Analytical Calculation of the DC-Link Current Harmonics in Active Power Filter Considering Coupling Effect

Current harmonic is an important factor in power converters’ dc-link design. Its accurate derivation is conducive to estimating the reliability of dc-link capacitors. In active power filter (APF), due to its wide output bandwidth, the dc link suffers current harmonics on a broader spectrum, and the coupling effects among compensated harmonics make the derivation in this application more complex. However, the traditional methods in voltage source inverters usually assume the output current to be a simple sinusoid, or only calculate the root mean square values. They cannot readily illustrate the coupling effect, nor discriminate between different modulation strategies. This article presents a general method for APF to solve the dc-link current harmonics under any operation condition. Using the double-Fourier theory, a new variable called “harmonic factor” is defined to describe the coupling effect among compensated harmonics. Then, the dc-link current spectrum is quantitatively derived with the low-frequency compensated harmonics and the switching-frequency ripple currents. Finally, the simulation and experiment of a 2-level 100-A APF verify the theoretical analysis.