Transformer connected as autotransformer

[hublanc]

Hi
We have a 300kVA Y - Y transformer. Primay is multi tap (220 to 600VAC), and secondary is 208VACy. We would like to use this transformer as an autotransformer, by connecting 600VAC line on the primay 600V tap, and connect the load on the 440VAC primary tap. We don't intent to connect anything on the secondary.
Is it something feasible, and if yes should we derate the transformer ?
Regardsm

 

[rcwilson]

Depends on the type of taps in the transformer. If it is really just one winding with taps coming out at 220-440-600 V then your idea will work. If the windings are configured like a mulit-voltage motor with different series-parallel winding connections for different voltages, there may be a problem. It still might work, but it would take some careful examination.

Look at the transformer nameplate. It should show how the windings are connected.

Connect the 208V neutral point to ground so the unused winding doesn't float.

No derate is needed. In fact, you may be able to uprate. The wire size or ampacity of the leads is a the limiting factor. At 300 kVA, the 600V leads are designed for 289 amps and the 440V leads for 394 amps. The transformer will only be transforming 160V (600V-440V = 160V) so it will be only loaded to 20% of its rating. If the windings and leads could take it, it could carry 1500 kVA.