When installing capacitors for PFC purpose, harmonics need to be taken into account when designing the PFC system in order to prevent resonance conditions that would damage the whole electrical system (fig. 2). When PFC capacitors are connected, the inductance of the transformer together with the capacitors forms a resonant circuit that could be excited by a harmonic current generated by the load. This resonant circuit has a resonance frequency, and if a harmonic current of this frequency exists, it will lead the circuit into a resonance condition where high current will flow through the branches, overloading them and raising the voltage across them and across the whole electrical system that is connected in parallel.

Detuned PFC filtering is a technique to correct the power factor and avoiding the risk of resonance condition performed by shifting the resonance frequency to lower values where no harmonic currents are present. This is achieved by modifying the basic LC circuit formed by the transformer and the capacitor bank, introducing a filter reactor in series with the capacitors. This way, we obtain a more complex resonant circuit but with the desired feature of having a resonance frequency below the first existing harmonic. Besides this main objective, the reactor connected in series with capacitors will offer a low impedance path. Filtering of harmonic currents and “cleaning” of the grid will be achieved.