Nonlinear Differential Flatness Control Of Distributed Photovoltaic-energy Storage Generation System

In recent years, a growing number of distributed photovoltaic(PV) systems are connected to the grid, which plays a positive role in promoting the development of clean energy. The main drawbacks of distributed PV systems lie in intermittent and random power output. Along with the continuous improvement of PV permeability, an energy storage(ES) device is needed. Distributed photovoltaic-energy storage(PV-ES) dc generation system, with fast regulation of power electronic converter, has become an effective means to solve the PV output power fluctuation, further enhancing the PV permeability.In the proposed system, the control objective of PV boost DC-DC converter is to achieve maximum power point tracking(Maximum Power Point Tracking,MPPT). The control objective of ES bidirectional DC-DC converter is to quickly respond to the changes of the PV output power, and to achieve the constant power output. The control objective of DC-AC converter is to achieve unit power factor and stable DC bus voltage under all conditions. For the low-voltage and high-current output characteristics of the PV module and the battery, four-phase parallel converters are adopted in the proposed system, which can effectively widen the output voltage range and suppress the PV output voltage fluctuation. For PV four-phase parallel boost converters, an improved extremum seeking algorithm(ESA) is designed to solve the MPP oscillation problem of traditional ESA and improve dynamics of the system. For nonlinear characteristics of ES four-phase parallel bidirectional converter and DC-AC converter, a novel nonlinear differential flatness control strategy is proposed, which directly compensate the nonlinear and provide a sustained high performance over a wide operating range. Comparing with traditional linear algorithm, it has a smaller steady-state deviation, higher stability and faster response, it even can achieve zero overshoot.Based on MATLAB / Simulink simulation platform and RTDS platform, the correctness and validity of the proposed control algorithm are proved. The results show that the proposed PV-ES system could achieve the above objective. Comparing with traditional linear algorithm, the novel algorithm has a faster dynamic performance and a smaller error.