Research On Modulation Strategy Of T-type Three-level Inverter

Compared with the two-level inverter,the multi-level inverter can improve the voltage level,and the output waveform has less harmonic content and is closer to the sine wave.The development of multi-level inverter modulation technology can further improve the conversion efficiency and performance of the inverter.In this thesis,the topology of T-type three-level inverter is studied,and on this basis,the SVPWM control algorithm,neutral point voltage balance control and other related problems are deeply studied.In this thesis,the traditional SVPWM control algorithm in 90° coordinate system is analyzed.Aiming at the defects of the complex calculation process of sector division and vector action time,a SVPWM algorithm in 60° coordinate system is studied.In the 60°coordinate system,a large number of trigonometric functions can be avoided to facilitate the subsequent digital implementation.Through the standardization of variables and the normalization of large sectors,the division of small sectors in other large sectors and the calculation of vector action time are transferred to the first large sector,which greatly simplifies the operation process.SVPWM algorithm has the problem of neutral point voltage unbalance in the control of T-type three-level inverter.This thesis analyzes the causes of neutral point voltage unbalance,and concludes that neutral point voltage is related to the polarity and size of neutral point current.In this thesis,a neutral point voltage control method based on redundant small vectors is studied in depth.However,this method has a limited regulating range and a large stable pressure difference under high modulation ratio,so it can only achieve better regulation effect under low modulation ratio.This thesis proposes a neutral point voltage balance control strategy based on a new virtual vector,the strategy constructs a new virtual space vector based on the principle that the current flowing into and out of neutral point is zero within a switching cycle.On this basis,a new switching state sequence is designed by adding the control of the regulator,and the action time of each basic space vector in the switching state sequence is combined with the regulator,so that the neutral point voltage balance control in the range of the full modulation ratio can be realized.Based on the Matlab/Simulink platform building simulation model,the simulation results verify the validity of the algorithm,and further set up with the DSP + FPGA as the core of the experimental platform,again by the experimental results of experiment in this thesis,the control algorithm is involved,including 60 ° coordinate SVPWM algorithm,neutral point voltage balance control based on redundancy small vector and neutral point voltage balance control based on a new virtual vector.