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Old 08-29-2012, 11:29 AM   #1
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Symmetrical components


Hello everyone

If this isn't the right place to ask, my apologies, but I am trying to build an abc to positive/negative/zero components transformation block (in LTspice), I did it, but sometimes I get erroneous behaviour: DC levels seem to be "omitted" at the output for an abc > 120 > abc chain.

The schematic I'm using is this:



and the waveforms are these:



You can see that all waveforms are identical, but b/b* and c/c* have a DC difference between them. Here are the differences:


(in1, in2 and in3 are the a, b and c in the previous screenshots)

The signals are these:

- all three phases have unity fundamental + 3rd/3, 5th/5 and 7th/7 all positive harmonics;
- phase a has twice the amplitude;
- phase b has 0.1Vdc;
- phase c has 0.3rad phase lead.

Is this normal? Are DC bias levels "left behind"?

In order to build the schematic for a practical use, I had to maneuver a bit through the equations to avoid the [+/-]2*pi/3 delay, and so I transformed exp([+/-]j*2*pi/3)=−1/2[+/-]j*sqrt(3)/2, which left only j to take care of, a minor leap for SPICE-kind.


Thank you in advance,
Vlad

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