Design of LCL filter for improving robustness of grid-connected voltage source inverter

Abstract

In this paper, a design method of the LCL filter for the robustness of grid-connected voltage source inverter (VSI) is proposed in the case of the grid-side current feedback control. A stable region is investigated for the robust operation of VSI with regard to stability margins without a passive or an active damping. The proposed design method presents a means to place the resonance frequency in a proposed stable region, and considers the harmonic currents injected into the grid. Also, the grid impedance is considered for the system stability. The inverter-side inductance and the filter capacitance performs an important role in the proposed design method. A grid-connected VSI with filter parameters designed by the proposed design method is experimentally verified. I. INTRODUCTION In grid-connected applications, LCL filters have been widely used for the better attenuation of the switching harmonics with lower inductance and cost than L filters to meet the harmonic limits in the grid (1)-(6). However, the stability problem is caused by the resonance phenomenon, and it should be solved for the stable operation of voltage source inverters (VSIs), which is the greatest concern in the LCL filter. In the case of grid-side current feedback control, a small stable region which does not require the damping emerges due to delays caused by a computation and a pulsewidth modulation (PWM) in the digital control (7), (8). This stable region depends on the ratio of the sampling frequency to the resonance frequency. If the resonance frequency is placed out of this region, the damping method is essentially needed for a stable operation. The damping methods which have presented in the other studies cause power losses (1)-(3) or increase a control complexity and costs (9)-(13). In this paper, a design method of the LCL filter for the robust grid-connected VSI without damping is proposed in the case of grid-side current feedback control. A stable region represented by the resonance frequency is investigated for the desired phase margin (PM) ensuring a certain gain margin (GM), and to fulfill the harmonic limits in the grid. This region guarantees the system stability without damping. An upper limit of the resonance frequency is suggested for the harmonic currents in the grid, and a lower limit is suggested to guarantee the GM affected by the resonance. The proposed design method presents a means to place the resonance fre- quency in the proposed stable region, and considers harmonic currents in the grid. Also, the grid impedance is considered for the robustness. The inverter-side inductance and the filter capacitance perform an important role in the proposed design method. The proposed design method of the LCL filter is an effective means to place the resonance frequency in the stable region with only the filter parameters. The grid-connected VSI with filter parameters designed by the proposed method is experimentally tested.