Single-bushing distribution transformers 2

[luffy2011]

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!

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[stevenal]

Pretty tough to disconnect neutral from ground, since the connection is internal. Wye do want disconnect the neutral? Unlike a wye-delta, the primary neutral on a wye-wye serving unbalanced loads cannot be floated.

 

[jghrist]

It would be more likely to inadvertently open the primary neutral-ground connection (or transformer bank neutral point to system grounded neutral to be more precise) in a bank of two-bushing transformers than in a bank of single-bushing transformers with internal ground connections. If the primary neutral is not grounded, the secondary voltages will not be stable even if the secondary neutral is grounded.

 

[prc]

In India we have both two bushing single phase transformer connected between lines and single bushing transformer connected between line and neutral(ground).Neutral is not earthed inside,but brought out through 1.1 kV bushing which is either directly earthed or through a resistor to limit faut current.

 

[luffy2011]

Thanks for the input...

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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[stevenal]

There is no primary neutral bushing on a single bushing transformer. The neutral end of the winding is attached internally to the case, and the case is connected to the neutral conductor and ground via the case ground lug. Assuming this is a 4 wire multi-grounded system, the loss of the local ground rod connection will be compensated for by the remaining neutral to ground connections. This is no different from double bushing transformers.

 

[luffy2011]

stevenal,

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...

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[jghrist]

Unless the internal primary connection to ground is opened (an unlikely event), opening the connection to ground on a single bushing transformer would also open the secondary connection to ground. It is no more likely to open the secondary connection to ground on a bank of 1-bushing transformers than it is on a bank of 2-bushing transformers. Using 2-bushing transformers provides an external connection of the primary neutral to ground and another point where accidental opening can occur. I fail to see how a concern about opening the neutral connection to ground would favor using 2-bushing transformers.

 

[stevenal]

Ground rod connections are low where they may be subjected to physical damage, copper theft, etc., but unless your neutral connection also takes this path (unlikely really bad design), the neutral connections will remain and keep your transformers at line to ground potential. It would take a double contingency to get to the line to line voltage scenario you are concerned with. Double bushing transformers are also stressed when subjected to overvoltage by the same contingencies. In addition, double bushing transformers are more expensive and are more likely to leak due to the added penetration in the lid.

 

[luffy2011]

jghrist and stevenal,

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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[stevenal]

Yes, you are wrong. The ratings will be the same unless for some reason you choose to otherwise change specifications.

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.

 

[potteryshard]

As far as I can tell, the greatest justification for double-bushing transformers is so that linemen can easily see and flip as needed the primary and secondary polarities when making up delta (HV or LV) connections.

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.

 

[stevenal]

It's not about the ease. You cannot properly float a wye point with a wye delta connected single bushing transformer. It would require floating the transformer tanks, which would violate codes and safety rules stating that tanks must be grounded.

 

[jghrist]

With either 1- or 2 bushing transformers, if the neutral connection were isolated from ground and the system neutral, and one phase was opened, the wye point voltage to ground would become half way between the two intact phases. Ø-ground voltage of the wye point would be 1/2 of the system Ø-grd voltage. The transformer tank, the part of the pole where the tank is bolted, the secondary wye point, the open primary phase, and the associated secondary phase would have 1/2 of the system Ø-grd voltage. Not a good situation. I think the voltage stress would be on the pole between the transformer and the primary system neutral wire (or a lineman who might be on the pole) instead of on the transformer windings.

 

[luffy2011]

Ok guys, thanks...

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[luffy2011]

stevenal,

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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[stevenal]

luffy2011,

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.

 

[jghrist]

Padmounted delta is tough to do, since elbows and cables use a grounded neutral.

What is the problem with grounding the system neutral at a delta primary padmounted transformer?

 

[stevenal]

I should have said padmounted wye-delta bank.

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.

 

[luffy2011]

stevenal,

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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