Active damping control of LLCL filters based on virtual resistor for T-type three-level PWM converters

This paper proposes an active damping control scheme of the LLCL filters for the grid-connected three-level PWM converters, which is based on the virtual resistor concept. The LLCL filter structure consists of connecting a small inductor in series with the capacitor in the conventional LCL filters, which can attenuate the switching-frequency-related current ripples more effectively than the LCL filters, resulting in a significant reduction in the grid-side inductor size. However, the resonance phenomenon is still an issue to solve in this filter. By adding a virtual resistor, the resonance can be damped out, which is less dependent on the variation of the grid impedance. The simulation and experimental results have verified the effectiveness of the proposed scheme for three-phase T-type AC/DC PWM converter.