On the secondary, the phase shift, in realtion to the primary can be shifted by any number of degrees.

But why is this important?

What does the load care, how much shift is done?

I am lost on this, anyone?

Power factor aside, it probably doesn't matter. But if you need synchronous sources, it can be a problem. For traction power, we use both delta/delta and delta/wye connections because we want the relative phase shift. This helps fill in the gaps on the outputs of the 6 pulse rectifiers.
Joe

" For traction power, we use both delta/delta and delta/wye connections because we want the relative phase shift. This helps fill in the gaps on the outputs of the 6 pulse rectifiers."

Joe
Please explain this. You can't be using
a dd and dy as a source for the same motor.

You could if the 6 pulse rectifiers were feeding the DC buss for a VFD.

Larry C

Bob, we do it 24/7. They are 600VDC traction power compound motors on all the truck assemblies. The 600VDC comes from rectified 480 3PH. When your rectifiers can put out over 10,000 amps each, you can't just slap a capacitor on to filter out the bumps. What you can do is take advantage of the relative phase shift of d/d Vs d/y secondaries. The rectifier outputs feed a common DC bus. The positive peaks are shifted, reducing the ripple amplitude. This along with rail inductance, smooths out the 600VDC supply. Add to this the fact that most rail sections are fed from both ends. Many substations have 3 rectifiers. If one has 2, d/d and 1, d/y, the next will typically have 1, d/d and 2, d/y. As you can probably imagine, there is absolutely no regulation. 600 is just what we call a voltage that usually isn't.
Joe

I've used them for harmonic mitigation, when used in pairs, but couldn't visualize any other applications.

Joe, that is an interesting application using phase shift.

I didn't realize you could do that.

Any other applications where these are used?

Theoretically, the phase shift can be any number of degrees. But in practice, it's 30 degrees in a standard delta-wye transformer. (Well, 150 degrees if you want to be technical.) This is rarely desireable, and just a by-product of 3-phase power.

The phase shift creates problems with UPS systems, as the UPSs generally take delta power and output wye power. The maintanance bypass circuit has to phase with this, and match it exactly. So, where the UPS is operating at a different voltage from the building power and input/output transformers are required, it creates a real engineering challenge to ensure everything matches.

[This message has been edited by SteveFehr (edited 10-21-2006).]

So, what exactly controls how many degrees of phase shift there are? Does this only occur on 3 phase, or single phase as well?

In delta/wye power, there is an inherent phase difference between whether you're measuring line-to-ground or line-to-line. Basically, the peak measured A-to-neutral is going to peak about 30 degrees after A-B peaks, and about 150 degrees after C-A peaks. Usually, this is described as the delta leading 30 degrees behind wye (or that the wye lags the delta by 30 degrees)

It helps to draw a phasor diagram to picture this- if you draw a wye system and then connect the peaks to create the delta vectors, it should make sense pretty quickly.

The problem is when you wire up a transformer delta-wye, as the transformer now ties in the wye to the phase angle of the delta, which bumps the whole thing by 30 degrees.

This only occurs in 3-phase. There's no phase shift in 1-phase.

Edit: had something backwards

[This message has been edited by SteveFehr (edited 10-26-2006).]

If I may add a small point to Steve's excellent post (and something it took me a while to grasp):

The difference in time between the line-to-line and line-to-neutral voltage peaks is also why 120 + 120 = 208, and not 240, in 208/120-Y systems. Unlike the hots of a single-phase 240/120 system (180 deg. apart), the opposing peaks are not rising and falling simultaneously (120 deg. apart).

Using any one phase's voltage peak as a reference point, the voltage peak of the previous (or next) phase is already on the way back down (or still rising), so the phase-to-phase voltage is not twice the phase-to-neutral voltage.

Since there are only two points used to measure a voltage difference, it still looks like a symmetrical sine wave, rather than a lop-sided wave similar to a 2-cylinder engine with an uneven firing order. This is the part I used to have trouble with.

Logic seemed to dictate that, with two points peaking 120 and 240 degrees apart, the waveform would be very un-symmetrical. However, electricity doesn't work that way. We instead get a sine wave with an algebraically-derived voltage.

This is why an understanding of mathematics is so important to electrical theory. We may not need to know the math to do the physical work, but, as we see here, it can indeed matter when we have to do our own engineering.

[This message has been edited by Larry Fine (edited 10-25-2006).]

As a lot of people on this forum are on-the-job trained electrians as opposed to engineers, I went ahead and drafted up a few phasor diagrams, hopefully they won't be too hard to understand!

These are Phasor Diagrams drawn in polar coordinates. (A phasor is basically a voltage represented as a vector.) Vectors represent voltage; angle represents the sine wave and phase shift.

1 full rotation represents 1 cycle, or about 1/60th of a second. If this were viewed in real time, you would see the vectors rotating counter-clockwise 60 times a second. The distance between the vectors is 1/3 of a cycle, or 120 degrees and represents the phase shift between each of the 3-phases.

This is what a normal Wye (star) looks like as a phasor diagram. Each leg is 120V long, and each is rotated 120 degrees apart.


Delta is derived by going line-to-line. The phasor diagram makes it easy to see the relationship between the wye and delta voltages. Mathematically, this is how the difference in voltages is derived.


In this diagram, the vectors we drew in the above diagram have been shifted on the graph so they're all measured relative to 0V. You can see that the delta voltages are rotated 30 degrees ahead of the wye voltages- this is what's referred to when people say delta is leading wye by 30 degrees or vice versa. The square root of 3 (1.73x) difference is easily visible here, too.


In this diagram, the blue represents the delta primary on a delta/wye transformer. The green phasors represent the wye created on the wye secondary. As the wye secondary is wrapped around the same coils as the delta primary, it essentially is taking the new wye and forcing it onto the delta. And since that delta was leading the old wye by 30 degrees...
Well, it's hard to explain this in a way that doesn't come off as incomprehensible gibberish, but hopefully staring at the pictures long enough will make sense


[This message has been edited by SteveFehr (edited 10-28-2006).]

Those sure are pretty, but all wrong down here in the great state of Alabama. We're ACB rotation.

Steve, thanks for the post.

It is memmbers like this, all of you, who make this forum great.

I am just "scrathing the surface" on these phasor diagrams, and the math involved, so if my questions seem stupid to some, I apologize.

Another way for this phase shift to make your life miserable is if some one rolls the primary rotation on one of two transformers that need to be paralleled. This will lead to a 60% phase shift on the secondary between the two transformers. Not a happy scenario when the high side is a welded 138 Kv bus.

Karma already caught up with me- I took the PE exam today, and there were 2 questions dealing specifically with this, which I was really not expecting! So taking the time to think it through again well enough to explain it has already paid off

I just wish there had been more NEC questions and less freaking high-voltage fault analysis... this fall's test did not play to my strengths, that's for sure.

WFO: we had that happen once a couple years ago on a 480V plant- breakers tripping left and right, popping like popcorn! Was bad news. Turns out, the contractor, despite reassuring us TWICE that he had metered and verified rotation, had indeed swapped phases at the UPS bypass switch. The only good news was it wasn't my fault, nor my problem, as I didn't work there yet. But oh, I hear about it every time anyone suggests an online test vice scheduling an outage...

[This message has been edited by SteveFehr (edited 10-27-2006).]

I remember my first OT callout to a substation. The utility claimed to be finished repairing our 12.6KV AC1 line. I called our Power Controller and stood clear as I asked him to close the AC breaker. KABOOM! Once my pulse settled, I asked him to open the AC Bus Tie and try closing AC1 again. There weren't any loud noises that time. Since each AC bus has its own 120/208Y aux transformer, I went to an ATS fed by both for answers. I measured A to A, B to B, and C to C, not expecting any value, just that they should all be about the same. They weren't. The next step was to jog a compressor fed by a panel off of that ATS, transfer, and jog again. Seeing the pulley rotating the wrong direction was the final proof I needed to call the LD (Load Dispatcher) and ask him to please have the lines switched back the way they were. As a new guy, I wanted to be really sure before I told the utility that they had things a tad backwards.
Joe

Steve,

A phase rotation check will never determine a "rolled" primary. All a rotation check does is look at combinations like X1->X2->X3 or H1->H3->H2 it never determines which conductor is actually A phase. The only way to determine a rolled primary is to physically trace it out or to connect it up and compare the ouput voltages to a known source.

I have a location with six transformer all verified (using a TTR) as Dyn01 conncted units. But after they were installed and phase rotation was corrected, voltage measurements discovered a rolled primary on two of the units.

What is a "rolled" primary?

It's when the 3 phases are still in order, but attached to the wrong terminals. EG, instead of ABC on consecutive terminals, they might have CAB. So, if you, say, had an undervoltage on phase A, that undervoltage would show up at the B terminal instead of the A terminal. Not an issue for most power distribution, but it can be an issue for parallel systems and networks.

And yeah, my example wasn't rolling the phases, it was transposed phases and reversed rotation.

[This message has been edited by SteveFehr (edited 10-28-2006).]

Is there such a thing as a "rolled" secondary?

Like most high leg deltas are?

Yes, but it's meaningless, as the connections on the secondary define which phase is which- it's on the other end that you have to worry about rolling or transposition.

[This message has been edited by SteveFehr (edited 10-29-2006).]

What is a high leg delta?

High leg delta is where you have a 4-wire 240V Delta, where instead of all 3 phase carriers being 139V to ground like they would be in 240/139V wye, one of the 240V legs is center-tapped at the transformer, and the center-tap grounded and connected to the neutral. (+120 and -120, just like at a residential pole pig). The other delta- the "high leg" is now at a higher potential, 207V to ground.

So, it ends up where instead of having 3 phases, each 240V between them and 139V to ground, you end up with 3 phases- each of which still have 240V between them, but two of these are now 120V to ground, and the third is 207V to ground.

The advantage with this is that you can supply 3-phase loads and 120V 1-phase loads from a single distribution system without any additional transformers.

[This message has been edited by SteveFehr (edited 10-29-2006).]

How many xfmrs (pole pigs) does it take for the high leg delta? Also, what in the world is 139V to ground? Is that peak, RMS, or Stevo measurement?

240V RMS / Sqrt(3) = 139V RMS. You could call it 220/127V or 230/133V if you wanted to, all depends where in the distribution system it is, what with voltage drop at all.

Pole pigs aren't high leg delta, as least not normal 1-phase residential ones. They're a single center-tap step-down transformer that drops the line voltage to 240V with a grounded center-tap for the neutral.


[This message has been edited by SteveFehr (edited 10-29-2006).]

I've never seen 139 VAC to ground anywhere or 220/127V, for that matter. I thought utilities don't like to go lower than 0.95 PU (per-unit)? Don't 2 or 3 pole pigs make up your 3-phase Delta? Also, why would you divide 240VAC by the sqrt(3) to get 139V, when from the high leg to ground you would get 208V to ground?

That's because nobody ever runs a 240V WYE, there's not much point. (Wouldn't surprise me if there are a few out there, though!) If it's (normal) delta or wye, it's going to be 480/277V or 208/120. But if there was a 240V wye, it would be 240/139V.

Gotcha!

The 240Y/139 (actually 230Y/133)construction is the standard voltage in 230V drive isolation transformers.

Let's say you are going to parallel two transformers, but you get the primary rotation on one different from the other.
So, one will give a 30 degree shift "leading" to the secondary and the other will give a 30 degree "lagging" shift. What happens is the phases of one end up being 60 degrees out from the other, and no amount of swapping on the secondary will correct it.


You can do it with two primary phases and two pots (open delta) or 3 primary phases and three pots (closed delta). The 3 pot bank has better voltage regulation than the open one.



[This message has been edited by WFO (edited 11-11-2006).]