Unbalanced open delta loads

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Joethemechanic said:
Is A-N utilized for anything? Is there a building at this pump station or just a control panel?

I'm wondering if xA or xB is loose inside the center tapped transformer
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They replaced the original transformer with 2 additional sets of transformers. I know new doesn't mean much anymore. Not much on the 120 volt side. Control panel only
 
synchro said:
Are you saying that you get these unbalanced current measurements when across-the-line, as well as when feeding the motor with a VFD?

A suggestion is to put a current clamp around all of the conductors feeding the VFD (i.e., A, B, and C) to verify that you don't get a significant current reading. That's just to make sure there isn't some other current path involved. It's not likely to be a problem, but it's probably easy to check.
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Done this with a Fluke data logger
 
kclark8799 said:
Control panel only
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That more or less eliminates any kind of hidden stuff. It's got to be something upstream of the utility power terminals of the transfer switch. On your end that doesn't leave much, I guess just a main breaker and some service conductors.

Is this an aerial drop, or underground from the pole?

Have you checked the transfer switch contacts?
 
kclark8799 said:
Done this with a Fluke data logger
Click to expand...

Joethemechanic said:
That more or less eliminates any kind of hidden stuff. It's got to be something upstream of the utility power terminals of the transfer switch. On your end that doesn't leave much, I guess just a main breaker and some service conductors.

Is this an aerial drop, or underground from the pole?

Have you checked the transfer switch contacts?
Click to expand...
Thanks for the ideas. I took the power company s feeder from the meter can Polaris some 1 kv gen. Cable and landed it to the main in the control panel (by passing everything on customer side.) Then had the Poco throw the jacks in on the pole. Stayed the same. Beginning to think Poco has a lead lag issue. Need proof.
 
It is not uncommon to have unbalanced VFD input currents when fed from an open-delta (see paper from Yasakawa below).
The line-to-line output impedances of the open delta will not be symmetrical. For one thing, the center-tapped transformer is typically larger and so the impedance across A and B will be lower than that across B and C from the smaller transformer. And the impedance across the "open" part of the delta between C and B will be the highest because the secondaries of both transformers are in series.
Of course, the open circuit voltage on the transformer outputs will also have a major affect on the balance between VFD input currents.

If you don't already have a line reactor and/or DC link choke in the VFD, they could help equalize the input currents.


kclark8799 said:
Everything was checked both ways. Start p reports 3 years ago showed issues. Now that we sized all gen sets using VFDs the dc buss keeps faulting the drives. Sometimes during the prime time they will not reset. Dirty power what I call it.
Click to expand...

Line reactors might also reduce voltage spikes and other distortion that could be causing such issues.

Paper frpm Yasakawa:
https://www.yaskawa.com/delegate/ge...PS.01&cmd=documents&documentName=PR.PS.01.pdf

This article is somewhat dated but still has some relevant info:
https://www.ecmweb.com/content/article/20891755/beware-of-linecurrent-unbalance-on-vfds
 
synchro said:
It is not uncommon to have unbalanced VFD input currents when fed from an open-delta (see paper from Yasakawa below).
The line-to-line output impedances of the open delta will not be symmetrical. For one thing, the center-tapped transformer is typically larger and so the impedance across A and B will be lower than that across B and C from the smaller transformer. And the impedance across the "open" part of the delta between C and B will be the highest because the secondaries of both transformers are in series.
Of course, the open circuit voltage on the transformer outputs will also have a major affect on the balance between VFD input currents.

If you don't already have a line reactor and/or DC link choke in the VFD, they could help equalize the input currents.




Line reactors might also reduce voltage spikes and other distortion that could be causing such issues.

Paper frpm Yasakawa:
https://www.yaskawa.com/delegate/ge...PS.01&cmd=documents&documentName=PR.PS.01.pdf

This article is somewhat dated but still has some relevant info:
https://www.ecmweb.com/content/article/20891755/beware-of-linecurrent-unbalance-on-vfds
Click to expand...
Supposedly all VFDs are set the same for operation with hi leg open delta if they are supplied by it.

Why just this motor when operated with the VFD, or ATL without it, on this feeder?
 
Joethemechanic said:
I thought he bypassed the VFD and did ATL running of the pump and the problem persisted
Click to expand...

I tried to clarify that with post #40 below, but I didn't seem to get a clear answer.
synchro said:
Are you saying that you get these unbalanced current measurements when across-the-line, as well as when feeding the motor with a VFD?

A suggestion is to put a current clamp around all of the conductors feeding the VFD (i.e., A, B, and C) to verify that you don't get a significant current reading. That's just to make sure there isn't some other current path involved. It's not likely to be a problem, but it's probably easy to check.
Click to expand...
Reply:
kclark8799 said:
Done this with a Fluke data logger
Click to expand...
 
synchro said:
It is not uncommon to have unbalanced VFD input currents when fed from an open-delta (see paper from Yasakawa below).
The line-to-line output impedances of the open delta will not be symmetrical. For one thing, the center-tapped transformer is typically larger and so the impedance across A and B will be lower than that across B and C from the smaller transformer. And the impedance across the "open" part of the delta between C and B will be the highest because the secondaries of both transformers are in series.
Of course, the open circuit voltage on the transformer outputs will also have a major affect on the balance between VFD input currents.

If you don't already have a line reactor and/or DC link choke in the VFD, they could help equalize the input currents.




Line reactors might also reduce voltage spikes and other distortion that could be causing such issues.

Paper frpm Yasakawa:
https://www.yaskawa.com/delegate/ge...PS.01&cmd=documents&documentName=PR.PS.01.pdf

This article is somewhat dated but still has some relevant info:
https://www.ecmweb.com/content/article/20891755/beware-of-linecurrent-unbalance-on-vfds
Click to expand...
Thank you. We have 5 percent line reactors. Do you think if the smaller transformer was changed to match the other one that this may help ? Thanks for the article, I ll get to reading.
 
kclark8799 said:
Thanks for the ideas. I took the power company s feeder from the meter can Polaris some 1 kv gen. Cable and landed it to the main in the control panel (by passing everything on customer side.) Then had the Poco throw the jacks in on the pole. Stayed the same.
Click to expand...

It sounds like the control panel in question is close to the service point. If there are other VFD's on the same transformer bank that are working but they are significantly further away, perhaps there's additional impedance in the conductors that's helping balance the currents, etc. That's just a guess without further info.
 
kclark8799 said:
Thank you. We have 5 percent line reactors. Do you think if the smaller transformer was changed to match the other one that this may help ? Thanks for the article, I ll get to reading.
Click to expand...
Increasing the size of the smaller transformer might help somewhat, but there will still be two transformer secondaries in series supplying one of the L-L voltages.
Also, it's possible that there could be issues on the MV side of the transformers in terms of voltage, impedance, etc.
I think load bank measurements could be useful to quantify how the L-L voltages and phase shifts change vs. load current.
 
synchro said:
Increasing the size of the smaller transformer might help somewhat, but there will still be two transformer secondaries in series supplying one of the L-L voltages.
Also, it's possible that there could be issues on the MV side of the transformers in terms of voltage, impedance, etc.
I think load bank measurements could be useful to quantify how the L-L voltages and phase shifts change vs. load current.
Click to expand...
I'll try a load bank. I believe Florida rural water has one. Thanks for all inputs and advices. Ken
 
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