Insulation test on Transformers
Insulation test on Transformers
(OP)
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?






RE: Insulation test on Transformers
RE: Insulation test on Transformers
RE: Insulation test on Transformers
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. www.netaworld.org
RE: Insulation test on Transformers
RE: Insulation test on Transformers
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.
RE: Insulation test on Transformers
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.
RE: Insulation test on Transformers
I agree interely with what edison123 suggested.
Regards,
Wolf
www.hydropower-consult.com
RE: Insulation test on Transformers
P.S. That 2*rated voltage +1000V is from an old standard (NEMA I think) that has been removed from recent revisions.
RE: Insulation test on Transformers
RE: Insulation test on Transformers
RE: Insulation test on Transformers
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)
RE: Insulation test on Transformers
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
RE: Insulation test on Transformers
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.
RE: Insulation test on Transformers