Research On Three-level Photovoltaic Grid-connected Inverter Technology

With the development of world economy and the deepening requirements of thescientific concept of development, growing energy demand and the global environmentalpollution and ecological deterioration because of the wide use of conventional fossil fuels,the humans are forcing continually to seek and explore renewable energy. Solar energy is akind of new and renewable energy and solar power to alleviate the serious environmentpollution and energy crisis has very important significance. Under this background, thetechnology of the PV grid-connected inverter is researched deeply in this paper. Themaximum power point tracking of solar cell, SVPWM modulation technology of threelevel inverter, grid inverter control strategy, system simulation and so on were also deeplystudied in this paper.The equivalent circuit of solar battery has been first got by analyzing the workingprinciple of solar battery. Based on the equivalent circuit, the battery simulation model wasgot, and then the output characteristics of solar battery can be acquired from the model. Byresearching and analyzing the common methods of maximum power point tracking, thevariable step size conductivity incremental method can be designed based on fuzzy controltheory to achieve maximum power point tracking.Analysis and testing the methods byestablishing the simulation model.Compared to the traditional two level inverter, the multilevel inverter has many advan-tages. First, it decreases voltage stress and the harmonic of the output voltage by increasingswitch tube quantity and the level number of main circuit. Second, the control mode is agile.Third, the switch tube of inverter is working in low frequency or work in frequency state toreduce the switching losses and electromagnetic interference, and it is more suitable for highpower applications.In this paper, The three level inverter was used as photovoltaic grid inverter circuit andthe topology of the three level inverter circuit was analysed. The key research is SVPWMmodulation technology of three level inverter circuit. It contains classification and judgmentof the sector, selection of the basic vector and decision of basic vector action sequence andcalculation of action time of basic vector in each sector, calculateing switching time of eachbridge arm of power tube, generateing twelve pulse waves and processing neutral potentialbalance. The simulation model has been found in the analysis of each parts, and theoretical analysis was validated through the simulation results.The control strategy of Photovoltaic grid inverter is one of the key technologies ofphotovoltaic system grid control, and it directly affects the steady and dynamicperformance of the system and the waveform quality of grid voltage and current. In thispaper, several control strategies of grid inverter were expounded, and the direct powercontrol strategy based on virtual magnetic chain orientation and PI control is the Keyresearch.There is a integral drift phenomenon in the virtual magnetic chain orientation.Theinaccurate problem of magnetic chain orientation caused by integral drift has been solvedthrough the design of the virtual magnetic chain observation model.