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transformer calculations, and a big phantom load problem

transformer calculations, and a big phantom load problem

transformer calculations, and a big phantom load problem

(OP)
First of all, a cluster of 3 200kva pole transformers gives me what kva capacity?  Secondly, on a generator supplied system we are getting phantom loads of up to 300 amps. Basically, we generate 480V, step up to 13.8kv, step down to 4160, and then  to 575 and 480 respectively.  I'm suspecting a set of ancient step up transformers.  Any thoughts?

RE: transformer calculations, and a big phantom load problem

First, I'm going to go out on a limb and say your three 200 pots should be good for 600Kva if the load is balanced.

On reading the second part of your post, my initial thought was "Rube Goldberg". If you can supply more specifics, it might help. Are you using more than one genset? Where/when do you see the 300A reading?  

RE: transformer calculations, and a big phantom load problem

(OP)
Not far wrong about the Rube Goldberg, but sometimes any port in a storm - I'm stuck with old stuff in a remote mine. Only using one genset, running about 300 amps, but spiking to 600 everyonce in a while (3 or 4 times a day) The step down trans in the field are dry type 3 phase, nitrogen filled, while the step ups are old oil filled.

RE: transformer calculations, and a big phantom load problem

If I understand correctly, you have an unknown load that shows up for some period of time, then goes away. How long does it last? Seconds, minutes, or longer? What is your largest motor load?   

RE: transformer calculations, and a big phantom load problem

(OP)
surge lasts a couple of seconds, sometimes repeats up to 3 times.  Biggest motor is 58hp. Don't think it's starting current - just too high!

RE: transformer calculations, and a big phantom load problem

The running current of a 58 HP motor will be about 75 Amps at 480 Volts. Regardless of the actual motor voltage 75 Amps is what the motor will draw from the 480 V generator.
800% staring current will give a 600 amp surge.
Probably starting current.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: transformer calculations, and a big phantom load problem

(OP)
Thanks for your thoughts - don't believe it's starting current - that would normally be max 300% and only a very short time - not enough to bog a generator down with 300 solid amps. Also, I can see starting surges as motors are started, and nothing so dramatic.  Something will let go before too long and I'll let y'all know what the gremlin was

RE: transformer calculations, and a big phantom load problem

doryman:
I have to second waross. Unless you post new information you, are chasing a problem that does not appear to exist. What you are describing does not constitute a problem.

Rafiq Bulsara
http://www.srengineersct.com

RE: transformer calculations, and a big phantom load problem

(OP)
And the saga continues - today, no phantom loads even though all systems were running normally. Kinda proves we're not looking at motor starting currents.  I'm still suspecting a set of 3, 200kva, old oil filled monster transformers used as step up. (getting some replacements ready just in case!)

RE: transformer calculations, and a big phantom load problem

What you are describing does not sound like a normal failure mode for a transformer.
I am reminding myself that you are on site and I am not. There may be some subtle indications that are difficult to express but never the less, lead you to believe that the issue is not motor starting. You're there and I'm not and you may be right. How is the condition of your generator? If you have a failing AVR or an intermittent high resistance connection in your voltage control scheme it may be causing trouble. If the voltage is being driven high at the generator, any large device being driven into saturation may draw unexpectedly large currents for the duration of the event.
If you are using three single phase transformers rather than one three phase transformer, you may see indication of an impending failure one one transformer by high currents on one (wye) or two (delta) phases but not a balanced three phase fault current. If the transformers have been in service for years, what are the odds that all three would have the same short term fault at the same time for the same duration?

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: transformer calculations, and a big phantom load problem

(OP)
Thanks for your continued interest - it is a hair tearer!  Here's some more detail: preferred generator is a little 350kva Volvo.  It's laboring under near capacity, and the mystery load almost stalls it and/or trips it's main. We switched to a big cat 1 meg unit for a short time (have overheating problems to iron out) While running, it held the phantom load, showing 600 amps, up from the normal 280. The two systems have a common bus, but otherwise don't share any controls. But - today like I said - no events. Just a dumb thought - how would a poor power factor figure in this?

RE: transformer calculations, and a big phantom load problem

Perhaps it is time to put current monitoring devices at strategic points (feeders) in the system and narrow down the part of the system causing the extra current.

 

Rafiq Bulsara
http://www.srengineersct.com

RE: transformer calculations, and a big phantom load problem

Are there any switched capacitors?

RE: transformer calculations, and a big phantom load problem

(OP)
nope, no capacitors at all.  Today I had couple of powerline guys up scratching their heads.  Best they could come up with is that two transformers are resonating somehow causing some phase shifting. Hmm, maybe. I asked them if I should check the flux capacitors.  Nevertheless, we did iron out our generator cooling problems and now have the 1 meg unit on line, which shrugs off a 300 amp extra load.  I also have some recording meters coming to pinpoint the problem.  Thanks y'all for your ideas, and I'll come back on with the solution of this puzzle, hopefully next week.  Thanks again, Alex

RE: transformer calculations, and a big phantom load problem

Look for a hidden motor on automatic control. Possibly a large sump pump or maybe a cooling tower fan.

Bill
--------------------
"Why not the best?"
Jimmy Carter

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