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Magnetic Balance Test on Transformer

03/14/2017 10:56 AM

The Transformer I am testing (132/33kV) 40MVA, tripped on Bucholtz gas. I have carried out a magnetic Balance Test by applying 380Vac on the high Voltage winding terminals as follows:

1. Applied 380Vac across Red and Yellow phases, and measured 189.5V across yellow and blue phases, 188.8V across red and blue phases.

2. Applied 380Vac across Yellow and blue phases, and measured 376.5V across blue and red phases, then 751V across Yellow and Red phases.

The first case looks ok as the sum of the measured values more or less balances the applied Voltage. The second case is where I need some interpretation.

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#1

Re: Magnetic Balance Test on Transformer

03/14/2017 11:28 AM

These numbers look to be expected to me. The magnetic balance test values are due to the core construction. The image below should help you visualize:

See how the outer 2 coils (R and B) have core material only on one side of them while the center coil (Y) has core material on both sides? That will affect the magnetic field. So if you apply voltage to R-B (in this case), then R-Y and B-Y both have similar physical core configuration around them. But if you energize B-Y, then B-R and Y-R have different core structures, so the magnetic fields for those windings will be different.

In regard to your problem, the magnetic balance test won't tell you a whole lot about the condition of your transformer. Buchholz relay operates on the generation of fault gases. You need to get a dissolved gas analysis done on the transformer oil to tell you what gases were found, and that will point you in the direction of possible causes (overheating, corona, arcing, etc.).

Other good, more valuable tests than the magnetic balance for diagnosing possible failure conditions include turns-ratio tests, DC winding resistance, Megger (DC insulation resistance) and Doble insulation power factor / dissipation factor tests. These will give you a much better idea of the health of the transformer than what you are doing.

If this is out of your league, then hire a good HV electrical testing consultant who does these tests on a regular basis and has the proper equipment , training, and experience. A good place to look is for members of the InterNational Electrical Testing Association (NETA) at the following web site:

http://www.netaworld.org/

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#3
In reply to #1

Re: Magnetic Balance Test on Transformer

03/14/2017 1:10 PM

Thanks Peter T.

Yes turns ratio Test has been done as well and it is only on the blue phase where I have a variance of more than 20% from the ideal/theoretical on some winding taps.DC resistance results look good, it is the tan delta that I am unable to perform.

However, going back to the magnetic Balance, I also have these results:

With Voltage applied to HV Yellow phase and neutral, I would expect a balanced Voltage in the two outer phases, that is the Red and Blue. But this is what I have:

Y-N 218V, applied. Then measured 217V between Red and Nuetral; and 1.38V between Blue and neutral.

Regards.

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#4
In reply to #3

Re: Magnetic Balance Test on Transformer

03/14/2017 2:14 PM

Suspect a turn-to-turn fault on the Blue phase. The winding resistance variance would possibly suggest this, as maybe also that last test you did.

Did you perform a Megger test on the winding to see if the insulation resistance to ground and to the LV winding is compromised? Of course since this looks like the HV winding is Wye, you can only do the whole winding (R-B-Y to ground , or R-B-Y to LV) tests. Any info there?

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#5
In reply to #4

Re: Magnetic Balance Test on Transformer

03/14/2017 2:29 PM

Yes indeed the transformer is Wye, all insulation results are good. With short cct test, all currents are balanced

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#6
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Re: Magnetic Balance Test on Transformer

03/15/2017 9:11 AM

Then you need to find out what gases caused the Buchholz to operate. Get a dissolved gas analysis (DGA) done on the transformer oil. You have told us nothing about the operating conditions of the transformer. If it has been running heavily loaded, overheating tends to cause carbon monoxide due to incomplete oxidation of the insulating paper. Methane can also be produced by this condition. As things get hotter (localized overheating, limited hotspot, sparking, arcing under oil), the hydrocarbon gases get more "energy-intensive," from ethane to ethylene, to acetylene. The relative ratios and absolute concentrations of these gases give a pretty good pointer to things happening in the transformer, but you need to have someone qualified interpret the results.

And if it comes back with high gases, DON'T just let someone try to convince you to just do oil processing. Yes, it will get the gases out, but it doesn't treat the cause of the gases. Gases are just a symptom; the cause of the gases is the problem itself, and you need to treat that.

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#2

Re: Magnetic Balance Test on Transformer

03/14/2017 1:07 PM

The magnetic balance test won't tell you much about an internal fault that does not leave any form of permanent interturn or ground fault. As Peter said, you need to get a DGA in order to have a better clue as to what happened internally.

You said the unit tripped on Bucholz gas. Depending upon the exact model and relaying philosophy, Bucholz relays usually have an alarm on high gas volume, and trip on high fluid flow. If that's your setup, then there was probably an internal flashover that caused a pressure pulse that was picked up by the Bucholz. Whatever you do, do not re-energize that transformer until you know exactly what happened and why!!

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#7

Re: Magnetic Balance Test on Transformer

03/15/2017 9:57 AM

Richard, . . . you are not providing sufficient information. The transformer's configuration is MOST important here . . . Yy?, Yd?, Dy, Dz? what is it?

The normal thing to do is to energize a "Y" connected winding to Neutral. Delta connected windings tent to equalize the phases so that any unbalance is more difficult to notice.

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#8
In reply to #7

Re: Magnetic Balance Test on Transformer

03/16/2017 5:22 AM

The transformer is a Yy with a stabilizing tertiary. The neutral terminals on either side are solidly grounded.

I agree I did not avail all important information, I should for example have mentioned that the transformer tripped on bucholtz simultaneously with a 33kV distribution feeder; when the distribution feeder developed a single phase to ground fault on blue phase, of a magnitude 7000 amps. The fault was Isolated within 0.194 seconds according to the fault record retrieved from the protection device on the distribution feeder; but despite this, the transformer tripped as well.

The transformer normally is loaded to about 80-90% average, and is relatively'new' about 6 years old. The transformer is Crompton Greaves manufactured in India

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#9
In reply to #8

Re: Magnetic Balance Test on Transformer

03/16/2017 4:36 PM

With a rated current of ~700 amperes on the 33kV side, and a 7,000 ampere fault, this would cause leads and windings to "whip-lash' and generate pressure surges in the oil. Pressure surges will trip a Buchhotz relay.

This size of fault lasting for 10 periods - is unacceptable, . . . get up-to-date relays.

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