low-impedance source, decent voltage regulation and the ability for downstream overcurrent devices to operate reliably may be seriously limited.
[This message has been edited by Bjarney (edited 03-14-2003).]
jdevlin Thanks for your question. The load bank will be attached to the output of the UPS. After the adjustments have been made, our computor loads will be reattached and the load bank will be removed for another test. Hope this one works, because I am tired of working all these sat nights
I am by no means a power expert. I do know that we do a load test on our 3 gensets once a year. The largest one is 1.025 Mw. I think the main reason is load up the deisel and make sure it is operating properly.
Other than my once a month 10 minute test, I call in in the generator service company if there is any problem. WE just spent 3 day in 0 degree F pumping cleaning and refilling the fuel tank that had been filled with contaminated fuel somewhere onlong the line.
Good luck with your test.
On a situation like this, would it be possible to tweak the Genny's Exciter, so it doesn't see such a distorted AC?
Tweak and filter out the offending distortion, then stabilize the VAC as seen by the Exciter? (to compensate for the highly Capacitive situation/leading P.F.)
Seems like I read the True Power load is only around 48KW, so is the GenSet rated for 50KW, more than 50KW, or less than 50KW?
There is a listing for Apparent Power (300KVA).
Sure seems like an awful lot of distortion being pumped back into the AC system from the UPS Equipment! Anyway to filter this and "Tame The Peaks" via notch filters?
Wow, what noisy components!!!
Scott35 S.E.T.
A very lightly loaded UPS will send tremendous harmonics upstream. Some new options on UPS equipment include an active input filter to take the filter off line at low load. I would be interested if a resistive load on the UP, at about 60-80% of the UPS rating would limit the upstream harmonics suffieciently to allow the gen to operate. See the articles below for more info.
http://www.powerware.com/About/technologies/tracy.asp http://www.mgeups.com/techinfo/techpap/articles/0246-e.pdf
Ron. It is strange that you sent me a powerware site, because they are the ones working on our ups. Being that it is 12 years old, the pwoerware rep dosen't thin we would be able to add on something to help, Of course it was mentioned that a newer ups would have these features already installed!
If I understand you correctly you have a 300KVA generator, and a 300KVA UPS. In my opinion your generator is way undersized.
Here are a few points:
1: A UPS of this type is probable a dual conversion type unit, meaning rectifier-to-dc, dc-to-inverter. Each stage at best will have 80 to 90% efficiency. So work the math backwards. 300 KVA out the UPS output to the dc stage through the inverter is 375 KVA. 375 KVA from the dc stage back to source through rectifier is 468 KVA at full load. Since you mentioned it is an older UPS the efficiency could be as low as 60% if they use SCR rather than switch mode rectifiers.
2: Another factor is the battery recharge current. Not only do the rectifiers have to supply the inverter power, but they must also have the capacity to recharge the batteries at the same time. This is usually an additionally 20 %. So now lets back up to the DC stage and see what the rectifiers must actually be able to produce and work backwards. 375 KVA from the last step plus 20% recharge current equals 450 KVA at the rectifier output. Assuming the rectifiers efficiency is 80% we get 562.5 KVA input power requirement at the UPS during recharge.
3. Now lets talk generators. Just because a generator is rated at 300 KVA doesn’t mean it can deliver 300 KVA.
Taking all this into consideration a 300KVA UPS would need a minimum 500 KVA generator depending on the efficiency of the UPS, actual load, and battery recharge demand. I design large scale UPS with generator back up and typically we double the generator output over the UPS depending on the above items. Sounds to me who ever designed it overlooked some things.
Good Luck
Dereck
[This message has been edited by DBC1 (edited 03-15-2003).]
Thanks DBC1 We shall see what happens in a couple of hours. I will either be all smiles or frowns this time tomorrow. I have learned a ton about this subject in the last several months, so all has not been lost. I'm printing all this out for my boss to keep just in case this dosen't work. Then it will be up to the people who interpret the wording in the contracts, and as was stated earlier, the electrician is on the bottom of the pile.
jbfan, One thing I was not clear on your UPS. Is the UPS rated 300 KVA output or input?
I did some checking on various UPS manufactures with 300 KVA output, and they all require an input from 440 KVA to 470 KVA.
Taking the highest of 470 KVA multiply by 1.25 gives you a 588 KVA generator to handle a 300 KVA output UPS.
Even if your UPS is rated 300 KVA input, a 300 KVA generator falls short. 300 x 1.25 = 375 KVA generator. And that may fall short if the UPS does not have a walk-in feature built in.
Dereck