ECN Forum Forums General Topics of the Trade General Discussion Area generator set

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