Testing Transformers

[10linepoet]

We are installing a 300 kVA transformer, 3 wire 3 ph 520 V/345 A in with 4 wire 3ph 440 V/394 A out.
Does anyone know if there any significant tests required or things to be aware of (pitfalls) when testing this type of equipment?
We propose to undertake an Insulation Resistance test of the cable & an Earth Loop Impedance test from the load end through the transformer back to the distribution panel.
Any info/advice regarding the above would be welcomed.
Thanks.

 

[mogga]

are you american per chance??

 

[10linepoet]

No, I am not American.
I guess you were phased by the voltages but its for a test cell for specialist equipment!
Any Help for me?

 

[markysparky]

How exciting!!

IR test to cable. Watch so you don't get reading through transformer. For the cable from the secondary winding to the load, is this any great length as you may have to alter the tap changes (if any) to make sure you get the correct voltage at the load end. I notice 520V on the primary, how do you obtain this? Presume primary delta connected and secondary star. Last one I did I think we bolted star point down to earth as we obtained some floating voltages. Make sure you do voltage checks on primary and secondary outputs on no load and then with load connected.

Markysparky

 

[10linepoet]

We have got about 17M from the D.B to the transformer & about 13M to the load isolating switch. The input is wired in Delta (3 wire) & the output is wired in Star (4 wire) but thats internally on the tranny. We will definitely carry out our IR test with the cables disconnected. My main concern was our earth loop impedance test through the tranny & whether this will throw up any rogue readings or is there anything to beware of or to take into consideration.
Thanks for your assistance.
For your information, our customer has an 11 kV transformer supplying the 520 VAC @ 60 Hz for their test cells (American Specialist Equipment).

 

[markysparky]

I presume your secondary star point is earthed then you shouldn't have any problems with earth loops. The reason for our step up TX and then Step down TX is that we had to supply a building which was 1.5km away from the supply. The building just housed lights, sockets and experiments but transformed up to 750V and then back down to 400/230 at the building so our earth loop tests were all ok. I presume that the line voltage is 440V secondary and the phase voltage is root 3 of that. About 254V single phase. Make sure you don't put 440V across the earth loop tester otherwise it WILL GO BANG!

Hope that helps

Markysparky

 

[Knobhead]

Regard the earth loop test as 2 seperate tests.
DB to 520V winding
440V to load.

525V is quite a common voltage for M&Q situations.
I've installed several transformers with similar specs, They were Star/Star to keep the vector angles the same, 50Hz and not 60Hz.

The beauty of the them being Star/Star was they could be used as either step or down depending on which side the earth/neutral link was fitted.