Research On Pulse Width Modulation And Application For Multilevel Z-source Inverter

The traditional DC/AC inverter includes the voltage-source inverter and current-source inverter. Due to the restrictions of topology, peak value of voltage on the AC side is either lesser or greater than the DC voltage. Z-source inverter couples a X impedance network between the DC source and inverter bridge, which can overcome shortcomings of conventional VSI and CSI with excellent characteristics of single-pole buck and boost, and it has has strong advantages in distributed power generation and other new energy applications.Based on introduction of the general topology and boost principles of two-level ZSI, and according to the requirements that ZSI presents to direct insertion, the conventional PWM method is studied including simple boost control, maximum boost control, maximum constant boost control, modified modulation controland SVPWM method. The rules and principles of inserting shoot-through state into the traditional PWM modulation method are mainly introduced with comparative analysis.In order to achieve the boost output of three-level inverter, voltage-boost principle and shoot-through method for NPC three-level Z-source inverter with a single Z-source network and dual DC sources are discussed, and the relationship between shoot-through time and boost ratio is derived.The method of shoot-through boosting PWM in single Z-source NPC three-level inverter is analyzed including synclastic carrier PWM with shoot-through, alternate reverse PWM and SVPWM algorithm based on the vectordecomposition of the reference voltage. Considering the problems of unbalanced midpoint voltage in single Z-source and single DC power supply of NPC three level inverter, a kind of neutral point potential balancing control method based on double modulation wave technique is discussed, and the insertion position of shoot-through state is reasonable designed, which can dynamically adjustthe middle voltage modulation wave, eliminate low frequency fluctuation of neutral point potential and inhibit the migration of neutral point potential without increasing the switching loss.According to traditional topological structure of the five-level, the topological structure of cascaded Z-source five-level inverter, Z-source NPC five-level inverter arederivedrespectively, voltage-boost principle and shoot-through method of each inverter are analyzed. It also discusses unipolar carrier phase-shift SPWM, bipolar carrier phase-shift SPWM and Z-source synclastic carrier PWM. With the superiorcharacteristics of single-poleboosting of Z-source inverter, the application of Z-source inverter in connecteddistribution generation grid system with unstable DC input is studied.Decoupled control of active current and reactive current achieves high power factor operation.