Power Utility Issue

Jdrais

Member
Location
Florida
Occupation
Electrical Contractor
I have a 480v 1600 amp service feeding a 600 hp VFD controlled potable water pump. The Power utility has recently shut down our substation and picked us up from another substation while they are performing work. When we tried to run out 600 hp pump after this swap over our VFD would trip out on “Fault Supply Phase” The Voltage coming from our Utility is within 3% but the Current on the Primary and Secondary side of the Utility transformer is off by more then 40% between 2 phases. This is what trips out our VFD. Luckily we have 3 other 50hp VFDs that our of a newer type and are able to handle this massive current imbalance.

When we questioned the Utility about this they told us that they are within the ANSI standards with what they are providing us :

Per ANSI Standard C84.1-1995, “Electric supply systems should be designed and operated to limit the maximum voltage unbalance to 3% when measured at the electric utility revenue meter under no-load conditions”. The supply voltage at the meter revenue point are well within the requirements even under full load conditions.

Question:
Are the Utilities only required to supply voltage balance to a certain amount but not current balance ? For the last 3 weeks we have been running a diesel pump and now it needs service and we had to bring in a 500kw generator to run our 600hp electric pump sucking 600 gallons of diesel a day. This is mind boggling to me. Yes when they switch back to their other substation which could be another month from now we will be back to normal but right now just seems like the Utility should be repairing the issue they are having that isn’t giving us the required current that we need. We are towards the end of the line downstream of their substation.
Thanks for the help
JDrais
 

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
You say the voltage is fine, (within 3%) but the current is off??
Power companies can’t deliver the right “current”.
With the transformers unloaded the voltage may be fine. But with a large load the voltage will sag to unacceptable levels.

what is probably happening is the current draw you require is pulling the voltage down to an unacceptable levels because of current draw

Did you check the voltage with the machines off or running? If it’s within 3% with them running it’s something with the motors.

My bet is if you would check the voltage with the motors running you would find the voltage goes way down on the afflicted phases. That’s the information you need to give the POCO. And not just the lineman that comes out. Ask to speak to an engineer when you call in, but have the readings at hand.
 

Jdrais

Member
Location
Florida
Occupation
Electrical Contractor
You say the voltage is fine, (within 3%) but the current is off??
Power companies can’t deliver the right “current”.
With the transformers unloaded the voltage may be fine. But with a large load the voltage will sag to unacceptable levels.

what is probably happening is the current draw you require is pulling the voltage down to an unacceptable levels because of current draw

Did you check the voltage with the machines off or running? If it’s within 3% with them running it’s something with the motors.

My bet is if you would check the voltage with the motors running you would find the voltage goes way down on the afflicted phases. That’s the information you need to give the POCO. And not just the lineman that comes out. Ask to speak to an engineer when you call in, but have the readings at hand.
Ok thank you for your reply but you unfortunately would lose that bet. Our voltage is good there it has no affect on the voltage at the station and it is within 3% while running or not running the vfd. We have 3 other VFDs running also with the same thing happening but they are able to handle the 40% imbalance on the supply. Besides manually checking all of these voltages while off and running we also have 2 power monitors trending through our PLC Scada system showing these results.
Thanks for your reply. Hopefully with this additional info someone else may have an answer to this problem.
 

Jdrais

Member
Location
Florida
Occupation
Electrical Contractor
https://www.ecmweb.com/content/article/20891755/beware-of-linecurrent-unbalance-on-vfds
What it suggests is check the load level, then if that allows it, change the protective settings in the drive. .
The ABB ACS800 does not allow to change the protective settings. It is monitoring the oscillation for more then 13% and when it has that it throws the fault.

Thanks for you reply. Any other ideas?

Focusing more on the Utility company. Possible Power Correction Capacitors ????? Is there anyone from a Utility in this board that has heard of this issue and are there no requirements for a Power utility in that regard to correct this issue when something like this is happening ?

Thank you
 

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
I have a 480v 1600 amp service feeding a 600 hp VFD controlled potable water pump. The Power utility has recently shut down our substation and picked us up from another substation while they are performing work. When we tried to run out 600 hp pump after this swap over our VFD would trip out on “Fault Supply Phase” The Voltage coming from our Utility is within 3% but the Current on the Primary and Secondary side of the Utility transformer is off by more then 40% between 2 phases. This is what trips out our VFD. Luckily we have 3 other 50hp VFDs that our of a newer type and are able to handle this massive current imbalance.

When we questioned the Utility about this they told us that they are within the ANSI standards with what they are providing us :

Per ANSI Standard C84.1-1995, “Electric supply systems should be designed and operated to limit the maximum voltage unbalance to 3% when measured at the electric utility revenue meter under no-load conditions”. The supply voltage at the meter revenue point are well within the requirements even under full load conditions.

Question:
Are the Utilities only required to supply voltage balance to a certain amount but not current balance ? For the last 3 weeks we have been running a diesel pump and now it needs service and we had to bring in a 500kw generator to run our 600hp electric pump sucking 600 gallons of diesel a day. This is mind boggling to me. Yes when they switch back to their other substation which could be another month from now we will be back to normal but right now just seems like the Utility should be repairing the issue they are having that isn’t giving us the required current that we need. We are towards the end of the line downstream of their substation.
Thanks for the help
JDrais
What your saying isn’t making much sense..
The utility can’t just “increase your current” like they can the voltage.
Power factor capacitors work back to the source for PF correction. They will increase the voltage a little ahead of them.
 

Flicker Index

Senior Member
Location
Pac NW
Occupation
Lights
What are the instantaneous voltages on all three phases unloaded and loaded measured on line voltage monitor? I would expect that things might be happening too fast for you to be able to take meaningful measurements with a normal DMM for voltage under load test. The other suggestions in that article are likely not practical given the temporary nature of your situation.

Have you asked ABB? Having the above values before you ask them would be a good idea.
 

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
The ABB ACS800 does not allow to change the protective settings. It is monitoring the oscillation for more then 13% and when it has that it throws the fault.

Thanks for you reply. Any other ideas?

Focusing more on the Utility company. Possible Power Correction Capacitors ????? Is there anyone from a Utility in this board that has heard of this issue and are there no requirements for a Power utility in that regard to correct this issue when something like this is happening ?

Thank you
There is, but he probably can’t help you with the information given...
 

hillbilly1

Senior Member
Location
Atlanta,Ga
Occupation
Field coordinator/ technical support
Just a shot in the dark, but could the fault be reversed supply rotation? I know all of the vfd’s I’ve hooked up sends CW regardless of input, and don’t care. Maybe it has an extra feature that may see it as utility fault after setup that the others don’t? Don’t know enough about vfd’s to give an informed answer, but it appears something has changed between the two substations. Just throwing it out there. I have had poco’s reconnect transformers backwards.
 

Flicker Index

Senior Member
Location
Pac NW
Occupation
Lights
The effect of voltage on current balance have similar effects as balancing weight distribution on table legs. It only takes very little unevenness in ground level or leg length before you have a wonky table with all the weight being carried by three of four legs with one barely touching the floor or even visibly lifted.
 

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
Can you measure the voltage and/or current waveforms on the VFD line inputs? That could help diagnose whether there's ringing from a resonance at a harmonic, a blown diode in the rectifier bridge, or some other factor that's causing the current imbalance.
 

Jdrais

Member
Location
Florida
Occupation
Electrical Contractor
What your saying isn’t making much sense..
The utility can’t just “increase your current” like they can the voltage.
Power factor capacitors work back to the source for PF correction. They will increase the voltage a little ahead of them.
What are the instantaneous voltages on all three phases unloaded and loaded measured on line voltage monitor? I would expect that things might be happening too fast for you to be able to take meaningful measurements with a normal DMM for voltage under load test. The other suggestions in that article are likely not practical given the temporary nature of your situation.

Have you asked ABB? Having the above values before you ask them would be a good idea.

Yes way past that ! And way past what your talking about. Voltages checked running and not running. Let’s get passed the problem being with our equipment... the 600 hp electric motor is working flawlessly right now for 3 days in a row on a 500kw generator. No issues.

Ok so this is what I’m looking to confirm. Is the Utility having an issue with their Power Factor Correction Capacitors ???

I never said anything about increasing current. I said we have unbalanced current. I said that the power company is claiming that they are within ANSI standards but that only speaks about voltage. I’m trying to get to the bottom of this problem which seems to be that the Power utility power factor capacitors are not working correctly.

Do you want me to post a picture of trending voltages and amperages. I’m not making this up. The amperage when the VFD is running on utility is over 40% unbalanced between 2 legs. The voltage is within 3%. It’s been tested with 2 power monitors on the service working all the time and I brought in a fluke portable power monitor to check the other 2 monitors over a 5 days span. Same thing. The focus should not be on the equipment my question is in regards to what’s required from a power utility and why they are not correcting this issue ? The issue is not with our equipment. The 3ph 480 AIr condition system Amperages have the same 40% unbalance !!!!!!! Voltage is good.
 

Flicker Index

Senior Member
Location
Pac NW
Occupation
Lights
There's a difference between all 3 phases being at 490v vs one being at 494v and the other two at 467v despite being "within 3%" in both cases.
How hard is it for you to just to share the simple set of values?

Then compare the values to when it's on the gen set. That's a good starting point. If you have the ability, can you catch the VTHD % while on genset vs the "temporary" utility power?
 

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
It's possible that there could be a resonance of the power factor correction capacitors with the transformer inductance at or near a harmonic frequency present in the VFD input current.

Also, PFC capacitors would provide a reservoir of charge (and therefore current) during the conduction interval of the rectifier diodes near the line voltage peaks, effectively lowering the source impedance driving each diode. Therefore, if there's asymmetry in the capacitance on the phases then the current drawn from the phases may vary from each other.
 

Besoeker3

Senior Member
Location
UK
Occupation
Electrical Engineer
Yes way past that ! And way past what your talking about. Voltages checked running and not running. Let’s get passed the problem being with our equipment... the 600 hp electric motor is working flawlessly right now for 3 days in a row on a 500kw generator. No issues.

Ok so this is what I’m looking to confirm. Is the Utility having an issue with their Power Factor Correction Capacitors ???

I never said anything about increasing current. I said we have unbalanced current. I said that the power company is claiming that they are within ANSI standards but that only speaks about voltage. I’m trying to get to the bottom of this problem which seems to be that the Power utility power factor capacitors are not working correctly.

Do you want me to post a picture of trending voltages and amperages. I’m not making this up. The amperage when the VFD is running on utility is over 40% unbalanced between 2 legs. The voltage is within 3%. It’s been tested with 2 power monitors on the service working all the time and I brought in a fluke portable power monitor to check the other 2 monitors over a 5 days span. Same thing. The focus should not be on the equipment my question is in regards to what’s required from a power utility and why they are not correcting this issue ? The issue is not with our equipment. The 3ph 480 AIr condition system Amperages have the same 40% unbalance !!!!!!! Voltage is good.
Interesting problem. If the 500kW and 600hp arrangement are operating correctly as you say, then the problem must be with the Utility supply.
I don't know where the PFC is connected but unlikely to be directly related to the VFD. The VFD is usually around 0.95 distortion PFC rather than displacement PFC.
 

Jdrais

Member
Location
Florida
Occupation
Electrical Contractor
Can you measure the voltage and/or current waveforms on the VFD line inputs? That could help diagnose whether there's ringing from a resonance at a harmonic, a blown diode in the rectifier bridge, or some other factor that's causing the current imbalance.
The VFD is working seamlessly right now. Purring like a little kitten for 72 hours in a row at 95% pushing millions of gallons of water. There’s nothing wrong with our VFD or our electric pump.

I’m going to share voltage and Amperages in reply to a post if I can find it in this thread
 

Jdrais

Member
Location
Florida
Occupation
Electrical Contractor
There's a difference between all 3 phases being at 490v vs one being at 494v and the other two at 467v despite being "within 3%" in both cases.
How hard is it for you to just to share the simple set of values?

Then compare the values to when it's on the gen set. That's a good starting point. If you have the ability, can you catch the VTHD % while on genset vs the "temporary" utility power?
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
 

Flicker Index

Senior Member
Location
Pac NW
Occupation
Lights
It's possible that there could be a resonance of the power factor correction capacitors with the transformer inductance at or near a harmonic frequency present in the VFD input current.

Also, PFC capacitors would provide a reservoir of charge (and therefore current) during the conduction interval of the rectifier diodes near the line voltage peaks, effectively lowering the source impedance driving each diode. Therefore, if there's asymmetry in the capacitance on the phases then the current drawn from the phases may vary from each other.
If that's the issue, I think it would be reflected in the form of excess VTHD at the input terminals. So voltage values for the current setup on (thread stareter's generator), and VTHD comparison between generator and temp power would be useful info for us to see.
 

Flicker Index

Senior Member
Location
Pac NW
Occupation
Lights
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
So, I don't know what you've got in between the point of measurement and the input terminals to the VFDs, but both your loads are showing a current imbalance of about 50%. The level of tolerance to imbalance depends on model and load level. Some are even capable of operating with a single phase input.

If those readings are average, VAB is excessive as it exceeds 5% utilization voltage limit.
 

Jdrais

Member
Location
Florida
Occupation
Electrical Contractor
So, I don't know what you've got in between the point of measurement and the input terminals to the VFDs, but both your loads are showing a current imbalance of about 50%. The level of tolerance to imbalance depends on model and load level. Some are even capable of operating with a single phase input.

If those readings are average, VAB is excessive as it exceeds 5% utilization voltage limit.
 
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