How to electrical test (PAT test) large transformers?

Um, firstly are these really portable appliances and not part of the fixed electrical installation? What actually is the scope of work?

Even if they are portable, to test it properly you'll end up with largely the same methods as fixed installs, so probably best starting your search / reading along those lines rather than trying to squeeze it into PAT. Others with more experience from the tools will no doubt be here soon to fill you in on methodology. I expect you'll need a new test device (or more than one)!

  I presume these are not really used like portable appliances and as you note the normal PA tester with a 13A socket on it won't quite cut it ;-)

The first thing is to decide what needs testing, what failure modes you hope to pre-empt and then what the test limits should be.

So for class 1 enclosures, the normal earth/ CPC connection can be verified but it needs to be good enough to operate the circuit fuse or whatever is in the supply - no good having an earth connection that breaks at a lower current than the supply fuse ;-) to that end certainly at the 90KVA end, I'd expect to see earthing of the same cross-section as the live cores, probably 25mm2 assuming that is a 3 phase unit, or rather more if its single phase. testing that with a meter is not really that easy or desirable - and if you do it will require a 4 terminal instrument. A quick buzz test with the lowest ohms range your normal meter can do,  and a thorough visual inspection may be more appropriate. Finding 'high' resistances on the scale of tens of milliohms is usually fraught with difficulty.

Insulation primary to core/frame and from secondary to both core/frame and primary should be confirmed (except autotransformers and variacs, where primary and secondary are the same coil). Expect several megohms, but don't test at more than 500V DC without checking with the makers. It should do a lot better than that, but how much better is not clear.
If you can measure the magnetising current without a load connected, and I realise that may not be so easy, where loads are wired in, as any increase in this between successive tests indicates a shorted turn or deterioration of the insulation between laminations.
M.

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.

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

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

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

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

Thank you!

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.

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.