Modeling and Delay Compensation Method for Improving the Stability of Grid-Connected Inverter With <i>LCL</i> Filter Under Weak Grid Conditions

In LCL-filtered inverters under weak grid conditions, the point of common coupling (PCC) voltage feedforward have been applied to mitigate the current harmonics caused by background harmonics in the PCC voltage. However, due to the control delay of digital controller, the system stability is significantly reduced. Moreover, since the grid impedance cannot be sensed in practice, conventional model-based advance prediction techniques fail to provide delay compensation any more. To overcome these limitations, a high-accuracy local model independent of the grid impedance instead of the overall system model is established and a novel PCC voltage feedforward with delay compensation is presented. First, the best placement of the delay compensator is investigated, which concludes that the PCC voltage feedforward compensation can achieve the best delay compensation effect. Then, based on the established model, the PCC voltage is calculated and predicted for one sampling period in advance. Thus, the control delay in the PCC voltage feedforward loop is compensated and the system stability is significantly improved under weak grid condition. Finally, the effectiveness of the proposed scheme is verified by experiments.