23 MVA Power transformer IR value

Sure you are not looking for I²R losses?

I think he refers to Insulation Resistance, (a megger test); in which case, I'd know the answer if I was an engineer in charge of such a costly project, OR I had read the manufacturers specs.

Regards

1. Doesn't it depend on the voltage?

2. Core, tank, frame...should they all not be earthed ? If so, no IR between them?

Not my field, so, sorry if this is stupid.

I'm not sure what you mean by "frame," but the core of the transformer in operation will be bonded to the (grounded) tank. The core must be bonded to the tank in only one place. A core grounding strap, often using a relatively easy to access low-voltage bushing, is provided so that during acceptance testing you can disconnect the strap and do an insulation resistance (IR or "Megger") test.

Per IEEE C57.12.00, the core to ground insulation resistance should be measured at 500VDC for 1 minute. It does not give a value for acceptable IR. Industry rule of thumb is 1 megohm + 1 megohm per kV equipment rating. Since the core is normally grounded, I would use 1 megohm as minimum IR, but typically you will read more.