voltage variation

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Thread Starter

panther

hi
our transformer which is feeding to an arc furnace 20MVA, primary volatage 34.5kV with 17 taps and secondary voltage 300-650V with open delta. Secondary is connected to electrods, through watercooled busbars which are supported on insulators.

The problem is, the open ckt voltage is found to be imbalance 750/450/400.
is same identical other transformer the voltage remains same 420/400/410.

what may the reasons for that and what should i do?
 
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Curt Wuollet

I'd be looking for winding problems like shorted turns or open connections. You know the balance is a result of the physical number of turns, so it can't change without major internal problems. One secondary that high points to shorted turns in a section of the primary, but these things can be tricky. Inductance measurements and magnetizing current measurements may help.

Regards
cww
 
is there any possibility of insulator failure/weakness of the supports on which the bus bar of the secondry have been supported?
i suppose as secondry is delta connected and if the insulator on one phase is weaking , can it lead in voltage drop in that phase while increasing the voltage of the others.
 
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Phil Corso, PE

Paru,
Yes, insulator failure will increase its the phase-to-earth capacitance, thereby resulting in a decrease in its capacitive reactance!

Regards, Phil C.
 
W

William Hinton

Voltage imbalance phase to ground and line to line is very common in an open-delta power system.

You will find a solution to your voltage imbalance at: http://www.Phaseback.com This website explains voltage issues with waveforms, videos and solutions.

I hope this helps,
William Hinton
 
hi
the only thing i could understood from the site given above is, the voltage imbalance is because of insulation failure of some perticular phase.
As it told that there will be phase shift causing voltage imbalance.

am i right ?
what should i do now?
 
hi friends,
i am still confusion. I add one more thing to earlier mail. hope after this i may get some solution of my problem.

the problem of unbalance voltage is only when i check in no load condition. as it is a furnace transformer, while running the volatages are perfectly balanced on all taps.

i checked the megger value of secondry windings it is in gigaohms.
 
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Phil Corso, PE

Paru... there is a simple test that can be carried out to determine where the insulation-to earth weakness is in the bus duct or coolant system!

If you are interested contact me!

Regards, Phil C.
 
hi let me ask my question in other way...and hope this time i would get my answer. if i measure the open ckt voltage with respect to ground in secondary of three phase delta/delta transformer.

it is sure the voltage will not be same. my question is why and on what factor the voltage difference depends.

second question: how much difference in voltages can be seen in max.

as i am getting phase1-ground-450v
phase2-ground 750v
phase3-ground 950v
 
W

William Hinton

Paru,

Yes, this is more common than you would expect. With very little system capacitance the voltages will typically be unequal. An arcing ground fault or even a slight imbalance of impedance can cause phase voltage to ground readings like this. The Video at Phaseback.com clearly shows a Phaseback balancing the phase voltages with respect to ground. You can watch the video, read about it and there is even a place for contact information. I have used these with great success.

William Hinton
Sr. Electrical Engineer Delphi Automotive
 
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Phil Corso, PE

Responding to Paru's 23-Jun-08 (10:35) query:

As suggested by William Hilton different phase-to-ground voltages are common on ungrounded delta-connected systems. However, what is unusual in your case is the large disparity in values. It suggests that there is a large discrepancy in phase-to ground capacitance. Such a large voltage could produce an arcing ground-fault resulting in severe damage! Thus, the cause, since it appears to be located on the same phase should be corrected.

It was stated earlier that the bus-bar and its associated cooling system are isolated with insulators. A weakness in insulation anywhere in the current path, including the electrode connections, can be found rather easily.

Typically, an insulator will be installed with a grounding strap. Of course, I'm speaking of the ground-end of the insulator.

Simply measure the current thru that strap with a clamp-on ammeter! Seek out those that have unusually large current, comparatively speaking! Oh yes, include the insulated water piping connections.

Regards, Phil Corso
 
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Phil Corso. PE

Paru, adding a caveat to my 24-Jun-08 (00:42) post:

DO NOT, REPEAT, DO NOT, DISCONNECT INSULATOR GROUNDING CONNECTION WHILE SYSTEM IS ENERGIZED!!!

Happy hunting,

Phil
 
Yes, today I checked it again. The line voltage, I mean phase to phase without load, is also normal, balanced. The designed phase voltage is 700 volts. So we can say that y phase is nearly good while phase r is less and phase b is more.

I will try to check it now in the way you people are suggesting.
 
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Phil Corso, PE

Responding to Paru's 25-Jun (13:34) observation...

A Megger won't work because "essentially" it measures resistance. The variation in voltage-to-ground problem is a capacitive phenomenon!

Please keep us informed of your findings.

Phil
 
hi phil
yes, as you told megger can measure the IR i.e. insulation resistance. but capictance is also the insulation between two conducting things. Isnt it?
 
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Phil Corso, PE

Responding to Paru’s 29-Jun-08 (16:24) query…

Insulation Resistance is determined when the Megger® applies a DC voltage and measures DC leakage-current through and over an insulation system. But, the Megger® can not measure AC leakage-current!

BTW, what were the associated B, Y, and R phase-to-ground voltage measurements mentioned in your 25-Jun post?

Phil
 
Hi,

As I told in running condition, I mean with load, the phase to ground voltages are perfectly balanced. Also, in no load condition, if I check phase to phase, these voltages are also balanced.

When I check voltage between phase and earth while secondary circuit onload and taking balance current... these voltages are equal to line voltages/root 3.
 
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Phil Corso, PE

Responding to Paru's 01-Jul-08 (01:00) query... I apologize for having misunderstood the information you posted.

My question was, "What were the no-load, Y, R, and B, phase-to-ground voltages, for the condition described in your 29-Jun post?" In that post you said, "So we can say that y phase is nearly good, while phase r is less, and phase b is more."

Regards, Phil
 
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