Power Utility Issue

Jdrais

Member
Location
Florida
Occupation
Electrical Contractor
is that loaded or unloaded voltages?

Thing is they possibly have blown fuse on a capacitor or maybe just need to connect certain single phase services to a different phase and it possibly clears up the balancing issue. POCO's around here would be willing to try to solve this problem.
Unfortunately for over a month now they have not even let us know any information in regards to what’s wrong. That’s why I need as much info I can find to bring to there attention in regards to the possibility of what they have wrong there.

Under load But voltage doesn’t move too much load or no load.

VFD % 89.6

VAB 487
VBC 475
VCA 488

IA - 433
IB - 385
IC - 272
 

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
What is it supposed to be within 3% of? Nominal volts of 480, or 3% or the differentials can not be more than 3% of one another?

Differentials here is huge, we have 1, 12 and 13.

Percent within nominal of 480 is low. Have two that are ~1.5% high and one that is ~1% low
The formula for percent voltage unbalance is;
100 × Maximum Voltage Deviation /Average Voltage

483.33 average voltage
Max deviation is 13V

(13/483.33)=.02689
.02698X100= 2.689%

I will agree with a suggestion earlier, look for a blown cap fuse on the POCO lines
 

mbrooke

Batteries Not Included
Location
United States
Occupation
Electricity
The formula for percent voltage unbalance is;
100 × Maximum Voltage Deviation /Average Voltage

483.33 average voltage
Max deviation is 13V

(13/483.33)=.02689
.02698X100= 2.689%

I will agree with a suggestion earlier, look for a blown cap fuse on the POCO lines
How do we know there isn't a voltage regulator? They will often correct the line to neutral voltages on long lines but produce uneven voltages or phase angle shifts line-line.
 

kwired

Electron manager
Location
NE Nebraska
How do we know there isn't a voltage regulator? They will often correct the line to neutral voltages on long lines but produce uneven voltages or phase angle shifts line-line.
Won't that mostly be because of unbalanced single phase loads and the solution would be to re evaluate which lines single phase loads should be connected to to attain better balance?
 

Flicker Index

Senior Member
Location
Pac NW
Occupation
Lights
This document is worth a read: https://www.pge.com/includes/docs/p...tatus/powerquality/voltage_unbalance_rev2.pdf

As it says, the allowable variation by utility, and what the drive manufacturer wants to set as the limit to guarantee maximum output are often different. So if the drive loading is right at the maximum I can see running into issues.

I think asking ABB if they have any suggestion is a good idea, giving the details on the applications, and specific values you're seeing. When it is running on the generators what are your voltage/amperage values like?
Having things being right at, or better than tolerance sometimes let other things out of tolerance get away with it, but when one factor deviates from it, things may start having problems.

If existing setup was working correctly with existing setup, but the new setup is within tolerance, it doesn't always mean that "new" is bad, and "old" was right. For example, a well used 13/16" (20.64mm) socket that you have now that was already at the largest allowed tolerance when it was new might fit nicely on a 21mm bolt around your facility. You lose the socket and get another 13/16" (incorrectly), but ends up not even fitting on the bolt. No, that doesn't mean the new socket is "defective", but it was user error all along.
 
Perhaps I've missed something along the way, but if the voltages are relatively correct and the currents delivered are very uneven, that really sounds like the impedance of the lines are not even closely the same. A simple load test ought to show this. (Of course, a resistive load won't show harmonic-related problems, but it might show something. Possibly a mismatched or fialing transformer in a bank.)

I'd look at doing a good-size load test, at least 250 amp/phase, and see how that measures out.
 

kwired

Electron manager
Location
NE Nebraska
Perhaps I've missed something along the way, but if the voltages are relatively correct and the currents delivered are very uneven, that really sounds like the impedance of the lines are not even closely the same. A simple load test ought to show this. (Of course, a resistive load won't show harmonic-related problems, but it might show something. Possibly a mismatched or fialing transformer in a bank.)

I'd look at doing a good-size load test, at least 250 amp/phase, and see how that measures out.
Isn't OP sort of doing a load test when he tries to run the motor driven by the VFD in question? The drive just happens to be conversion equipment between the source and the load. If it weren't for built in protection in the drive it would still continue to power the motor in an unbalanced manner, but will overload portions of the front end rectifier if left this way for too long. Output to the motor is very likely balanced, but that is derived from two wire DC bus regardless of conditions on the front end of the drive.
 
Isn't OP sort of doing a load test when he tries to run the motor driven by the VFD in question?
Sort of, but that's not an independent test; think of this as a gigantic case of the 'hair dryer" test for a loose neutral.

Not mentioned, or I missed it, is whether the VFD itself has been checked. That it works on a generator suggests that it probably does, but wouldn't hurt to check the input section anyway. (If you're going to point at the utility, need to really check everything else.)
 

kwired

Electron manager
Location
NE Nebraska
Sort of, but that's not an independent test; think of this as a gigantic case of the 'hair dryer" test for a loose neutral.

Not mentioned, or I missed it, is whether the VFD itself has been checked. That it works on a generator suggests that it probably does, but wouldn't hurt to check the input section anyway. (If you're going to point at the utility, need to really check everything else.)
Kind of agree that OP needs to check everything else, possibly even asking POCO why such and such capacitor bank has a blown fuse (if something like that is found). Seems this POCO isn't willing to help much, even though what evidence there is so far leans to them having issues.

attachment in post 35 showing the error and potential issues causing it sort of point to input power issues. If it was rectifier problem it should still give same error on alternate supply.
 

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
This is the voltage

VAB 487
VBC 475
VCA 488
Assuming this is a grounded wye secondary, the L-N or L-G voltages corresponding to the L-L voltages above should be about:
289V A-N
274V B-N
274V C-N

It would be helpful to measure these L-N voltages to confirm this. If so then then following may help, assuming that the transformer is delta-wye with independent 2.5% taps on the primary (and you can get the POCO to cooperate):
1.) Select 2 taps (i.e., 5%) higher voltage on the primary winding that corresponds to the A-N secondary. This should get you about 275V A-N and therefore about 476V L-L across all phases.
or
2.) If you want closer to 488V L-L and 282V L-N on all phases then select 1 tap higher voltage on the primary winding that corresponds to the A-N secondary. Also, select 1 tap lower voltage on the two primary windings corresponding to the B-N and C-N secondaries.
 
Last edited:

mbrooke

Batteries Not Included
Location
United States
Occupation
Electricity
Won't that mostly be because of unbalanced single phase loads and the solution would be to re evaluate which lines single phase loads should be connected to to attain better balance?
Haven't forgotten about you :)

My understanding is that most of it will be from unbalanced single phase loading, however it is not always practical or possible to balance out a feeder.
 

Open Neutral

Senior Member
Location
Inside the Beltway
Occupation
Engineer
These voltage and amperage reading are taken in the front of the service after the transformer at this location while trying to run large VFD on Utility Power

VAB- 506.33
VBC- 491.48
VCA- 504.94

IA- 237.33
IB- 373.91
IC- 410.69

Small much newer VFD thats working:

VAB- 506.73
VBC- 492.74
VCA- 504.14

IA- 136.43
IB- 109.72
IC- 83.72

Rotate all three; i.e. what load was on A is now on B, B->C, C->A
Measure again.
 

kwired

Electron manager
Location
NE Nebraska
Haven't forgotten about you :)

My understanding is that most of it will be from unbalanced single phase loading, however it is not always practical or possible to balance out a feeder.
If enough variance in single phase loading then it will be difficult to balance. If it remains somewhat constant, some carefully selected changes to other phases likely can get somewhere.

Since the feed is temporarily being routed through a different path the balance may be different at OP's point of use than it is via the normal path.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Took me a while to read everything, but I agree with you (Jdrais) that this is a problem on the UTILITY side. I have come up against this sort of thing several times over the years and unfortunately, I can tell you that you are going to be into a fight with them. The upshot of each situation I have been involved in is that the ANSI regs that they spout are true, and yet incomplete, because as you suspect, they ONLY address voltage imbalance. This ASS-U-Me(s) that if the voltage is balanced, the current will be relatively balanced, so from a design spec standpoint, it SHOULD work. So there are no standards for current balance, because current is DRAWN by the user, not PUSHED by the utility. But these ANSI standards were developed BEFORE we had large power electronic devices like VFDs that can draw lots of current in ways that do NOT follow the rules.

VFDs draw current from the line in "chunks" only at the TOP of the sine waves, which is why we refer to it as "non-linear" current draw. Diodes have what's called a "forward conduction voltage" threshold, so they only conduct when the sine wave increases to that level, then ceases to conduct when the sine wave drops. If you have a properly designed system and fully healthy transformers, that is somewhat irrelevant, but when you have old failing equipment, it looms large. What utilities do, and NEVER admit to, is to "boost" the voltage with line capacitors. This is basically a way for them to satisfy the ANSI specifications without having to spend a lot of money replacing a bad transformer somewhere just for one customer. Is this a "cheat"? Some think it is, hence the utilities not wanting to EVER admit it. But the problem then with VFDs is that because of the non-linear current draw, they suck all of the energy from those boost capacitors instantly at the first point of forward conduction (how a diode works) and the voltage drops on that phase to where the next diode on the VFD front end fails to conduct. That then causes a large DC bus ripple, which is what the VFD is actually concerned with, and it shuts down. But when you measure VOLTAGE on that line, it APPEARS normal because the meter is averaging the voltage (RMS), thus allowing the utility to claim they are meeting the ANSI specs. If you could look at it with a high speed scope, you could probably see the issue more clearly. But from the things you describe and the troubleshooting you have already done, this is exactly what it sounds like to me, because I have been there.

In one fight I got into with the utility near Las Vegas, the plant was far out in the desert and the powerlines ran along the only road going there. So on my way to the site for the 4th time to meet with the owner and utility reps, I photographed the capacitors on their pole and when they again denied they existed, I showed them the photo. That was the final blow, they admitted to it then (claiming they were unaware) and paid for the damage they had caused (blowing up some large Toshiba VFDs).
 

Open Neutral

Senior Member
Location
Inside the Beltway
Occupation
Engineer
It just occurred to me that while a scope is the go-to tool for this, wouldn't a True RMS voltmeter also reveal the cause?

Also, on my last post, I was not very illuminating as to why I suggested rotating the feed. By doing that, you can bifurcate the issue. If the bad leg moves to the next phase, then it is the load. But if it stays on the same phase, that nails it down to a POCO issue.
 

kwired

Electron manager
Location
NE Nebraska
It just occurred to me that while a scope is the go-to tool for this, wouldn't a True RMS voltmeter also reveal the cause?

Also, on my last post, I was not very illuminating as to why I suggested rotating the feed. By doing that, you can bifurcate the issue. If the bad leg moves to the next phase, then it is the load. But if it stays on the same phase, that nails it down to a POCO issue.
Just to clarify I think you mean - if after rotating feed the current problems remain on same load terminals it is the load that has a problem if they follow the incoming lead to the next load terminal then it is POCO issue.
 

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
Took me a while to read everything, but I agree with you (Jdrais) that this is a problem on the UTILITY side. I have come up against this sort of thing several times over the years and unfortunately, I can tell you that you are going to be into a fight with them. The upshot of each situation I have been involved in is that the ANSI regs that they spout are true, and yet incomplete, because as you suspect, they ONLY address voltage imbalance. This ASS-U-Me(s) that if the voltage is balanced, the current will be relatively balanced, so from a design spec standpoint, it SHOULD work. So there are no standards for current balance, because current is DRAWN by the user, not PUSHED by the utility. But these ANSI standards were developed BEFORE we had large power electronic devices like VFDs that can draw lots of current in ways that do NOT follow the rules.

VFDs draw current from the line in "chunks" only at the TOP of the sine waves, which is why we refer to it as "non-linear" current draw. Diodes have what's called a "forward conduction voltage" threshold, so they only conduct when the sine wave increases to that level, then ceases to conduct when the sine wave drops. If you have a properly designed system and fully healthy transformers, that is somewhat irrelevant, but when you have old failing equipment, it looms large. What utilities do, and NEVER admit to, is to "boost" the voltage with line capacitors. This is basically a way for them to satisfy the ANSI specifications without having to spend a lot of money replacing a bad transformer somewhere just for one customer. Is this a "cheat"? Some think it is, hence the utilities not wanting to EVER admit it. But the problem then with VFDs is that because of the non-linear current draw, they suck all of the energy from those boost capacitors instantly at the first point of forward conduction (how a diode works) and the voltage drops on that phase to where the next diode on the VFD front end fails to conduct. That then causes a large DC bus ripple, which is what the VFD is actually concerned with, and it shuts down. But when you measure VOLTAGE on that line, it APPEARS normal because the meter is averaging the voltage (RMS), thus allowing the utility to claim they are meeting the ANSI specs. If you could look at it with a high speed scope, you could probably see the issue more clearly. But from the things you describe and the troubleshooting you have already done, this is exactly what it sounds like to me, because I have been there.

In one fight I got into with the utility near Las Vegas, the plant was far out in the desert and the powerlines ran along the only road going there. So on my way to the site for the 4th time to meet with the owner and utility reps, I photographed the capacitors on their pole and when they again denied they existed, I showed them the photo. That was the final blow, they admitted to it then (claiming they were unaware) and paid for the damage they had caused (blowing up some large Toshiba VFDs).
Nice piece.
I fail to see why it’s so hard for other places to keep up a system. I’m amazed sometimes by the way other POCOs treat customers..

for the record, we balance circuits to keep the same (as close as possible) draw equal at the substations.
And yes, we have to rebalance about every three of four years

capacitors.. we use them mainly to keep our stations at or as close to unity as possible.
for voltage boost, they are set to 118V to close. In reality these only close (if at all) during CVR events
A capacitor though isn’t a good answer. It only boosts a couple of volts on a 120 base.

We read each and every meter twice a day for voltage anomalies and I get an email every night showing every meter under a given threshold. 5% of nominal.
Sometimes I know a customer is out of power before they do.
(Vacation place or hunting cabin usually)
what it usually shows me are heavily loaded XFs with too many houses on it for the size or service drops which are too small.

Not all utilities are the same...
 

Open Neutral

Senior Member
Location
Inside the Beltway
Occupation
Engineer
Just to clarify I think you mean - if after rotating feed the current problems remain on same load terminals it is the load that has a problem if they follow the incoming lead to the next load terminal then it is POCO issue.
Yes. The issue is the "Is it live, or Memorex?" type question. I, at least, often do such steps to isolate the issue to one area.
Does it show that in decades past I would swap vacuum tubes in two-way radios on the bench?
 
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