Capacitive Reactance

When you see a reactor in series with a capacitor it is usually a tuned circuit that is installed to handle harmonics that are generated within the system.

a short note on reactors

Electrical energy is a significant production factor for industry, and its efficient use should be a primary objective. Reducing the reactive current component by PFC correction helps to save energy.

The increasing use of modern power electronic apparatus (drives, uninterruptible power supplies, etc) produces nonlinear current, influences and loads the network with harmonics (line pollution). The power factor correction or capacitance of the power capacitor forms a resonant circuit in conjunction with the feeding transformer. the self-resonant frequency of this circuit is typically between 250 and 500 Hz, i.e. in the region of the 5th and 7th harmonics.Resonance can lead to the following undesirable effects:

- overloading of capacitors,

- overloading of transformers and transmission equipment,

- interference with metering and control systems, computers and electrical gear,

- resonance elevation, i.e. amplification of harmonics,

- voltage distortion.

These resonance phenomena can be avoided by connecting capacitors in series with filter reactors. Detuned systems are scaled so that the self resonant frequency is below the lowest line harmonic. The detuned PFC system is purely inductive seen by harmonics above this frequency. For the 50 Hz line frequency, the detuned system acts purely capacitively, thus correcting the reactive power.

for calculation please go through the last 5 pages of this pdf.

http://www.epcos.com/web/generator/Web/Sections/Publications/PDF/PFC__Katalog,property=Data__en.pdf;/PFC_Katalog.pdf