A robust design strategy for resonant controllers tuned beyond the LCL filter resonance frequency

LCL filter is one of the most prevailing approaches for harmonic filtering due to its premium switching order harmonic attenuation performance. However, resonance issues, which may cause system instability, are also induced by the LCL filter. Thus, bandwidth of the inner current loop is required to be sufficiently lower than the filter resonance frequency to ensure system stability. In this paper, a robust resonant controller design strategy for converters with LCL filters is proposed to enhance the harmonic control capability. With the proposed design method, it is possible to tune the central frequency of the resonant controller above the LCL filter resonance frequency without triggering instability issues. In contrast with conventional techniques, the LCL filter can be designed with lower resonance frequency by utilizing the proposed approach. This way, a better switching order harmonic attenuation can be achieved without sacrificing high order harmonic tracking capability. Experimental results are presented to demonstrate the effectiveness of the proposed design strategy.