Voltage Unbalance

[rockman7892]

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?

 

[Skogsgurra]

One percent is quite common at the LV level, but shouldn't be accepted at MV levels.

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

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[ePOWEReng]

NEMA MG1 states that motors shall operate successfully under rated conditions at rated load when the voltage unbalance at the motor terminals does not exceed 1%.

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

http://www.ess-engineering.com

 

[slavag]

Hi.
Please see attached thread on the topic.
thread238-201894
Hope that help.
Best Regards.
Slava

 

[rockman7892]

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?

 

[dpc]

Keep in mind that the NEMA MG-1 definition is based on the maximum deviation from the average voltage, not between the max and min voltages.

 

[ePOWEReng]

dpc

You are correct. That is a common misconception. Thank you for the clarification!

ePOWEReng

http://www.ess-engineering.com

 

[rockman7892]

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?

 

[dpc]

I don't think I'd be too concerned about 1%. I'd start being concerned at 1.5% to 2%. At any event, at the MV level, a lot of this unbalance may be coming from the incoming power from the utility.

(I wouldn't make any assumptions about how a digital meter calculates imbalance.)

 

[stevenal]

The ANSI C84.1 recommendation is to limit unbalance at the meter to 3% under no-load.

 

[busbar]

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