How does a notch filter mitigate harmonic distortion in a distribution system, and what are its limitations?

A notch filter, also known as a band-stop filter, is designed to attenuate or block a specific range of frequencies while allowing frequencies outside that range to pass through with minimal attenuation. In the context of harmonic mitigation, a notch filter is typically tuned to the frequency of a dominant harmonic, such as the 5th or 7th harmonic, which are often generated by nonlinear loads like variable frequency drives (VFDs) or electronic ballasts. The filter consists of an inductor (L) and a capacitor (C) connected in series, creating a low impedance path at the tuned frequency. This low impedance path shunts the harmonic current away from the system impedance, thus reducing the harmonic voltage distortion. Ideally, the notch filter would completely eliminate the targeted harmonic, but in practice, it reduces the harmonic to an acceptable level. A limitation of a notch filter is that it is only effective at the specific frequency to which it is tuned. If the harmonic frequency shifts or if other significant harmonics are present, the notch filter's performance degrades. Also, the filter's effectiveness is sensitive to changes in system impedance. Furthermore, notch filters can create resonance at other frequencies if not properly designed, potentially amplifying other harmonics. Therefore, careful design and tuning are essential to avoid unintended consequences.