Current Imbalance Definition 2

[peebee]

Please confirm:

Current Imbalance is defined as "(Imax-Imin)/Imax" ?

Shoud Iavg be the denominator instead of Imax?

Is the preferred term "Imbalance" or "Unbalance"?

 

[dpc]

Not sure of your application, but I believe NEMA, in MG-1, defines unbalanced **voltage** as the maximum percentage deviation of any one phase from the AVERAGE of the three phases. I assume NEMA would define unbalanced current the same way.

This is the basis for the NEMA limitations on "voltage unbalance" in MG-1.

 

[Bung]

I have never heard or seen the term "imbalance".

Another way to define unbalance (from the IEC world) is to define it as the ratio of negative phase sequence voltage to positive phase sequence voltage. Makes better engineering sense to me, but the American way is aesier for "back of the envelope" calcs using magnitudes only. So you pays your money and takes your choice!


Bung
Life is non-linear...

 

[electricpete]

I am familiar with dpc's definition from NEMA standards.

Note the difference between using max and average is small.

There is a big difference between "max - min" and "max deviation from average" Typically almost a factor of 2 difference.

With unbalance as defined by dpc (max deviation from average over average), unbalance is typically a close approximation to negative sequence as fraction of positive sequence. Unbalance can be calculated with magnitudes only. Negative sequence requires phase information as well.

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[DanDel]

ANSI MG 1-1987 list 'percent imbalance' of voltage or current as: %imbalance = 100% x (max deviation from average/average)
IEEE Recommended Practice for Monitoring Power Quality Std 1159-1995 lists "Voltage Imbalance(or unbalance)" in Part 4.4.4 as equal to 100 x (max deviation from average voltage)/average voltage. It does not list parameters for current imbalance.
I have seen the terms 'imbalance', 'inbalance', and 'unbalance' used interchangeably.

 

[peebee]

Red star for DanDel and his valuable post!

 

[buzzp]

I believe all the terms are correct, imbalance, unbalance. Current imbalance is defined the same way as voltage imbalance although I have never seen a formula for this in any MG standard. It is assumed to be calculated the same as the voltage imbalance.

 

[cuky2000]

The ANSI/IEEE Std 242 recommends for motor/generator protection against unbalance phase current or single phase detection, the Balance Current Relay (Device 46). There are two ways to operate this device:
a- Difference in magnitude of rms phase current that exceed a given percentage.

b- Measuring the magnitude of negative sequence current, but calibrated in term of I^2t characteristics of the machine.

Relay manufacturer suggest the following calculation methods for unbalance current:
UB% = |Imax-Iav|/Imax.100
_{See site below:}

http://www.automatictiming.com/pdf_div/3phase_current_unbalance.pdf

Other manufacturers suggest the following calculation method with somehow different than above.

Case a: Iav > I_flc; ==> UB% =|Imax-Iav|/Iav.100
Case b: Iav < I_flc; ==> UB% = |Imax-Iav|/I_flc .100

_{See page 1-4 for Calculation Method:}

http://www.geindustrial.com/products/manuals/mm3/mm3-a4_1.pdf

_{Where:
UB% = Unbalance phase current in %
Iav = average phase current
Imax = current in a phase with maximum deviation from IAV
Iflc = motor full load current setting}

 

[peebee]

No wonder I was confused. I Googled this thing pretty hard before I gave up and posted this thread here, too.

Thanks cuky for your input. Red star for you too.

 

[DanDel]

Thanks, peebee.

 

[eemotor]

I have also seen current unbalance calculated using:

Iavg = (Imax + Imin)/2

%Ia = ((Ia - Iavg) / Iavg) * 100
%Ib = ((Ib - Iavg) / Iavg) * 100
%Ic = ((Ic - Iavg) / Iavg) * 100