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# Power calculation from Voltage and current waveforms

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Stefano Albertini on 17 Feb 2021
Hi to everybody!
I'm trying to calculate active and reactive power starts from current and voltage waveforms.
To do that i've prepared a simulink model and a matlab script, i'll attached to this post.
With simulink is really easy to obtain datas I'm looking for, but it's just a model. My goal is to write analytic formula to do that and after implement into a FPGA for a real calculation in AC/DC application.
Theory said that : with t2-t1=period T.
So in my code I've multiplie voltage vector for current vector and integrated it during 1 period. The result is not god, but I've no idea why.
Maybe use trapz as integral is not the best way?
Could you help me?
##### 2 CommentsShowHide 1 older comment
David Goodmanson on 4 Mar 2021
Hi Stefano, nothing I or anyone else can really say without seeing the data. Certainly if the simulink data contains more than one frequency component, then things are different.

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### Answers (1)

David Goodmanson on 24 Feb 2021
Hi Stefano,
The biggest issue is that when using trapz, you have not taken into account the width of the time steps when doing the integration. WIth only one input provided to trapz, that program assumes each width to be 1. For proper results the time array must be inputted as well.
Starting where your code leaves off, the following integration agrees with powerP = 1.5256e+03
Note that the line
Icarico_rms = rms(abs(Icarico));
is highly misleading. Since abs(Icarico) is a real scalar, and since rms(scalar) = scalar), the rms function does not actually do anything. Fortunately Icarico is basically an rms quantity already.
% go to amplitudes since the integral will take care of the rms adjustment
V0 = Vmod_rms*sqrt(2);
I0 = Icarico_rms*sqrt(2);
phi = angle(Icarico);
t = linspace(0,1/50,100001);
V = V0*cos(2*pi*freq*t);
I = I0*cos(2*pi*freq*t + phi);
% plot, and multiply I by 10 for visual purposes
plot(t,V,t,I*10)
legend('V','I*10')
title('ELI the ICEman')
PowerPint = (1/(1/50))*trapz(t,V.*I)
##### 3 CommentsShowHide 2 older comments
Stefano Albertini on 3 Mar 2021
Hi David
I referred to simulated ones.
In my script I lunch a simulink simulation, and I'm trying to calculate power (with trapz) from data imported in workspace from Simulink.
With original value of Rcarico (sorry I've writed Rload) power calculated with trapz is similar to the one I've calculated by formula.
If I change Rcarico value (for example divided by 2) power with trapz is really different from Power calculated with formula.
For this reason I think there still be some problems with trapz.
Thanks!
Stefano

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