Y to Delta transformers.

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powerpete69

powerpete69

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Professional Electrical Engineer
I noticed on site that we have several Y to delta transformers in our substations. Obviously, we see delta to Y the most often. What is going on with a Y to delta transformer and why would they use that? 4160V to 480V is the norm in the substations.
 
powerpete69 said:
I noticed on site that we have several Y to delta transformers in our substations. Obviously, we see delta to Y the most often. What is going on with a Y to delta transformer and why would they use that? 4160V to 480V is the norm in the substations.
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Are they being used as grounding transformers? If so they would typically have minimal or no loading on the delta side.
 
4160 V to 480 V. Step down. It should be noted that several of the substations have "floating grounds". Perhaps that may have something to do with it.
 
synchro said:
Is the center tap of the wye connected or left floating?
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Utility companies typically they will be bonded, NEC applications they should be left floating, or at least run a separate EGC if you want to connect the neutral to the primary coils.
 
kwired said:
Utility companies typically they will be bonded
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Kwired have you (or anyone else who would like to comment) seen a wye- delta utility transformer? Seattle city light used the open wye/open delta connection with pole pigs commonly for their high leg delta services in the past, but that is the closest I have seen (http://www.seattle.gov/light/engstd/docs2/0125.02.pdf). I would be curious what a Y-D padmount fed from an MGN distribution would look like. I worked in a building that had an ungrounded 600V delta service but I never got the see the serving transformer.
 
191120-2156 EST

In some large manufacturing plants, a few hundred thousand to millions of square feet, in the past they have run with ungrounded floating deltas at 480 V to avoid shutdown on a single line to ground fault. Down time is too expensive.

.
 
gar said:
191120-2156 EST

In some large manufacturing plants, a few hundred thousand to millions of square feet, in the past they have run with ungrounded floating deltas at 480 V to avoid shutdown on a single line to ground fault. Down time is too expensive.

.
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I sort of don't see what that has to do with OP's question. you can leave any delta secondary ungrounded doesn't matter if the primary side is wye or delta.

That said they do similar with a wye secondary by using a high impedance ground system when they don't want shutdown on a single line to ground fault.
 
powerpete69 said:
I noticed on site that we have several Y to delta transformers in our substations. Obviously, we see delta to Y the most often. What is going on with a Y to delta transformer and why would they use that? 4160V to 480V is the norm in the substations.
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Such transformers might be serving only motor loads, not requiring a neutral.
 
Does the 4160V side have an auto or motorized tap changer? On large power transformers, they are often designed to operate on the neutral and maybe that's why it is on the primary side.
 
More information: You can parallel a delta-wye with a wye-delta because the secondary side is lagging the primary side by 30 degrees in either case. You can parallel delta-delta with wye-wye transformers, zero phase change on the secondary side. But you can’t parallel any type of (delta wye or wye-delta) with a (delta-delta or wye wye.).
We will get to the bottom of this as I will keep digging.
 
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191122-2354 EST

kwired:

One other reason to use a closed delta is to eat 3rd harmonic current. Thus, the 3rd harmonic transferred to the primary lines is reduced, and if you also want a floating source, then two functions are accomplished with the closed delta.

.
 
gar said:
191122-2354 EST

kwired:

One other reason to use a closed delta is to eat 3rd harmonic current. Thus, the 3rd harmonic transferred to the primary lines is reduced, and if you also want a floating source, then two functions are accomplished with the closed delta.

.
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Aren't most the issues of third harmonics located in the neutral conductor of a wye system?

If you don't have line to neutral loads, which you won't have with a delta system nor will you power such loads without additional transformation, you don't have those issues anyway do you?
 
powerpete69 said:
More information:You can parallel a delta-wye with a wye-delta because the secondary side is lagging the primary side by 30 degrees in either case.
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lagging by 30 degrees if the primary side is connected correctly.
connect the primary c to h1, b to h2, and a to h3, the secondary will lead by 30 degrees.
that being said, if you have one connected this way, and one connected by “normal” convention, you will have 60 degrees between the circuits.
 
Thanks for the additional information. I have a model in EasyPower that I am looking at. When I run a 1500 KVA system with a delta-wye, I get a certain short circuit rating and calorie rating for arc flash. When I run the same simulation but switch the system to a wye-delta, I get the same exact short circuit rating and calorie rating. Perhaps this may lead to a conclusion that there is much difference between the two. Perhaps they run the delta secondary on loads that they know they need to mitigate harmonics. Perhaps they use the wye secondary for 277V loads. Incidentally, some of the wye secondaries are grounded, and some are not. Some of the delta secondaries are grounded, some are not. Interesting single line for sure.
 
Hv&Lv said:
lagging by 30 degrees if the primary side is connected correctly.
connect the primary c to h1, b to h2, and a to h3, the secondary will lead by 30 degrees.
that being said, if you have one connected this way, and one connected by “normal” convention, you will have 60 degrees between the circuits.
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This was pointed out to me by someone else as well. There are apparently charts that tell you which wires to switch depending on how much they are out of phase when a qualified electrician measures the difference in phase angles with a meter. Thanks for the additional information.
 
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