Wye-Wye transformers

[Dsg319]

Even with a 480/277 wye power source from the poco. When needing to step down to 208/120, in my head a delta-wye transformer would still make the most sense. With not having to pull along a neutral conductor on primary side of the transformer. What are some examples and reason where a wye-wye could be utilized and used and why?

 

[jim dungar]

For your application, IMHO there are no reasons where a wye-wye would make sense.

The few times I have used them involved micro-grids, or similar on site inverter usage, or medium voltage distribution.

 

[paulengr]

Wye wye is useful for two reasons. First is cost. The highest voltage seen on delta is line to line voltage. On wye it’s line to neutral. Makes a slight difference in MV insulation.

Second is utility just has to stock single phase transformers and it’s very simple to mount 3 on a pole and replace them one at a time as needed.

These two factors alone are why utilities almost automatically use them but brings up a third reason. Customers can often buy spare transformers from smaller local utilities.

 

[Hv&Lv]

Even with a 480/277 wye power source from the poco. When needing to step down to 208/120, in my head a delta-wye transformer would still make the most sense. With not having to pull along a neutral conductor on primary side of the transformer. What are some examples and reason where a wye-wye could be utilized and used and why?

Wye-Wye on a utility line, even pad mounts are used for costs and they can be used behind three single phase protection devices. A delta-wye behind three OCRs could backfeed primary voltage back over the miles of overhead line that could be laying on grass or tree limbs.

floating wye primary can really cause some problems on a down line also.

 

[jim dungar]

Makes a slight difference in MV insulation.

Wye-Wye on a utility line...

The OP was asking specifically about 480V primary with a 208V secondary.
For general non-utility applications I cannot think of any advantage offered by wye-wye.

 

[paulengr]

The OP was asking specifically about 480V primary with a 208V secondary.
For general non-utility applications I cannot think of any advantage offered by wye-wye.

Agreed. But the question was asked when if ever wye wye makes sense.

In instrument transformers, again not the same thing, the most common “full” installation is 3 CTs and two PTs in broken delta. Once in a great while I’ve run into three PTs in wye but it’s very rare. I’ve only ever seen it twice in 25 years and both times in power generation.

 

[Hv&Lv]

The OP was asking specifically about 480V primary with a 208V secondary.
For general non-utility applications I cannot think of any advantage offered by wye-wye.

From the OP
What are some examples and reason where a wye-wye could be utilized and used and why?

You were saying...?

 

[winnie]

AFAIK the only time a wye-wye makes sense at low voltages such as 480 to 208 also involves utility requirements, for example 'effective grounding ' issues for things like buildings with installed solar power.

 

[jim dungar]

From the OP
What are some examples and reason where a wye-wye could be utilized and used and why?

You were saying...?

The OP said they had 480V primary and a 208V secondary. They mentioned they were considering standard delta-wye configuration and wanted to know if they should investigate wye-wye instead.

My intent was to prevent most of the useful information from getting lost amid sidetrack discussions. I believe conversations regularly jump off track before the OP's question is properly addressed.

 

[Dsg319]

Thanks for all the replies, are the primary neutrals generally left floating or are they also bonded on the transformer? I could see the utility doing such but not a customer transformer.

 

[GoldDigger]

Thanks for all the replies, are the primary neutrals generally left floating or are they also bonded on the transformer? I could see the utility doing such but not a customer transformer.

If the secondary is a delta or there is a tertiary delta winding, then the primary wye point must be left open in customer installations. Connecting it can cause damaging circulating currents if there is any vector imbalance in the source voltage.
But no similar considerations apply to wye wye.
In the utility context ferroresonance is also a potential problem, but I do not think that applies to consumer scale line lengths.

I see no reason to leave the secondary neutral floating, as this would produce an ungrounded system. Or are you talking about creating an SDS with the secondary neutral grounded but not connected to the primary neutral directly?

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[Dsg319]

If the secondary is a delta or there is a tertiary delta winding, then the primary wye point must be left open in customer installations. Connecting it can cause damaging circulating currents if there is any vector imbalance in the source voltage.
But no similar considerations apply to wye wye.
In the utility context ferroresonance is also a potential problem, but I do not think that applies to consumer scale line lengths.

I see no reason to leave the secondary neutral floating, as this would produce an ungrounded system. Or are you talking about creating an SDS with the secondary neutral grounded but not connected to the primary neutral directly?

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Yes I was just wondering in the situation of a wye-wye poco transformer If the primary and secondary neutrals are bonded together or if they leave the primary floating.

I guess trying to figure out in my head also Theoretically if a primary neutral could or would accept secondary fault current, or it would just simply return to secondary windings.

 

[paulengr]

A wye wye not only acts like but IS just like 3 single phase transformers. Ground faults on either side for instance pass right through. One of the best features of delta-wye is that ground faults stay on “their own sides”.

 

[Dsg319]

A wye wye not only acts like but IS just like 3 single phase transformers. Ground faults on either side for instance pass right through. One of the best features of delta-wye is that ground faults stay on “their own sides”.

Gotcha. Haven’t had a lot of experience with transformers and been a couple years since had to be around one other than small control transformers.

I know this is way over thinking it but even with a delta/wye transformer, in a since isn’t the nuetrals still tied together from primary to secondary since you bring an egc ( that’s bonded to primary neutral at the source) and bond to the transformer case, than also have secondary side neutral and egc bond to the case? In this situation current only returns to the secondary neutral winding, but what keeps objectional current from going on the bonded primary egc back to the primary neutral. Or does the current simply only want to return the the place it’s originated in (secondary windings)?

 

[GoldDigger]

It wants to return to the winding it originated from, but the path to get there may involve some combination of EGCs, neutrals, and earth.

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[Dsg319]

It wants to return to the winding it originated from, but the path to get there may involve some combination of EGCs, neutrals, and earth.

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Thanks

 

[paulengr]

Normal practice is to bond the case to the GROUND of the primary side, not the neutral. Bonding to the neutral makes it multigrounded and violates the rules on main bonding jumper.

On the secondary side we have a couple choices. At this point X0 (wye center) is electrically isolated entirely from the primary side because there is no neutral in a delta. To complete the circuit any fault on X1-X3 must flow back to X0, X1, X2, or X3. So one option is to set up an entirely new grounding electrode and main bonding jumper to it from X0. This makes it a separately derived system independent of the primary side as far as grounding and bonding is concerned. Do not tie to the case or the grounds are not independent and you are subject to ground potential rise. The ground electrode needs to be some distance from the primary. At least 1-2 ground rod lengths but the mining rule is 50 feet.

The second option is to tie X0 to the case/bonding on the primary side. Thus sharing grounds. Again neutral connects to X0 as well as the new system bonding jumper. Landing “H0” (primary side neutral) on X0 makes it a second main bonding jumper and a code violation. Grounds will not be independent. Noise/GPR on one affects the other. But typical faults are still mostly isolated.

The third option is to tie X0 to both neutrals and continue the ground but do not tie them together. This is similar practice to control power transformers. It makes the ground path longer since a fault has to travel via the main bonding jumper but it’s no longer a separately derived system.

In mining in particular they use the first option and even go as far as putting lightning arresters and all housings in the “substation” on the primary side. The goal is to keep lightning and other power disturbances out of the active area of the mine down in the pit/underground which by its nature usually sees very little of that. The same principle applies in industrial facilities. If the building and the primary side utility feed are all tied to one grounding system it will be subject to disturbances, switching surges, and lightning on the primary side. Using a separate ground for the secondary side makes it truly a separately derived system and external effects on the neutral/ground are greatly reduced if not eliminated entirely, for the cost of some extra wire and two ground rods.

 

[winnie]

I can see the possible benefits of paulengr's first option, but don't believe it is permitted under the NEC.

Per the NEC all grounding electrodes must be tied together. Mining and utilities follow different codes to best meet different needs.

I believe that option 2 is most common for installs covered by the NEC.

Is the third option permitted under the NEC? For some situations where making a connection to a proper GEC is difficult it might make a transformer install easier. I'd think at a minimum that if the _primary_ neutral is expected to carry _secondary_ fault current, that the primary neutral would need to be sized to the minimum secondary EGC size.

Thanks
Jon

 

[electrofelon]

I can see the possible benefits of paulengr's first option, but don't believe it is permitted under the NEC.

Per the NEC all grounding electrodes must be tied together. Mining and utilities follow different codes to best meet different needs.

I believe that option 2 is most common for installs covered by the NEC.

Is the third option permitted under the NEC? For some situations where making a connection to a proper GEC is difficult it might make a transformer install easier. I'd think at a minimum that if the _primary_ neutral is expected to carry _secondary_ fault current, that the primary neutral would need to be sized to the minimum secondary EGC size.

Thanks
Jon

I was going to say the same thing about Paul's #1. That is not done in typical <600v wiring per the NEC.

It is important with these discussions to note the context, i.e. utility, NEC, voltage level - note the NEC does have an MGN provision if MV that would allow HO to be bonded.