Transformer Insulation Resistances

The acceptable limits are set by ANSI/IEEE standards for here in the US. I would suggest your getting a copy of these standards. It really varies some for the different transformer types with the various BIL ratings.

You cannot use standards of different country, you can either obtain datasheet from IEEE(thats international standard as you know) website or find out what is used in your country.

testing a 1250 KVA transformer, how many M ohms must be the insulation resistance of this transformer?

Dear Muhammad A. Razzaq

its depend on the X-mer voltage rating, cooling system (dry or oil),

Use this formulae:

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

If oiled x-mer, when winding alone without oil, you should calculate "C = 16"

Good Luck...

Rgds

Siswanto

Thank you very much Mr. Siswanto.. but what is the symbol between CE and KVA?

C = a constant

" C " for Dry x-mer or winding alone without oil = 16

" C " for winding in the oil = 0.8

KVA : X-mer capacity, your xmer is 1250 kva

E : is voltage rating, you are not mention what is voltage your x-mer, letsay : 20 000 volts

R : is insulation resistance value in M ohm

Good Luck

This formula doesn't work for us. For example, in the electric utility where I work, for our 14400/240 kV distribution transformers 500 kVA and below, we have noticed that at 1500 MegOhms, the transformer will not last long (more than 2 years) if installed and energized. At 1000 MegOhms, the transformer will usually fail within the week it is energized. So, we set our minimum insulation at 2000 MegOhms.

In my opinion, if you have no historical records to based on, the limits set by NETA is a more appropriate guide.