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For grid-connected inverters, switching harmonics can be effectively attenuated through an LCL-type filter. In order to suppress resonance and guarantee good performance, many strategies (e.g. active damping (AD), harmonic resonant control, repetitive control and grid feedforward) have been proposed. However, the wide variation of grid impedance value challenges system stability in practical applications. The aforementioned methods need to be investigated. This study evaluates the applicability of each part of the overall control in a weak grid case with the use of a stability criterion. It has been demonstrated that the feedback-based AD control can work well in a wide range of grid conditions. However, the resonant and repetitive control methods meet constraints. The grid feedforward method brings in an extra positive feedback path, and consequently results in high harmonics or even instability. Finally, a recommendation for system design has been presented. Simulations and experiments have been provided to verify the analysis.

In grid-connected LCL-filtered inverters, the dual-loop current control is widely used. The LCL resonance is highly damped by proper feedback of the capacitor current. To suppress low-frequency current harmonics, a grid-voltage feedforward is commonly used. However, the system performance with such control is declined when connecting to a weak grid. Phase and gain margins are largely reduced while the proportional feedforward is used. The margins get worse and instability is aroused if the derivative feedforward is further implemented. Besides, a negative impact on the rejection of gridvoltage-induced harmonics is produced. To improve the control performance in the weak grid case, an adaptive control has been proposed. Adopting the estimated grid impedance, the signal for the grid-voltage feedforward is modified and the controller is adjusted with an adaptive rule to maintain a good phase margin or a high bandwidth. Performances of the inverter with the typical and the proposed methods are compared. Simulation and experimental results have demonstrated that the grid-connected LCL-filtered inverter with the proposed method produced a high-quality current while large grid impedance existed.

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