Current Control of a Three-Phase, Grid-Connected Inverter in the Presence of Unknown Grid Parameters Without a Phase-Locked Loop

Three-phase inverters for grid-connected applications typically require some form of grid voltage phase detection in order to properly synchronize to the grid and control real and reactive power. This phase detection is usually based upon some type of grid voltage sensing. However, in this work, a method is proposed, whereby the phase angle of the grid can be accurately identified solely via the grid current feedback. In the proposed current-control scheme, the only measurement is the output inverter current, and no phase-locked loop (PLL) exists within the scheme. This phase observer is incorporated into a current controller, which can manage the injected power to the grid. The design of this combined observer/controller system is motivated and validated via a Lyapunov stability analysis. The performance of the proposed control scheme has been compared with the conventional PI PLL-based control scheme that needs both a grid voltage and a current sensors. Experimental results utilizing a Controller+ Hardware-in-the-Loop test bed have been presented in order to validate the proposed observer/controller scheme under normal operation, weak grid, distorted grid, and faulty grid conditions. To summarize, this scheme archives current control for a grid-connected inverter using only current feedback. In comparison, traditional methods require voltage and current feedback along with a PLL to synchronize with the grid creating a cascaded closed-loop control system.