Research On Control Strategy Of Three Level Photovoltaic Inverter

In the application field of high-voltage and high-power photovoltaic inverter,the three level inverter is widely applied because of its small voltage stress,good quality of output waveform and low harmonic content.However,more control technology is involved in the application of three level photovoltaic inverter.There are still some shortcomings in the existing control technology.Therefore,this paper makes an analysis and Simulation of some control techniques in the control of three level PV inverter,and draws the corresponding conclusion.These control techniques include the maximum power point tracking(MPPT),the three level SVPWM modulation technology and the mid point potential balance control technology.The main research work of this article is as follows.Firstly,the topology and working characteristics of the traditional three level inverter are analyzed.A ANPC type three level topology is proposed.The bidirectional clamping of the topology is more reliable than the NPC type unidirectional clamp,providing redundant channels for the output zero level.This provides a possibility for the balance of the loss of the switch device and the flexible control of the midpoint potential.Secondly,the basic principle of photovoltaic cell is analyzed.The mathematical model and simulation model of the photovoltaic cell are set up.The characteristic curve of the photovoltaic cell is studied in detail,and the variation law of the characteristic curve of the photovoltaic cell under different light intensity and ambient temperature is obtained.The basic principle of the maximum power point tracking is analyzed,and the essence of the maximum power point tracking is the dynamic impedance matching process.Four common methods of maximum power point tracking are studied in detail,which lays a foundation for the simulation research of the latter.Then,the three level space voltage vector principle and the traditional three level SVPWM control algorithm are analyzed.A simplified control algorithm is proposed.The simple logic operation is used in the sector judgment,which solves the problem of the high complexity of the three level SVPWM algorithm and the large calculation burden of the controller.The expression of the mid point voltage of three level inverter is derived.The influence of the three level voltage vector on the midpoint voltage is analyzed,and the influence of the space voltage vector on the middle point voltage is obtained.Based on this analysis,a neutral point potential balance control strategy based on virtual space voltage vector is proposed,which realizes the dynamic control of the midpoint potential closed loop.Finally,In MATLAB/Simulink environment,three simulation models of maximum power point tracking,three level SVPWM algorithm and mid point potential balance control strategy are built.The control effect of disturbance observation method and three point comparison method is compared.It is concluded that the three point comparison method is superior to the perturbation observation method in tracking speed and steady state accuracy.The correctness of the three level SVPWM simplification algorithm is verified,and the simulation waveform is consistent with the theoretical analysis.It is also proved that the three level SVPWM modulation has the characteristics of high DC voltage utilization and good waveform quality.The control effect of midpoint potential is compared before and after the mid point potential balance control strategy using virtual space vector.The conclusion that the medium point potential fluctuation can be effectively suppressed by this strategy is obtained.