Delta and Wye

[jonny1982]

How can I tell if a service is Delta or Wye? Is it safe to say that most 480 volt services are delta and most 208 are wye? What are the possible outcomes of hooking up a 480 to 208 3 phase transformer (delta to wye) if the 480 side is truly wye and not delta?

Thanks

 

[Strathead]

How can I tell if a service is Delta or Wye? Is it safe to say that most 480 volt services are delta and most 208 are wye? What are the possible outcomes of hooking up a 480 to 208 3 phase transformer (delta to wye) if the 480 side is truly wye and not delta?

Thanks

Not even close. Delta and Wye refer to how the individual phase transformers are hooked together. In a Delta configuration, for say a 480/240 delta each transformer has a 2:1 turn ratio, so each transformer steps voltage from 480 to 240. They are hooked up in a triangle configuration, the ends of one transformer connect to the ends of each of the others.


In a 480/120 Wye configuration, each transformer has a 4:1 turn ratio so each transformer steps voltage from 480 to 120. The configuration as shown in the drawings all connected together at one end, causes a current flow across two coils from say A to B. The phase angle of the transformers means that the usable voltage is not directly additive so the actual voltage is 1.73 times the voltage of one coil (120 x 1.73 = 208 volts) Generally in a three phase application it makes sense to utilize a delta configuration since you have no need for a line to common voltage. It is not by any stretch necessary though. It is all about math.


Anyway, 277/480 Wye is a very common service (secondary) voltage. More common than 480 delta which could also be 480/240 if it has a stinger leg. But it totally depends on the need. If you were feeding a single piece of 480 equipment that had no need for 277 then why add the expense of a Wye wound secondary on a transformer? But most of the time you want to run your lights at 277 so the Wye is a better bargain.

 

[kwired]

How can I tell if a service is Delta or Wye? Is it safe to say that most 480 volt services are delta and most 208 are wye? What are the possible outcomes of hooking up a 480 to 208 3 phase transformer (delta to wye) if the 480 side is truly wye and not delta?

Thanks

a 480 to 208 transformer does not care if the supply system is derived via wye or delta, it is only looking for 480 volts between each input lead.

How to tell if the service is delta or wye if you can't see transformer configuration? For a 480 volt system if it is a wye system you will have a 277 volts to all three phases neutral. Any other voltages to grounded conductor is going to be a delta system, but they can vary some depending on where it is grounded, or if it is grounded at all.

 

[jonny1982]

a 480 to 208 transformer does not care if the supply system is derived via wye or delta, it is only looking for 480 volts between each input lead.

How to tell if the service is delta or wye if you can't see transformer configuration? For a 480 volt system if it is a wye system you will have a 277 volts to all three phases neutral. Any other voltages to grounded conductor is going to be a delta system, but they can vary some depending on where it is grounded, or if it is grounded at all.

So if each individual phase reads 277 to ground or 277 to neutral, it is a wye? Because I'm looking at an existing setup and the transformer is Delta (doesn't say delta, but has the triangle on it for primary, so that means delta right)? The voltage is 277 to ground on each phase. Is this an incorrect installation? Just to clarify it is a 277/480 step down to 120/208. Primary seems to be delta, secondary is wye. The actual voltage of panel feeding the transformer has 277 volts to ground on each phase

 

[Strathead]

So if each individual phase reads 277 to ground or 277 to neutral, it is a wye? Because I'm looking at an existing setup and the transformer is Delta (doesn't say delta, but has the triangle on it for primary, so that means delta right)? The voltage is 277 to ground on each phase. Is this an incorrect installation?

Please read my explanation. kwire answered your question, but I tried to explain to you how the terminology works. A transformer, as in one phase, is two coils in magnetic proximity to each other that transfer power inductively. As such that is two wires on (for this purpose) on one side and two wires on the other side. When you put three transformers for this purpose) together to form a three phase transformer bank (which we still refer to as a transformer but is Three individual transformers also) You can wire EACH side in a different way. So, if you read the voltage on the low side and it is 277/480 then the low side is hooked up in a wye configuration. Any time you have a 1/1.73 ratio of three transformers hooked up together it is a wye configuration. However, this tells you nothing about the primary. There are many ways to tell if the primary is delta or wye, but the easiest is to look at it and see how the primary coils are hooked up. But you could take resistance readings and if you were one of the real wizards here (not me, one of those who I put the parenthesis above for) and figure out the coil ratios and come up with how it is configured, or you could inject a voltage in to the secondary and take voltage readings on the primary to determine the output given my more extensive explanation in the first post.

But it doesn't really matter from an installation perspective. Either a Wye-Wye or a Delta Wye would still be 480-120/208 for the customer. Each would draw the same current on the primary for an equivalent secondary and you would pull the same number of wires to it. I just assume a Wye-Wye would be far more expensive because they wouldn't make many of them.

 

[kwired]

So if each individual phase reads 277 to ground or 277 to neutral, it is a wye? Because I'm looking at an existing setup and the transformer is Delta (doesn't say delta, but has the triangle on it for primary, so that means delta right)? The voltage is 277 to ground on each phase. Is this an incorrect installation?

The transformer primary windings are probably configured in a delta fashion - it does not care if the supply originated from a wye or delta secondary it only wants to see 480 volts input and preferably at 120 degree phase angles which either system will be.

 

[jonny1982]

The transformer primary windings are probably configured in a delta fashion - it does not care if the supply originated from a wye or delta secondary it only wants to see 480 volts input and preferably at 120 degree phase angles which either system will be.

I totally understand now, good explanation, thanks all

 

[GoldDigger]

One more very important point not covered above, which is subtle but has a large potential effect:
If the secondary is wired in delta, the primary must also be wired with only three wires whether the actual source is wye or delta.
Because wye to delta transformers are very uncommon, the problem comes up most often when reversing a delta to wye step down transformer to get a step up transformer.
You must not make a connection to the wye point (neutral) on the input side when you do that.
If you are interested in the physics behind this, let us know. Otherwise just take it as given.

 

[augie47]

A vast majority of the three phase transformer installs that I see are identical to what you have.
The transformer primary (480) is connected to the 3 phases regardless of the supply being 480 delta or 480/277.. as golddiger states ignore the neutral on the 480 side if there is one.....
The secondary is commonly 208Y/120 with the neutral grounded per 250.30.

 

[Strathead]

One more very important point not covered above, which is subtle but has a large potential effect:
If the secondary is wired in delta, the primary must also be wired with only three wires whether the actual source is wye or delta.
Because wye to delta transformers are very uncommon, the problem comes up most often when reversing a delta to wye step down transformer to get a step up transformer.
You must not make a connection to the wye point (neutral) on the input side when you do that.
If you are interested in the physics behind this, let us know. Otherwise just take it as given.

I did not know that, and I am interested in the physics if I can follow it.

 

[GoldDigger]

I did not know that, and I am interested in the physics if I can follow it.

OK, here goes, and if any members come up with a more understandable explanation, please jump in.

First we recognize that in a real world power system the voltages and phase angles may not be perfectly balanced to start with or may end up imbalanced because of unbalanced load or transmission conditions.
Now the voltages from line to neutral in a wye source will be whatever they are and the line to line voltages will be whatever the geometry of those vector voltages makes them. They will always be such that as you make a complete loop around the triangle of the three line terminals the voltages will add exactly to zero (Kirchoff's Laws, among other things.)
If you connect a balanced delta wired load to a wye source, the currents will be what they need to be, not necessarily balanced if the wye voltages are not balanced.
All good so far, but here comes the rub:
At the wye the line to neutral voltages can be unbalanced by a small amount with no consequences to speak of.
But when you add a wye to delta transformer, those unbalanced line to neutral voltages are transformed to unbalanced line to line voltages. And if the vector sum of those voltages around the triangle do not add up to zero substantial currents will flow in both secondary and primary, even with no loads attached.
Similar to what would happen if you tried to parallel the outputs of a 120 to 240 transformer and a 120 to 208 transformer with the same input voltage.
Very bad! In addition to overheating the transformer it can cause an overload of the unprotected neutral wire on the primary side.
As a practical matter, note that an open delta (two transformer) connection will NOT have this problem.

 

[Strathead]

OK, here goes, and if any members come up with a more understandable explanation, please jump in.

First we recognize that in a real world power system the voltages and phase angles may not be perfectly balanced to start with or may end up imbalanced because of unbalanced load or transmission conditions.
Now the voltages from line to neutral in a wye source will be whatever they are and the line to line voltages will be whatever the geometry of those vector voltages makes them. They will always be such that as you make a complete loop around the triangle of the three line terminals the voltages will add exactly to zero (Kirchoff's Laws, among other things.)
If you connect a balanced delta wired load to a wye source, the currents will be what they need to be, not necessarily balanced if the wye voltages are not balanced.
All good so far, but here comes the rub:
At the wye the line to neutral voltages can be unbalanced by a small amount with no consequences to speak of.
But when you add a wye to delta transformer, those unbalanced line to neutral voltages are transformed to unbalanced line to line voltages. And if the vector sum of those voltages around the triangle do not add up to zero substantial currents will flow in both secondary and primary, even with no loads attached.
Similar to what would happen if you tried to parallel the outputs of a 120 to 240 transformer and a 120 to 208 transformer with the same input voltage.
Very bad! In addition to overheating the transformer it can cause an overload of the unprotected neutral wire on the primary side.
As a practical matter, note that an open delta (two transformer) connection will NOT have this problem.

I didn't know that, but it makes common sense to me.

 

[GoldDigger]

Just to provide another look from a different angle, think about what would happen if you connect a 1:1 wired 120V transformer to L1, N, and L2 of a single phase service.
If the voltages from POCO are balanced, no problem. But if they are not, high currents will flow until the various IR voltage drops bring the voltages into balance.
In the three phase situation, leaving the wye point disconnected on the source side is like leaving the common point unconnected to the source N in the above example. That point will simply float to where the voltages on both sides balance.

 

[meternerd]

The transformer primary windings are probably configured in a delta fashion - it does not care if the supply originated from a wye or delta secondary it only wants to see 480 volts input and preferably at 120 degree phase angles which either system will be.

Maybe I'm missing something. A Wye winding will read 120 deg phase to phase, but a Delta winding will read 60 deg phase to phase. Sum of angles in any triangle always add up to 180 deg.

 

[GoldDigger]

Maybe I'm missing something. A Wye winding will read 120 deg phase to phase, but a Delta winding will read 60 deg phase to phase. Sum of angles in any triangle always add up to 180 deg.

meternerd, you have missed the fact that regardless of the orientation of the coils in space you have put the vectors tail to tail to measure the angle.
In a wye all of the coil tails are connected.
But in a drawing of a delta the tail of one vector is at the head of the previous vector.
You are just looking at the wrong angle of the four that the two lines make when extended.

 

[ggunn]

Maybe I'm missing something. A Wye winding will read 120 deg phase to phase, but a Delta winding will read 60 deg phase to phase. Sum of angles in any triangle always add up to 180 deg.

On a wye three phase, if you compare voltage A to N with B to N or C to N, the waveforms will be 60 degrees apart. On a delta three phase if you compare voltage A to B with B to C or C to A, they will likewise be 60 degrees apart.

If on a wye you compare voltage A to N with A to B they will be... hmmm, I dunno. I never thought about it. Somebody?

 

[meternerd]

In the utility metering and relay world, Delta and Wye refer to the phase relationship produced by transformer winding configuration. A phase angle meter connected A-phase to B-phase on a Delta source reads 60 deg for voltage. On a Wye it reads 120 deg. Vectorally the three phases total zero deg. We stick to Phase to Phase relationships, because sometimes you can have a 3P 3W service. You need to know if it's 3W Delta or 3W Wye.
I'm sure we're talking the same thing, but phase relationship is everything when talking kw metering, so I guess that's the perspective I'm coming from.

 

[GoldDigger]

I just do not see where you are getting 60 degrees.
If AB, BC and CA each differ by 60 degrees, AB will be 180 degrees out of phase with itself.
It may be 60 degrees between waveform zero crossings, but it is 120 degrees between positive-going zero crossings!
And for power measurements it really makes a difference between 60 and 120.

 

[GoldDigger]

I think I may have at least a partial explanation.
When looking at a transformer, you need to know the phase relationship between input and output.
Within the input, and within the output, you can have inly multiples of 120 degrees.

 

[Smart $]

On a wye three phase, if you compare voltage A to N with B to N or C to N, the waveforms will be 60 degrees apart. On a delta three phase if you compare voltage A to B with B to C or C to A, they will likewise be 60 degrees apart.

That's incorrect. AN, BN, and CN are 120? apart.

On both delta and wye, AB, BC, and CA are 120? apart.

As GD stated, the vectors must be arrange tail to tail.

Don't confuse phase and line. Phase requires two points, N excluded... but many use the first letter of the two points in discussions, e.g. A? is actually AC. Line on the other hand is just one point, N excluded.

If on a wye you compare voltage A to N with A to B they will be... hmmm, I dunno. I never thought about it. Somebody?

The angles formed at a common vector point are 30? and its supplement, 150?, with the former being the most prominent in discussions and calculations.