Negative VARs
Negative VARs
(OP)
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!
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!






RE: Negative 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[I]}, 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)' ?
RE: Negative VARs
should've been:
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(2B)+(2B)' ?
RE: Negative VARs
The point is, if you have more capacitive reactance than inductive reactance, your meter will show negative KVARs.
Bill
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"Why not the best?"
Jimmy Carter
RE: Negative VARs
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.
RE: Negative VARs
aso, how relative is this value compared to the real power? are you seeing 4kw charge and +/-0.004kvar? Or what values of each?
RE: Negative VARs
Sometimes your load is inductive, sometimes your load is capacitive, that does seem to happen in many situations. Not a big deal.
RE: Negative VARs
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
RE: Negative VARs
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)' ?
RE: Negative VARs
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
RE: Negative VARs
http://ie.utcluj.ro/Contents_Acta_ET/2003/Number%2...
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)' ?