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Discussion Starter #1
I have some submersible pumps that are fed by some Siemens Sirius soft starts. Pumps are 125HP @ 600vac. When we run the pump, the softstarts gets up to speed like normal (5s ramp) and the bypass contactor pulls. Within about 10 minutes they start overloading and tripping (some trip within 30 seconds). I checked the current on each leg supplying the starter and get the following readings:

L1: 119A
L2: 151A
L3: 128A

Quite the imbalance. Just for the fun of it, we swung the transfer switch over and ran the station on genset power. All phase currents were perfectly balanced with no issues. Pumps didn't overload or trip and were very happy.

Swung back to the utility and major current imbalance again. Measured phase to ground on each leg and got within 5vac which is very close...so doesn't seem like a voltage issue. The problem is the same for all pumps in the station.

Anyhow, power company came up and said everything was fine. Clearly it's not. What do you think could be the cause of this?
 

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Hackenschmidt
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Swung back to the utility and major current imbalance again. Measured phase to ground on each leg and got within 5vac which is very close...so doesn't seem like a voltage issue. The problem is the same for all pumps in the station.
I am just curious, did you check the voltage under load?

Am I right in assuming this is corner grounded delta?
edit: oops, never mind, wasn't paying attention
 

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I am just curious, did you check the voltage under load?

Am I right assuming this is corner grounded delta?
I'm with Splatz here, check voltage at the contactor terminals under load and phase to phase. A little voltage imbalance makes for a much larger current imbalance.
 

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Utilities has a problem under high load. Its a pain to prove it as your equipment will shut down as soon as it senses the voltage drop which reduces the load allowing the voltages to return to normal.

After proving to poco it was there problem using a voltage recorder at the pad mount low voltage side (we rented the pad mount from them) they put more effort into finding the problem.
They claimed that lightning had stuck a cable causing a thin spot and they used a mechanical clamp (no idea what that is) to fix the problem. After that the problem went away and voltages were stable under load.
 

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They claimed that lightning had stuck a cable causing a thin spot and they used a mechanical clamp (no idea what that is) to fix the problem. After that the problem went away and voltages were stable under load.
That's pocotech talk for tightened a loose connection.
 
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I've had similar issues and POCO refused it was their issue. What I usually do to troubleshoot if it is equipment or unbalance of the panel or POCO is to shift all 3 phases over . IE. ABC becomes BCA after the soft start, check current on each phase and then do the same shift before the soft start and check again. If the imbalance is always on the same legs(coming from the source) before the equipment I know it's not my stuff.
 

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I may be repeating myself (again), but I had some soft starters constantly tripping on 400HP motors and as reported, the voltage drop went away as soon as they tripped, so the PoCo said it was all good. Finally I went outside of the pump station to look at the transformer and it looked way too small. It was supposed to be 750kVA, but it was a tiny little pad mount. My suspicion was that someone dropped a zero and put in a 75kVA. Long story short, after several calls and visits and the utility gaslighting me by INSISTING that was a 750kVA transformer because it had a sticker saying so, they finally send out an engineer who knew what one was supposed to look like and admitted the mistake. Took over a week though.
 

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Chief Flunky
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1% voltage imbalance causes 6-8% current imbalance. Motors are generally delta and even if they’re wye you can’t tell externally anyways. So carefully measure voltages and currents. If you see a current imbalance but not voltage look downstream. Otherwise upstream.

Real examples.

Some utilities are notorious for using wye-wye transformers because they’re cheap. Same ones won’t have balanced single phase loads (houses) so the voltage imbalance comes and goes. Solution is delta-wye.

Utility checked cutout fuses on load side and voltage not under load both sides. Turned out it was a bad Bay-O-Net primary fuse. Had to train lineman where they are at.

Good voltage balance, big current imbalance on wye-delta starter. Delta contactor tips were worn out.

Current imbalances and voltage with any phase converter.

Current imbalance startup only. Cheap ass soft start that only has SCRs in two phases.

Current imbalance line side of VFD but not load side trips breaker. Increases with load. Line side SCR blown.

I use imbalance as per NEMA.
 

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I may be repeating myself (again), but I had some soft starters constantly tripping on 400HP motors and as reported, the voltage drop went away as soon as they tripped, so the PoCo said it was all good. Finally I went outside of the pump station to look at the transformer and it looked way too small. It was supposed to be 750kVA, but it was a tiny little pad mount. My suspicion was that someone dropped a zero and put in a 75kVA. Long story short, after several calls and visits and the utility gaslighting me by INSISTING that was a 750kVA transformer because it had a sticker saying so, they finally send out an engineer who knew what one was supposed to look like and admitted the mistake. Took over a week though.
After proving the voltage imbalance poco told me it was the transformer. I told them no problem go ahead and replace it.
They got all excited and asked if i had the authority to make that call considering the cost.

I said no problem we rent it from you.

Now there all "lets get the engineer it might not be the transformer"
 

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Modérateur
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I have a simple question to ask the OP .,,

Was this source is wired in Delta or Wye set up ?

You should grab a data recording to see where this goes so that way you can able nail it. get one that can read both amps and volts at the same time so you know what it shaking on that.
 

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Like mentioned above, I always roll the phases when chasing current imbalances. You need to narrow down it either being a pump or supply issue.

I may be repeating myself (again), but I had some soft starters constantly tripping on 400HP motors and as reported, the voltage drop went away as soon as they tripped, so the PoCo said it was all good. Finally I went outside of the pump station to look at the transformer and it looked way too small. It was supposed to be 750kVA, but it was a tiny little pad mount. My suspicion was that someone dropped a zero and put in a 75kVA. Long story short, after several calls and visits and the utility gaslighting me by INSISTING that was a 750kVA transformer because it had a sticker saying so, they finally send out an engineer who knew what one was supposed to look like and admitted the mistake. Took over a week though.
This makes me chuckle.

A power co that can't by sight tell the difference between a baby padmount 75kva and it's big brother 10 times it's size. Here's your sign....
 

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Like mentioned above, I always roll the phases when chasing current imbalances. You need to narrow down it either being a pump or supply issue.
I agree but OP has already ran it on the back up generator and its balanced and running with out tripping so that really narrows it down to transfer switch and poco side.
 

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Bilge Rat
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Measured phase to ground on each leg and got within 5vac which is very close...so doesn't seem like a voltage issue.
Motors don't see phase to ground, they care about phase to phase.

Depending on the type of loads on the PUCO lines, phase to ground can be pretty close but phase to phase can be off by quite a bit.

You'll need to check phase to phase voltage as close to the motor as you can get, in this case it'll likely be at the load side of the bypass contractor.
 

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I agree but OP has already ran it on the back up generator and its balanced and running with out tripping so that really narrows it down to transfer switch and poco side.
You're right, overlooked that part.
 

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This makes me chuckle.

A power co that can't by sight tell the difference between a baby padmount 75kva and it's big brother 10 times it's size. Here's your sign....
I forgot to mention that we blew the primary fuses on the transformer. The technician that was there to replace the fuses was the first one that I told about the transformer not being a 750 kVA. He kept pointing to the fact that there was a set of stick-on house numbers inside the door that said 750 kVA. I wanted to go buy some that spelled out “Ham Sandwich” to make the point that just because someone puts a label on something doesn’t mean you can ignore the evidence in front of your face that it’s obviously incorrect. But 4 more levels of people kept agreeing with him because the drawings said it too. So you have to imagine that some idiot got out there with that 75 kVA transformer, saw the drawings calling for 750, but went ahead and installed it anyway, probably assuming the 750 on the drawing was what was incorrect, maybe because it was a small pump station building and he had no idea what was inside.

It also made me wonder if somewhere, someone else who ordered a 75kVA padmount ended up with a 750...
 

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Discussion Starter #17
Motors don't see phase to ground, they care about phase to phase.

Depending on the type of loads on the PUCO lines, phase to ground can be pretty close but phase to phase can be off by quite a bit.

You'll need to check phase to phase voltage as close to the motor as you can get, in this case it'll likely be at the load side of the bypass contractor.
Forgot to mention I checked phase to phase with my multimeter, but all within about 5vac.

I have a suspicion based on some conversations with others that the phase angles may be off. Going to put a power meter on the utility supply on friday and have a look at the phasor diagrams. This is the only way we can get to the bottom of this.
 

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Just trying to get home
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It also made me wonder if somewhere, someone else who ordered a 75kVA padmount ended up with a 750...
Seriously undersized feeders...:surprise:
 
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