[mekinik]

Hello all, and to the point, I uncovered a rather strange issue recently that I could use a little advice on. While doing a study on phase balance on a 750 KVA, 480 volt, 3 phase transformer; I was measuring the amperage on the wires entering the building. There are 5 wires per phase and five for nuetral, I found 1 wire in one phase carrying over 200 amps while all other wires in that phase and the other phases carried only 70 to 80 amps. I had the power shut down and we inspected all wires and tightened all lugs etc. There was no obvious problem with that line or any other. After powering up the problem was still there. Local electricians and the power company just scratch thier heads. Any thoughts or comments would be appreciated.

 

[MDShunk]

I think you should start by doing a fall of potential test from the conductor to the lug (if you can get to bare conductor) at each end and go from there. You have a termination issue someplace. My guess would be that the one with the higher amp draw is the only good one in the batch.

Are these overhead or underground? Reason I ask is that maybe if they're underground, the higher one might be faulting to ground a bit and ready to have a blowup.

 

[wwilson174]

I think you should start by doing a fall of potential test from the conductor to the lug (if you can get to bare conductor) at each end and go from there. You have a termination issue someplace. My guess would be that the one with the higher amp draw is the only good one in the batch.

Are these overhead or underground? Reason I ask is that maybe if they're underground, the higher one might be faulting to ground a bit and ready to have a blowup.

I have seen this "load hogging" phenomenon before and if memory serves me well it was related to the effects of inter- conductor inductive effects due to the physical routing of the conductors .BillW

 

[wwilson174]

I think you should start by doing a fall of potential test from the conductor to the lug (if you can get to bare conductor) at each end and go from there. You have a termination issue someplace. My guess would be that the one with the higher amp draw is the only good one in the batch.

Are these overhead or underground? Reason I ask is that maybe if they're underground, the higher one might be faulting to ground a bit and ready to have a blowup.

I have seen this "load hogging" phenomenon before and if memory serves me well it was related to the effects of inter- conductor inductive effects due to the physical routing of the conductors .

 

[mekinik]

Thanks for the input, I did a potential test when we tightened the lugs, all wires tested equal to each other and were all good. These wires are routed through a duct just above ground and are fairly easy to inspect. Inductive effects of the wires was something that I had not considered and is a very interesting possibility. Right now the wires are held in a loose bundle with zip ties and are not routed very well as some wires may actually be crossing and even wrapping around other wires, I will check this the next time I can shut down the facility. Thanks again for the input, this was becoming a very "Hmmmm" problem, with alot of head scratching.

 

[brian john]

I did a potential test

Did you do a fall of of potential test, across the connectors?
Did you perform an IR scan?
When you had the power off did you use a micro-ohm meter/ductor/ DLRO?

 

[bobelectric]

Hello all, and to the point, I uncovered a rather strange issue recently that I could use a little advice on. While doing a study on phase balance on a 750 KVA, 480 volt, 3 phase transformer; I was measuring the amperage on the wires entering the building. There are 5 wires per phase and five for nuetral, I found 1 wire in one phase carrying over 200 amps while all other wires in that phase and the other phases carried only 70 to 80 amps. I had the power shut down and we inspected all wires and tightened all lugs etc. There was no obvious problem with that line or any other. After powering up the problem was still there. Local electricians and the power company just scratch thier heads. Any thoughts or comments would be appreciated.

750 kva,full load @480 volts=900 Amperes

 

[mekinik]

The local power company did the fall of potential test, I am not really sure how it was performed as I was rather busy testing and tightening conductors at the time. I did test from lug to lug for any conductor resistance but found nothing unusual. IR scan is being scheduled for the entire facility, should take place in about two weeks, I think I will have my hands full following that. I know that this transformer is no where near fully utilized, just trying to solve an issue before it becomes an issue.

 

[brian john]

The local power company did the fall of potential test,

That would be a first, few if any linemen (IN MY EXPIERENCE) know anything about testing, other than checking for voltage. The FOP test is performed with the system energized carrying load.

I did test from lug to lug for any conductor resistance but found nothing unusual

What did you use to measure the resistance?

PREVIOUSLY POSTED BY ME

Fall of Potential test (FOP)

With any electrical procedures that involve making measurements with energized conductors care must be taken and proper PPE must be worn

Fall of potential is a measurement of voltage drop (VD), in a circuit that is the voltage lost to heating due to resistance in the circuit. To perform this test one measures voltage from the line side connections to the load side connections, the readings are typically in the millivolts range and require load.

For example with a 3-phase Circuit Breaker that is nuisance tripping, measure current (balance loads are beneficial but not necessary), then measure from the line bus to the load conductors (if bare conductor is exposed), for all three phases. If one phase has a higher that average millivolts reading, then try to isolate this issue. Measure from line bus to the Circuit Breaker bus stab, if all 3-phases have millivolts readings that are the same (or close to the same) measure from the bus stab to the load side conductor termination connector (through the Circuit Breaker), if all these readings are close, next take measurements from the load side termination connector to the load conductor. In this case you can determine if it is a line bus connection issue, bad Circuit Breaker or a load conductor termination issue. Any accessible portion of the device can be tested in this method.

You could do FOP on the individual conductors to see if there is a conductor issue. Though if all checks out with testing I lean towards Wilson post. I have seen some wide variances in current sharing.

 

[nick]

feeder problem

if conductors are parralled in tray ? one is drawing more current than the others all taps are good ? you said three phase 480v ? if you tell me more i like problems . but one wire almost twice the amps as the four others same phase , whats the distance from end to end ? what is amp reading on that leg at no load on transformer meaning service disconnected just the impedance of the transformer at no load . as 750 kva trans is large for just only 200amps on one conductor in a semi fault condition to ground ,you were busy tighten lugs while the power company was testing thats a little shocking got alot of trouble shooting been shocked few times tell me more. maybe that conductor is separated so far from the others it will not cancel itself out induction wise . or the four others are just about burnt or cut damaged maybe , but induction at 277 volts to ground induced 100 amps , i dont think so unless its a mile long . what size is the conductors copper or alum? also put your amp probe around the grounding electrode or one of the rods on transformer, tell me what you got there ?how many amps? it will detect a fault in your transformer ? you may have a bad transformer phase . insulation going on windings megg the windings . i changed my mind when i read your post again found my mistake in reading it .

 

[brian john]

your power company must be nice guys florida power would not touch the secondary side . if conductors are parralled in tray ? one is drawing more current than the others all taps are good ? you said three phase 480v 120 /208 load ? if you tell me more i like problems . but one wire twice the amps as the four others , whats the distance from end to end ? whats the load actual load meaning normally . as 750 kva trans is large for just only 200amps on one conductor in a semi fault condition to ground ,you were busy tighten lugs while the power company was testing thats a little shocking got alot of trouble shooting been shocked few times tell me more i like to help you out ? id say someone in the plant tapped it. you know how somewhere down the line to feed something or its seperated so far from the others it will not cancel itself out induction wise . or the four others are just about burnt or cut maybe , but induction at 208 volts i dont think so unless its a mile long . what size is the conductors and copper or alum? also put your amp probe around the grounding electrode tell me what you got there? fall test was it a ground rod test or what kind of test did you mean ?its hot down here in florida

Did I miss something?

 

[randomkiller]

Did you do a fall of of potential test, across the connectors?
Did you perform an IR scan?
When you had the power off did you use a micro-ohm meter/ductor/ DLRO?

I doubt any of the three were done. As for "did I miss something?, I just read through and I must have missed it too.:001_huh:

 

[nick]

strange issue

no you guys didnt miss anything at all in fact you covered every test that could be made on this problem , but in my limited time in the electrical trade if the connection drawning 200 amps one conductor out of five on one phase you must have four connections not in good contact or at main brk you have a problem like connection to it , and at 200 amps you might see discolor on terminals like burning , or four conductors are cut or damage down stream and one conductor is fine . let me add induced inductance would be higher on the four conductors not the one with the 200 amp load on it or someone has the amprobe not clipped on correctly you know closed all the way i think they have more than a balance problem on this job . i hope to hear the final fix on this problem was it 2 weeks for the infa red test our company would fix it that day , ill be waiting to see what happens cant wait to find out what it was ?go to http://www.pecplc.com go to tech papers armour currents in single core cables this will help you out . explained in detail.