Wye connected transformer primary

[coulter]

I didn't want to hijack a thread. And 5e likely wants to keep focused until he gets an answer. So ....

jt (and others) -
I am absolutely not familiar with Wye connected xfm primaries, advantages, when one would want to use one. Could you recommend some references?

carl

Carl, most utility transformers these days are Y-connected. Most are single-bushing units, or single-ended, meaning that one end of the transformer is connected to the system neutral.

Next time you have the opportunity, look up at the transformers on a pole and especially the primary connections.

Carl

Wye primaries have lower insulation levels than comparable delta primary transformers, they're cheaper.

Another problem is ferroresonance anytime you have a delta in an underground system. The high capacitance of underground MV cables combined with the reactance of the transformers can create a resonant condition.

Jim T

Most of the primaries I deal with are 25kv, 69kv, 138kv, all overhead - and one 38kv UG (10 years ago). And I deal with a lot of 13.8kv secondaries as well as the standard 480 secondaries.

The one 38kvUG xfmr was provided by the utility and is wye primary. A couple of the small 25kv are pole mount xfmrs and I'm not sure about those. I'll look at them nest time I'm out there. The rest are above ground services and the primaries are all Delta.

I took a class in 1985 about transmissions lines and I recall issues with stability on UG lines above 30kv with any significant length - we studied one that was 30kv and 30 miles (I'm guessing) - Goat Island Norway maybe. And that was a big deal.

Of the xfms in the plants, fed with 13.8kv, all are Delta primaries.

So, I'm understanding what you are saying about insulation resistance and resonance, but now I'm wondering why this doesn't apply to all of the systems I deal with - Why aren't they all Wye primaries? - even the 13.8kv.

I'm guessing there is a downside.

carl

 

[steve66]

I think odd harmonics are canceled out in the primary of a delta-wye.

Also, one advantage of a wye secondary is the line to neutral voltage you get. But that's not much use on the primary side.

Just a guess here, but I also think it has something to do with voltage balance. The delta's tend to stay more balanced when the phase currents aren't all equal. But with the center point of the wye may drift a little toward one phase or another. (After all, the utilities don't want to run a 4th wire for miles to give a low resistance neutral return line.)

There might also be some fault current and Overcurrent protection issues here, but they are beyond my understanding.

I think you will get some more complete answers from others who are more familiar with utility and HV design.

By the way, ECM has a high voltage cable seminar that is about to start in 40 min. I wish i had time to listen in. I am hoping they record it and leave a link. (I think they usually do.)

Steve

 

[charlie b]

I think odd harmonics are canceled out in the primary of a delta-wye.

I think that would be the triplens harmonics (3, 6, 9, 12, 15, etc.).

Separate from this concept, the evens (2, 4, 6, 8, etc.) disappear from power systems for reasons I no longer remember. I think it has to do with the cosine curve being "even" (i.e., the path to the right of the vertical axis is the mirror image of the path to the left). In any event, when we talk of "triplens harmonics," we usually omit the evens, and list them as 3, 9, 15, 21, etc.

 

[jtester]

Of the xfms in the plants, fed with 13.8kv, all are Delta primaries.

So, I'm understanding what you are saying about insulation resistance and resonance, but now I'm wondering why this doesn't apply to all of the systems I deal with - Why aren't they all Wye primaries? - even the 13.8kv.

I'm guessing there is a downside.

carl

Steve hit on two big benefits of delta primaries. They tend to be a balancded high side when there is a load imbalance on the low side, and they trap the triplen harmonics.

Insulation levels aren't too expensive in 15 kv class equipment. I specified transformers for a utility for many years and we used delta primaries on padmounts at 15 kv and wye on 25 kv class padmounts. That was the general standard for overhead banks as well.

You can somewhat control ferroresonance with 3 pole switching, so things aren' t really bad in the 15 kv class.

Jim T

 

[micromind]

I'm not an engineer, so this is mostly from a practical standpoint.

A closed delta (3 xfmers) tends to balance voltage at the expense of current. Thus a delta-wye is easier on both sides of the system.

If one phase of the secondary were loaded more than the others, with a wye-wye the current imbalance would carry through to the primary as well, causing an even worse condition. A delta-wye spreads the imbalance out a bit more evenly among all 3 phases.

Alot of xfmers are fed from overhead lines through cut-outs. If the wye-wye is one 3 phase unit in the same can, (not 3 separate pots), closing the cut-outs one at a time would be quite interesting.....no, make that frightening!

also, a delta connection needs only 3 wires, a grounded wye needs 4.

One advantage is differential protection is extremely simple, there's no phase shift involved.

In my 18 years as an electrician, I've only seen 2 wye-wye systems. One was installed last winter, it was 60kv to 277/480. Around 2000kva, I'm not sure. One thing I do remember is this unit was built in 1964! As far as I know, it still works. The other one was about 4 or 5 years ago, 69kv to 4160, about 2000kva as well. It was a temp for construction of a power plant.

 

[coulter]

...Alot of xfmers are fed from overhead lines through cut-outs. If the wye-wye is one 3 phase unit in the same can, (not 3 separate pots), closing the cut-outs one at a time would be quite interesting.....no, make that frightening! ...

Well, I only have a statistical sample of one. The installation was an overhead 38kV line to a pole with cutouts, then UG about 100 ft to a pad mount 1000kva xfm (all three phases in one steel box). The utility crew went up in their bucket truck and closed the cutouts with a hook stick. I didn't see any extraneous energy emmission and it didn't seem to bother the crew.

...One advantage is differential protection is extremely simple, there's no phase shift involved. ...

I asume you mean with Y-Y. I don't think this is a problem with D-Y for today's protective relays

I just finishing up commissioning a 20MVA 138kV D primary, 13.8kV impedance grounded Y sec. The differential relay is a Schweitzer 387. You just tell it the configuration, xfm ratio, CT ratios and it applies the correct phase shift and vector addition.

I suspect it was not a problem with yesterday's equipment either. One would have to install the correct phase shift xfm on ome set of CTs - the physics seems straight forward, but I don't know this.


carl

 

[coulter]

Here's an example: (just picked from the blue - but could be typical)

The engineer is designing a service to the plant. The company is buying the equipment (and paying for the design) The utility will go along as long as the proposed installation does not cause heartburn with their other customers. The contract is set up for: (none of this may matter)
Demand charges
Prohibits harmonics outside of the IEEE spec
Requires pf to be .95 lead to .95 lag

Xfmr is to be 20MVA. Secondary will be 13.8kV impedance grounded Y

Utility service is 138kV Y.

1. Does the engineer want to spec a Y or D primary?
2. Would this change if the utility service was 69kV?

I've got a few others, but this will do for a start.

Like I said, I certainly don't mind doing the research, and would certainly appreciate references for a jumpstart.

carl

 

[templdl]

It is of my opinion that one has to separate power distribution is a utility application and those of industrial/commercial applications. Sometimes the two get muddied up together.

In industrial/commercial applications the D-Y are most common.
Even though a distribution system may be feed with a Y, such as a 12470Y/7200, one would specify a 12470D-480Y/277 step down xfmr. and then a 480D-208Y/120.
It is interesting when I get a request to quote a transformer when a Y primary voltage is specified. Why? Because that is what the supply voltage is.
There are legitimate engineering reasons such with a Y-Y there is no phase shift. With a D-Y it?s a 30deg from pri to sec., which normally who really cares. If you had a UPS or another need to have different systems in phase a 30deg phase shift could be a significant issue.
Another interesting thing that must be considered is that there is an additional cost to manufacturing a Y-Y transformer. They must be designed with either a tertiary winding of a 5-legged core to deal with the circulating magnetic that occur if a common 3-legged D-Y core design is used. This adds about 25% to the cost of the transformer.

 

[micromind]

The SEL 387 is an excellent relay, I've seen them used for generator differential protection as well.

To compensate for the 30 degree phase shift on a delta-wye xfmer using the old style disc type relays, you simply hook the delta side CT's to the relays in a wye configuration, and vice-versa. Cancels out the phase shift completely.