Transformer IR Measurement

[NCTHAI]

Question of testing the transformer: EHV Trasnformer:

220KV / 15 KV // YNd11 // 250MVA // ONAN / ONAF

Is it normal to test PI for Oil filled transformer?? I guess not. PI (IR 600 Sec / IR 60 sec) is not valid measurement for Oil Filled transformers due to the oil inulation.

PI is valid for dry insulation.

Any comments on this opinion??

Can someone provide good reference for trasnformer IR measurement techniques and result evaluation??

Thanks in advance!!

 

[NCTHAI]

What should be the normal PI if at all measured you can expect for large oil filled transformers??

I have rarely seen higher than 1.

Min acceptable IR for Oil Filed X'mers is given by CE/Sqrt(KVA), C=30, E = phase to phase voltage and KVA is ratign of x'mer.

PI is more valid for dry / solid insulation - dry X'mers or rotating machines - motor or generators.

Please provide you opinion on subject.

 

[Zogzog]

ANSI/NETA 2007 MTS requires the following tests on oil filled transformers:
IR at min of 5kV, including PI. IR should be >5G, PI should be>1.0
TTR
Pf/Df on each bushing
Excitation test
Winding R on each tap
And oil sample including DGA

 

[NCTHAI]

ZogZog,

These are commissioning tests or maintenance tests or acceptance tests? Can you please clarify??

NCTHAI

 

[prc]

It is correct-PI is more relevant for dry insulation,say that of generator or motor than for oil immersed transformers.But some customers are very fussy about it and in India there are utilities who insist for a PI of 2.0 Min and absorbtion ratio ( IR at 120/IR at 20 seconds) of 1.5 Min as acceptance test.These are extremely difficult to achieve but we struggle as customer is still the undisputed king in many parts of the world.A more realistic demand will be as in IEEE std 62-1995 Guide for diagnostic field testing of oil filled transformers and reactors where it is said" For small transformers,PI will be 1 or slightly higher.Larger trfs may exhibit PI of 1.1-1.3......A PI of less than 1,indicates that immediate corrective action is required.( clause 6.1.5.2).

A more sensible test for checking the dryness of trf internal insulation is power factor testing.

 

[DTR2011]

In most cases, the megger readings will be at Infinity (max scale) after 1 minute and are not likely to change after 10.

I have found (for HV oil equipment), the PI test to be a waste of time, even using modern test equipment (Tera Ohm range). Infinity/infinity = ????

As a quick test, prior to AC PF/DF/TD tests, the megger test is OK.

Just my 2 cents