what is the minimum acceptable insulation resistance?

Dear Colleagues. There is a mix up.The question is what is the minimum acceptable insulation resistance value for a vacuumed Power Transformer (7.5 or 15MVA 33/11Kv) before fresh transformer oil is poured into its tank ?

What are you measuring and what is the rated operating voltage ?

Any other nameplate data ?

Looks like my cell phine reply did not go through.

The drying process should have begun with an inital megger reading before any vacuum is applied. During the drying process, 12 or 24 hour or ... take timely readings, every 30 minute, 1 hour ... take values after 1 minute and 10 minutes. Graph the data or tabulate and the drying process basically ends when there is no material change in the insulation resistance values. You can calculate Polarization Index which is a measure of the dryness of the equipment.

As far as what values are required, there are no current industry consensus on Insulation Resistance values. If you have data from similar units, or factory / commissioning doucments, that would be bonus for comparison. The rule of thumb or general fall back number is 1 megohm per KV plus 1 megohm, but only a guideline at best.

So for your 33 KV equipment we like to see minimum 34 Megohm.

Oil filled equiment at you MVA size and KV class, with new oil and dry transformer would likely see a reading in the hundreds of Giga ohm for HV/LV to ground and Tera ohm.

it depends on the electrical equipment (transformers, motors) and voltage ratings (LV/MV/HV).

Minimum acceptable insulation resistance is minimum allowed insulation resistance under appropriate voltage in according with appropriate standards...
I think that the values which are exposed by sir Raymond in his own previous comment are good...

Its recommended 1kilohmio/Voltio, it is to say 1Megohm/KV

The minimum allowable insulating resistance is the value of the insulating resistance under the most violent case of voltage rise circumstance during an abnormal situation and extremely violent weather changes.and other abnormalities.

if you mean inuslation resistance between active part and tank .it depends on your technology and previous back ground for same design . what is most imoprtanat is to be sure of drying process and do not measure insualtion resistance under vacuum