ABB VFD delta high leg

[kwired]

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.

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.

 

[herding_cats]

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.

 

[wncjefftino]

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.

 

[wncjefftino]

"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.

 

[kwired]

"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.

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.

 

[HeavyDuty123]

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!

Could be single phase load imbalance on the system, usually supposed to be within 5%.

 

[HeavyDuty123]

Could be single phase load imbalance on the system, usually supposed to be within 5%.

5% of kva

 

[jim dungar]

Could be single phase load imbalance on the system, usually supposed to be within 5%.

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.

 

[HeavyDuty123]

It's supposedly still a common problem with open delta

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. Advantages: Open-deltas only require the utility to install two transformers. Future Capacity can be increased by simply installing a third similar sized...

americas.hammondpowersolutions.com

"Unbalanced single phase loads can cause voltage fluctuations and additional, uneven transformer heating."

 

[kwired]

It's supposedly still a common problem with open delta

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. Advantages: Open-deltas only require the utility to install two transformers. Future Capacity can be increased by simply installing a third similar sized...

americas.hammondpowersolutions.com

"Unbalanced single phase loads can cause voltage fluctuations and additional, uneven transformer heating."

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.

 

[HeavyDuty123]

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.

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.

 

[jim dungar]

It's supposedly still a common problem with open delta

"Unbalanced single phase loads can cause voltage fluctuations and additional, uneven transformer heating."

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?

 

[HeavyDuty123]

"Open-delta systems often exhibit increased voltage fluctuation as single-phase loads are applied and removed from the sometimes unevenly sized transformers."

https://www.idc-online.com/technical_references/pdfs/instrumentation/Open_Delta_Systems_Affect_Variable_Frequency_Drives.pdf

I'm not saying it is the issue, it would be extremely easy to check though.

 

[kwired]

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.

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.

 

[HeavyDuty123]

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.

Okay.

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

 

[jim dungar]

Okay.

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

Actually they were talking about two single phase transformer where one of them has a center tap.

 

[HeavyDuty123]

Actually they were talking about two single phase transformer where one of them has a center tap.

Yep.....