Phase Imbalance on 3 Phase Service to Pumps

[Jon456]

They saved cost of one transformer, especially if load is not there to warrant the third transformer. You have added a 100 amp motor that was not there when transformer bank was originally constructed, it may be worth having it now.

That's a very good point. But the new 100A motor was there when the transformers were replaced and upgraded following the lightning strike. PoCo replaced both transformers, and did so with larger units. The reason for the upsized transformers was specifically to help resolve our pump power issues. If we have three primary phases at the pole, I don't understand why they didn't switch us over to a 3-transformer "true' 3-phase system at that point, unless it's because they didn't recognize the issues that were raised by Augie (for those who haven't read every post in this thread, Augie described a similar problem with 3-phase power to submersible pumps that was solved by supplying them with "true" 3-phase power).

The only time they install one large transformer and one or two smaller ones is when there is significant line to neutral loads because it is that particular transformer that will carry those loads. If majority of load is line to line this is not an issue.

What is considered "significant". Our current 120V line-to-neutral loads are roughly 10A, the majority of which are from two 120V exterior lighting branch circuits that are on two separate phases. We will soon be adding an additional 1,000VA lighting load, but that will be 240V so it shouldn't affect our line-to-neutral loading.

So is 10A of 120V line-to-neutral current (rougly split across the two phases) considered significant compared to our smallest 3-phase pump load of 60A (old pump running alone)?

 

[mivey]

Ok, these are the best photos I could get of the transformers and connections on the pole. I could not read any numerical markings on the transformers other than the stenciled "25" on both cans which I believe to be a PoCo identifier for this location.

You have a 3-phase primary delta system with no neutral. The transformers are both 25 kVa units. They are connected open delta - open delta. The secondary has a center-tap ground for the 120/240 volt loads and that also gives you a high leg.

If both pumps run at the same time, you will definitely have a higher unbalance as the units will be loaded above their nominal rating. The POCO should supply you with a 3rd 25 kVA unit (they could also give you two 37.5 kVA units but I see no reason why they would). I could give you a whole blurb on negative sequence impedances that will be produced even if the transformers are perfectly matched and the primaries are perfectly balanced but it really does not matter since these two transformers are being pushed too hard anyway if you are sensitive to unbalance.

I wonder why they did not put in two 37.5s instead of the two 25's? I am guessing the did not expect you to be at 160 amps or that the motors would be that sensitive to unbalance. Although the unbalance figures you have shown are within normally acceptable ranges, I would plead your case based on you having the transformers at 133% nominal (66.5 kVA on a 50 kVA bank).

 

[mivey]

That's a very good point. But the new 100A motor was there when the transformers were replaced and upgraded following the lightning strike. PoCo replaced both transformers, and did so with larger units. The reason for the upsized transformers was specifically to help resolve our pump power issues. If we have three primary phases at the pole, I don't understand why they didn't switch us over to a 3-transformer "true' 3-phase system at that point, unless it's because they didn't recognize the issues that were raised by Augie (for those who haven't read every post in this thread, Augie described a similar problem with 3-phase power to submersible pumps that was solved by supplying them with "true" 3-phase power).

The open-delta is not the deal-killer. The deal-killer is when they load the open-delta close to its nominal rating. The usual method to solve open-delta unbalance issues is to over-size the transformers. In your case, the open delta appears to be under-sized.

So is 10A of 120V line-to-neutral current (rougly split across the two phases) considered significant compared to our smallest 3-phase pump load of 60A (old pump running alone)?

Not really since the bank can handle more than ten times that amount at the nominal rating. The problem is that it will only tend to aggravate your unbalance issue.

 

[mivey]

...I could give you a whole blurb on negative sequence impedances...

make that current

 

[Open Neutral]

As explained to me; open delta is the choice when most of the load is single phase; and there's some 3-ph.

When it's mostly 3-ph, and a little single-phase; use conventional 3-xfmr wye or closed delta.

 

[Jon456]

First off, I'd really like to thank Mivey and all others for your knowledge and assistance. This thread has been very educational and useful.

The transformers are both 25 kVa units.

Even though the one on the left is physically larger than the one on the right?

The open-delta is not the deal-killer. The deal-killer is when they load the open-delta close to its nominal rating. The usual method to solve open-delta unbalance issues is to over-size the transformers. In your case, the open delta appears to be under-sized

Based on your observation that we have a 50kVA bank, I agree that the transformers are undersized to handle both pumps running simultaneously (66.5kVA). But when the pumps are run singularly (41.5kVA or 24.9kVA), the bank is not undersized. Yet we're still seeing a current imbalance on one leg when running either pump alone. Why would that be, if not an issue related to the way the 3-phase service is derived?

Either way, I think we have a case for getting the PoCo to install a third transformer. Particularly when considering, as "Open Neutral" pointed out, our requirement is predominantly for 3-phase power, with only small single single-phase loads.

 

[mivey]

First off, I'd really like to thank Mivey and all others for your knowledge and assistance. This thread has been very educational and useful.


Even though the one on the left is physically larger than the one on the right?


Based on your observation that we have a 50kVA bank, I agree that the transformers are undersized to handle both pumps running simultaneously (66.5kVA). But when the pumps are run singularly (41.5kVA or 24.9kVA), the bank is not undersized. Yet we're still seeing a current imbalance on one leg when running either pump alone. Why would that be, if not an issue related to the way the 3-phase service is derived?

Either way, I think we have a case for getting the PoCo to install a third transformer. Particularly when considering, as "Open Neutral" pointed out, our requirement is predominantly for 3-phase power, with only small single single-phase loads.

Maybe I was not clear. With balanced loads and equal impedances, the 3- pot bank will have balanced voltages but the open delta will not. By over-sizing the open delta, the impact of the open delta configuration can usually be reduced to acceptable levels.

They could increase the size of your 2-pot bank enough to get the unbalanced down to a level that is acceptable. The fact that you have a 3-phase primary instead of v-phase makes the obvious solution to be adding the 3rd pot.

The size of the tank is not always an indicator of relative kVA size. TTwokV Aerelative k

 

[kwired]

As explained to me; open delta is the choice when most of the load is single phase; and there's some 3-ph.

When it's mostly 3-ph, and a little single-phase; use conventional 3-xfmr wye or closed delta.

Amount of load is also a factor.

We have hundreds of open delta three phase services around here that supply limited three phase loads such as irrigation machines. The max load is often 15 amps or so @ 480 volt. POCO would rather only supply two transformers for these services to keep their costs down and 2 - 10KVA transformers is more than enough to supply the load. If there is an electric well over 25 hp they almost always provide 277/480 wye. There are some places where there are exceptions, sometimes they bring one phase from one direction and another phase from another to get two phases plus MGN to create an open delta instead of building a long run of three phase primary. The wild leg on 480 delta is about 415 volts to ground, other two phases are 240 to ground.

 

[Ralph B. Hoffman]

Voltage Unbalance Gives Rise to Very Large Current Imbalances and Motor Overheating

Voltage Unbalance Gives Rise to Very Large Current Imbalances and Motor Overheating

General Remarks:
1. A very small voltage imbalance will give rise to large current imbalances in three phase induction motors. This is noted in NEMA MG 1 14.35.
2. The transformer connection shown, an open delta, produces inherently unbalanced voltages under load. An equivalent circuit for the transformer connection will show this but is tedious to reduce to formulas.
3. You are probably being flubbed off by a non engineer, i.e. a field tech who does not appreciate the technicalities.
4. The solution is a three phase transformer bank for the three phase loads and a single phase transformer for the single phase load.
5. There is an interesting derating curve for motors subject to unbalanced voltages at Joliet Technologies web site from the EASA handbook.
6.There are solid state starters with current balancers that are suited to this situation. However, you cannot use a bypass contactor when using them to phase balance currents. The AB SMC dialog plus with 825 sensor, if I recall correctly, offers this current balancing feature.

 

[bearsfan]

This is very typical for those of us out here in Communist Edison rural area, basically most of northern Illinois that is west of the burbs. Edison only EVER delivers open delta transformer banks because they are too cheap to run the 3rd primary wire. And in many, many, many locations they won't even supply 2 primary phases, and instead provide a Ronk rotary phase converter and only single phase service for farmers, who have been building larger and larger grain systems. In fact, just finishing an upgrade top a farm that now has 3 phase converters next to an 800 amp single phase service.

With that said, we have been fighting larger and larger imbalances in our open delta banks for several years now. We have learned that upstream load can change the voltage on down stream transformer banks quite drastically. One farm last fall the imbalance was so great that the 75HP fan motors in a new grain dryer wouldn't even start because the electronic motor overloads were too sensitive for the imbalance of current on the 3 phases. Million dollar grain system, and couldn't start the dryer, my customer was really impressed.

We have been searching for any clue how to help these imbalances. ComEd's solution is to have the farmer pay for the line extensions themselves, which on average run $1000 per wire span from the source. We have had several of these that had no choice but to pay, and wrote checks for $100,000.00 just tog et reliable power.

I welcome any ideas that you have to help this pump house, or any of my grain systems.
Mike

 

[kwired]

This is very typical for those of us out here in Communist Edison rural area, basically most of northern Illinois that is west of the burbs. Edison only EVER delivers open delta transformer banks because they are too cheap to run the 3rd primary wire. And in many, many, many locations they won't even supply 2 primary phases, and instead provide a Ronk rotary phase converter and only single phase service for farmers, who have been building larger and larger grain systems. In fact, just finishing an upgrade top a farm that now has 3 phase converters next to an 800 amp single phase service.

With that said, we have been fighting larger and larger imbalances in our open delta banks for several years now. We have learned that upstream load can change the voltage on down stream transformer banks quite drastically. One farm last fall the imbalance was so great that the 75HP fan motors in a new grain dryer wouldn't even start because the electronic motor overloads were too sensitive for the imbalance of current on the 3 phases. Million dollar grain system, and couldn't start the dryer, my customer was really impressed.

We have been searching for any clue how to help these imbalances. ComEd's solution is to have the farmer pay for the line extensions themselves, which on average run $1000 per wire span from the source. We have had several of these that had no choice but to pay, and wrote checks for $100,000.00 just tog et reliable power.

I welcome any ideas that you have to help this pump house, or any of my grain systems.
Mike

As soon as one customer pays for the three phase line you will have several customers gladly upgrade their services to three phase and connect to said line. Kind of happens somewhat around here also.

As far as alternatives a VFD would supply balanced phases to the motor, even if input is not balanced, it also gives you benefits of soft starting the load, and no sudden voltage changes when starting motors. Would also make good phase converter at places that do not have three phase. Units need to be oversized when only single phase input is used.

 

[Open Neutral]

As far as alternatives a VFD would supply balanced phases to the motor, even if input is not balanced, it also gives you benefits of soft starting the load, and no sudden voltage changes when starting motors. Would also make good phase converter at places that do not have three phase. Units need to be oversized when only single phase input is used.

I had never pondered same, but there is no reason a VFD can not be smart enough to balance load currents. Not seen it in data sheets but was never looking for it either. And yes, with enough oversizing, they'll get you out of needing three-phase service period. But you will need at least DC line chokes and load reactors to protect the motors.

I know the rule¹ but in the job I'm working; PG&E wants $15K less for 120/240 open delta than 208/120 wye... and they are feeding us with all three primaries, not two.




1] Of course it makes no sense; it's a tariff..