Analysis of the DC-Link Current for the Single-Phase H-Bridge Inverter Under Harmonic Output Currents

This paper investigates the dc-link current to facilitate the dc-link capacitor sizing for the single-phase H-bridge inverter under harmonic ac currents. As the capacitor equivalent series resistance varies significantly until the frequency is higher than 1 kHz, the expressions of the dc-link current individual low-order harmonics (LOHs) and the switching harmonic currents (SHCs) root-mean-square (rms) value are derived, respectively, to accurately evaluate the dc capacitor losses. The dc-link LOH expressions reveal that each harmonic ac current induces two harmonic currents in the dc-link. When the ac current has multiple combinations of LOHs, the dc-link current envelopes take infinite possible unusual forms, so after somewhat complicated derivation, the SHCs rms equations are successfully derived and further simplified into one equation for easy worst case design as demonstrated by the actual case study. A single-phase H-bridge inverter lab prototype is developed to inject arbitrary combinations of harmonic currents of different orders, amplitudes, and angles into the grid. The good match between the computational, simulation, and experimental results validate the effectiveness of the proposed method. In particular, the worst case analysis is verified by the experiment and the differences between the experimental and computational results are formulated and explained definitely.