How to electrical test (PAT test) large transformers?

This is very true - thinking back I too have seen this sort of thing done, though only as a customer witness, with something like 230 or 110V on the 7/11kV side of things. Rather like running a current transformer with no load one has to know exactly what one is expecting and in my defence I was being a bit quick as I wanted to keep the explanation simple, but just mention it in case the OP had seen both inductance terms somewhere and to point out that there are 2 very different parameters not to be confused, and different tests.

A fall in magnetising inductance - an increase in no load primary current is a good indicator of trouble cooking.
M.

Leakage inductance may be measured by short circuiting one winding of the transformer, and applying a much reduced voltage to the other winding. Adjust this voltage until normal load current passes.

I have done this, years ago, a few hundred volts was applied to the 11kv side of a distribution transformer, whilst the LV side was short circuited.

basically, without a secondary load the only current that flows is that required to magnetise the core back and forth on alternate cycles - the  no-load primary current. this should be a lot less than the on-load current, or some fault in the transformer is acting as an extra load.

It is also referred to as the primary inductance. The similar sounding but complementary quantity, the "leakage" inductance, is the input inductance when the secondary is shorted rather than open circuit, but for most transformers you want to keep operational for more than a few cycles, there are very good reasons for calculating that from voltage droop tests, not trying to measure it directly !

Mike.

To measure magnetising current, connect the transformer to a supply of the correct voltage and frequency, and disconnect all loads from the secondary side. Measure the input current. Basic testing is usually done at nominal voltage and frequency, more detailed testing may be done at the upper limit of permissible input voltage such as 253 volts (110% of 230 volts) and at the lower limit of frequency, such as 49.5 cycles, the lower limit of UK grid frequency.

Magnetising current increases at high input voltages and low input frequency. Excessive magnetising current may indicate an internal fault.

Thank you!

Hi, not yet, I thought I'd get some other opinions first but that's a good idea

Hi, the TX's are indeed portable but yes, I think a fixed install approach and test equipment is required for this one.

Very useful, thanks.  How could one measure the magnetising current?  Never heard of that before

Have you spoken to the manufacturer and see what they offer in terms of training for these units?  They could offer some insight.  

BS ISO 18095 and the relevant parts of IEC 60076 for the transformer type are perhaps the standards to look at, as well as the relevant manufacturer's guidelines.