Combined DC-Filter and optimized Modulation to Absorb DC-Link Oscillations of Cascaded H-Bridge Converters

Cascaded H-bridge converters can be employed suitably for applications that need higher voltages, avoiding series connection of semiconductor switches. However, DC-link oscillations are a practical issue when converters operate under three-phase unbalance condition and/or single phase operation. Resultant low frequency oscillations would affect the harmonic performance as well as efficiency of the converter. This paper proposes four various external DC active filter circuits, aiming at cancelling these oscillations. Proposed circuits are simulated, and their performances on compensation of oscillations are compared to select the best choice. Furthermore, an optimized switching pulse train is developed to attenuate the harmonics objectively. The solution is then combined with the selected DC active filter circuit to limit the peak of the oscillations. The combined method can be applied to modular AC-AC multilevel converter as an application of the suggested technique. Simulation results confirm that the combined method limits the DC-link oscillations of cascaded H-bridges more suitably compared to other proposed circuits, and can be employed for single-phase applications as well as unbalanced three-phase systems.