Phase Imbalance on 3 Phase Service to Pumps

[Jon456]

While I am committed to doing whatever it takes to diagnose this problem, in the back of my head I still remember the words of Augie:

Some of the far more learned than I can explain what I am referencing, but over the years I have encountered several pump situations which have 2 pot 240/120 delta 3 phase supplies which for reasons beyond my comprehension reek havoc with pumps, especial submersible pumps. We had a water-crest farm in our area that had 20 or 30 pumps that were constantly going out. The brainiacs from TVA were called in and concluded that due to the nature of the loads, only a "true" 3 phase system with 3 phase primary and 3 pots would alleviate the problem. At much expense POCO changed the system and the problems disappeared. The engineers had a great explaination which was way over my head.

I sure wish I knew what the explanation of the engineers were. :?

So if it does turn out that our problem is caused by the 2-transformer delta 3-phase supply, my original questions still stand:

1. Does the PoCo have an obligation to do upgrade our service to a "true" 3-transformer 3-phase system?
2. If the PoCo does upgrade us to "true" 3-phase service, will we still be able to run our 120V panel off one of the 3 hot legs?

 

[Open Neutral]

1. Does the PoCo have an obligation to do upgrade our service to a "true" 3-transformer 3-phase system?
2. If the PoCo does upgrade us to "true" 3-phase service, will we still be able to run our 120V panel off one of the 3 hot legs?

I'm sure they will be HAPPY to do whatever you need..just $ign here.

The usual wye would be 208/120. But you already have 240V not 208 motors. Not sure what they would do here. I can see transformers tapped to be 240 P-P but then you'd get ~140V P-N. You could use a tiny 240-120V stepdown, or maybe a buck. [The technical answer is easy; it's what the Code requires that I'll pass on...]

But I really agree with the post above:

Rotate one motor feeder but not the other and see what happens.

Take careful notes on what phase voltages/currents are before and after.

Also, do you have a true RMS meter? VFD's/soft starters are not symmetric loads and that can make meters lie to you. Headscratchers are even less phun when your test equipment is lying to you.

 

[Jon456]

But I really agree with the post above: Take careful notes on what phase voltages/currents are before and after.

I will do that, but I'll have to wait until we start getting some rain.

Also, do you have a true RMS meter? VFD's/soft starters are not symmetric loads and that can make meters lie to you.

My meters are Fluke, but they're in the truck and I can't recall the model numbers. They are true RMS. Also, as I stated earlier, the soft-starter for the new pump has a built in electro-mechanical contactor that bypasses the SCRs after ramp-up. So for the new motor at least, the soft-starter is not a factor.

 

[mivey]

This may be the issue...Here are the measurements...

That may be your issue since the current unbalance can be 6-10% higher than the voltage unbalance but if you are below 2% voltage unbalance at load, I would not expect an issue as long as you are only about 95% loaded.

Your voltage unbalance is:
Unloaded = 0.66%
Pump 1 operating = 1.62%
Pump 2 operating = 0.54%

While I am committed to doing whatever it takes to diagnose this problem, in the back of my head I still remember the words of Augie:

Get me the transformer sizes (the lettering on the tanks may show them..10, 15, 25, 37.5, 50, etc) and we'll see what the calcs show.

Does the PoCo have an obligation to do upgrade our service to a "true" 3-transformer 3-phase system?

No. There is no requirement to give you three transformers. It would help your plea if there is a 3-phase primary line. If you only have a v-phase line you are probably out of luck unless the three phase primary is REALLY close (hundreds of feet).

They should give you an adequately sized bank and from what you have shown, the voltage unbalance falls within what they would probably consider acceptable limits. 2-3% unloaded is considered acceptable by many utlities but you could make a plea if above 2%. If you are below 2% it is a tougher case but it will just depend on the utility.

If the PoCo does upgrade us to "true" 3-phase service, will we still be able to run our 120V panel off one of the 3 hot legs?

You have a delta service so you have 120/240. They can provide you with a 3-phase, 3-transformer delta bank and you will have the same 120/240 available. I don't see why they would replace the bank with a 120/208 3-phase bank if you tell them you need 120/240.

How many primary wires do you have? From your picture, I can't tell if the center conductor is a neutral that was just dead-ended on suspension insulators or what because I don't see the neutral elsewhere on the pole (kind of fuzzy).

Can you take a clearer picture of all four sides? Also of the lettering on the transformers?

 

[mivey]

It seems that with a 15% phase imbalance...

Your data does not support that. Do not use the L-G voltages but use the L-L voltages instead. As for the current, I saw a max of 8.72% unbalance.

% Unbalance = max(reading - average reading)/average reading * 100

 

[kwired]

I'm sure they will be HAPPY to do whatever you need..just $ign here.

The usual wye would be 208/120. But you already have 240V not 208 motors. Not sure what they would do here. I can see transformers tapped to be 240 P-P but then you'd get ~140V P-N. You could use a tiny 240-120V stepdown, or maybe a buck.

Or they could just add one more transformer to what is already there and have a full delta bank. Then you still have exactly what is there plus the capacity of the third transformer. They will need to bring in the third primary phase.

 

[ptonsparky]

Or they could just add one more transformer to what is already there and have a full delta bank. Then you still have exactly what is there plus the capacity of the third transformer. They will need to bring in the third primary phase.

Most likely not for free, unless it is real close.

 

[Open Neutral]

Or they could just add one more transformer to what is already there and have a full delta bank. Then you still have exactly what is there plus the capacity of the third transformer. They will need to bring in the third primary phase.

My bad; I was thinking 3ph wye was the upgrade. I'm working on a job where PG&E will charge us $60K for 25KVA 3ph wye; but we can get delta for a mere $43K. The irony is -- they are pulling all three primaries for the delta....

My guess from the photo is he has all three phases.

But I don't see a MGN below the primaries???

 

[Jon456]

Your data does not support that. Do not use the L-G voltages but use the L-L voltages instead. As for the current, I saw a max of 8.72% unbalance.

% Unbalance = max(reading - average reading)/average reading * 100

I was using the L-L voltages and L-L currents (I posted the L-G voltages for completeness). But I was not comparing against the average current on all three legs because I thought the high leg would improperly bias the average; I was comparing the high leg against the average of the other two legs. In other words:

% Unbalance = (L2 - (L1+L3)/2) / ((L1+L3)/2) * 100

(where L2 is the high leg)

Using the numbers I posted earlier, my calculation (without averaging in the high leg) showed an 11% - 13% current imbalance, but I have taken other measurments where the current imbalance was as high as 15% (again, using my formula). If I use your formula (averaging all three legs), then I see the same numbers as you: 7% - 9%. Which still seems too high to me.

 

[Jon456]

Get me the transformer sizes (the lettering on the tanks may show them..10, 15, 25, 37.5, 50, etc) and we'll see what the calcs show.

How many primary wires do you have? From your picture, I can't tell if the center conductor is a neutral that was just dead-ended on suspension insulators or what because I don't see the neutral elsewhere on the pole (kind of fuzzy).

Sorry, I took that photo with my phone's camera (and then scaled it down to post here). I'll go back out today and take some better photos. I'll try to get the markings, and other connection details. But it's difficult because of the viewing angles: the pole is mounted at the top of a levy so I can step back too far or I'm down the embankment and in the mud/water. I should have a bucket truck available next week so then I can get much better photos.

In any case, that is probably one of the obstacles to getting the PoCo to pull another phase (if indeed we need another phase line): the high tension lines coming to that pole are spanning about 200' from a pole on the opposite side of a creek.

I can't imagine that the transformers are undersized considering the PoCo just up-sized them after our previous transformers were knocked out in a storm. The PoCo was aware of our power quality problems before that, and there were already discussions about upgrading the transformers before the storm occurred.

There are 3 primary wires. And it looks like all three make connections to the transformer. There is no fourth wire so one of those three may be an MGN.

 

[Open Neutral]

There are 3 primary wires. And it looks like all three make connections to the transformer. There is no fourth wire so one of those three may be an MGN.

I see what appears to be a substantial ground connection running down the pole.

Could it be the primaries are delta & there is no neutral carried along with them?

 

[Open Neutral]

I will do that, but I'll have to wait until we start getting some rain.

Any way to open an output bypass so the water spills back into the pond?

 

[Jon456]

Any way to open an output bypass so the water spills back into the pond?

Unfortunately, no. The outflow pipes are 16" diameter and there's no accommodation for a bypass. I was out at the location this afternoon taking more photos (to be posted soon) and the collection pond is so dry, I doubt we could run the pumps now even if we had a bypass.

 

[Jon456]

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.

(Click on the thumbnails for full-sized images)

 

[kwired]

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.

(Click on the thumbnails for full-sized images)

Looks to me like all three primary phases are there. They would not put a fuse in the MGN. Shouldn't be too big of a deal to put in one more transformer and have a full delta. Doesn't mean POCO will not want some money to do so.

 

[Jon456]

They would not put a fuse in the MGN.

That's an interesting point. But if we have three primary phases on the pole, why do all three connect to the two transformers for 3-phase high-leg delta service?

 

[kwired]

That's an interesting point. But if we have three primary phases on the pole, why do all three connect to
the two transformers for 3-phase high-leg delta service?

That still works, transformers just need to be rated for line to line voltage instead of line to neutral voltage. One phase is common to both transformers and the other two are only connected to one transformer. Switch the input leads around in any configuration you want the only thing that changes is the phase rotation. With line to neutral transformers the neutral must remain common to both.

 

[Jon456]

That still works, transformers just need to be rated for line to line voltage instead of line to neutral voltage. One phase is common to both transformers and the other two are only connected to one transformer. Switch the input leads around in any configuration you want the only thing that changes is the phase rotation. With line to neutral transformers the neutral must remain common to both.

So why, if there are 3 primary phases available, would the PoCo install high-leg delta transformers instead of "true" 3-phase transformers? Just to save the cost of the third phase transformer? I would think the cost difference would not be significantly higher since, with the high-leg delta system, they need to install one large transformer to produce two of the phases.

Also, if our three high-tension lines are carrying 3 phases, what happens to the neutral current? I know the neutral is derived from a center tap on the transformer and is grounded, but isn't an MGN needed to carry back any residual current that's the result of unbalanced loads on the phases?

 

[kwired]

So why, if there are 3 primary phases available, would the PoCo install high-leg delta transformers instead of "true" 3-phase transformers? Just to save the cost of the third phase transformer? I would think the cost difference would not be significantly higher since, with the high-leg delta system, they need to install one large transformer to produce two of the phases.

Also, if our three high-tension lines are carrying 3 phases, what happens to the neutral current? I know the neutral is derived from a center tap on the transformer and is grounded, but isn't an MGN needed to carry back any residual current that's the result of unbalanced loads on the phases?

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.

Whether or not there is two or three transformers it can still be connected so you have high leg on the secondary.
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.

With three primary phases and no neutral to the primary windings there is no current for the the neutral to carry, I think it would be better decision to at least ground the transformer case to MGN but it will work otherwise. The grounding on the secondary doesn't care if there is a MGN or not, it is a separately derived system and whatever conductor happens to be grounded is the grounding reference. You can take same transformers and ground a phase instead of the center tap of one phase and now you have a corner grounded system.

 

[Open Neutral]

That's an interesting point. But if we have three primary phases on the pole, why do all three connect to the two transformers for 3-phase high-leg delta service?

I got this drawing from PG&E. Short answer is it is Delta-Delta vs Delta-Wye.

[Hmm; I need to go read the HTML on other posts on how to display, not just link, an image. Sorry.]