A harmonic filter is a circuit or device that reduces selected harmonic frequencies in a power system. Harmonics are extra frequency components created by non-linear loads such as drives, inverters, and converter stations, and they distort the normal 50/60 Hz AC waveform. Filters may be passive, using tuned LC components, or active, measuring distortion and injecting compensating current. Their job is to keep voltage and current within acceptable limits, protect equipment, and reduce stress on the grid.
In cyber security, harmonic filters matter because many critical systems depend on power quality. Industrial control systems, substations, and data centers can all be affected if harmonics grow large enough to trigger faults, damage equipment, or confuse monitoring tools. Attackers targeting OT environments may abuse or disable power-conditioning gear, while defenders watch harmonic signatures as part of anomaly detection and maintenance. In practice, filter performance is part of the security and resilience of the electrical environment, not just an electrical detail.