Displacement power factor correction systems are made up of switched banks of capacitors adding capacitive reactive current to neutralise the inductive reactive current of the load.
In the presence of high harmonic currents, the woltage waveform becomes distorted with the distorted voltage applied to the terminals of the capacitors.
Harmonic voltages increase the currents flowing through the capacitors and can cause premature capacitor failure.
The impedance of the capacitor reduces with increasing frequency. The addition of a series reactor causes the impedance to rise with increasing frequency and reduces the harmonic currents through the capacitors.
The addition of detuning reactors to power factor correction capacitors causes the terminal voltage of the capacitors to rise in the presence of harmonics.
It is important to increase the voltage rating of the capacitors condsiderably.
Capacitors designed for use with detuning reactors are typically rated at 525V or 565 volts for a 380/400 volt system. The KVAR rating of the capacitors must be based on the actual supply voltage and the continuous applied voltage rating must be much higher.
The detuning reactors are designed to be used with special high voltage capacitors and are rated in KVAR where the KVAR rating is the KVAR of the capacitor that they are designed to be used with. You use a 25KVAR detuning reactor to detune a 25KVAR capacitor.
The series circuit of reactor and capacitor forms a resonant circuit. It is important that the resonant frequency of this tuned circuit is not near a harmonic frequency. This is why the detuning reactor must match the detuned capacitor.