how does the power values VI calculates active power, reactive power and power factor if the voltage and current waveforms have distortion?

[Hao_Liu]

Hi,

I am currently set up an VI to monitor the power values based on the voltage and current acquired by NI hardware. The voltage are basically mains voltage at 240V and 50Hz while the current could be drawn from different household appliances(say fan, fridage, heater and so on). In terms of calculating the power values, the basic formulas, for instant, P=Vrms*Irms*cosµ, S=Vrms*Irms, are all based on the condition that the waveforms are all sinusoidal. In some of my cases, the current has obvious distortion. My question is that:

1. how does the power values VI calculates them?
2. is it still applicable when the voltage and current waveforms are not purely sinusoidal?
3. how can i get the accurate power values for distorted waveforms if the VI doesnt work in such case?

Many Thanks,

Hao

[johnsold]

Hao,

That is a difficult problem for which the answer is not well defined. Part of the problem is that as soon as waveform distortion is introduced, the signals have higher harmonic content and it is no longer possible to assign a single value to the phase shift between voltage and current.  Some authors consider each harmonic and determine a phase shift for each. Even then it is not clear how to assign a value to terms such as power factor so that it is rigorously correct in the same sense as when applied to sinusoidal signals.

Provided (1) that you can accurately determine the beginning and ending points of one cycle or period of the fundamental and (2) the distortion does not change very fast, you can get a pretty good measure of the power (real power) by multiplying the voltage and current sample by sample and integrating over one period (or an integer number of periods during which the waveforms do not change).  The reactive power is not so easily defined or measured.

Lynn

[Hao_Liu]

Hi, Lynn

Thanks for your reply.

I totally agree with you. I read some material which mentioned that the general definition for real power is calculated as you say in (2), mutiplying V and I, then accumulating the product sample by sample and finally taking the average of the accumulation over one cycle. I think this method accords with the basic meaning of real power which generally decribes the real work done by the current drawn I under voltage V no matter what shape the waveforms of V and I are. Apart from this, I also came across some reading mentioned that the reactive power can be defined in the similar way. The only difference is that the current I should be shifted by 90 degree, then do the same process (multiplying, accumulating and averaging). Do you think this method is fairly reasonable?

Many Thanks,

Hao