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Step-Up Step-Down Transformer Inrush Question

Step-Up Step-Down Transformer Inrush Question

Step-Up Step-Down Transformer Inrush Question

(OP)
I am looking for some advise on MV transformer connections and inrush currents. We are connecting a small DG solar project using MV step-up/step-down transformers. The inverter is 375kW with a 3-Phase 480v output. The electrical service is 4000A/480/277V. We have two 500kva/480y/277:4160D PM transformers. When we energized the system from the primary breaker at the main electrical service the breaker trips as if there is a dead short. When the ground strap and neutral conductor is removed from the XO bushing we are able to close the breaker and energize the system.

Is there any issues with floating the neutral on a Delta-Wye transformer?
Are we seeing substantial increased inrush current because of the neutral connection or is this a ground fault issue for the breaker?

Any insight would be greatly appreciated.

RE: Step-Up Step-Down Transformer Inrush Question

Ground and neutral are two different functions that are often used interchangeably. If your system is wired so that the ground fault protection sees the neutral current it will indeed trip on ground fault. There will be an energization surge that returns through the neutral not the ground.
A wye delta transformer may have circulating currents in the delta secondary if the primary voltages and/or phase angles are not equal if the neutral is connected.
A wye delta transformer may have switching transient over voltages if the primary neutral is floating. Take your pick. I would used a connected and grounded primary neutral if I was confident of equal voltages. Others prefer floating the neutral.

Bill
--------------------
"Why not the best?"
Jimmy Carter

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