Research On A Three-phaze T-type Three-level Power Conversion System Based On Power Storage

With the rapid socio-economic development and the expansion increasingconstantly of demand for energy, those potential problems of the traditionalcentralized grid, such as high costs, the difficulty of operation and maintenance, thepoor security and reliability are becoming more severe. The distributed generationsystem can effectively utilize wind, solar and other renewable energy sources bysending the energy into the grid or working as an independent power source to supplythe load, with the advantages of high energy efficiency, flexible installation, lowpollution, which is a good resolution to solve the current energy shortage andenvironmental issues.The battery energy storage is a common storage technology.It can realize theconversion between the electric energy and chemical energy efficiently and offer theflexible power for the load.When introduced in the power system,it can make acompensation for the grid peak to improve the reliability and the controllability forsystem.What’s more,this technology has the great application prospects with theadvantages such as good characteristics, high energy storage density, no specialrequirements on the installation site. As the interface between the energy storage unitand the grid or the load, the performance of power conversion system (PCS) willdirectly affect the stability of the grid or micro-grid. This paper studies a bi-directionalpower conversion system (PCS) which is suitable for high-capacity battery storage.This system takes the lithium battery as energy storage carrier and use the three-phaseT-type three-level topology structure. The completion of experimental on thegrid-connected mode and the islanded mode verifies the performance of the system.The main contents of this paper include: Firstly, this paper describes some common multi-level converter topologies andseveral guideposts of three-level converter. The principle of T-type three-level circuitis introduced and the loss of T-type three-level circuit is analyzed. Then the methodsto calculate the conduction losses and the switching losses are introduced and theformulas are deduced.Secondly, by modeling the mathematical model of the T-type three-level converter,the control scheme of the system is designed. With the use of synchronous vectorcurrent proportional-integral (PI) control technology, taking the strategy of the gridvoltage oriented current closed-loop vector control as the grid-connected modecontrol method and the grid voltage and current double closed loop strategy is adoptedon the islanded mode, the controller of this system is designed.Then, this paper analyzes the three-level space vector modulation algorithm andanalyses the reason of the middle point voltage fluctuation. By combining with thethree-level space vector modulation, the method to suppress the midpoint voltagefluctuation is introduced. The simulation results show that the algorithm is simple andfeasible to implement and the midpoint voltage has been inhibitory effectively.Finally, after the completion of the system’s main circuit device selection andparameter tuning, the design of the digital control circuit and IGBT drive circuit andthe software design and programming, a full digital prototype based on DSP control isconstructed. Then the proposed theories and methods are comprehensively, theexperimental results show the feasibility and effectiveness of the control strategy ofthis system.