HOW TO CONDUCT A BACK ENERGIZATION TEST ON A POTENTIAL TRANSFORMER? 7

[juanpedro]

HI! I'VE BEEN A FREQUENT VISITOR OF THIS FORUM BUT NOW I JUST BECAME A MEMBER.

MAYBE SOMEONE HERE CAN ANSWER MY QUESTION.

QUESTION: HOW TO CONDUCT A BACK ENERGIZATION TEST ON A POTENTIAL TRANSFORMER?

PLEASE HELPME ON THIS...

THANKS IN ADVANCE...

 

[oldfieldguy]

Safety, safety, safety...

If I understand your question, we called this a "reverse excitation" test on PT's. It proved quite useful in finding faulty PT's that showed proper results for ratio and DC insulation tests.

In this troubleshooting mode, the test required a variable voltage power source with voltage and current metering, capable of rated secondary voltage and at least rated current for limited time. With the high voltage terminal(s) clear and safely barricaded to prevent inadvertent contact by nearby personnel, connect the low voltage source to the secondary terminals and begin raising the voltage on the secondary winding. I problem with the high voltage winding will be readily apparent as increasing voltage is induced in the HV winding and it begins to break down. This may occur at relatively low values of secondary voltage.

If this is not what you had in mind, I apologize, but this test has worked very well for me in many past incidents.

old field guy

 

[juanpedro]

THANKS A LOT old field guy for giving me some ideas. But I can't imagine as of now what sort of signs for breakdowns in the primary winding may appear if the PT is defective and how to determine these breakdowns? sorry for my ignorance, but i hope you can give me a clear idea on this matter.

thank you very much

 

[rasevskii]

Yes, as oldfieldguy said, back energize it using a variac and measure the voltage and current in the LV winding that you are injecting. If the PT is actually installed in switchgear, the HV side of that will become energized. All safety precautions have to be applied, and signage, fencing, is needed. Use maybe a 1Kw 110V heater element in series with the injected voltage to limit the current if something fails suddenly, or a very small fuse.

Let it soak at rated voltage for an hour or so. This does not mean that it will not fail in service, however. If in doubt about the PT, best is to replace it. They usually fail in the middle of the night or some other inconvenient time according to Murphy's Law.....

rasevskii

 

[prc]

Back energisation test will be useful to check the insulation strength of the HV winding. In case you want to check for any interturn faults in HV or LV or defects in core, a low voltage excitation current from HV side may be useful.

 

[rcw retired EE]

To answer your question on how to tell when it breaks down- you will see the amp output of your test supply suddenly jump. The test power supply breaker/fuse will probably trip. Depending on the type of VT, you may hear the partial discharge sparking as you hit the flashover voltage.

What is the purpose of your test? To verify the ratio? Checking for winding failures? Commissioning test to verify the unit is ready to energize? The purpose may have an impact on how you conduct the test. I the unit is already suspect and are trying to confirm that the HV winding has internal problems, the test procedure will be different than if you are trying to verify some nameplate ratio data.

 

[oldfieldguy]

Juanpedro-

While I haven't dissected a failed PT to see, I suspect turn to turn shorts in the primary winding. The examples I had to remove from service had NOT visible damage and had passed DC insulation resistance (hi-low, hi-gnd, lo-gnd), DC winding resistance, and ratio.

At the low voltages associated with normal ratio testing, the shorts don't show up. Nor do they show up in DC winding resistance tests, They don't even show up in DC tests where the voltage is evenly distributed over the winding and the fault is between one turn and another and not from the run to ground.

However, when the winding is excited and the voltage raised, the impressed voltage between windings increases to the point of breakdown, effectively shorting the turn, and as you know, shorting one turn of a winding effectively shorts the whole winding. The excitation current jumps to high values, pointing out the problem.


old field guy

 

[juanpedro]

Hi! thanks a lot to the following wonderful individuals who enlightened me
a. rasevskii
b. prc
c. rcwilson
d. oldfieldguy

But as mr. rcwilson said "The purpose may have an impact on how you conduct the test. If the unit is already suspect and are trying to confirm that the HV winding has internal problems, the test procedure will be different than if you are trying to verify some nameplate ratio data."

So I was wondering on how to conduct the test for commissioning purposes? Mr. rcwilson please help me on this & to all the other guys in this great forum.

thanks in advance

thanks in advance

 

[scottf]

To pick up a turn-to-turn short in the primary winding, you need to perform an excitation test and compare versus like units. A unit with a shorted turn in the primary will show a noticeably higher excitation current and lower knee-point voltage than a good unit.

 

[lz5pl]

I haven't done this kind of tests from many years, but in the past I used it almost with every new installation. Injecting voltage from low voltag side gives opportunity to set part of your installation (in switchgears up to 220 kV usually it includes also CT and CB of the same phase) under real 50 Hz voltage, what is not practically possible with test van equipment for installations over 33 kV.
In addition with VT's up to 110 kV I used this test also for check of VT-ratio - with temporary connection phase-to-phase with neighbouring VT, injecting in LV-side of one and measuring on LV-side of the second VT and for real phasing of secondary VT circuits up to check-synchronizing instruments and relays.
As excitation data for VT's are usually not available, for VT-diagnostics I would suggest to perform same test on another VT from the same type and compare results with these of suspected one.

This is a good test, although it is not very common. But with CVT's it is not possible, so I cannot remember we have done it in recent years.

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It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant