Distribution Transformer Bank Configuration 1

[luffy2011]

All,

Good Day!

I would like to inquire what Transformer Bank Configuration is currently practiced by your utility for three-phase distribution to customers.

For example, we are using Y - delta(center-tapped) but are planning to change it since we are encountering voltage problems during single-phasing.

Thanks...

http://www.powerqualityworld.com

 

[waross]

Delta:wye, wye:wye and open delta for small, special applications.
Common voltages in North America are 120/208V, 277/480V and in Canada, 347/600V.
Three phase transformers may be delta:wye and single phase banks are most often wye:wye.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[stevenal]

Wye-wye is preferred for new services, but wye-delta is maintained for existing. Open wye-open delta is used where only two out of three primary phases are available.

Changing is rather difficult because except for the 480 services, configuration change would cause a change in the service voltage.

Not sure how you can avoid a voltage problem during single phasing, since by definition you have a voltage going to zero for that condition. Phase loss protection is always advised for three phase machinery.

 

[luffy2011]

waross,

Does your utility use DELTA-wye(grounded) using single-phase transformers? It's relatively costly but would just want to know...

http://www.powerqualityworld.com

 

[luffy2011]

stevenal,

Ironically, we are not worried with phase loss during single-phasing but with voltage swells. We have made actual tests using additive single-phase transformers and have confirmed that overvoltages/swells occur especially with unbalance loading.

Obviously, WYE(G)-wye(g) is the most stable configuration in terms of voltage but we're a bit worried (should we?) about harmonics and SLG protection issues.

http://www.powerqualityworld.com

 

[waross]

No we don't use delta wye with individual transformers. All of our single phase transformers are purchased for our phase to neutral voltage of 7620 Volts.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[luffy2011]

I see... Most utilities think similarly when it comes to cost...

Hey waross,you're a utility guy right? (based on how you answered)

Any bad experience with the WYE(G)-wye(g) bank? Voltage wise, it is very stable. But how about other issues such as harmonics and SLG fault protection concerns?

Thanks...

http://www.powerqualityworld.com

 

[waross]

I am not with the utility anymore but still am called in from time to time to consult. Our's was a very very small system. We wouldn't have known if we had harmonics (but our customer base was not sophisticated and we really didn't have anything on our system that would generate significant harmonics. Mostly residential and a few large plants with DOL induction motors.)
When I started with the utility we had some serious issues with wye:delta with a four wire primary connection. This was a part of the world where the only wye:delta connection used was a four wire primary connection. On the mainland served by the Government owned utility it was interesting to drive around and observe the wye:delta banks. Most had a fuse blown and one cutout hanging down and were operating open delta with one transformer idling on the secondary connection. It took several years to have the industrial users changed from wye:delta to wye:wye to eliminate these issues.
We had a non-symmetrical sub-sea feed to a small island that caused voltage issues but no issues that we knew of as a result of the transformer connections.
All our distribution protection was by fuses, nothing sophisticated.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[luffy2011]

waross,

Thanks for the info...

It's of big help as I'm currently inquiring other utilities with regards to their common practice on transformer banking...

We are on the verge of changing our WYE(FLOAT)-delta(center-tapped) into a less problematic transformer configuration with WYE(G)-wye(g) and DELTA-wye(g) as the likely candidates.

To other forumers, please do not hesitate on sharing your experiences and I would really appreciate it.

Thanks...

http://www.powerqualityworld.com