Transformer Inrush current

[82angelfan]

Good morning all. Sort of a question.
I had a couple of transformers on a new installation. I have 100amp 277/480 feed to a panelboard that then serves these two 30KVA transformers. With no load on the system, if both 60 amp transformer breakers are switched on the feeder breaker trips due to inrush. Schneider suggested turning up inrush (I of i) dial based on their coordination study.

This got me thinking. A transformer is physically a dead short phase to phase.
Is it the magnetic field that opposes the short circuit current flow or the configuration of the phases?
Is the inrush caused by the build up of this magnetic field?

Hope these don't sound like stupid questions.

 

[winnie]

It is the changing magnetic field in the transformer core which induces a voltage which opposes the applied voltage and prevents the short circuit.

The magnetic flux in a transformer builds up and collapses twice each AC cycle (ignore the 3 phase bit for now )

When the transformer is running in steady state, the flux goes from -max to 0 to +max to 0 over and over again. The swing of 2x max is _required_ over the course of a half cycle.

When a transformer is being started, depending upon the timing of the switching event, there is a possibility that the flux swing could go from 0 to 2x normal max, rather than from -max to +max. If this happens, then the transformer core becomes severely saturated and lots of excess current flows to create the required flux.

So the inrush is not the build up of the magnetic field (which is continuously alternating) but rather the transient where the alternating magnetic field balances itself around zero.

If you can do something to get the magnetic field alternating in proper step with the applied voltage (say with some sort of resistive precharge) you can turn a transformer on with no inrush. If you can time the breaker closing correctly you can also turn a transformer on with no inrush. I don't think either of these approaches is ever used for small transformers.

-Jon