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Breaker trpping upstream a transformer while grounding its neutral

Breaker trpping upstream a transformer while grounding its neutral

Breaker trpping upstream a transformer while grounding its neutral

(OP)
In our system, we have a 2km, 600V distribution line, where we have a 150 kVA Y/D step-up transformer 600/4160V at its beginning and a 150 kVA  D/Y step-down transformer 4160/600V at its end, to lower the voltage drops across the line. A SQD 200A breaker TM with an instantaneous of 1000A protects the primary of the step-up Transformer 600/4160V. Whenever we try to ground any of those transformers neutral, the 200A breaker trips. We do not know what is the reason go that.

RE: Breaker trpping upstream a transformer while grounding its neutral

If the step-up transformer is at the source end, you can't ground its neutral or you will create a grounding bank.  You should be able to ground the neutral of the step-down transformer.

If the source is at the other end, then you can'y ground the neutral of the step-up transformer for the same reason.

RE: Breaker trpping upstream a transformer while grounding its neutral

(OP)
In fact, we tried to ground either the step-up or step-down transformers and still the main breaker trips.

RE: Breaker trpping upstream a transformer while grounding its neutral

How about a more detailed description?

What is source to the 600V system?

Breaker is at (600V?) _____

Then transformer
    600V winding = ________ (wye, grounded wye, delta)
    4160V winding = ________ (wye, grounded wye, delta)

Protection on transformer secondary/sending end of 4160V line = ______________

Then line
    3 wire, 4 wire, or _________

Protection on receiving end of 4160V line/transformer primary = ________________

Then transformer
    4160V winding = ________ (wye, grounded wye, delta)
    600V winding = ________ (wye, grounded wye, delta)

Then what?

RE: Breaker trpping upstream a transformer while grounding its neutral

(OP)
Hi Davidbeach,
I will pick up all details soon and come back to you with it.

Thanks,
     

RE: Breaker trpping upstream a transformer while grounding its neutral

First you have radically different transformer connections although they may seem the same.
Step-up.
If the primary neutral of a wye/delta bank is connected to the supply neutral (as usually happens when a connection is grounded) any primary voltage unbalance or primary phase angle error will cause circulating currents in the delta winding. These currents are limited by the impedance of the delta winding.
This connection may be used for large scale power transmission but power transmission loads are generally well balanced and single phasing is rare. (Single phasing is an exreme form of voltage unbalance.) Also the typical PU impedance of a power transformer is typically much more than the impedance of a distribution transformer.
Bottom line, don't ground the wye point of the wye/delta step-up transformer. This is not a code violation in North America and is the RUS recommended connection for a wye/delta transformer or transformer bank.
Step-down.
Delta/wye with the wye point grounded is a commonly used connection for distribution circuits. Look for a ground fault in either the transformer or in the system that it is feeding.  

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

RE: Breaker trpping upstream a transformer while grounding its neutral

I can see a possible problem with grounding the yD transformer at the beginning.  If there is unbalanced load on the source 600 V system, the transformer will act as a grounding bank and feed the unbalance.  I don't see a similar problem with grounding the Dy transformer at the end.  This is a normal connection of a Dyn transformer to a load.  

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