Grounding of 3 Phase Wye Transformer

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eeeee

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I would like to confirm whether the following grounding scenario is confirmed in the NEC. I could not find it when I read the NEC handbook:

I have a 12,4000/7,200X208/120v Wye-Wye connected 300KVA transformer that is has the following items connected to the transformer ground ring: Cable Shields, Chassis Ground, Surge Arrestor. The secondary neutral is connected to a separage ground rod a minimum of 6000mm from the ground rod per this installation practice that has been suggested to me in the referenced drawing.

What would the reason be for connecting the secondary neutal to a separate ground rod separate from the ground ring of the transformer since it is apparently not covered by the NEC.
 
We'd need more info to make definitive comments, but most utility systems, as well as others, are multi-grounded systems with a common neutral. Primary and secondary neutrals are usually bonded at each point that they can. There is no separation of high and low side neutrals in this kind of a system.

You'd need to determine what type of system you are working in before we could condem or bless the grounding.

Jim T
 
This transformer is a "separately derived system" and should be grounded per NEC 250.30. All equipment associated with this system (cable shields, surge arresters, etc.) MUST be connected to this ground so that all equipment maintains the same reference for "ground".
 
Unless there is an internal jumper, I'd add one. I can't think of one condition where nearby electrodes should not be bonded.

Not bonding the neutral to the EGC system could create a hazardous step-potential condition around the transformer.
 
jt (and others) -
I am absolutely not familiar with Wye connected xfm primaries, advantages, when one would want to use one. Could you recommend some references?

carl
 
Carl, most utility transformers these days are Y-connected. Most are single-bushing units, or single-ended, meaning that one end of the transformer is connected to the system neutral.

Next time you have the opportunity, look up at the transformers on a pole and especially the primary connections.
 
Carl

Wye primaries have lower insulation levels than comparable delta primary transformers, they're cheaper.

Another problem is ferroresonance anytime you have a delta in an underground system. The high capacitance of underground MV cables combined with the reactance of the transformers can create a resonant condition.

Jim T
 
Grounding Transformer

Grounding Transformer

The primary shields, surge arrestors, and chassis ground is connected to the transformer ground ring. The secondary neutral is connected to a separate ground rod.
 
transformer ground ring.

transformer ground ring.

The primary shields, surge arrestors and chassis ground are connected to the transformer ground ring. The secondary neutral of the transformer is connected to a separate ground rod, 20 feet from the transformer ground ring.
 
eeeee said:
The primary shields, surge arrestors and chassis ground are connected to the transformer ground ring. The secondary neutral of the transformer is connected to a separate ground rod, 20 feet from the transformer ground ring.
Click to expand...

What are the characteristics of your high voltage system, delta or wye, grounded or not, and how is it grounded?

As I said earlier, a single ground rod is not much of an electrode in many locations.

Jim T
 
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