Why use Detuned reactor?

Presence of Harmonic distortion due to the non-linear loads within the network or due to import of harmonic from grid or power source will increases the current flowing through capacitors as the capacitive reactance is inversely proportional to the frequency, consequently capacitors will be subjected to overload. The overload on capacitors can cause premature failure in capacitor due to increased voltage and thermal stress on dielectric.

On the other hand, the inductive reactance XL of a Reactor is directly proportional to frequency: The magnitude of inductive reactance will increase with high frequency harmonics thus blocking the harmonic current. Hence, use of detuned reactor in series with capacitor will offer higher impedance for harmonics, thus eliminating risk of overload in capacitors.

The inductance value of detuned reactor is selected such that the resonance frequency is less than 90% of dominant harmonic in the spectrum. For example: if 5th harmonic is dominant in the spectrum, any series LC circuit having resonance frequency 90% of 250Hz (for 50Hz system), i.e. if the natural resonance frequency of LC is less than 225Hz, it is categorized as detuned filters or detuned capacitors.

In a detuned filter application, the voltage across the capacitors will be higher than the nominal system voltage due to Vector sum of voltage drop, hence the capacitors must be designed to withstand higher voltages. Typical capacitor voltages for 400/415V network are 480V, 525V etc.