Passive-Damped LCL Filter Optimization for Single-Phase Grid-Tied Inverters Operating in Both Continuous and Discontinuous Current Mode

This paper proposes a filter design method for a passive-damped LCL filter in single-phase grid-tied inverters operating in both continuous current mode (CCM) and discontinuous current mode (DCM). Low-inductance filters leads to two main problems: zero-crossing current distortions and filter resonance at weak power grid. An operation in both CCM and DCM is introduced to eliminate the zero-crossing current distortions. Meanwhile, a design flow chart is proposed to show step-by-step how to design the LCL filter based on root loci. Current control performances are compared between a well-known proportional-resonant (PR) controller employed with the only-CCM operation, and the PI controller with the proposed CCM/DCM operation. A 4-kW prototype is constructed in order to analyze the control performance. Compared to the conventional CCM control, the proposed CCM/DCM control reduces the current total harmonic distortion (THD) by 75.9%. Even at weak power grid (i.e. a high grid inductance of 2000 μH), the proposed control can obtain a low current THD of 0.8% with the filter inductance reduced to only 160 μH, which is 0.5% of the total inverter impedance.