Single-bushing distribution transformers
Single-bushing distribution transformers
(OP)
Guys,
In a WYE(G) - wye (g) transformer configuration, some utilities simply utilize single-bushing units.
However, the transformers could be subjected to full line voltage once neutral is disconnected from ground and one line opens.
I need your experiences and comments here... Thanks!
In a WYE(G) - wye (g) transformer configuration, some utilities simply utilize single-bushing units.
However, the transformers could be subjected to full line voltage once neutral is disconnected from ground and one line opens.
I need your experiences and comments here... Thanks!






RE: Single-bushing distribution transformers
RE: Single-bushing distribution transformers
RE: Single-bushing distribution transformers
RE: Single-bushing distribution transformers
However, I am more concern of the external conductor from the neutral bushing to ground rod, which can be accidentally disconnected. Any thoughts?
This is because in our utility, we have been using the double bushing for any transformer configuration. But since we are planning to standardize 3-phase transformer bank to YG-yg, the single-bushing unit has caught my attention.
Previous studies have shown that on average the single-bushing is 10% less expensive. Yet, I want to know whether the risk of being subjected to line voltage is high or not.
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RE: Single-bushing distribution transformers
RE: Single-bushing distribution transformers
Thanks for the clarification, but then again from the case it will be connected to a neutral conductor going to ground, which could be accidentally opened. Although it's rare, there are already several reports of loose ground in our utility.
If that happens, the single-bushing transformer will be subjected to full line voltage. This may not concern the system in general since it is multigrounded, but the transformer units could be damaged. In addition, an ungrounded wye in the primary even with grounded wye secondary may cause overvoltage in the secondary especially if loads are unbalanced.
Please correct me if I'm wrong... Thanks...
http://www.powerqualityworld.com
RE: Single-bushing distribution transformers
RE: Single-bushing distribution transformers
RE: Single-bushing distribution transformers
I thought double bushing transformers will be able to handle the increase in the neutral-to-ground voltage as they are rated to handle such. During accidental disconnection, neutral-to-ground voltage may rise close to line voltage. Am I wrong?
Anyway, I appreciate your comments as they imply that based on your experience the risk of a single bushing transformer getting damaged due to voltage stress is very low. And... you are using or have used single bushing units in your utility or facility. Right?
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RE: Single-bushing distribution transformers
The only way accidental disconnection will cause line to line voltage to occur across a primary winding is for the wye point to become disconnected from both the system neutral and ground while the wye point itself remains intact. I doubt this voltage will bother either transformer type very much (I expect saturation would cause the fuse to blow), but the downstream equipment might be affected. This is a double contingency for both transformer types.
Yes, we use both types. Double bushings are more flexible since they can be used in wye-delta banks and wye-wye banks, so a single transformer may be stocked. We've added the single bushing since all new banks are wye-wye, and maintain only enough double bushing to maintain existing wye-delta services.
RE: Single-bushing distribution transformers
I caught quite a bit of flack from the linemen for attempting to do away with double-bushing transformers. (Delta primaries were never used; delta secondaries can be done, but require more attention to polarity.)
In my mind the slightly lower cost was a side benefit; I was more interesting in making it easier to successfully sheild the connections from wildlife faults.
RE: Single-bushing distribution transformers
RE: Single-bushing distribution transformers
RE: Single-bushing distribution transformers
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RE: Single-bushing distribution transformers
What were the reasons why you shifted from wye-delta to wye(g)-wye(g)?
We are also planning the same due to voltage problems encountered when one line is accidentally opened.
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RE: Single-bushing distribution transformers
Padmounted delta is tough to do, since elbows and cables use a grounded neutral. So open delta is the reasonable option to get 120/240 three phase. This results in underutilized transformers, and poor power factor and phase balance.
To avoid ferroresonance when energizing an overhead bank one phase at a time, one temporarily grounds the primary wye point. Not too hard when energizing locally, but much difficult if performed at a distant tap point. Either risk ferroresonance or delay restoration until all the wye points on the tap are grounded.
Wye-wye makes things easier all the way around.
RE: Single-bushing distribution transformers
RE: Single-bushing distribution transformers
Just like the single bushing overhead cans, the wye point is connected to the grounded tanks. It should be floated instead for better voltage balance.
RE: Single-bushing distribution transformers
How about with overhead installations?
In our case, we encountered several high voltage problems when one line is accidentally opened. Also, ferroresonance during switching.
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