Compensation of DC-link oscillations of cascaded H-bridge converters

Single-phase AC applied voltage of an H-bridge converter produces second harmonic on top of the DC-link voltage. Three-phase unbalanced voltages make similar effects on the DC-link voltages, as well. Nevertheless, for applications that need higher voltages, series connection of H-bridges could lower the amplitude of the oscillations. Low-frequency oscillations are considerable when the number of cascaded H-bridges is less than four, introducing the worst case oscillations for a single H-bridge converter. This paper proposes various external DC active filter circuits, aiming at cancelling these oscillations of cascaded H-bridges up to three. Proposed circuits are simulated, and their performances on compensation of oscillations are compared to select the best choice. Simulation results confirm that the proposed methods limit the DC-link oscillations on DC-link of H-bridges. Also, the presented methods are compared in terms of both their advantages and disadvantages.