Adaptive Control of Grid-Connected Inverters Based on Online Grid Impedance Measurements

Stability of a grid-connected inverter depends on the ratio of the grid impedance to the inverter impedance. Since the grid impedance changes during normal power system conditions, this paper proposes a gain-scheduling adaptive control system that uses online grid impedance measurements. For grid impedance measurement, an impulse-response analysis method is programmed in the digital-signal processor (DSP) of the grid-connected inverter. For adaptation, a Routh–Hurwitz stability analysis approach is used to derive, analytically, the stable operation boundaries of the interconnected system. To simplify the analytical derivations, the grid impedance is assumed inductive at low frequencies and curve fitted to the online impedance measurements. Experimental measurements demonstrate the improvement in system stability, when the impedance identification and adaptive control algorithms are programmed together in the DSP of a three-phase inverter, which is connected to a grid with a variable feeder impedance.