Windings in a Wye vs Delta Generator, Need some help understanding 1

[RobsVette]

Hey Guys,

I'm having a little trouble with a couple of things and was hoping you guys could clear up my mis-understandings with the whole Wye and Delta arrangements. I'm going to use a standard 277/480 system as the basis for discussion.

In a Wye arrangement the Phase to ground voltage is 277V. While the phase to phase is 480V. To me that makes perfect sense because phase to ground you are looking at a positive voltage to a non-potential ground and getting some voltage. When you look phase to phase you are seeing a positive voltage to a negative voltage in the other phase, thus you are seeing the 480V. Mathematically you can derive why the phase to phase voltage peaks before the phase to ground and whey the phase to phase voltage is Sqrt of 3 times more. Intuitively you can see this by looking at a three phase wave form as well.

In a delta arrangement, I don't understand this quite so well.

What do you see if you look at the phase to ground voltage of a delta system? I assume you would see 480V, because teh absolute magnitude of the voltage in teh system with reference to gound would be 480V.

Also if this is true I understand why a Delta to Wye transformer needs more winding on the Wye side. This is because in the winding the effective voltage on the delta side is 480 volts and you are only gettign 277 on the wye side.

Given this, would a delta generator have a different number of windings rather than a wye generator? Intuitively I would say no, because the connection to teh other phases would provide the absolute voltage (between the positive in one winding and the negative in the other)

If anythign I am asking is unclear or incorrect please let me know and I will try to explain further. I am a mechanical engineer trying to learn electricity. Please take pity on me.

Thanks,
Rob

 

[electricpete]

I think the question you are asking gets into the characteristics of ungrounded systems. Even if truly ungrounded, there is some capacitive coupling which will generally maintain line-to-neutral voltage near 277vac in steady state. What happens during a transient is another story, ungrounded systems may be more susceptible to voltage transients, where line to ground voltage can go far beyond 480vac.

=====================================
(2B)+(2B)' ?

 

[electricpete]

Correction in bold:

I think the question you are asking gets into the characteristics of ungrounded systems. Even if truly ungrounded, there is some capacitive coupling which will generally maintain line-to-neutral voltage near 277vac in steady state. What happens during a transient is another story, ungrounded systems may be more susceptible to voltage transients, where line to ground voltage can go far beyond 480vac.

should have been...

I think the question you are asking gets into the characteristics of ungrounded systems. Even if truly ungrounded, there is some capacitive coupling which will generally maintain line-to-ground voltage near 277vac in steady state. What happens during a transient is another story, ungrounded systems may be more susceptible to voltage transients, where line to ground voltage can go far beyond 480vac.

=====================================
(2B)+(2B)' ?

 

[cranky108]

On a pure delta system with a 480 V generator, the capactiance of each phase to ground would hold the phase to ground voltage at 277 V. Given this, at some point there will be a phase to ground fault, and then one phase will be at zero voltage to ground while the other two phase will be at 480 V. But there would be no current to speak of.

On a grounded wye generator a phase to ground fault the phase to ground voltages of the other two phases will not increase to 480 V because of the solid ground connection. But there will be much more current. Which is the reason for impedance grounded generators.

Given that two generators are the same size (electrically), each of the three windings would produce about a third of the power produced. On a delta winding, each coil will have a larger number of windings because of the higher voltage, but smaller current capacity. On a wye winding, each coil will have a smaller number of windings because of the lower voltage, but a larger current capacity.
The design differences reflect the price differences.

 

[waross]

Let's talk about some specific generator applications.
In Europe, wye:delta connections are popular. A generator (or a transformer) may have 230 volt windings. The windings may be connected in delta for a 230 Volt output, or connected in wye for a
400 Volt output.
Suppose that the windings are rated at 100 Amps.
The delta connection produces 230 Volts and 100 Amps x 1.73 = 173 Amps.
The wye connection produces 400 Volts (230 Volts x 1.73) and 100 Amps.
Common voltages in Europe are in the ratio of 1.73:1.
Note that I am illustrating the difference between voltages and/or currents and there is another factor of 1.73 to calculate KVA output of a three phase machine.
In North America voltage ratios are more likely to be in the ratio of 2:1 This is achieved by connecting windings in series or parallel.
A typical generator will have 138 volt windings, but they are often labelled as 120 Volts.
These windings are connected in parallel for 120:208 Volt service and in series for 240;415 volt service. Also the voltage on many 240:415 Volt sets may be safely pushed up to 277:480 Volts.
The North American sets may be connected in series delta for 120/240;240 Volt four wire delta systems.

It is important to know what voltage and configuration is needed when sizing a generator. I have had to curtail loads on generators when a customer needed a certain KVA at 120:208 volts and was sold a set rated for 120/240:240 Volts.
Lets take a set with six coils rated at 100 Amps and 120 Volts (138 Volts)
Used for 120:208 Volts this set will produce 200 Amps x 120 Volts x 3 Phases /1000 = 72 KVA.
Used for 120/240:240 Volt four wire delta this set will produce about 83 KVA.
Used for 277:480 Volts this set will produce 83 KVA.

And there are some single voltage sets designed for 480 Volts or 600 volts that may designed and wound for iether wye connections or delta connections

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[RobsVette]

Hey guys,

thanks for all of the replies. I actually think I am starting to understand this much better. However, one thing is still unclear to me.

In a Delta configuration generator (transformer would apply as well, but I will use the generator as my example) there is a rotor rotating in the middle of the generator with 180 degrees postive field and 180 degrees negative field.

I understand now why a delta generator makes 1.73 times as many line amps as a wye. you are getting the added combination of 2 winding together at 120 degrees apart so you get the one winding amps * 1.73.

What I dont understand is, with the delta winding are you not putting two different voltages (that are different and any instant) in parrallel with one another? Ie two winding are connected to one phase and they are 120 degrees apart so the voltage would be off by 120 degrees.

If you try to put two batteries of different voltages together it doesnt go well. Why would the same not happen with a delta winding?

I wye winding I can understand becasue with all three winding joined together at one spot, if you look from phase to phase you actually see two voltage in series, which is fine and makes perfect sense to me.

Please help, I'm confused.

 

[electricpete]

What I dont understand is, with the delta winding are you not putting two different voltages (that are different and any instant) in parrallel with one another? Ie two winding are connected to one phase and they are 120 degrees apart so the voltage would be off by 120 degrees.

If you try to put two batteries of different voltages together it doesnt go well. Why would the same not happen with a delta winding

Remember, a voltage always requires 2 locations to define it. Tgenerator windings provide a voltage between A and B, between B and C. That does not create any circulating current. In fact, if you add up the vectors around the loop Vab + Vbc + Vca, it is zero.

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(2B)+(2B)' ?

 

[waross]

The shape of the delta is the clue. As a mechanical engineer you are familiar with forces acting at angles other than perpendicular and the use of trig functions and vector diagrams to resolve the resultant forces.
When two alternating voltages are added, the result is not the numerical sum but the vector sum. The vector sum of three equal voltages or three equal forces each displaced 120 Degrees is zero.


Bill
--------------------
"Why not the best?"
Jimmy Carter