Return Ratio Shaping Approach to Stabilize Inverter-Weak Grid System

The article presents a corrective control philosophy of a grid tied inverter for adapting the instability incurred at the Point of Common Coupling (PCC) due to uncertainty in the utility grid. It has been observed that the converter impedance which is dependant on the control system loop have a considerable impact on the stability. Therefore, the proposed philosophy explores the participation of the control parameters in shaping the return ratio trajectory which consequently modifies the stability margins of the system. A sensitivity function has been formulated which gives the gradient of each point in the return ratio trajectory to changes in the control parameters. The formulated function has an additional advantage of being grid parameter independent, thus increasing robustness and reducing complexity. An adaptive multi-variable, multi-degree of freedom feedback philosophy using the sensitivity gradients has been proposed to dynamically reach the optimum value of the control parameters in case of grid impedance change. The result shows that the proposed philosophy successfully brings stability to the system in the presence of a weak grid. As well as it preserves the dynamic performance requirements of the converter control.