Feedback linearization control of instantaneous complex power injected into a weak grid by a self-synchronized inverter

This paper presents a nonlinear, observer-based controller for governing the power injected into the grid by a Voltage-Source Inverter (VSI) with an inductive filter. The main feature of the proposed controller is that the observer eliminates the need for measuring the Point of Common Couplilng (PCC) voltage, thus avoiding the adoption of a Phase-Locked Loop (PLL) algorithm for synchronization. This results in a self-synchronizing algorithm, preventing instability problems that can arise with PLLs. The control algorithm considers the nonlinear dynamics introduced by the VSI and is based on a flatness strategy. Since the design does not use cascaded decoupled loops for control, the secondary loop dynamics does not need to be relatively slow, allowing the DC link to include a relatively small capacitor. The algorithm behaves robustly, even when the inverter is connected to a weak grid. For this purpose, a state observer is designed that allows estimating the voltage at the PCC from the measurement of the output inductor current. A start-up controller is also included, which allows synchronization even in the case of system start-up. Simulation results are presented for different operating cases, including start-up, normal operation, and grid-voltage sags and swells. In all these cases, it is considered that the exact parameters of the grid to which the inverter is connected are unknown. Additionally, the results are validated through experiments.