Modular Control Design and Stability Analysis of Isolated PV-Source/Battery-Storage Distributed Generation Systems

A distributed generation (DG) system with a photovoltaic (PV) source supported by energy storage devices and feeding dc- and ac-loads in islanded-mode operation, is considered and analyzed. As all the DG parts are interfaced through power electronic dc/dc or dc/ac converters, a control strategy is introduced which is applied directly on each individual duty-ratio converter input. The aim of the control design is to drive the PV-array energy production at the maximum power and to ensure instantaneous power balance in the limits of the storage capacity. In this scheme, critical quality demands are fulfilled, such as operation with constant ac- and dc-voltages at the load sides, independently from the power consumed. The particular controllers are implemented by applying the standard local cascaded structure with the inner-loops being fast nonlinear proportional-integral current-mode controllers. To avoid adverse impacts on the system performance, caused by contradictory actions between the individual controllers, the complete accurate DG model is considered as an isolated microgrid with the fast inner-loop controllers incorporated. Adopting a common modular inner-loop nonlinear controller form, a rigorous novel stability analysis is developed by constructing the appropriate Lyapunov function in a new sequential manner. Finally, the stability and convergence to the equilibrium are verified by simulation and experimental results.