When a sinusoidal voltage is applied to a non-linear circuit, both the resulting current and the voltage waveform become non-sinusoidal. Through Fourier series decomposition of these non-sinusoidal waves, the component with a frequency equal to the power system frequency is identified as the fundamental wave, while components with frequencies higher than the fundamental are termed harmonics. A vast majority of loads in widespread use today—such as rectifiers, variable frequency drives (VFDs), UPS systems, elevators, air conditioners, energy-saving lamps (fluorescent lights), photocopiers, and household appliances—are non-linear. These loads generate significant harmonic currents that are injected into the power grid, causing voltage distortion; such harmonic "pollution" poses serious risks to both the grid and end-users. Furthermore, impulsive and fluctuating loads—such as electric arc furnaces and welding equipment—not only generate substantial high-order harmonics during operation but also cause power quality issues like voltage fluctuations, flicker, and three-phase unbalance, thereby jeopardizing the safe operation of the power grid.

Impact of harmonics on the power grid. Capacitors used for reactive power compensation are widespread in power grids; at certain frequencies, conditions for series or parallel resonance may be met. When harmonic levels are sufficiently high, dangerous overvoltages or overcurrents can occur, often damaging electrical components and equipment and seriously compromising the safe operation of the power system.

Impact of harmonics on rotating electrical machines. Harmonics primarily cause additional losses and overheating in rotating machines, by mechanical vibration, noise, and harmonic voltages. These effects shorten the machine's lifespan and, in severe cases, can even cause damage.

Impact of harmonics on transformers. When harmonic currents from harmonic sources flow into a transformer, the primary impact is an increase in copper and iron losses, leading to elevated heat generation and noise.

Interference of harmonics with communication systems. Harmonic interference introduces noise into communication systems; severe interference can result in signal loss and may even pose risks to equipment and personal safety.

To address the aforementioned complex issues, comprehensive mitigation using grid filtering devices is required to resolve the problem of grid harmonic pollution. This led to the emergence of the Active Power Filter (APF). As a power electronic device capable of dynamic harmonic current compensation, the APF differs from traditional passive filters by being unaffected by system impedance; it offers rapid response, automatic tracking and compensation, high harmonic filtering efficiency, and a compact footprint. The deployment of active filtering devices plays a vital role in ensuring power quality, reducing grid losses, and minimizing the occurrence of accidents.