Low Voltage Power Factor Correction Capacitors - Inrush Current Limiting - An interesting discussion

There was an interesting query from Mr. Sajid Munir, a LinkedIn member, in a private LinkedIn message to me, as below:

In APFC Panel, we normally connect the cables between capacitor and contactor in the ring form like a coil. What is its impact on the system and why it is mandatory? More what is its design criteria. For instant, if we have 50KVAR or 25 KVAR stage, how many rings are required?

My answer:

By coiling the interconnecting cable between the contactor & capacitor, you are creating a cost-effective inductor, in series with the capacitor.

You must know that a fully discharged capacitor is akin to a dead short circuit and thus any capacitor's switching impedance would be almost zero. Thus, the capacitor would draw a very high inrush current at the time of switching, limited only by the source impedance. Also, the frequency of this switching inrush current is very high.

Now, when you connect an inductor in series, it offers a high impedance to this inrush current. As the inductive reactance is directly proportional to frequency, the inductive impedance would be more for this high frequency inrush current, in particular.

But, providing an actual inductor would not be very cost effective for low voltage capacitors. (Whereas, high voltage capacitors are supplied necessarily with an in-built current limiting reactor in series).

Now, to make a 'cost effective' reactor for limiting the inrush current of the LV capacitor, we coil the interconnecting cable between the contactor and the capacitor.

There are precise formulae for calculating the magnitude & frequency of the capacitor inrush current as well the value of the series reactor for current limiting, which are beyond the scope of this discussion.

However, as for the number of rings to be made in the cable between the contactor & capacitor, for the given rating of an LV capacitor, the attached table may be taken as a rough guide:

Source for the table: EPCOS AG Application Note