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High Resistance Grounding

High Resistance Grounding

High Resistance Grounding

(OP)
I have a small industrial customer that would like to change from a solidly grounded to a high resistance grounded 480V service.  They are presently served by a three phase wye-wye transformer, but do not have any phase to neutral loads. My initial thought was to change the transformer to a delta-wye, but I have to wonder if the existing transformer is also suitable.  The 480V windings do not have reduced neutral insulation.  The high and low voltage neutrals are tied together internally and brought out through a single bushing.  I would tie the primary neutral to the grounded side of the NGR so that the fault current in the secondary would have a path in the primary(to balance the amp-turns).  The transformer is a 1500 kVA, so the only protection will be high side fuses.

This is not a connection you typically see in articles, so I have some concern that there is something I am overlooking.

RE: High Resistance Grounding

Two issues that I see:

1. The neutrals are connected internally so you cannot do what you are proposing.  If you could somehow separate the neutrals and continue to solidly ground the primary, it would probably work, but there is only one bushing available to do two tasks.

2. A small industrial customer, in my experience, is usually unable to benefit from a high resistance grounded system.  It takes someone knowledgeable about how the system operates and the discipline to apply that knowledge aggessively after a ground fault occurs to realize any benefit.  I see many such systems installed by customers large and small, and, particularly for the small ones, the benefits are usually not obtained because of a lack of knowledge and/or maintenance department follow-through.  I was a big advocate of this type system 25 years ago but have been disappointed by their "real life" application.

RE: High Resistance Grounding


Low-voltage resistance grounding has been extensively discussed a this site.  Suggest using the search function.
  

RE: High Resistance Grounding

As metioned by PWR, you cannot use the common neutral transformer for this application. If the primary neutral is not grounded or connected to your system neutral, the zero sequence impredance seen by the customer will be infinite (theoreticaly), so there will be no ground fault current.

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