Reversing a Delta-to-Delta Transformer


Thread Starter


We have wired a 3phase 480v Delta to 240 4w delta transformer backfed to up the voltage to 480v from a 3ph 208v highleg panel with nothing connected to X4.

When we turned on the 3 pole beaker it tripped. There are no shorts on the conductors and the transformer ohm reading were consistent

Any ideas what the problem can be?
Please mention, what is the trigger of 3-PH breaker trip...??

What type of electrical relay operated..?

If it is a numerical relay , which triggered the tripping...??

Short CKT / UV / reverse power / Residual voltage.

I think your 280V side short circuit current setting need to be re-evaluated.

Make it at least 7 times of full load current with 50 milli sec delay.

If I have interpreted your dilemma correctly, you are trying to feed a 480V, 3-ph, delta-connected load from a 240V, mid-phase-grounded (Hi-Leg) power supply!

The interposing Xfmr is Delta-Delta, wired as hi-leg, but the X4 terminal (presumably used as the mid-phase ground connection) has been isolated! I suggest a simple resistance check to confirm that fact!

Secondly... Barindra raises a very good point. Are you sure the breaker powering the interposing xfmr can handle its inrush-current?

Phil Corso