Auto-transformer - Earth Leakage Problem

[CanadianTaff]

We have a piece of equipment which was designed to be supplied by 1000VAC, the customer informed us late on that the supply would be 1100VAC.

As a solution an auto-transformer was used to drop the supply to the designed level.

Now when the power is applied the main breaker trips due to the transformer centre tap being connected to the earth instead of neutral.

There is no available neutral to the equipment which is remote and we cannot install a new earth.

What would be the result of disconnecting the earth and leaving the unbalanced load floating?

Is there another solutuion apart from replacing with larger, heavier, expensive alternatives to the auto-transformer?

 

[waross]

Use an open delta auto transformer. There is no neutral, no ground connection and no phase shift.
Step 1: What is the current draw?
Step 2: Choose two transformers with a)
A rating of 1000:100 Volts.
A current rating on the 100 Volt winding that equals or exceeds the current draw of the load.
Rated for buck boost service.
Connect the two transformers in open delta auto transformer configuration.

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

 

[busbar]

In applying an autotransformer, it’s important that both windings be insulated for full line voltage; i.e., in this case, the 100-volt winding also needs to be intended for 1100-volt operation.

 

[waross]

What busbar said. A conventional transformer rated for buck-boost service will meet this criteria and may be connected as an autotransformer.

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

 

[CanadianTaff]

Thanks for replies, we're tallking with the manufacturer about wiring as Delta now.

Just as a footnote though, we're dropping 100V, from 1100 to 1000, not dropping to 100V.

Its only a small amount, but apparently a big difference in that at 1000V it can be catagorised as a low voltage machine.