Design of LCL-filters with LCL resonance frequencies beyond the Nyquist frequency for grid-connected inverters

This paper proposes a novel LCL-filter design method and its current control for grid-connected converters. With the proposed design method, it is possible to set the resonance frequency of the LCL-filter to be higher than the Nyquist frequency, i.e. half of the system sampling frequency, and this observation is so far not discussed in the literature. In this case, very cost-effective LCL-filter design can be achieved for grid-connected converters whose dominant switching harmonics may appear at double of the switching frequency, e.g. in unipolar modulated three-level full bridge converters and 12-switch based three-phase pulse-width modulated (PWM) converters. Moreover, a single-loop current control strategy is proposed for the designed LCL-filter, and the control system is inherently stable without introducing any passive or active damping. Experimental results, showing the high quality output current and excellent resonance attenuation are presented in the paper, which are also in very good agreement with those simulated ones. These results successfully verify the feasibility of the proposed LCL-filter design and its current control.