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
#130682 - 10/18/0603:43 PMRe: Phase shift on transformers
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
#130685 - 10/19/0606:47 AMRe: Phase shift on transformers
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).]
#130687 - 10/21/0601:05 PMRe: Phase shift on transformers
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).]
#130689 - 10/25/0612:14 PMRe: Phase shift on transformers
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).]