Harmonic filter

[Gauss2k]

I am currently reading a report talking about the test and sizing of an harmonic filter.

I have a couple of questions about how they managed to get their results.

Here we go. The filter is made of one 3-phase reactor, with a value on the plate of 115 uH. In series with this reactor are four 3-phase capacitors (identical) in paralel.

The specifications of each of the 4 capacitors are: 72 kVAR, 525V.

On the report, they actually measured the capacitance (line-to-line) of the capacitors and got:

AB=351uF
BC=351uF
CA=352uF

(about the same result for each of the 4 capacitors)

They then say that the capacitance per phase is thus 702uF.
First of all, how did they concluded that? Ok, that's twice the measured value, but I don't see why they say that?

Then, the measured value of the 3-phase reactor, on each of it's coil, is 66uH. How can they conclude that this value is ok compared to the value of 115uH on the plate? What is the relationship between the two?

Finally, they conclude that the tuning frequency is thus 280 Hz, on which I agree if we consider a 115 uH reactor in serie with 4x702uF capacitor (total of 2808uF per phase).

I just want to know how we can use those values from the measured values.

Thank you!

 

[Runsor]

Having three 700 uF capacitors in a Y-connection gives 350 uF for the line-to-line capacitance. There are two 700 uF capacitors in series: 700/2 = 350.

 

[Gauss2k]

Thank you for your answers!

Yes, indeed, for the capacitor. I also thought about that later yesterday, it was obvious after all!

As for the measured value for the reactor, 66mH instead of the 115mH mentioned by the manufacturer, I think it is because the value measured on a single phase does not consider the mutual inductance of the 2 other coils. That mutual inductance only affect the value when all 3 coils are feeded. Thus, unless I'm wrong with my little calculation, the mutual reactace when the 3 phase reactance is live, is the measured value on a single coil, 66 mH, + 2/3 of the value for the mutual reactance, which would result in 110mH, which would make more sense.