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Old 04-08-2018, 05:09 PM   #1
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Default Metering and Phase Imbalances Questions

Hi there

Recently my organization had me install a Schneider metering system on one of our facilities. This particular facility has a main building, and several outbuildings for services such as septic, water treatment and the like.

The design as installed is located in our incoming disconnect switch before the 600-120/208 transformer in the main buildings electrical room. So we are collecting a 600V delta reference voltage and amperage reading from the three phases on the delta side of the service.

I'm a fairly green journeyman, and have no experience in metering design or installation, and our senior facilities guy who is not an electrician was receiving advice from a similar installation when it came time to purchase a meter. I had prepared a proposal of a 36 input meter system to allow us to narrow down which panels and even individual loads were potentially causing us problems (eg. peak loading of generators). This was dismissed as too expensive and installation to complicated so we ended up with the simpler, but in the end far more expensive Schneider system. I advised management at that time that I did not think the system they were purchasing was appropriate or would allow us to gather information to make the decisions they required.

Now with the meter installed, we are receiving load imbalance warnings constantly, with the load imbalance ranging from a 6-14%. They are consistently higher on A than C, regardless of time of day or what is going on at our facility.

I'm curious if anyone can speak to the overall generating efficiency of the system running with this phase imbalance if we're running of diesel generators, and how do I go about determining which single phase loads are appropriate to swap phases on to try to balance it out?

Or is 6-14% phase imbalance ok and I should just set the warning threshhold higher to avoid the warnings popping up.

Thanks in advance
r.o
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Old 04-08-2018, 05:38 PM   #2
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You have a single Meter?

This meter is located on the primary of a single transformer feeding your facility?

What size is the transformer?

What are the phase current readings?

What are the phase voltage readings?
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Old 04-08-2018, 06:04 PM   #3
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Quote:
Originally Posted by brian john View Post
You have a single Meter?
Yes, a single meter.

Quote:
This meter is located on the primary of a single transformer feeding your facility?
It is located at the 600V service disconnect upstream of the main buildings transformer.

Quote:
What size is the transformer?
225 kVA 600V - 120/208V

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What are the phase current readings?
Currently:
A:58A
B:62A
C:38A

Average values from the meter:
A:58A
B:57A
C:48A

Quote:
What are the phase voltage readings?
A:596V
B:603V
C:610V
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Old 04-08-2018, 06:37 PM   #4
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You happen to have the no load voltage readings?
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Old 04-08-2018, 06:41 PM   #5
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At that load for that size transformer hardly worth worrying about. But I would look into that voltage imbalance.


Do you have secondary voltage readings?
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Old 04-09-2018, 03:37 AM   #6
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Have you tried shuffling some of the smaller circuits around ?
To try and balance it a bit better ?
But in the end it's just a difference of 15%.
I doubt that there is many perfectly balanced loads out there.
So the system should cope ok !

Last edited by dmxtothemax; 04-09-2018 at 03:40 AM.
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Old 04-09-2018, 09:58 AM   #7
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Quote:
Originally Posted by brian john View Post
You happen to have the no load voltage readings?
Quote:
Originally Posted by brian john View Post
At that load for that size transformer hardly worth worrying about. But I would look into that voltage imbalance.


Do you have secondary voltage readings?
This was a fairly quiet day, I will grab some max readings today and find a time to measure no load and secondary voltages.

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Originally Posted by dmxtothemax View Post
Have you tried shuffling some of the smaller circuits around ?
To try and balance it a bit better ?
But in the end it's just a difference of 15%.
I doubt that there is many perfectly balanced loads out there.
So the system should cope ok !
I did try that - moving some constant single phase loads around did not change the issue or actually made it worse. It is hard to organize time to measure and modify circuits because of the nature of the business.

Thanks for all the help so far - one question is this: is the measured A phase amperage on the delta side of the transformer indicative of a higher load on the A phase of the wye side? Or is it correlated to both the A-B and A-C coils.

Part of me thinks I should know this from school and another part thinks its a question above my pay grade.
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Old 04-09-2018, 10:17 AM   #8
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That amount of imbalance is not unreasonable at all. I wouldn't give it a second thought.

As noted, I'd look into the voltage imbalance, 2% is not all that bad but it can cause trouble with motors operating at full load.
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Old 04-09-2018, 01:31 PM   #9
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Read this

https://library.e.abb.com/public/0c4...low%20res).pdf
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Old 04-20-2018, 09:39 PM   #10
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Are your CTs centered on the wiring and perpendicular? Are they measurement grade or protection grade? It makes a big difference. An easy way to tell is swap CTs or PTs (or PT inputs). If the problem moves too then itís a measurement accuracy problem. If it doesnít itís real.

Also if you did it again, I wouldnít buy the Square D system. Those were top of the line 20 years ago but havenít kept up with the times and theyíre overpriced. Iíd look at the SEL 735 or the EIS Shark 200, both with a mid grade option. They both have free software. The SEL data is best downloaded and viewed while the Shark one has a built in web server with the data on that. Either way except for long term use your second meter should be portable and runs around $5,000. You just clip it onto a line where you need it and close the door. Dranetz is top of the line but expensive. Flukes suck. If you had the SEL system they sell it in a box so you can move it around and the data integrates between both systems.

The meter you have assumes balanced three phase in the default settings which is bad for 3 phase loads at 15% but not bad at all for single phase loads. Motor current imbalance trip settings are usually 10-15% so thatís the accepted trip limit, often alarming at 5%. Voltage imbalance in a motor of 1% acts like a 6-8% current imbalance so if the 2.5% is real, it is causing excessive motor heating on three phase motors.


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Old 05-22-2018, 10:21 AM   #11
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After seeing those readings I would wonder what my harmonic structure looked like at the panels serving my heaviest loads. With proper drives the three-phase loads shouldn't add to the mix, but if you have direct power (no controllers/reactors/cap banks) to full-speed motors or other inductive loads it may be causing voltage imbalance and/or harmonics. Phase harmonics can help out to track down problems. The harmonics won't necessarily be a problem, but they can help with other areas of interest.

There should be other clues that your metering can give you. Are you running Struxureware or Ecostruxure?

Hopefully you don't have some type of static PF correction solution running in your facility. If you are monitoring upstream of one of those setups, and you are trying to effect fixes downstream the results can be confusing without taking that into account. If your facility manager bit the bullet on someone else's recommendation rather than his own electrician, he may have been sold on static or active PF correction (I think they are rather like snake oil myself...but that's just me).

How long is your trend data good for?

My facility only runs an 800A bus, so rather smallish around the forum. But, my PF is nearly always .99 to 1, and I have no voltage imbalance to deal with. This is achieved at each panel and rounds out nicely in the end.

Last edited by cuba_pete; 05-22-2018 at 10:25 AM.
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