Robust Control Scheme Based on PCC Voltage Feedforward and VPI Controller for LCL-type Grid-Following Inverters Under Weak Grid Condition

The control performance of the LCL-type grid-following inverters is challenged by the non-negligible grid impedance and the various background harmonics at the point of common coupling (PCC) under weak grid condition. The PCC voltage feedforward (PVF) is widely used to cancel the effect of the grid disturbance. However, due to the inevitable time delay, it fails to satisfactorily suppress the current harmonics and tends to destabilize the inverter system when the classical resonant controllers are added for a higher current quality. In this paper, this stability issue is explicitly elaborated by the impedance-based stability criterion, and the vector proportional integral (VPI) controller, which is previously used for harmonic regulation in synchronous (dq) frame, is introduced with the combination of a proportional part to form the proposed proportional-VPI (P-VPI) controller in stationary frame. The system stability of the PVF-based control scheme with the VPI-based controller is proved to be much better compared to that with the classical resonant controller, and it is found to be interestingly associated with the LC resonance frequency of inverter side inductor and filter capacitor. Appling the proposed control scheme with the combination of the PVF and the P-VPI controller, the good transient performance under grid disturbance, the low current harmonics, and the high stability-robustness against grid impedance variation can be achieved simultaneously. The theoretical analysis results are verified by experimental results.