Stability analysis and fail-safe operation of inverters operated in parallel

In this paper, the stability in the sense of boundedness of inverters operated in parallel is proven and the fail-safe operation is achieved for generic linear or nonlinear loads described in the generalised dissipative Hamiltonian form. The robust droop controller (RDC), recently proposed in the literature for achieving accurate proportional power sharing and tight voltage regulation, is implemented in a nonlinear matrix form to achieve a bounded inverter voltage and inherit a fail-safe capability. Using nonlinear Lyapunov methods, it is proven that the RDC with fail-safe approximates the original RDC, generates a bounded inverter output voltage within the given technical limits and guarantees nonlinear closed-loop system stability in the sense of boundedness. When the prescribed limits are violated, e.g. due to sensor failure, the proposed method rapidly shuts down in a continuous-time manner, thus disconnecting the inverter to prevent a complete system failure even when the protection circuit fails. Extensive simulation results are presented to demonstrate this approach for two single-phase paralleled inverters feeding a linear and a nonlinear load under a sensor failure scenario.