wye wye transformer

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Wyeman94

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Hello all,

1) Customer owned padmount Wye-Wye primary transformer: Primary was not served a neutral. I know I can leave X0-to-case strap, install another MBJ at the load's main disconnect & run PVC with no ground to the load's main disconnect per 250.30(A)(1)Ex2. I believe H0 should still be strapped to X0 because of the lack of primary neutral. Would this still be a SDS?

2) If the primary neutral IS served to the transformer, should H0 and X0 still be connected? Since a primary neutral is served, would THIS be a SDS as H0 would be direct tied to X0? Also, I don't think you would ever want to bond H0 to the case.

3) What differences would it make if this transformer were utility owned? Can I still invoke the same 250.30(A)(1)Ex2 load-side installation technique?

Thanks for reading.
I used the word 'neutral' rather than 'grounded conductor' for ease of reading.
 
Wyeman94 said:
Hello all,
Click to expand...
Hello... and welcome.

1) Customer owned padmount Wye-Wye primary transformer: Primary was not served a neutral. I know I can leave X0-to-case strap, install another MBJ at the load's main disconnect & run PVC with no ground to the load's main disconnect per 250.30(A)(1)Ex2. I believe H0 should still be strapped to X0 because of the lack of primary neutral. Would this still be a SDS?
Click to expand...
Yes.

Strapping H0 is not required, and depending on who you talk to, not recommended either.

2) If the primary neutral IS served to the transformer, should H0 and X0 still be connected? Since a primary neutral is served, would THIS be a SDS as H0 would be direct tied to X0? Also, I don't think you would ever want to bond H0 to the case.
Click to expand...
In most cases, H0 is not connected to X0. That said, the NEC does not prohibit such connection. If connected, it would not be an SDS. If you connect H0 to X0, and X0 is grounded, you effectively are connecting H0 to the case.

3) What differences would it make if this transformer were utility owned? Can I still invoke the same 250.30(A)(1)Ex2 load-side installation technique?
Click to expand...
As such, the secondary would be a service. Same technique can be employed, though an additional grounding electrode is required [250.24(A)(2)].
 
On a similar note I am performing a study for a plant which uses several wye-wye transformers for distribution within the plant. The distribution voltage in the plant is 12.47kv and is stepped down at several unit substaions to 480V.

Why would a facility use wye-wye transformers as opposed to the more traditional delta-wye transformers? I understand on of the negatives is that these transformers will pass zero sequence currents. Is there any advantages however?
 
Thanks for the clarity Smart $.
I believe POCOs use this transformer because it has fewer windings (cheaper). They may also like the lack of phase angle displacements for interconnected systems. I always thought that Delta/Wye was a more stable system, hence more common with user-side distribution systems.
 
mull982 said:
On a similar note I am performing a study for a plant which uses several wye-wye transformers for distribution within the plant. The distribution voltage in the plant is 12.47kv and is stepped down at several unit substaions to 480V.

Why would a facility use wye-wye transformers as opposed to the more traditional delta-wye transformers? I understand on of the negatives is that these transformers will pass zero sequence currents. Is there any advantages however?
Click to expand...

Are there a bunch of interconnections between busses? Could be they don't want to worry about inter-tie's mismatches which could be created by multiple 30 deg shifts. Just a guess.
 
kingpb said:
Are there a bunch of interconnections between busses? Could be they don't want to worry about inter-tie's mismatches which could be created by multiple 30 deg shifts. Just a guess.
Click to expand...

No there are not really any interconnects.

Are you saying though in cases where there are interconnects such as tie-breakers etc... that could parallel feed a bus they dont want to worry about having a phase different between two of the paralled sources due to a 30deg phase shift in a Delta-wye transformer?
 
POCO Transformer

POCO Transformer

Most rural electric utilities use wye-wye padmount transformers and standard practice is to bond HO to XO and ground them and connect the system neutral through the cable concentric neutral. The only time you do not ground HO is if the transformer secondary is a delta (480D, 240D/120V) because then the transformer will act as a grounding bank.
 
mull982 said:
No there are not really any interconnects.

Are you saying though in cases where there are interconnects such as tie-breakers etc... that could parallel feed a bus they dont want to worry about having a phase different between two of the paralled sources due to a 30deg phase shift in a Delta-wye transformer?
Click to expand...

That is correct. To parallel to different feeders, depending on the transformers they go through, it could be mismatched at the switchgear.

Say you have one go through a Y/Y and another goes through a D/Y, then the two sources are 30 deg out of phase and you could not parallel.
 
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