Investigation of Filter Requirements and Three-Element Filter Candidates for Cascaded H-Bridges to Achieve Harmonic Compensation with Staircase Modulation

The cascaded H-bridge (CHB) can achieve selective harmonic compensation (SHC) using the harmonics generated from its staircase modulation. However, the maximum harmonic voltages of CHBs with SHC are smaller than those of an H-Bridge (HB) with sinusoidal PWM modulation because of the slower dv/dt and lower switching frequencies. Therefore, the maximum injected current of CHBs with SHC is limited. Moreover, the injected current of CHBs with SHC is sensitive to the undesired perturbations and parameter variations in actual filters and dc bus voltages. Therefore, the applications of CHBs with SHC are limited. This paper develops models to explain why the CHB with SHC is susceptible to perturbations and parameter variations, and why the conventional single-inductor or the T-type LCL filters are not the best to minimize the susceptibility. One-element, two-element and three-element filters are compared in the paper. The T-type CCL filter is identified as an optimal filter topology to achieve good compensation capacity and small size. The parameter design procedure is developed for a CHB with SHC using a CCL filter. Simulation and experiments with an 11-level CHB validated the effectiveness of the proposed CCL filter.
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