Help me understand 3Phase high-leg open delta voltages

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jvanick

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Crystal Lake, IL
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Building Engineer
I'm trying to spec out buck boost transformers for some sensitive printing equipment that requires voltages less than 240V. Our Power Company has intermittently been sending high voltages to us (phase to phase as high as 259V at times)

High leg delta, the "B" phase is high.

Recent Measurements

A-Ground: 127
B-Ground: 219
C-Ground: 127

A-B: 254
B-C: 232
A-C: 254

2 questions:

1. Is the B-C voltage being unbalanced on a open delta service a concern - or is that just the way it is?

2. when wiring the pair of buck boost transformers, does where the high leg connect matter? The transformer installation manuals say nothing about where to connect a high leg.

My electrician said "this is a good question for an online forum" as he was under the impression that the phase-to-phase voltages should be in balance.

Thank you!
 
If the power company has given you a "High-Leg" service you have little choice but to live with it. It is not a "concern" but needs to be kept in mind.
What are the requireemtns for your equipment ? 240 3 phase or 240/120 ie: is a neutral required ?
 
No neutral required, this is just a 3-wire + Ground connection.

Printing manufacturer spec'd the transformers-- thye're 1kva units.

Just need to know if it matters where to have the electrician wire the high leg.
 
Open delta services have higher impedance and higher voltage drop across the 'open jaw'.

The fact that the L-L voltages are so different suggests that the supply transformer is very heavily loaded. IMHO the unbalance is large enough to be a significant issue for things like across the line three phase motor loads.

Three phase buck-boost with two transformers is also called 'open delta'. With respect to L-L voltages, it doesn't matter which terminal of the buck-boost pair goes to which supply terminal.

IMHO it is good practice to connect the B-B transformers to minimize the output voltage relative to ground and to directly load the supply transformers.

Looking at figure CC below, I select terminals as follows:
Terminal 1 input to terminal B of your system, the 'high leg'
Terminal 2 input to terminal A of your system, the 'common leg' between the two supply transformers
Terminal 3 input to terminal C of your system, the 'far 120V leg'


Buck-Boost+1.jpg
 
Thank you, that's extremely helpful.

Our electrician was seeing the same inbalance even with all the loads disconnected.. he turned off all the main breakers to the facility and measured directly across the busses where the 500mcm cable is tied down from the transformers.

What's really interesting is these problems JUST started occurring about a month ago, it's been fine the past 3 years in this facility.
 
Are there any other customers on that transformer? If you are seeing that much imbalance with no loads connected, then IMHO the utility company needs to be looking at their hardware.

-Jon
 
jvanick said:
Thank you, that's extremely helpful.

Our electrician was seeing the same inbalance even with all the loads disconnected.. he turned off all the main breakers to the facility and measured directly across the busses where the 500mcm cable is tied down from the transformers.

What's really interesting is these problems JUST started occurring about a month ago, it's been fine the past 3 years in this facility.
Click to expand...
You might contact the POCO to come and check voltages and their transformers.
 
jvanick said:
Recent Measurements

A-Ground: 127
B-Ground: 219
C-Ground: 127

A-B: 254
B-C: 232
A-C: 254
Click to expand...
Something is off in the above, the voltages are not internally consistent.

Ground is the midpoint of the A-B voltage as expected. Then the voltage B-Ground is consistent with all the L-L voltages being the same, 254V. Which is true for A-B, and B-C, but the B-C voltage is 22V less at 232V.

Another way of putting this is that if you hadn't measured B-Ground, we could calculate it from all the other measurements. The B-C voltage being less than A-B and A-C means the voltage angle BAC is less than 60 degrees, it turns out to be 55 degrees. And then the calculated voltage B-Ground comes to 208V, not the 219V measured.

Cheers, Wayne
 
wwhitney said:
Something is off in the above, the voltages are not internally consistent.

Ground is the midpoint of the A-B voltage as expected. Then the voltage B-Ground is consistent with all the L-L voltages being the same, 254V. Which is true for A-B, and B-C, but the B-C voltage is 22V less at 232V.

Another way of putting this is that if you hadn't measured B-Ground, we could calculate it from all the other measurements. The B-C voltage being less than A-B and A-C means the voltage angle BAC is less than 60 degrees, it turns out to be 55 degrees. And then the calculated voltage B-Ground comes to 208V, not the 219V measured.

Cheers, Wayne
Click to expand...

Good catch.

I'd simply assumed that the phase angles were screwed up, but I just played with the geometry and I agree you can't get that 219V B-N with 254V A-B _and_ 232V C-B. At least not with pure 60Hz sine waves.

-Jon
 
I just re-read the voltages:

A-G: 125
B-G: 217
C-G: 125

A-B: 252
A-C: 252
B-C: 230

I also put the meter in frequency mode...

A-G: 60hz
B-G: 150hz?! < could that even be close to correct?
C-G: 60hz

I don't have anything to see how pure the sinewave is, or if my meter is truly correct or not.

The plot thickens?
 
Now I'm totally guessing here:

Significant 3rd harmonic distortion (possibly from the primary side of things). Normally 3rd harmonic 'circulates' in a delta, but this is an open delta, so the 3rd harmonic voltage shows up more prominently on the 'open jaw'. The 150Hz is really 180 Hz confusing the meter, and the voltage measurement is from a composite waveform with huge 3rd harmonic component.

Sorry, I don't have anything other than guesses for you. I think you need to invest in an oscilloscope.

-Jon
 
winnie said:
Now I'm totally guessing here:

Significant 3rd harmonic distortion (possibly from the primary side of things). Normally 3rd harmonic 'circulates' in a delta, but this is an open delta, so the 3rd harmonic voltage shows up more prominently on the 'open jaw'. The 150Hz is really 180 Hz confusing the meter, and the voltage measurement is from a composite waveform with huge 3rd harmonic component.

Sorry, I don't have anything other than guesses for you. I think you need to invest in an oscilloscope.

-Jon
Click to expand...
Jon
My choice would be a Fluke 43B PQ Analyzer. chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/https://xdevs.com/doc/Fluke/Fluke%2043B%20Service.pdf
 
ATSman said:
Jon
My choice would be a Fluke 43B PQ Analyzer. chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/https://xdevs.com/doc/Fluke/Fluke%2043B%20Service.pdf
Click to expand...
I forgot to mention, I picked up mine on ebay, used for a very reasonable price.
 
I'm gonna see if the electricians that I called have one of these I can borrow or rent. $700-$2500 is a lot of money if it's just a power company issue.

Lineman that was just out here says it is bad programming in the smart capacitors they installed a month ago, so at least I guess we have something to go on.

Also interesting, A true-rms multimeter (Fluke 336) shows different results than my ancient Fluke 75 when measuring phase to phase. the phases are much more balanced with the true-rms meter (a-b: 248, a-c: 248, b-c: 253) vs (a-b: 254 , a-c: 254, b-c: 231).
 
Sounds like a secondary issue
Is you drop over head from the utility transformers. If so how many wires. Did they bring a netural to your service disc. Even though you don't need it.

See if there is two primary lines feeding two transformers. With a three wire drop.
 
Tulsa Electrician said:
Is you drop over head from the utility transformers. If so how many wires. Did they bring a netural to your service disc. Even though you don't need it.

See if there is two primary lines feeding two transformers. With a three wire drop
Click to expand...

it's underground, but yes, there's 2 transformers with 3 wires underground to the transformers. And it definitely is an open delta service according to the power company.
 
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