Negative VARs

[kagen327]

Hi Everyone,

I have a power meter that measure PF, Real (kW) and Reactive Power (KVar), etc. Sometimes some of the meters measure negative KVars. I don't understand why some select moments there would be select measurements the are negative. The only downstream load is a electric vehicle.

Any ideas?

Thanks!

 

[electricpete]

I'm not very familiar with the type of system you're talking about, but I assume it's all sinusoidal ac (because you're talking about vars).

I assume there is a generator and a load.

It is logical to assign a reference direction for REAL power flow. For example from generator to loads is usually the positive reference direction for real power flow.

The same reference direction is also selected for current.

If you follow through the math using Q = Re {V conj}, then you would conclude the following:

[*]If the load fed by the generator is inductive, then the reactive power [Q] flowing from the generator to the loads is positive.

[*]If the load fed by the generator is capacitive, then the reactive power [Q] flowing from the generator to the loads is negative (or alternatively we say the [positive] vars flow in a direction from from load to generator).

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(2B)+(2B)' ?

 

[electricpete]

Sorry... obvious correction in bold as follows:

Q = Re {V conj}

should've been:

Q = Im {V conj}

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(2B)+(2B)' ?

 

[waross]

Inductive loads draw KVARs. Capacitive loads supply KVARs, or vice-versa if you prefer.
The point is, if you have more capacitive reactance than inductive reactance, your meter will show negative KVARs.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[kagen327]

So no answers to my question yet, I understand what you all have sad and very much appreciate the response. Can you all expand a little more?

What I am saying is that the load (EV) is the same load every time and most of the time the kVars are positive (inductive) and a few select times the kVars ate negative.

My question is why would it ever switch from positive to negative and then back to positive?

The EV is feed with AC and has an onboard charger for charging the battery.

 

[mikekilroy]

did you ask the charger manufacturer this question? seems they would know what their charger is doing to cause what you see.

aso, how relative is this value compared to the real power? are you seeing 4kw charge and +/-0.004kvar? Or what values of each?

 

[sibeen]

As there is no international standard for what a negative kVAR is (as waross was alluding too) it's realistically a moot point.

Sometimes your load is inductive, sometimes your load is capacitive, that does seem to happen in many situations. Not a big deal.

 

[waross]

I have seen banks of discharge lighting (4 kW lighting on a 6 kW generator) supply enough VARs to raise the voltage about 10%. On a large system you would not see a voltage change but you would see a change in VARs when the lights were on. In some cases this may be enough to show negative VARs on a VAR meter.
You have to consider all the loads connected past the EV. The EV may not have anything to do with the negative VARs.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electricpete]

Do you know if the voltages and currents are sinusoidal and steady state?

If not, perhaps look for some waveforms in your equipment, or better yet put a scope on it at the locations where your installed meter senses. That will probably reveal the details. If not, that would be time to investigate the meter details.

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(2B)+(2B)' ?

 

[rasevskii]

Presumably the EV has its own microprocesser controlled charger which is some kind of thyristor or IGBT circuitry, probably a proprietary system. THe AC current waveform as mentioned by electricpete is likely quite distorted, not a sinewave.
THe KW/KVAR meter that you are using is evidently some type of electronic circuit that does not like a distorted waveform. Possibly giving some very odd readings. Or was it something supplied with the EV? But if the EV is in fact fully charged then there may be no KW but only KVARS flowing in the AC circuit. Take all this fancy technology with a large bit of skepticism. (can we say "over-digitalization" )

rasevskii

 

[electricpete]

Here is a paper with some discussion of vars on non-sinusoidal system

http://ie.utcluj.ro/Contents_Acta_ET/2003/Number 2/Paper03_Cepisca.pdf

Can get quite complicated quickly. One thing to note - there are 5 different definitions given (sections 3.1, 3.2, 3.3, 3.4, 3.5).

As noted by Raveski, you have complicated non-passive load. It is probably adjusting firing cycles as the voltage varies, so not truly steady state and not truly periodic.


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(2B)+(2B)' ?