Primary Bond of a WYE Transformer

[JGirard]

While teaching transformer connections to H.S. vocational students we found diagrams in two different books by different publishers, showing a bond from the center connection primary of a WYE transformer connected to ground.
I have always connected the secondary of a WYE to ground but never the primary. Most times the primary is Delta anyway.
Questions are; is this actually ever done? Don?t sound legal to me. The real question that we were thinking about is; what would happen? It was just funny that this same error is in both books.

 

[Smart $]

While teaching transformer connections to H.S. vocational students we found diagrams in two different books by different publishers, showing a bond from the center connection primary of a WYE transformer connected to ground.
I have always connected the secondary of a WYE to ground but never the primary. Most times the primary is Delta anyway.
Questions are; is this actually ever done? Don’t sound legal to me. The real question that we were thinking about is; what would happen? It was just funny that this same error is in both books.

Is that ground connection to EGC, GEC, or grounded neutral?

I don't believe it is a violation.

I've never witnessed such a connection. I can't speak for the myriad of transformers I haven't seen

Hmmm... second thought is that it could put objectionable current on the "ground", which would be a violation if not dealt with.


What are the book titles, authors, publishers, or ISBN #'s?

 

[Smart $]

PS: Welcome to the forum

 

[infinity]

If you had a WYE primary and grounded the XO wouldn't the current flow back on the smaller EGC in the primary feeder?

 

[roger]

I have hooked up a number of Wye / Wye transformers, the primary neutral connects to "HO" tap which has an internal jumper to the "XO" tap where the secondary neutral connects, these are typical installations on NC state jobs.

Why wye/wye, I don't know. :grin:

Roger

 

[kwired]

This is done all the time by POCO's on transformers that are individual core and coil for each phase (typical pole top installed transformer bank).

It is never done in indoor dry type transformers where the coils are installed on a common core, and likely not done on liquid filled transformers that also have a common core.

I have never found out why but I think the individual vs common core has something to do with it.

There are many threads in this forum where the poster had trouble with reverse feed delta-wye dry type transformers when connecting to the XO on the (reversed) primary. Let the XO float and there are no problems. Yet POCO's do this all the time on pole top transformer banks.

 

[jim dungar]

There is no single answer.

With utility Wye-Wye transformers, both H0 andX0 bushings are normally bonded together and to earth.

With Wye-Delta transformers the X0 bushing is never connected to ground, when used for general step up applications.

 

[jtester]

I have hooked up a number of Wye / Wye transformers, the primary neutral connects to "HO" tap which has an internal jumper to the "XO" tap where the secondary neutral connects, these are typical installations on NC state jobs.

Why wye/wye, I don't know. :grin:

Roger

With a wye/wye, you must connect the high side neutral to ground or you may have excessively high low side voltages.

Jim

 

[roger]

With a wye/wye, you must connect the high side neutral to ground or you may have excessively high low side voltages.

Jim

Probably so but that wasn't my question. The question I would ask is why a design would opt for a wye/wye verses a delta/wye.

Roger

 

[StephenSDH]

Probably so but that wasn't my question. The question I would ask is why a design would opt for a wye/wye verses a delta/wye.

Roger

Especially when a delta / wye helps to balance your load.

 

[BILLY101]

Depends on the utility Co. transmission and distribution.
For example say we are distributing 13,200 volts P-P but we also tap some URD from (1) leg at 7600 volts. This is a WYE system.
So we feed some 3 ph transformers with WYE even though we don't need the 1 ph voltage at that location.
The WYE primary transformer is less costly to build because the winding only needs to drop the 7600 volts not 13,200 volts.
Bottom line is always the bottom line.

BILLY

 

[roger]

.
The WYE primary transformer is less costly to build because the winding only needs to drop the 7600 volts not 13,200 volts.
Bottom line is always the bottom line.

BILLY

Okay, now provide some data to back up this part of your post.

Roger

 

[kwired]

Depends on the utility Co. transmission and distribution.
For example say we are distributing 13,200 volts P-P but we also tap some URD from (1) leg at 7600 volts. This is a WYE system.
So we feed some 3 ph transformers with WYE even though we don't need the 1 ph voltage at that location.
The WYE primary transformer is less costly to build because the winding only needs to drop the 7600 volts not 13,200 volts.
Bottom line is always the bottom line.

BILLY

If you build the same transformer bank and not connect to the neutral on the primary you will still have 7600 volts drop across each winding to the center point of the wye. A transformer intended to drop 13,200 across the winding would have to be used for a delta primary connection because you would be attaching a phase conductor to each end of the winding to get 13,200 - I don't doubt that it may cost more.

 

[RUWired]

Probably so but that wasn't my question. The question I would ask is why a design would opt for a wye/wye verses a delta/wye.

Roger

Interestingly we are currently involved in replaceing 3 wye/wye step downs with a common ho-xo connection. In the bottom of the existing transformer there is an instruction sheet that specifically says not to connect the xo to ground.

In each of the three sets there is not a main ocpd on the secondary and the panelboards are main lug.

Maybe the installation costs are less than the standard delta/wye.

Rick

 

[fredfriarpe]

Interestingly we are currently involved in replaceing 3 wye/wye step downs with a common ho-xo connection. In the bottom of the existing transformer there is an instruction sheet that specifically says not to connect the xo to ground.

In each of the three sets there is not a main ocpd on the secondary and the panelboards are main lug.

Maybe the installation costs are less than the standard delta/wye.

Rick

I'm in the middle of a review of a project with several 12,470/480 Y-Y transformers in unit substations. I haven't had a chance to discuss the design intent with the designer, although some have said that the X0-X0 connection allows the transformer to not be considered a separately derived system, sparing the expense of establishing a grounding electrode system in the unit substation.

 

[raider1]

I'm in the middle of a review of a project with several 12,470/480 Y-Y transformers in unit substations. I haven't had a chance to discuss the design intent with the designer, although some have said that the X0-X0 connection allows the transformer to not be considered a separately derived system, sparing the expense of establishing a grounding electrode system in the unit substation.

Welcome to the forum.

A wye/wye transformer is still a separately derived system.

Here is the 2011 NEC definition of a separately derived system.

Separately Derived System. A premises wiring system
whose power is derived from a source of electric energy or
equipment other than a service. Such systems have no direct
connection from circuit conductors of one system to
circuit conductors of another system, other than connections
through the earth, metal enclosures, metallic raceways,
or equipment grounding conductors.

Chris

 

[RUWired]

Welcome to the forum.

A wye/wye transformer is still a separately derived system.

Here is the 2011 NEC definition of a separately derived system.



Chris

The transformers being replaced on this site are possible autotransformers. The primary and secondary conductors are of the same coil. These are acme electric transformers.

View attachment 5195

 

[fredfriarpe]

Welcome to the forum.

A wye/wye transformer is still a separately derived system.

Here is the 2011 NEC definition of a separately derived system.



Chris

Thank you for the welcome; I've been lurking for quite a while and finally joined. This forum has been an indispensable resource in my consulting career.

The Neutral is already established upstream at the service entrance disconnect and is simply "passing through" the subject substation transformer; does not the neutral "jumper" from the primary X0 to the secondary X0 on the wye-wye transformer qualify as the direct connection that is required in the NEC 2011 definition? It doesn't seem to fit the bill as any of the "other than" cases.

 

[raider1]

The transformers being replaced on this site are possible autotransformers. The primary and secondary conductors are of the same coil. These are acme electric transformers.

View attachment 5195

Thanks, I did not look into that.

Chris

 

[raider1]

Thank you for the welcome; I've been lurking for quite a while and finally joined. This forum has been an indispensable resource in my consulting career.

The Neutral is already established upstream at the service entrance disconnect and is simply "passing through" the subject substation transformer; does not the neutral "jumper" from the primary X0 to the secondary X0 on the wye-wye transformer qualify as the direct connection that is required in the NEC 2011 definition? It doesn't seem to fit the bill as any of the "other than" cases.

Yes if the neutral does pass through then you would be correct that this would not be a separately derived system.

Chris