Winding Insulation Resistance

400 volts ÷ 100,000 ohms = 0.004 amps = 4 ma. This is a ground fault (not exactly a short yet) that might not be sensed by any ground-fault detection you may have. But it may be an accident waiting to happen. The next thing it might turn into is an arcing ground fault (not so easily detected, either). Then, when some more insulation burns away--full-bore short.

Does the "winding insulation resistance test" can replace the " Hi-Pot test " ?

The terms "winding insulation resistance test" and "hi-pot test" seem to overlap each other.

For example, see this link: http://cirris.com/testing/guidelines/hipot_testing.html.

This describes 3 sorts of hi-pot testing (dielectric breakdown, dielectric withstand, and insolation resistance). The last of these would be a typical megohmmeter ("Megger^(TM)" test. The other two use even higher voltages, I think; thus the winding resistance test would not substutute for the more stringent types.

It is not a good reading you have a problem somewhere. what was the insulation test for the primary winding, one would expect similar readings for both windings .Have you checked the oil moisture content it could be a problem?

This unit is heading for a failure.

i could advise you to check you protection relay for correct operation because definetly you should have expirenced a trip due to the leakage fault current.

Does the secondary have a ground tap, if so did you isolate it?

The standard formula in finding the minimum winding insulation resistance is:(Vrated + 1000)/1000.Therefore, the mininmum secondary winding resistance of your transformer should be 1.40 Mega-ohm.Before taking a winding insulation resistance, the windings per phase should be isolated or disconnected from each other(whether it is connected in wye or delta).

May this answer will help you,merry Christmas!

I think .1 MegOhm is too low value.

The transformer insulation in not good.

Re: Winding Insulation Resistance

we have measured the value winding insullation resistance of secondary-ground of our transformator and the value was 0.1 MegOhm,

the transformator is 1600 KVA, primer 20 KV to secunder 400 Volt,

Dear jaeson

You can used the following formulae

( Ref : A Guide to Transformer Maintenance by. JJ. Kelly. S.D Myer )

XFMER INSULATION RESISTANCE

R = Insulation Resistance of Winding to Ground at 20^oC

C = 0.8 for oil filled transformer at 20^oC

C = 16 for dry compound filled or un tanked oil filled transformer

E = Voltage rating of winding under test

KVA = Rated Capacity of winding under test

Insulation Resistance Calculation that still permissible for this XFMER

HV Side

R = ( 0.8 x 20000 ) / √ 1600

= 16000 / 40 = 400 MΩ at 20⁰C

If temperature measurement at 30⁰C ( check the temperature winding indicator) the Insulation Resistance result should be corrected to with based temp 30⁰C, see table.

R at 30⁰C = 1.98 x 400 MΩ = 792 MΩ

For LV Side with V = 400 V

R = ( 0.8 x 400 ) / √ 1600

= 320 / 40 = 8 MΩ

R at 30⁰C = ± 16 MΩ

Those values are the minimum Insulation resistance that still acceptable for safe operations.

Note ;

- These Insulation resistance value valid when the winding in the tank with oil filled

- When the winding at out side the constant "C = 16"

PI / Polarization Index

Polarization Index can be determined by the ratio of 10 minutes resistance to the 1 minute resistance value. Since the leakage current is increases at a faster rate with moisture present that does the absorption current, the polarization index is lowered for insulation in poorer condition.

Polarization Index guide for evaluation of transformer condition

Be careful your XFMER status in the dangerous zone, check also by step voltage test with voltage megger® with Ratio 1:5, ==> used 500 V and 2500 V megger with reverse polarity (negative to copper winding, positive to ground), each test do 1 minute. Its useful to identified the water absorption in the insulation. Result of these measurements 25 % (decrease or increase) at the higher test voltage is presence of excessive moisture.

I used this standard since 1980, to repair / rewind of Power Transformer up to 150 kV, and success....

Rgds

Siswanto