ABB VFD delta high leg

wncjefftino said:
If my memory serves me right there are 2 transformers on the pole. There is 1 may be 2 other business besides us tapped to the larger of the two transformers and the other small one going to us.

I emailed our POCO rep., and he said that the voltages I gave him were within their voltage +/- 10%. Asked if we were seeing equipment issues. I replied with the forementioned VFD and speaking to the refrigeration guy the phase monitor is tripping our glycol system as well. Waiting on his reply.
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222 is within 10% of 240. But is still too much imbalance, particularly for three phase motor that is at/near rated loading. I think most motor manufacturers recommend 1-2% max voltage imbalance, particularly at full load rating. Derating the motor is recommended if imbalance is expected to be more than 3% for any significant time period.

Google suggests many utility companies consider up to 3% imbalance as being acceptable. Your no load voltage likely is rather balanced though. Still have a feeling the high leg transformer is overloaded or is at certain load conditions anyway. There likely is more load now than when the bank was first put into service.
 
We recommend a line reactor in front of the drive size to amperage of load, then the drive must be at least 1.73x larger. I go a solid 2x. go 2.5 on loads that are constant torque like compressors.
 
kwired said:
222 is within 10% of 240. But is still too much imbalance, particularly for three phase motor that is at/near rated loading. I think most motor manufacturers recommend 1-2% max voltage imbalance, particularly at full load rating. Derating the motor is recommended if imbalance is expected to be more than 3% for any significant time period.

Google suggests many utility companies consider up to 3% imbalance as being acceptable. Your no load voltage likely is rather balanced though. Still have a feeling the high leg transformer is overloaded or is at certain load conditions anyway. There likely is more load now than when the bank was first put into service.
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"There likely is more load now than when the bank was first put into service."

100% agree.
All I've heard from the POCO is crickets since the initial response to my inquiry.
 
wncjefftino said:
"There likely is more load now than when the bank was first put into service."

100% agree.
All I've heard from the POCO is crickets since the initial response to my inquiry.
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That's too bad. Most POCO's I deal with would respond at very least. Most would also not really hesitate to increase the size of that high leg unit or even both units if there were more evidence that they both were too small.
 
wncjefftino said:
Hi,

I got to checking the system without using the VFD. Here's the following:

A phase 10.7A
B phase 8.7A
C phase 8.7A

AB phase 240V
BC phase 230V
AC phase 240V

So that voltage difference is 4.2%

I went over to 2 other 3 phase systems and measured voltage and found the same as above.

cooler outside unit
AB 238V
BC 226V
AC 238V

chiller
AB 240V
BC 230V
AC 240V

Do I need to call the POCO?

Thanks!
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Could be single phase load imbalance on the system, usually supposed to be within 5%.
 
Devin Hanes said:
Could be single phase load imbalance on the system, usually supposed to be within 5%.
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The 5% value is for closed delta transformer wound on a single core. It is not applicable to open delta configurations using separate transformer. Open delta single phase loading restrictions needs to come from the utility.
 
Devin Hanes said:
It's supposedly still a common problem with open delta

americas.hammondpowersolutions.com

What are the advantages and disadvantages of an open delta transformer configuration ?

Open-delta configurations are typically deployed by utilities, typically to loads with little single-phase requirements.
americas.hammondpowersolutions.com americas.hammondpowersolutions.com
1751995472655.png
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Devin Hanes said:
"Unbalanced single phase loads can cause voltage fluctuations and additional, uneven transformer heating."
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I have to disagree with the information mentioned before even opening that link.

A lot of open delta, particularly 120/240 systems, have large 120/240 transformer for supplying 120/240 loads and have a small transformer to create the "third phase" that was only intended to supply a limited amount of VA to be able to run three phase loads.

If the need is truly for higher power (typically when the majority of load is motors or other high power three phase connected loads) then they generally will go full delta and all three transformers are same size.

A somewhat rare situation is remote locations where they decided to save a little $$ by running one less primary conductor and use an open delta. In that case if you have significant amount of three phase load then you may need to derate the transformers, but they will still get the job done if sized correctly.

One other situation is single/limited load applications. Say a pumping station that doesn't have too high of a VA rating. Those they may opt for open delta to save $ on equipment, but there comes a point where going full delta is still better choice overall if all three primary phases are present. This point not only looks at the VA rating of the load but duty cycle of that load as well. Something that doesn't run all that often may be determined to be ok to go with the open delta where something that runs nearly constantly may be determined is best to be put on full delta.
 
kwired said:
I have to disagree with the information mentioned before even opening that link.

A lot of open delta, particularly 120/240 systems, have large 120/240 transformer for supplying 120/240 loads and have a small transformer to create the "third phase" that was only intended to supply a limited amount of VA to be able to run three phase loads.

If the need is truly for higher power (typically when the majority of load is motors or other high power three phase connected loads) then they generally will go full delta and all three transformers are same size.

A somewhat rare situation is remote locations where they decided to save a little $$ by running one less primary conductor and use an open delta. In that case if you have significant amount of three phase load then you may need to derate the transformers, but they will still get the job done if sized correctly.

One other situation is single/limited load applications. Say a pumping station that doesn't have too high of a VA rating. Those they may opt for open delta to save $ on equipment, but there comes a point where going full delta is still better choice overall if all three primary phases are present. This point not only looks at the VA rating of the load but duty cycle of that load as well. Something that doesn't run all that often may be determined to be ok to go with the open delta where something that runs nearly constantly may be determined is best to be put on full delta.
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It's Hammond transformer company, pretty major transformer company.

It seems you may have taken what I said to mean you can't run much of single phase loads, but the issue raised is just imbalance.
 
Devin Hanes said:
It's supposedly still a common problem with open delta

"Unbalanced single phase loads can cause voltage fluctuations and additional, uneven transformer heating."
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That is an FAQ written by a transformer manufacturer that wants you to purchase one of their units. It is written as if they are talking about all open-deltas when thry are specifically referring to high leg open deltas.

Yes, unbalanced loading can cause unbalanced voltages on most transformers. But, properly sized and applied transformers can handle unbalanced loads.
The supposed loss of capacity ignores the fact of the actual component selection. It is not a surprise that 2 is less than 3, but who cares if you only need 1.75?
 
Devin Hanes said:
It's Hammond transformer company, pretty major transformer company.

It seems you may have taken what I said to mean you can't run much of single phase loads, but the issue raised is just imbalance.
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It is a problem with single core units. Most open delta is made up of two individual single phase transformers. If sized correctly for the load, they are pretty stable. Most of them are POCO transformers but doesn't have to be. What I do see is something that once was sufficient for the load has had more load added over time and now isn't what would be used if doing a new install, particularly when it is POCO transformers. If it is customer owned units, there may be more chance they are tripping overcurrent devices if things are well protected. POCO overcurrent protection is often rather conservative and will allow a certain amount of overloading, even if voltage drops as a result.
 
kwired said:
It is a problem with single core units. Most open delta is made up of two individual single phase transformers. If sized correctly for the load, they are pretty stable. Most of them are POCO transformers but doesn't have to be. What I do see is something that once was sufficient for the load has had more load added over time and now isn't what would be used if doing a new install, particularly when it is POCO transformers. If it is customer owned units, there may be more chance they are tripping overcurrent devices if things are well protected. POCO overcurrent protection is often rather conservative and will allow a certain amount of overloading, even if voltage drops as a result.
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Okay.

Just to clarify though, Hammond was specifically talking about two single phase transformers in open-delta. It's a brief page.
 
Devin Hanes said:
Okay.

Just to clarify though, Hammond was specifically talking about two single phase transformers in open-delta. It's a brief page.
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Actually they were talking about two single phase transformer where one of them has a center tap.
 
Hey folks,

Wanted to update you with the latest.
The problem continues. Operators have let me know that the "single phasing" of the vfds has spread to 2 other brands of VFDs. I went there Tuesday and the voltage is the following:

AB 247V
AC 246V
BC 228V
This is an imbalance of 5.1%

I come back on Friday to shut the building down to get a voltage reading from the pole without the building load and voltage is

AB 245V
AC 244V
BC 243V
Imbalance of 0.5%

Turn power back on
AB 243V
AC 242V
BC 240V
Imbalance 0.7%

I check a VFD and I still have it single phasing on input. Zero current on A phase.

I decide to let some time lapse and recheck. About an hour and recheck a couple more times.

AB 243V
AC 241V
BC 239V
Same problem on different VFD

POCO has not been helpful, stating the voltage is within specs.
 
wncjefftino said:
Hey folks,

Wanted to update you with the latest.
The problem continues. Operators have let me know that the "single phasing" of the vfds has spread to 2 other brands of VFDs. I went there Tuesday and the voltage is the following:

AB 247V
AC 246V
BC 228V
This is an imbalance of 5.1%

I come back on Friday to shut the building down to get a voltage reading from the pole without the building load and voltage is

AB 245V
AC 244V
BC 243V
Imbalance of 0.5%

Turn power back on
AB 243V
AC 242V
BC 240V
Imbalance 0.7%

I check a VFD and I still have it single phasing on input. Zero current on A phase.

I decide to let some time lapse and recheck. About an hour and recheck a couple more times.

AB 243V
AC 241V
BC 239V
Same problem on different VFD

POCO has not been helpful, stating the voltage is within specs.
Click to expand...
So with the power off the imbalance goes away, that leads me to suspect the customer is incorrectly loading the bank or you have some hi resistance failing connections. Some photos would help.
When I have fixed this exact problem it was by either moving the loads around or having the POCO close the delta;
Consider there are 4 main types of loads you can have on a typical utility 240 hi-leg delta made from 3 transformers on a pole
(In theory 5 types but I digress)
  • 120V loads (two wire),
  • 120/240 3-split phase loads (three wire),
  • single phase 240 loads (two wire)
  • three phase 240 loads (three wire).
Where B is the wild leg;
A-C is the split phase lighting pot,
A-B is a power pot; and B-C is a power pot
the first two kinds of loads
120V loads and 120/240 3-wire split phase loads
are only on the A-C winding so I calculate them differently

Where as the third type, straight 240 single phase loads, can be on any of the pots A-B, A-C, or B-C,
these loads used to be less common but are becoming very common, loads such as server racks or single phase 230V equipment.

The last type 240 three phase loads (the most common) evenly load all three pots.

Now if we eliminate the B-C, pot and make it a open hi-leg delta the kVA rating of the entire bank degrades.
Any non-three phase loads that use both transformers such as single phase 240 loads connected 'B-C' across the open set or the 5th type a 208V load running B-Neutral degrade the kVA rating even more and can cause voltage instability.
I have never seen the 208V L-N done intentionally but I do come across single phase 240 loads on the open set (B-C), moving those loads to the power transformer A-B improves the voltage stability.
 
Maybe I missed something, but didn’t the op say the phase loss moved with the changing of the phase? Sounds like it’s in the drive, possibly surge protection on the front end blown and open.
 
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