Research On PWM Optimal Control Of Multilevel Inverter

Multilevel inverter is widely used in AC speed regulation,photovoltaic power generation and other medium voltage high-power systems.Moreover,it has many advantages,such as low harmonic content of output voltage waveform,low voltage stress of power devices,and low switching loss.In this paper,five switch H-bridge multilevel inverter and capacitor clamped multilevel inverter are taken as the research objects.The main contents and innovations are as follows:Firstly,aiming at the problems of traditional cascaded five-level inverters which need many switching devices,a five switch H-bridge inverter topology is introduced,which is composed of an auxiliary switch circuit and an H-bridge circuit.According to the characteristics of the topology,the PWM strategy is studied,and an improved PWM control method based on carrier phase-shift PWM is proposed.According to the H-bridge cascade of two units,the waveform of five-level phase voltage is obtained by the single pole frequency doubled cps-pwm modulation method,and the five-level voltage waveform is taken as the output target waveform of the five switch H-bridge inverter,and then the PWM pulse signal is derived by combining the four working modes of the inverter.Secondly,in order to avoid the complex problem of neutral point clamped(NPC)/ H-bridge Five-Level Inverter Topology,which is caused by the existence of two clamping capacitors on the DC bus,a Five-Level Inverter composed of three-level topology with single capacitor clamped and half bridge is introduced.The Five-Level Inverter Topology only needs to ensure that the charging and discharging time of a clamping capacitor is equal in each carrier cycle to realize the voltage balance control of clamping capacitor.In this paper,a new SPWM control strategy is proposed,in which two modulation waves with opposite directions are used to modulate the positive and negative half cycle interleaved carriers,so as to realize the charge discharge balance of clamp capacitor.Finally,aiming at the problem of power balance in cascaded five switch H-bridge nine level inverter,based on the modulation strategy of in-phase disposition(IPD),the carrier degree of freedom is improved,and a novel control method of carrier in-phase staggering is proposed.However,this method can only realize the power equalization of 2n cascaded units,so combined with the characteristics of self equalization of power distribution of carrier phase-shifting modulation strategy,this paper improves the control method of triangle carrier co crossing staggered stack,and further proposes the same crossing staggered stack triangle carrier phase-shifting modulation strategy,which can realize the power equalization of n cascaded units and retain good harmonic elimination characteristics.