WYE - WYE?

[buckyhaa]

From an engineering standpoint, what if any are the drawbacks of a wye-wye transformer configuration?
The reason for my question is I ran across an article about a university in New York that had several wye-wye connected transformers fail catastrophically, contaminating several buildings with PCBs. The accident was caused by a car striking a pole several miles from campus. There is not much information out there about the specific cause of the failure.
What could possibly be the root cause of this type of failure ?

My daughter is an EE student and has told me her theories, now I would like to hear from the experts to see if my money has been well spent!!
Thanks

 

[iwire]

A bump to keep this up front, I am interested as well.

 

[Jraef]

To be clear, the wye secondary has many benefits, you are likely wondering why they used a wye primary. There could be several reasons for the original decision. Some have to do with being able to attenuate 3rd order harmonics, but if the transformers had PCBs in them, it's likely they were installed at a time when harmonics mitigation was even thought of yet. A more common reason might have been to limit the available fault current in the system without affecting the transformer efficiency as much (as opposed to increasing the impedance for example) where the facility is close to a substation and therefor has less line resistance to help with that, something you see more often in densely populated areas. Another reason may have simply been the convenience of the utility. They have some systems where the primary is 25kV, others where the primary is 13.8kV, so they can stock the same transformer and connect the primary in delta for a 13.8kV feed, or wye (with taps) for a 25kV feed.

 

[kingpb]

The transformers were all filled with fluid containing PCB's. The oil can create an explosive gas when transformers malfunction.

The malfunction was most likely caused by the voltage spike/surge on the line due to the external short circuit caused by the car hitting the pole.

This type of malfunction is the leading cause in transformer failures.

These transformers were likely old, and due to introduction of air and water over time, thus allowing it to interact with the PCB oil caused it to degrade which allowed the gases to form.

 

[mbrooke]

Here are some advantage:


> resistant to ferroresonance
>No phase shift (0 degree vector group)
>insulation can be gradually reduced as the winding heads toward the star point.
>single phase cans only need one bushing per can


Disadvantages:

>inductive tank heating with one phase open when using 3 phase 3 and 4 core design
> zero sequence currents pass through
> harmonics and power quality imperfections pass through (a tertiary can in theory help)
> phase angle imperfections also pass through where as they tend to circulate in delta primary.
> primary neutral shift results in secondary neutral shift



There is more but its not coming to me.

 

[steve66]

I think Jref has one thing backwards: Delta primaries tend to cancel the 3rd harmonics, and they keep the harmonics from appearing on the primary feeders, and the next transformer at the utility substation. (To explain it as simply as possible, the 120 deg phase shift between the secondary currents becomes a 360 degree shift at the 3rd harmonic. When these add together in the closed delta primary, they tend to cancel.)

So Wye primary transformers are rarely used today since harmonics are so prevalent. But back when those transformers were installed, nobody probably cared about harmonics.

 

[mbrooke]

To be clear, the wye secondary has many benefits, you are likely wondering why they used a wye primary. There could be several reasons for the original decision. Some have to do with being able to attenuate 3rd order harmonics, but if the transformers had PCBs in them, it's likely they were installed at a time when harmonics mitigation was even thought of yet. A more common reason might have been to limit the available fault current in the system without affecting the transformer efficiency as much (as opposed to increasing the impedance for example) where the facility is close to a substation and therefor has less line resistance to help with that, something you see more often in densely populated areas. Another reason may have simply been the convenience of the utility. They have some systems where the primary is 25kV, others where the primary is 13.8kV, so they can stock the same transformer and connect the primary in delta for a 13.8kV feed, or wye (with taps) for a 25kV feed.

Curious, how does a wye-wye limit fault current? Brain fart :happysad:

 

[mbrooke]

I think Jref has one thing backwards: Delta primaries tend to cancel the 3rd harmonics, and they keep the harmonics from appearing on the primary feeders, and the next transformer at the utility substation. (To explain it as simply as possible, the 120 deg phase shift between the secondary currents becomes a 360 degree shift at the 3rd harmonic. When these add together in the closed delta primary, they tend to cancel.)

So Wye primary transformers are rarely used today since harmonics are so prevalent. But back when those transformers were installed, nobody probably cared about harmonics.

You are correct the exception of utilities. Utilities use wye-wye almost exclusively because of the cost saving and ferresonance mitigation even though other issues come with wye-wye.

 

[Bugman1400]

You are correct the exception of utilities. Utilities use wye-wye almost exclusively because of the cost saving and ferresonance mitigation even though other issues come with wye-wye.

In my experience, 3ph distribution loads are almost always supplied by a DY1 xfmr. Having a G-wye on the dist primary would be a ground current source and frequently causes confusion in the ground relays back at the substation. The exception I see is when the customer has more than one service point at different voltage levels (ie 12.47kV and 4.16kV) and they need the phase angles to match in their switchgear for switching.

I don't see the use of wye xfmr as a cost savings or controlling Ferro resonance problems. To control Ferro resonance, I have only seen TRV caps back at the substation.

 

[mbrooke]

In my experience, 3ph distribution loads are almost always supplied by a DY1 xfmr. Having a G-wye on the dist primary would be a ground current source and frequently causes confusion in the ground relays back at the substation. The exception I see is when the customer has more than one service point at different voltage levels (ie 12.47kV and 4.16kV) and they need the phase angles to match in their switchgear for switching.

I don't see the use of wye xfmr as a cost savings or controlling Ferro resonance problems. To control Ferro resonance, I have only seen TRV caps back at the substation.

You are correct it would nuisance trip ground relays, but pocos just set them higher.

Your utility may use Dy1, however most others use wye-wye exclusively.

 

[Iron_Ben]

You are correct it would nuisance trip ground relays, but pocos just set them higher.

Your utility may use Dy1, however most others use wye-wye exclusively.

When I worked for the POCO, we installed several hundred three phase oil filled padmount transformers every year. Primary voltage was either 13.2 kv or 34.5 kv. Secondary was 208Y/120 or 480Y/277. Ratings ranged from 45 kva to 2500 kva. Every last one was grounded wye-grounded wye.

 

[mbrooke]

When I worked for the POCO, we installed several hundred three phase oil filled padmount transformers every year. Primary voltage was either 13.2 kv or 34.5 kv. Secondary was 208Y/120 or 480Y/277. Ratings ranged from 45 kva to 2500 kva. Every last one was grounded wye-grounded wye.

That my point, nearly all POCOs go for wye-wye. The exception is California.

 

[Bugman1400]

You are correct it would nuisance trip ground relays, but pocos just set them higher.

Your utility may use Dy1, however most others use wye-wye exclusively.

Care to disclose which POCOs do that?

 

[mbrooke]

Care to disclose which POCOs do that?

First tell me which POCO uses a delta primary. I am not saying your does not. Some do. Then I will give you a list of 100 POCOs which is only a fraction of those in the US that use wye-wye.

 

[steve66]

You are correct the exception of utilities. Utilities use wye-wye almost exclusively because of the cost saving and ferresonance mitigation even though other issues come with wye-wye.

The utilities around here only use delta-wye transformers for the final "customers" transformers - at least for transformers with 208V or 480V secondary's.

Those transformers will cancel out most of the harmonics, so I suppose that leaves the utilities free to use Wye-Wye transformers for all the larger and higher voltage substation transformers. Is that what you are referring to when you say "Utilities use wye-wye almost exclusively"?

 

[big john]

The utilities around here only use delta-wye transformers for the final "customers" transformers - at least for transformers with 208V or 480V secondary's.

Those transformers will cancel out most of the harmonics, so I suppose that leaves the utilities free to use Wye-Wye transformers for all the larger and higher voltage substation transformers. Is that what you are referring to when you say "Utilities use wye-wye almost exclusively"?

I do a lot of work in Emera, CMP, National Grid, and Eversource territory: I'd say the majority we see are Y:Y. About the only time I see delta primaries is when they can set a gang-switch in front of it which is obviously gonna be a lot more costly than a few cutouts.

 

[mbrooke]

I do a lot of work in Emera, CMP, National Grid, and Eversource territory: I'd say the majority we see are Y:Y. About the only time I see delta primaries is when they can set a gang-switch in front of it which is obviously gonna be a lot more costly than a few cutouts.

Ferroresonance?

 

[big john]

Ferroresonance?

That's the ticket. Apparently some places will drop in individual cutouts with a chain ground, but I've never actually seen that in person.

 

[mbrooke]

That's the ticket. Apparently some places will drop in individual cutouts with a chain ground, but I've never actually seen that in person.

Especially with Eversource using 23 and 34.5kv.

 

[buckyhaa]

OK-
Going back to my original post,
When these transformers failed, say because of a phase loss on the HV side or primary arcing externally, what actually would cause them to explode and burn up? I understand over pressurization and flammable gas...
But what happened to the transformer itself?:?

Is this type of failure more common in a wye vs delta primary?