Detuned Filter

Detuned Filter:Detuned filter are used, in supply networks having a high level of harmonic distortion, when the final objective is reactive power compensation at the fundamental frequency. Their purpose is to avoid the harmonic currents overload to the capacitors by diverting them to the mains. Protection filters are made by connecting reactors in series with capacitors, in such a way that the tuning frequency of the whole unit is set at a value between the fundamental frequency and the frequency of the lowest present harmonics, which is usually the 5th order harmonic. In this the filter has a high inductive impedance for all the harmonic frequencies.Connection of a reactor in series with a power capacitor, makes the capacitor to work at a voltage higher than the supply voltage. Because of this, capacitors need to be connected to protection reactors and need to be designed to work at higher voltages than standard capacitors. The choice of the tuning point of the filters is to balance between the quantity of harmonics rejected by the filter and the voltage increase produced in the capacitor at the fundamental frequency.It is to be also kept in mind that reactive power supplied by the filter at rated frequency (50 or 60 Hz), is different to the one that the capacitor would supply without the reactor. Taking all the above into account, the reactor is normally chosen in such a way that its impedance is about 7% of the impedance of the capacitor that protects. This will give a tuning frequency, for example at 50 Hz, of 189 Hz. Other tuning frequencies are also available.