Robust CCFAD method for suppressing harmonic resonance of LCL grid-connected inverter in weak grid

LCL filter grid connected inverter adopts capacitive current feedback active damping for network access control under weak current network. When the system resonance frequency is higher than 1 / 6 of the sampling frequency, the digital control delay will lead to poor robustness and even instability of grid connected inverter under the influence of wide-range varying grid impedance. Through analysis, it is pointed out that the capacitive current feedback active damping method is equivalent to the virtual impedance Zeq(s) connected in parallel at both ends of the filter capacitor, and the system instability is mainly caused by the negative resistance characteristic of Zeq(s). A robust method was proposed by using negative first-order inertia link for capacitive current feedback active damping. The inertia link was introduced into the capacitive current damping loop, and the frequency stability analysis method was used to discuss it in detail, and the design process of related parameters was further given. Theoretical analysis shows that this method can ensure that Zeq(s) is always in the range of positive resistance characteristics within the effective range of the resonance frequency of the LCL filter, which not only improves the stability margin of the system, but also suppresses the harmonic resonance of the grid-connected system. In addition, this method has better flexibility. If capacitive voltage feedback active damping control is used and phase-locked, the use of a set of current sensors is saved. Finally, effectiveness of the proposed method is verified through experiments. © 2023 Editorial Department of Electric Machines and Control. All rights reserved.