Wye Grounded Primary and Wye Grounded Secondary Transformer

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bobby ocampo

bobby ocampo

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What should be the grounding procedure for Wye-Grounded Primary and Wye Grounded Secondary TRANSFORMER to be considered as Separately Derived System?
 
The H0 and X0 are common (they can be isolated if need be) so they both connect to the GES and from there all bonding is as normal.

Roger
 
LarryFine said:
Bobby, just so it gets mentioned, the primary's neutral should be left floating, not grounded or "neutral'ed", as if it was a delta.


Grounding Electrode System
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Maybe, I think it depends on what voltages we are talking about and why this particular transformer is being used and what for. IF the primary is an MV MGN, primary neutral would be grounded.

More details Bobby. Is this just an academic question?
 
We had a long discussion on this last fall. I dont remember now what was decided or who won ;) Grab some popcorn and lemonade and have a read:

Wye to wye questions

Attached sketch shows from left to right: 600VAC inverters feeding customer owned 600V, 3000A service rated switchboard which has neutral to ground bond, GEC to ground grid and feeds 2000kVA 600VAC to 13.2kVAC customer owned transformer wye to wye. The transformer has no neutral to ground bond...
forums.mikeholt.com
 
My understanding is that three wye-connected individual transformers can have the primary neutral grounded, but a single, three-phase unit can not.
 
LarryFine said:
My understanding is that three wye-connected individual transformers can have the primary neutral grounded, but a single, three-phase unit can not.
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I dont think that is the case. Most utility pads are Wye-Wye and always have the primary neutral grounded.

In general, wye-wye transformers have some problems, such as ferroresonance, harmonics, and L-N voltage imbalance, and need to be carefully selected. My understanding is landing the primary neutral alleviates these issues. A tertiary winding is usually necessary in absence of a primary neutral. This is general, and im not sure what changes if both sides are LV or its a bank built from separate units. Maybe one of the gurus will chime in and learn us good.
 
Not sure if it was me.

The issue is that the line-neutral voltages on the wye side are reflected as line-line voltages on the delta side. Unbalanced voltages on the wye side cause circulating currents on the delta side trying to force the wye side neutral into balance. If the primary neutral floats, then its voltage can shift slightly to avoid these large circulating currents.

This is desirable, for example if you use a wye-delta transformer to derive a neutral to use as a grounding transformer.

But for most wiring covered by the NEC, the neutral of a wye primary delta secondary transformer is essentially a derived neutral which you don't want to connect to anything because now you will have two neutrals 'fighting' each other, with large circulating currents flowing to try to force small voltage differences to balance.

I believe that utilities intentionally use grounded wye primary transformers specifically because they want the balancing effect and the improved ground referencing of the distribution lines ('effective grounding'), however I am not well up on the theory as it applies to utility distribution.

-Jon
 
I have usually said; do not ground the primary side of wye-wye transformers unless you are a utility.

The reasoning behind this statement is that most industrial/commercial electricians would be involved with <600V on the primary side while utilizes would be using medium voltage primaries. I kind of skipped the situations where a facility use MV as its distribution system, like the OP has.

To answer the OP, the primary side neutral point being connected to ground/earth/building steel is not related to the secondary neutral point being connected to the same reference points.
 
jim dungar said:
I have usually said; do not ground the primary side of wye-wye transformers unless you are a utility.

The reasoning behind this statement is that most industrial/commercial electricians would be involved with <600V on the primary side while utilizes would be using medium voltage primaries. I kind of skipped the situations where a facility use MV as its distribution system, like the OP has.

To answer the OP, the primary side neutral point being connected to ground/earth/building steel is not related to the secondary neutral point being connected to the same reference points.
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Please elaborate. I don't want to thread jack but I don't think I am.
 
ActionDave said:
Please elaborate. I don't want to thread jack but I don't think I am.
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I am curious too. There is that often cited paper "the whys of the wyes", but it is quite technical in nature, Ive never been able to slog thru it without my brain hurting.

My understanding is POCOs typically use GrdY-GrdY due to ferroresonance.

I understand ferroresance is typically only a concern with MV primaries, but I dont see why the other potential issues are only a concern at MV voltages.
 
electrofelon said:
I understand ferroresance is typically only a concern with MV primaries, but I dont see why the other potential issues are only a concern at MV voltages.
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There are some little quirks that are not as much of a concern with a medium voltage grounded wye primaries (like multi-grounded neutrals) and others that are much worse (like ferroresonance).

My general warning deals more with the use of LV wye primaries especially those with delta secondaries, or high resistance grounded wye secondaries with grounded wye primaries.
 
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