Study On Hybrid Carrier PWM Control Method Of Cascade H Bridge Multilevel Inverter

With the rapid development of new energy utilization in recent years,multi-level inverter systems are required in many high voltage and high power applications.In order to improve the utilization rate of electric power,the requirement of multi-level inverter technology is higher and higher.Because of its various inverter topologies and modulation methods,the multi-level inverter technology has a lot of room for improvement.For the structure and modulation method of the multilevel inverter,the following major researches are made in this thesis:Firstly,the working principle of the topology structure of the multi-level inverter is analyzed.By comparing the structure of a variety of multi-level inverters,the cascade h-bridge multi-level inverter with the best practicability is selected as the research object.Analysis of various PWM modulation methods,mainly the application and development of various methods,especially the sinusoidal pulse width modulation SPWM method has a deeper understanding,for the following research to establish the basis.Secondly,the switching state of each switch tube of the asymmetric 7-level inverter is analyzed,and the improved control of the same polarity output level of the switch tube of the h-bridge unit is carried out to solve the power circulation problem that may occur in the output of the high-low voltage unit.Using the SPWM modulation strategy of mixed carrier cophase cascade,the asymmetric 7-level inverter was simulated on the MATLAB simulation platform,and the simulation results before the improvement were compared to verify the feasibility of the improved h-bridge inverter unit switch tube copolarity output level switch state.Then,in order to improve the quality of the total output voltage waveform of the inverter system,the unipolar frequency-doubling SPWM modulation method was improved.By increasing the number of carriers of the low-voltage unit,the hybrid carrier overlapping PWM control method was used to simulate the asymmetric h-bridge 7-level inverter.Simulation results show the validity of the mixed carrier overlap type PWM control method,and proposed a compromise m H-bridge inverter units cascaded multilevel inverter extended structure,through constructing 3 H-bridge inverter units composed of 9 level inverter simulation model validation,illustrates the feasibility of this cascade structure and can greatly improve the waveform quality advantages.Finally,the mathematical model of the three-phase multi-level inverter is established,and the relationship between the switching function of the three-phase h-bridge three-level inverter and the output phase voltage and line voltage is deduced theoretically.On the basis of this theory,the three-phase seven-level inverter is simulated,and the simulation results of the hybrid carrier overlapping PWM control method and the frequency-doubling SPWM modulation method without overlapping carrier are compared.The results show that the output waveform of the three-phase seven-level inverter system with the former is better and the output waveform quality is improved.It is verified that the hybrid carrier overlapping PWM control is still suitable for three-phase multi-level inverter system,and the inverter effect is better,which can improve the power quality.