Technical Articles
Tuned harmonic filters are used in various installations to supress harmonics at their source. The use of tuned filters ensures that harmonics do not spread throughout the system and to other neighbouring installations.
Tuned harmonic filters are passive filters that consist of an inductor (L) and a capacitor (C) connected in series. The value of L and C is selected in such a manner that the tuning frequency of the combination (ft=1/2π√(LC)) is as close to the frequency of the harmonic that needs to be cleansed from the system as possible.
Based on the tuning frequency, the LC combination forms a series resonant circuit and provides a low resistance path for the targeted harmonic frequency. Depending on the harmonic spectrum of the installation, a tuned filter may consist of several LC branches tuned at different frequencies.
As the tuned filter needs to sink in large currents, it is important that the inductor used in the filter has a high linearity and that the capacitor is able to withstand this higher current. Typically derated ultra-heavy duty capacitors with higher voltage ratings are employed in a tuned filter along with inductors having linearity in excess of 1.8. The impedance characteristics of a tuned filter are shown below:
The above arrangement consists of three tuned filter branches tuned for the 5th, 7th and 11th harmonic frequencies.
Tuned harmonic filters are placed close to the source of harmonic currents. The filter is sized primarily to provide the reactive power requirement of the load and should be capable of handling an incremental current to account for the harmonic currents that are expected. Once the capacitor is sized as per the reactive power requirements of the load, the tuning frequency needs to be arrived at based on the harmonic order that needs to be filtered. The tuning frequency should be close to the targeted harmonic order. Once the tuning frequency has been arrived at, the reactor value L can be calculated using the formula Ft = (1/(2π√(LC)). Care should be taken that the capacitor is able to withstand the overvoltage caused by the series connected reactor. When multiple branches are being used, the distribution of the load KVARs across different branches is iterative and is meant to optimize the filter efficiency. This needs to be simulated before finalization.
The tuned filter will improve the load power factor. Tuned filters are susceptible to voltage and frequency variations and changes in load profile. The filtering efficiency can change dramatically with changes in load profile.
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