Study On The Nonlinear Control Strategy Of Grid-connected Photovoltaic System Based On Z Source Inverter

Along with the global energy increasingly tenser, solar energy is favored withadvantages such as inexhaustibility, cleanness, safety and no contamination. And manycountries have solar energy development and utilization gradually commercialized. Thetechnology of photovoltaic (PV) generation is considered as one of the most potentialpower generation. Therefore, the study of PV generation system has far-reachingpractical significance. The gird inverter control is the key in grid generation system. In2002, Professor Peng Fangzheng put forward the impedance source inverter—Z sourceinverter. Its unique passive network allows the inverter upper and lower bridge armshoot-through to buck or boost voltage. It is better applied to the PV, wind power andother distributed generation or micro-grid system whose input voltage range is larger.This paper firstly analyzes the voltage boost theory of Z source inverter accordingto the equivalent circuit when in the shoot-through state and non-shoot-through state.And it summarizes its advantages and disadvantages, introduces the voltage boostimplementation methods of zero vector. Next, by analyzing the state space averagingmodel of Z source inverter, this paper obtains the corresponding transfer function. Itsnon-minimum phase characteristic increases the design difficulty of output voltagecontroller. Therefore it presents the control strategy by controlling the inductor currentindirectly to stabilize the output voltage.Almost all of the power electronic converters are nonlinear, which are usuallymade the Taylor series expansion near the system equilibrium point and thenapproximate linearization. But it has errors and follows the deviation distance from theequilibrium point increasing. Besides, the actual system always inevitably subject tosome unmodeled dynamics and external interferences during operation, which causesthe system to deviate from the quiescent operating point. But now the model fromTaylor expansion approximate linearization is no longer effective. In this papernon-linear method is proposed to control the Z source converter. Based on the affinenonlinear model of Z source converter, it studies its inner dynamic stability of thesystem taking capacitor voltage or inductor current as feedback. The conclusion thatinner dynamic of the system taking capacitor voltage as feedback is unstable is obtained.Thus the strategy is raised that taking the inductor current as feedback to indirectlycontrol the output voltage and carrying on its input-output linearization design, which combined with optimal tracking control technique to determine its feedback coefficientso as to achieve the nonlinear control of Z source converter.Then give a theoretical analysis on the grid-connected current loop and Z sourcecapacitor voltage loop. In view of PV array output voltage loop, this paper presents thenonlinear zero dynamics design method in order to achieve maximum power pointtracking. At the same time, it elaborates the parametric design principles of PV arrayoutput capacitor, the filter inductor as well as the Z source inductor and capacitor, andgives the related design formulas. Finally, it chooses the main circuit parameter basedon the system performance and the system simulation is carried out. Simulation resultsshow that the proposed method can realize the PV maximum power point track and theunity power factor running. And the RT-LAB experiment verifies the correctness of thecontrol strategies.