Transformer testing

[shocker9]

Hi Guys

Can somebody please advise me as to what voltage can a SGB 33/6.6 kv transformer be tested at during a pressure test?
I would normally test any 33kv transformer at abt 15kv dc between phase and earth as it will be working voltage, but phase to phase i'm not sure abt. I'm using a T&R pressure test kit.
thanks

 

[Kiribanda]

shocker9,

For acceptance or routine maintenance test,in my opinion the best reference is NETA Acceptance Testing spec or NETA Maint Testing Specification.

 

[Zogzog]

OK, I will ask. What is a pressure test and what is a SGB. I have tested hundereds of transformers and am not familiar with those terms. And judging by the lack of respomse to your questions I don't think I am alone.

 

[mgtrp]

Pressure is a slightly old-fashioned word for voltage. Not too sure about the SGB though.

 

[ScottyUK]

SGB are a German manufacturer. mgtrp is correct about the 'pressure test', it's an old term which lingers in the UK power industry. It's a DC voltage withstand test.

Can't help on the test voltage but will have a look when I get in to work and post any information I find.


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If we learn from our mistakes I'm getting a great education!

 

[Zogzog]

OK, makes more sense now. Next question, why would you want to do a overpotential (Pressure) test phase to phase, I would expect those results to be very low in a 3 phase transformer, you know with the windings connected.

Usually the overpotential test is done High to Low with low side grounded. Low to high with high side grounded, and high to low. A megger test is done followed by a PF test. The 33kV side should be meggered at 5000VDC or more and the 6.6kV side at 2500VDC or more. The minimum spec depends on dry or oil type as do your specs for the PF test.

I do not know of any standard that recommends a DC hipot test on transformers.

 

[shocker9]

hi guys, thanks for the prompt response. I used the term pressure test thinking that it is a worldwide term, however
the purpose of a voltage withstand test is to prove that the insulation is in good order and it is safe to connect up the working voltage. I appreciate the insulation resistance tests with the megger and the low voltage of 5kv but it is sometimes not to great to prove a potential fault.
Example, i tested a transformer at 5kv with a megger and it proved ok, when I performed a 'pressure test' the insulation broke down. That was a 11kv transformer that I injected 12.5kv, a new cable was terminated to it and the recommended test was 2,5 times working voltage per phase.

So in that respect, how do we prove that the 33kv transformer insulation is safe enough to energise. Does your megger on 5kv give you the satisfaction.
Thanks again.

 

[ausphil]

In terms of high voltage (or pressure, or highpot if you like) tests on transformers of this size and larger, we perform an applied test from each of the windings to each of the other windings and earth. This will test the bulk insulation of the windings.

We then perform an induced high voltage test, which overstresses the insulation between the turns of the windings.

The 2 tests perform very different functions. These are in addition to the insulation resistance tests.

Don't forget about all the low voltage tests that need to be performed, which act as good benchmarks for the future, as well as giving more confidence for energising it - resistance, ratio, FRA etc

Without these tests, you can really cross your fingers for energisation.

ausphil

 

[mshahwan]

NETA is fine for North American operation, but you need to consult with your XFMR vendor on specific testing and procedures required before energization.

 

[Zogzog]

Mshahwan,

Why would you limit NETA to North America? It is an international standards and has member companies worldwide. That said, NETA specs always say to follow manufactures recomendations and provide guideance in the absence of them.

Shocker,

As I said before, the megger test is done before the higher voltage test, the PF test, that is a much better test than a hipot (Pressure, dielectric, whatever you want to call it) test, it gives a truer indication of the insulation condition.

 

[shocker9]

Mshahwan, I didn't qute understand your last response about the megger test, did you mean to say that the megger testing is the best of all that you have mentioned above, just need some clarity.
This is the first time that I have dealt with the power side of a 33Kv transformer, I normally concentrate on the protection part.

 

[Zogzog]

I think you are asking me, no the megger test is basically just to make sure it is safe to do the PF test (Although your PI/DAR values are important). PF testing is required by ANSI and NETA standards and recommended by IEEE C57.

 

[bashar2008]

i think according to IEC standards
we apply 70KV (for 33KV winding) in power frequency withstand test for one minute and check transformer
and we apply abt. 25 KV after that for partial dischage test in order to chech the 33kv winding insulation .
thanks