Passivity-Based Virtual Damping Control of Three-Phase Grid-Tied PV Inverters

Apart from the classical control theory, the interconnection stability of a grid-tied inverter system can be investigated from the energy dissipation point of view. If a critical resonance occurs within regions where the system equivalent impedance/admittance behaves passively i.e. has only nonnegative real part, the system stability can be guaranteed. This paper presents the passivity-based stability analysis of a three-phase grid-tied photovoltaic inverter system. Passivity properties are observed over a wide frequency range for different control loops. Due to the implementation of a phase-locked loop and a direct-voltage control, passivity features are asymmetric between d- and q-axis. To enhance the system stability, a passivity-based control method is proposed by implementing virtual damping in d- and q-axis asymmetrically. By means of that, non-passive regions can be reduced over a specific frequency range. The validity of the proposed virtual damping control is investigated with both simulations and experiments.