Utility / Service Entrance Transformers - Wye or Delta?

[mbk2k3]

My projects are always on the service/consumer/industrial load end user.

Often I have to run by the utility whether the main transformer I'm procuring is WYE or DELTA, and utilities will give me their preferences / requirements.

Can someone walk me through why a utility would want a WYE on the primary? or why in some cases they would want a DELTA on the primary?

Are there use cases / applications for both?

I'd say 90% of the transformers I've seen are WYE on the primary/utility facing side.

 

[dpc]

Wye-wye - least expensive, most flexible in terms of utility purchasing and stocking. Least concern about ferro-resonance. Downside: no isolation of ground faults between primary and secondary.

Delta-wye - some risk of ferro-resonance. Slightly more expensive. Main advantage - ground faults on secondary do not show up as ground faults on primary due to the delta winding. If the customer is purchasing the transformer, I'd push for delta-wye.

 

[prc]

In most of the countries(like India) it is always Dyn ie delta on utility side. Practically there is no price difference between Y&D connections at MV voltages ie 11-33 kV. Advantages -dpc already covered. One more advantage is D prevents the load third harmonics (from home electronics etc) from entering in to grid as D filters them out.
dpc, can you explain how ferro-resonance with Dyn? I thought it is immune.

 

[dpc]

Loss of one or two phase on the primary side with relatively long underground primary feeder and ungrounded primary (such as delta). If you can't turn up some references, let me know and I'll see what I can dig up.

It's pretty rare, but exciting when it happens, I guess.

Agree about triplen harmonics.

 

[Mbrooke]

@DPC: At 13.8kv- at 25kv and especially at 35kv you do not need underground cables for the possibility to go way up of it happening.


Delta vs wye primary is also driven by the system and the secondary. Ie a 3 wire line will always be delta or wye ungrounded primary, a delta secondary would necessitate that as well.

 

[dpc]

Mbrooke:

Just curious - have you ever experienced a ferro-resonant event like this in the real world?

Also, I've never seen a three-phase utility distribution circuit without a neutral in my part of the world. 95% of utility-owned pad-mount transformers I've come across are wye-wye.

 

[mbk2k3]

some good points here.

in most of my projects the transformer is actually customer owned (not owned by the utility).

what about ground faults? having a wye on the utility side would mean that the transformer neutral would see a ground fault somewhere out in the grid.
we wouldn't want to trip unnecessaril (settings would have to be chosen carefully).

any other considerations about ground faults?

what advantage/disadvantage is it to a utility if the utility side of the transformer is WYE or Delta when it comes to ground faults?

 

[wroggent]

I think a wye winding is easier to protect since for a delta the full load line current will be 3^0.5 time higher than the full load winding current.

 

[jghrist]

what about ground faults? having a wye on the utility side would mean that the transformer neutral would see a ground fault somewhere out in the grid.

This would only be a concern if you had significant generation connected in parallel with the utility. A grd wye - grd wye transformer is not a ground source but generation on the secondary could be a ground source to utility ground faults. Current from the utility source would not feed through the transformer for a utility ground fault. A grd wye - delta transformer could present a problem with utility ground faults because this connection is a ground source.