Nonlinear analysis of a grid-connected photovoltaic/dc-load system driven by local current-mode controllers

A distributed power generation system controlled through the duty-ratio inputs of the power converters used to interface a photovoltaic source and the ac-grid to a common bus is considered. For the control design, the standard local cascaded scheme is adopted for each individual input while nonlinear stability analysis is deployed for the complete system. To this end, a model integration is proposed that includes all the power system units along with suitable current-mode, inner-loop fast controllers. Taking into account some basic features of the complete nonlinear forced model, the analysis it is proposed to follow a new sequential method for proving stability. In the first step, an easily obtained storage function is utilized as it is constructed from the passivity property. In the second step, taking into account the cascaded system form, a complementary storage function is provided that substantially upgrades the initial storage function in a manner capable for proving input-to-state stability. Additionally, using an advanced LaSalle-based technique, convergence to the equilibrium is proven. Hence, in this paper, a novel nonlinear method is developed to prove that slightly modifying the conventional current-mode controllers these can be used in a decentralized local scheme without creating contradictions on the system performance since stability is guaranteed. Simulation and some experimental results are finally conducted to confirm the controller design and system analysis.