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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?

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.

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

Regards, Phil C.

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.

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.

Dear Phil C.

Please tell me the simple test to find the place of insulation weakness.

Regards,
paru

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

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

Paru... a Megger won’t find the problem!

Regards, Phil Corso (cepsicon@aol.com)

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

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

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.

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?

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.

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

Hi,

As I said, as per design in all three phases with respect to ground voltage should be 700v. But in R phase it is 450 and Y phase 750 and B phase 950v.

Paru, following is information that supports the conclusion that the insulators are the source of the severe unbalanced voltage-to-ground problem:

Using an unbalanced, Y-connected, capacitive-load, to represent leakage-current through the insulator, I was unable to reach the phase-to-ground voltages you cited. However, when a lossy-capacitor, that is, a resistance in series (not in parallel) with a capacitor, was modeled, then the values you measured were reached!

This suggests that conductive pollutant from your process has unevenly contaminated the insulators!

Note, that because the resistive component is in series with the capacitor, the Meggar doesnt work!

Reards,
Phil Corso (cepsicon@aol.com)

Hi Phil,

Finally I have decided to just watch as the system with me is very bulky. The insulators are plate type mica between live part and earth. There is no earthing to the insulator's other side.

So it is true that this unbalanced voltage is due to uneven capacitance between the phases and ground.

I am worried only that some day some of my insulators will get damaged due to this problem. I got a lot of answers from other people saying that this is a normal phenomenon in furnace transformers with delta secondary.

Still I will try to find out if there is any possibility why this capacitance is uneven.

One more question I would like you to clarify...
As you said, the the opposition for AC current to flow through insulator is the impedence. That is the resistance + capacitive reactance. Ok.
Now say if some how the capacitive reactance has become less, still resistance is in circuit. Which will save the insulator.

Am I right?

Responding to Paru’s 10-Jul-08 (12:40) comments:

1) I strongly disagree with those convinced your EAF unbalance is a "normal phenomenon!" While some unbalance is present in ungrounded delta-systems, the voltage excursion you’re experiencing is far from "normal!"

2) The displaced-neutral (earth in this case) resulting from the observed 450, 750, 950V voltage-to-ground measurements is well outside of theoretical neutral (~400V) of the delta system! In any ungrounded, delta connected power system, a displacement of much more than 5% is considered worrisome!

3) What would be learned from an exercise to manipulate resistance and capacitance representing the insulator pollution? The problem is still the high degree of unbalance! And No! You are not right!

4) What metal(s) are processed in the EAF?

5) Insulator pollution is a result of the dust emitted by the EAF process!

6) BTW, pollution is exacerbated when water is present!

7) When were the insulators last cleaned!

8) Have you compared the two EAF circuits for physical anomolies or differences in their installations? Perhaps, busbar or cable lengths? Separation of phases? Etc?

Regards, Phil

hi phil
just few days back i had also same thinking like you and i was worried a lot.

the furnace is running perfectly without any problem. the arc voltages are also balanced when the furnace is running. the main insulators were recently changed which are between conducting arm and supporting structure i.e ground. some more insulators are there to support the delta closure.

It is very difficult to check the insulator other than visually checking as the system is very bulky. all insulators are plate shaped. there are big water cooled cables.

i dont know how to get rid of this problem.