Insulation test on Transformers

[a10jp]

We have a transformer rated at 13.2kV / 4160V, and specification asked for insulation resistance test be performed during field testing. But a test voltage was not specified. So test voltages (13.2kV and 4160V AC) which match the ratings of the primary and secondary windings were applied. The question is is that acceptable test voltages for field testing purposes? I would think the test voltages should be higher to stress the insulation of the core windings. I understand the hi-pot test would be a good stress test, but perhaps IR test is not considered such? Is there a standard that covers this testing requirement? Any comments?

 

[unclebob]

The standard procedure for transformers is to apply 1000 Vdc for 1 minute. You can also do a Polarization Index test which is 10 min per coils, but always at 1000 Vdc.

 

[a10jp]

Hi, do you mean this IR test voltage is the same, regardless of the voltage rating, primary or secondary? so IR test is truly not a stress test, correct?

 

[Zogzog]

NETA specs say at least 2500VDC on the 4160V side and 5000VDC on the 13.2kV windings. Assuming this is dry type LV should be at least 5G after 1 minute and 25G on 13.2kV side after 1 minute. Values need to be tempature corrected. Depending on the kVA rating a DAR and or PI test is also required.

IR readings should be taken with the opposite side windings grounded. (HV-LV/G and LV-HV/G.

I recommend you have a certified testing company test this for you if you are not sure what to do or dont have the right test equipment.

http://www.netaworld.org

 

[a10jp]

Hi Zogzog, thanks for the helpful input. This is for acceptance test, so it is a field test. Other than IR tes which you mentioned above, do you perform hipot test on the windings as well? If so, should you use the 2*rated voltage + 1000 criteria for the test voltages? Both primary and secondary?

 

[edison123]

a10jp

IR test is a condition assessment, trending test. It does not test the dielectric strength of the winding. I agree with zogzog on test voltage levels but that would require you to have a megger with multiple output voltages. For a routine assessment in service, 2.5 KV for both the windings would suffice.

AC Hipot level at 2U+1 (I think it is a bit more for trafos) is done only for new / rewound trafos at the OEM/repair shop. For a field acceptance test, the test voltage will be 80% of 2U+1.

For a regular assessment test, I would suggest 1.5 times the rated voltage for each side.

 

[a10jp]

Hi, thanks for your comments. You hit upon soemthing I vaguely remember. In one of the reference (maybe NETA) I read, it uses DC hipot as a reference, and it says the following:

dc accepotance test voltage = 1.6*(0.8*(2U+1)); and
dc maintenance test voltage = 1.6*(0.6*(2U+1))

Has anyone seen this before? I wish I could remmeber where that came from.

 

[wolf39]

a10jp:

I agree interely with what edison123 suggested.

Regards,

Wolf

http://www.hydropower-consult.com

 

[Zogzog]

No hipot, but a PF test is required per NETA ATS. Is this dry or liquid filled? What kVA?

P.S. That 2*rated voltage +1000V is from an old standard (NEMA I think) that has been removed from recent revisions.

 

[a10jp]

It's oil filled, 25MVA, 13.2Kv-4160V.

 

[prc]

Once upon a time,transformer engineers used to do DC voltage withstand test at site similar to being done on cables.Voltage applied was similar to the formula mentioned.This is no more done today.When insulation resistance test is asked for at site, use a voltage (megger normally have 500V,1KV,2.5KV,5KV ratings)not more than the rated phase voltage of winding.Preferene will be for as high a voltage as the winding voltage rating permits.

 

[prc]

Let me update on the topic a little more:

DC IR meghometer was introduced in England in 1904 and in US in 1910. for many decades that was the sole instrument for electrical engineers to check the soundness of electrical equipment till Doble came up with a portable power factor meter in 1929.

The major problem with DC voltage test is that it cannot detect partial degradation of insulation.(ie a bad insulation in series with good insulation)

NETA MTS -2001 and IEEE C57.90(1999)Transformer Test code give recommendations on the voltage for diffrent rated transformer windings.IEEE said Dc voltage shall not exceed the low frequency one minute separate source test voltage of winding(agaist my recommendation of working phase voltage)But IEEE also mentions that it is the max,but working voltgae is to be prefered( clause 10.8.3)

But latest edition of IEEE (2006) removed all that and said" insulation resistance of electrical apparatus is of doubtful significance compared with the dielectric strength......It amy be useful to indicate whether the apparatus is in suitable condition for application of dielectric test"! (clause 10.11)

 

[Zogzog]

Well said prc.

NETA ATS says IR test 2500V (Minimum) test on 4160V side, should be >1G. At least 5000V test on 13.2kV side, should be at least 5G. (temp corrected).

PI>1.0

TTR

Pf or DF test on winding and bushings

Exicatation current test

Tao changer resistance test on all taps

And Oil samples, including DGA

 

[edison123]

zogzog

About those Gohms of NETA, how well are they realistic about old, oil filled trafos ? I seriously doubt you could get those nos. even when you replace the old oil with new oil with more dielectric strength.

 

[Zogzog]

Very realistic corrected to 20 degrees C. 100M at 55 degrees C oil temp is 1.12G