Voltage Unbalance
Voltage Unbalance
(OP)
What is a typical acceptance value for voltage unbalance in an industrial plant? I have read that a 1% voltage imbalance can lead to a 6% current balance so I was wondering what an acceptable level is.
For instance if I am looking at the voltage coming in at my main breaker, what is a max unbalance that should be deemed acceptable and what implifications does this unbalance have throughout the rest of the plant. What are possible causes of this unbalance?






RE: Voltage Unbalance
VFDs are sensitive to unbalance, a couple percent can cause them to trip because the low voltage phase doesn't charge the DC link at all. So the other phases have to do double duty - which they are not always willing to do.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Voltage Unbalance
The voltage imbalance that is realistic depends on the setup of the plant. Is the plant fed from a distribution feeder? If so 1% maybe unachievable.
Is the plant fed directly from a large tranmission system? Then 1% is probably realistic.
Remember there is voltage imbalance cause by YOU and the voltage imbalance already present in the system.
Some causes for voltage imbalance are:
*Load imbalance in the plant, which causes voltage imbalance.
*Utility feed imbalance (the utility will maintain voltage unbalance within 1-3% typically)
Some implications:
*motors running hotter
*lower efficiency
*poor voltage regulation
ePOWEReng
www.ess-engineering.com
RE: Voltage Unbalance
Please see attached thread on the topic.
thread238-201894: blown fuse detection
Hope that help.
Best Regards.
Slava
RE: Voltage Unbalance
Thanks for the responses. I'm in the process of bringing a new plant online and therefore a new distribution system. The plant is fed from a transmission system with a utility transformer (owned by us) stepping the voltage down from 230kv to 4.16kV.
The plant is lightly loaded right now as we continue to bring loads online as they become avaliable. I noticed on my incoming metering that the voltage unbalance is 0.6%. I am keeping an eye on this number as the plant becomes more loaded and wanted to know what number would be a cause for alarm or action.
It sounds like 1% is the number not to exceed?
RE: Voltage Unbalance
RE: Voltage Unbalance
You are correct. That is a common misconception. Thank you for the clarification!
ePOWEReng
www.ess-engineering.com
RE: Voltage Unbalance
dpc
yes thank you for that clarification. I have alreay been through the exercise of learning how this is calculated and I'm assuming that most metering equipment uses this average calculation for unbalance.
With an MV system should I set 1% as my limit, or should I strive to acheive some lower percentage if possible?
RE: Voltage Unbalance
(I wouldn't make any assumptions about how a digital meter calculates imbalance.)
RE: Voltage Unbalance
RE: Voltage Unbalance
ANSI Std C84.1 fig D1 plots %-voltage imbalance versus per-unit motor-horsepower derating as follows:
1% 0.98
2% 0.95
3% 0.88
4% 0.82
5% 0.75