Can someone explain this graphic

[Dennis Alwon]

What is the calculation used to get 381 v. As you can see this is a Mike Holt graphic which I am sure is correct but I don't know how it was derived.

 

[zbang]

Hmm....
277 + 120 = 397 so we're missing 16 volts...
Maybe if the phase angles between two SDSs were just right those could add up to 381, but right now my money is on Artistic License and they're simply using a vaguely credible number over 300..

 

[don_resqcapt19]

The closest I get is 386.

You have to remember there will be a 30° phase shift between the hots on the 480Y/277 system and the 208Y/120 volt system because the typical transformer use to obtain the lower voltage is a delta/wye with a 30° phase shift between the primary and the secondary. That phase shift does not permit you to just add the voltages. You have to do a calculation

 

[wwhitney]

What is the calculation used to get 381 v.

Based on the 30 degree phase shift as Don described, the law of cosines gives the voltage via this calculation:

sqrt(120² + 277² + 2*120*127*cosine(30 degrees)) = 386V

Cheers, Wayne

 

[zbang]

Maybe that's really a 115v circuit .

 

[topgone]

A 277V circuit is the neutral voltage of a 480V, 3-phase system while the 120V circuit is the neutral voltage of a 208V, 3-phase system. The utilization voltages of 480 and 208V systems are 460V and 200V respectively. Therefore, 460/1.732 + 200/1.732 = 265.58V + 115.47 ~ 381V. Crystal!

 

[roger]

From Smart$

 

[Fred B]

You have to remember there will be a 30° phase shift between the hots on the 480Y/277 system and the 208Y/120 volt system

Why the 30 deg phase shift? Just mfg, or an actual difference created by the transforming to the lower voltage? (Can you get a transformer that doesn't create the phase shift referenced).
With the 30 deg shift would you be able to simply meter between the 2 separately derived systems?
What issues (if any) would arise from even attempting to meter between the 2?

 

[don_resqcapt19]

Why the 30 deg phase shift? Just mfg, or an actual difference created by the transforming to the lower voltage? (Can you get a transformer that doesn't create the phase shift referenced).
With the 30 deg shift would you be able to simply meter between the 2 separately derived systems?
What issues (if any) would arise from even attempting to meter between the 2?

It is the construction of the transformer and I think all delta/wye have the same 30° shift.
There would be no issues connecting a meter between a phase of the 208 volt system and a phase of 480 volt system.

The only time the shift really comes into play is parallel transformers or parallel systems.

I did a project for glass manufacturer where they were disconnecting their onsite generation because of the costs to bring their boiler into compliance with the EPA rules. As part of the project, they were increasing the size of the service. The utility brought in another feeder and were intending to do a closed transition between the old and new service on the 34kV side. The engineer I was working for asked the utility if they were sure that there was no phase shift between the two feeders. They said their wasn't but the engineer did not seem convinced and he kept asking. The utility guys were getting pissed, but they finally did put a meter between the two systems...they were 30° out of phase and closing together would have tripped both feeder lines. The plant generation system was still functional, so we put all of the load on the plant generators, disconnected the original feeder and then synced the plant generation with the new feeder. That was the last time they ran their generators as the next phase of my work was to physically disconnect them.

 

[don_resqcapt19]

A 277V circuit is the neutral voltage of a 480V, 3-phase system while the 120V circuit is the neutral voltage of a 208V, 3-phase system. The utilization voltages of 480 and 208V systems are 460V and 200V respectively. Therefore, 460/1.732 + 200/1.732 = 265.58V + 115.47 ~ 381V. Crystal!

That doesn't really work...you have to do vector addition...see post #7.

 

[wwhitney]

Why the 30 deg phase shift?

In the diagram in post #7, if the 208Y/120V is created by a transformer with a 480V delta primary, then its primary side coils will be connected between the tips of the yellow, orange, and light brown vectors. The triangle defined by those tips isn't drawn in, but you can see that the blue, red, and black vectors are parallel to sides of that triangle. Which is what the transformer does, it creates a voltage on the secondary coil in phase with the voltage on the primary coil.

So the phase shift is the same phase shift you see on the primary side between L-L and L-N. Since the transformer is using the primary L-L to generate the secondary L-N, the secondary L-N is in phase with the primary L-L, rather than the primary L-N.

Cheers, Wayne

P.S. The arithmetic in post #4 is shown in post #7 as the brown vector plus the blue vector (actually the difference).

 

[Fred B]

In the diagram in post #7, if the 208Y/120V is created by a transformer with a 480V delta primary, then its primary side coils will be connected between the tips of the yellow, orange, and light brown vectors. The triangle defined by those tips isn't drawn in, but you can see that the blue, red, and black vectors are parallel to sides of that triangle. Which is what the transformer does, it creates a voltage on the secondary coil in phase with the voltage on the primary coil.

So the phase shift is the same phase shift you see on the primary side between L-L and L-N. Since the transformer is using the primary L-L to generate the secondary L-N, the secondary L-N is in phase with the primary L-L, rather than the primary L-N.

Cheers, Wayne

P.S. The arithmetic in post #4 is shown in post #7 as the brown vector plus the blue vector (actually the difference).

I See the illustration post #7, makes sense. Your description sounds as if indicating a Delta grounded primary.
But isn't the OP illustration a 480/277 wye and a 208/120 wye?
Is there still a phase shift?
Any difference in the calculations?

 

[wwhitney]

I See the illustration post #7, makes sense. Your description sounds as if indicating a Delta grounded primary.

Yes, the assumption is one of the two wye systems is derived from the other via a delta-wye transformer, as is typical.

If instead a wye-wye transformer were used, the diagram in post #7 would be different, each 120V phasor would be in line with one of the 277V phasors, no apparent phase shift.

But given the ~380V indicated in the original diagram, we can infer that a delta-wye transformer was used.

Cheers, Wayne

 

[wwhitney]

Based on the 30 degree phase shift as Don described, the law of cosines gives the voltage via this calculation:

sqrt(120² + 277² + 2*120*127*cosine(30 degrees)) = 386V

Cheers, Wayne

Typo: that should be 2 * 120 * 277 * cosine(30 degrees) inside the square root.

Cheers, Wayne

 

[LarryFine]

Think of it as a phase difference, rather than a shift. Nothing is actually changing.

 

[jaggedben]

Why the 30 deg phase shift? Just mfg, or an actual difference created by the transforming to the lower voltage? (Can you get a transformer that doesn't create the phase shift referenced).
With the 30 deg shift would you be able to simply meter between the 2 separately derived systems?
What issues (if any) would arise from even attempting to meter between the 2?

Another recent thread on this...

Explaining Phase Shift in Delta Wye circuit

Hello… I was hoping someone could help confirm my understanding of the theory behind what is happening in a circuit… and hopefully explain it better than I can? The question comes from this circuit Which can be summarized as follows: 480/277 distribution system (277 3ph Y voltage source) To...

forums.mikeholt.com

 

[ramsy]

What issues (if any) would arise from even attempting to meter between the 2?

If separately derived systems have a common primary, can that Phase-Rotation meter find distribution voltage potential that blows up in your face?

 

[Strathead]

From Smart$

View attachment 2569422

I miss him.

 

[topgone]

That doesn't really work...you have to do vector addition...see post #7.

With all the neutral effectively grounded, there must be a common reference point. Why take the complicated way when the simplest explanation is the best one? Occam's Razor tells me that should be the way, or isn't it? ("The best explanation of any phenomenon is the one that makes the fewest assumptions.")

 

[jim dungar]

With all the neutral effectively grounded, there must be a common reference point. Why take the complicated way when the simplest explanation is the best one? Occam's Razor tells me that should be the way, or isn't it? ("The best explanation of any phenomenon is the one that makes the fewest assumptions.")

It is the easiest, but the values require vector addition.
Keeping track of transformer phase shifts is a common issue when paralleling secondaries as Don pointed out in post #9. I had a similar issue when I had to explain to a customer that they wired their underground primary wrong and there was nothing we could do to our transformer to fix the problem. It was going to be another three years before they could take the required outage so they would have their backup and be able to commission their ATO scheme.