Research On Z-source Three-level Bidirectional Converter And Its Control Method

As essential electrical equipment for the microgrid,the bidirectional converters can accomplish the electrical power transmission between the unity grid and the energy storage device.Most distributed generation units connect to the bus by bidirectional converters which are also widely used in the microgrid system as interconnection equipment.The bidirectional converter plays a key role in the power conversion,transmission and storage and its performance has an important effect on the steady operation of the microgrid system,so the research of the bidirectional converter is very important to the stable operation of the microgrid.Taking account of the buck-boost peculiarity of the Z-source network and the three-level converter circuit which has high voltage-resistance and better output waveform,this paper presents a new topology of the three-level Z-source bidirectional converter by combining the Z-source network and the three-level converter circuit.The main research content and goal are as follow:Firstly,this paper elaborates the topology of the tree-level Z-source bidirectional converter.Based on it,this paper analyzes the operation modes of the converter in different shoot-through states in detail.The mathematical modeling of the converter and its equivalent circuit is used for the deducing of the mathematical relation between the Z-source voltage,dc side voltage and the ac side voltage in different operation modes which illustrates the buck-boost principle of the three-level Z-source bidirectional converter.Secondly,this paper researches the modulation method of the three-level Z-source bidirectional converter.This paper improves the traditional SPWM method of the Neutral-point-clamped(NPC)inverter and applies it to the bidirectional converter.Based on the difference of the carrier waves,this paper compares the phase opposite disposition(POD)method and in-phase disposition(IPD)method and analyzes their different features.Based on the different ways to add the shoot-through signals,this paper analyzes four modulation methods for the bidirectional converter and compares them by the simulation result.Then this paper presents the simple open-loop and close-loop control method for the bidirectional converter in inversion mode and rectification mode.Thirdly,this paper researches the neutral-point unbalance problem for the three-level converter in detail.Based on it,this paper presents a neutral-point potential balancing method for the converter by adding the shoot-through offset which utilizes the up and low shoot-through peculiarity.To suppress the low frequency oscillation of the neutral-point potential,this paper refers to the double modulation wave carrier-based PWM method and combines it with the method which adding the shoot-through offset to solve the neutral-point unbalance problem for the three-level Z-source bidirectional converter.Fourthly,this paper researches the application of the three-level Z-source bidirectional converter in the distributed energy storage system and hybrid AC/DC microgrid.This paper ascertains several operation modes just as constant voltage charging mode,constant current charging mode and constant power energy exchanging mode of the bidirectional converter.Then it designs the closed loop control method to different operation modes.To the hybrid AC/DC microgrid,it designs the AC side voltage control method in the independent operation mode and the DC side voltage control method in the grid-connected mode.The Matlab/Simulink results verify the working performance and the control effect of the converter.Finally,this paper designs the hardware part and the software part of the bidirectional converter and set up its experimental platform base on the DSP controller.Then this paper tests part of the researches which mentioned above by experiment.