#### PsychicViking

##### Member

% voltage imbalance = (maximum imbalance)/(average voltage)*100

I think I read there were two formulas mentioned in ANSI c84.1. Two formulas does not help. I need one.

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- Thread starter PsychicViking
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% voltage imbalance = (maximum imbalance)/(average voltage)*100

I think I read there were two formulas mentioned in ANSI c84.1. Two formulas does not help. I need one.

- Location
- Horsham, UK

that is a very good report by me

that would no leave question about your unbalance problem to the local utility.

i think that your formula should be this way:

% voltage imbalance = (average of the unbalance voltage)/(voltage that you should expect)*100

- Location
- way north

How do you calculate "(average of the unbalance voltage)" number?--- % voltage imbalance = (average of the unbalance voltage)/(voltage that you should expect)*100

cf

- Location
- Horsham, UK

i do would change the name "(average of the unbalance voltage)" for "(average operation voltage)", i was victim of my own "low budget english".

if it is bigger than 5 % is bad or whatever the criteria that you use.

No.i think that your formula should be this way:

% voltage imbalance = (average of the unbalance voltage)/(voltage that you should expect)*100

The correct ANSI formula (C84.1-2006-C.3) is:

% Voltage unbalance = 100 * (max deviation from Mean of {Vab, Vbc, Vca}) / Mean of {Vab, Vbc, Vca}

where

Mean = (|Vab|+|Vbc|+|Vca|)/3

and the deviations are given by:

|Vab - Mean|

|Vbc - Mean|

|Vca - Mean|

If a utility is following ANSI (C84.1-2006-C.2), the maximum unbalance is 3% at the service point with no load. Most utilities I am familiar with try to stay at 2% or less and are usually below 1%.

NEMA does not consider less than 1% unbalance to be an unusual service condition for a motor and you would normally not have to derate the motor (actually about 99% of rated HP at a 1% unbalance). Operation above a 5% unbalance is not recommended.

FWIW, the IEC formula is (not really applicable in the US):

Negative sequence voltage unbalance = V2/V1 = sqrt{[1-sqrt(3-6*B)] / [1+sqrt(3-6*B)]}

where B = (Vab^4+Vbc^4+Vca^4) / (Vab^2+Vbc^2+Vca^2)

- Location
- Horsham, UK

because the Vab, Vcb and Vca that you use are puntual or instantaneous data. my data is stadistic. my data have less error than yours. and i can prove it

and what if is a single phase Voltage? V-N? or the client dont need or want to know whats happening in the other phases but just the one that he need?

- Location
- way north

May -agree with you mivey. but that doesnt mean that my method is wrong. in fact i bet my method is more accurate that the method that you show. ---

If you are trying to describe voltage imbalance, your method doesn't do that.. So, the accuracy may not matter much. :-?

cf

I think you would lose that betagree with you mivey. but that doesnt mean that my method is wrong. in fact i bet my method is more accurate that the method that you show.

says who?because the Vab, Vcb and Vca that you use are puntual or instantaneous data.

I would love to see the proof.my data have less error than yours. and i can prove it

You do know that unbalance is referring to the unbalance between phases, right?and what if is a single phase Voltage? V-N? or the client dont need or want to know whats happening in the other phases but just the one that he need?

- Location
- Horsham, UK

is unbalance? or imbalance? :S

is there a diference? what does my method does then?

is there a diference? what does my method does then?

Use unbalance.is unbalance? or imbalance? :S

is there a diference? what does my method does then?

I'm not sure what you are doing as you have some undefined terms in your equations. Your proof would clear that up.

- Location
- Horsham, UK

what i was trying to say about the Vn voltage, is that if i buy a single phase output service for V= 1 p.u, and i have a V = 0.8 p.u something is wrong with the service, (it can be an unbalance problem or a Vd problem, or neutral problem), i think that was the issue, how to prove that you have a bad voltage service.

excuse me if im wrong.

didnt mean to offend you mivey.

Don't think that. I'm not offended in the least. Short replies just mean I'm being lazy.didnt mean to offend you mivey.

Sounds like you are trying to address the voltage rating and operating limits, not the voltage unbalance. The voltage ratings and acceptable voltage ranges are the main thrust of ANSI C84.1.i cant prove nothing if i dont know how are you getting the values of Vca, Vab and Vbc (or how the op will obtain this data).

what i was trying to say about the Vn voltage, is that if i buy a single phase output service for V= 1 p.u, and i have a V = 0.8 p.u something is wrong with the service, (it can be an unbalance problem or a Vd problem, or neutral problem), i think that was the issue, how to prove that you have a bad voltage service.

BTW, Using the average of the individual readings over an hour may not help you. The average voltage unbalance over an hour might not be too bad, but the motor/chiller/etc can trip long before that.i cant prove nothing if i dont know how are you getting the values of Vca, Vab and Vbc (or how the op will obtain this data).

How I get the values depends on the situation but I use a snapshot of the three phase meter readings if I have my three phase meter with me. This is not quite "instantaneous" as you would need to capture the waveform to get that because the meter reading is damped. I usually take several readings. The preferred reading, of course, is when there is an anomaly.

A single phase meter will work fine if the voltage is fairly stable. If it is moving, several readings may be required to get an average reading, but I would normally take the set of readings in less than a minute.

- Location
- Horsham, UK

agree with you mivey in 100%.

hope the op had come to a conclusion and his doubt had been resolved.

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