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

With the development of human society and the improvement of industrial technology, the problems of traditional grid are becoming more severe. The distributed generation can solve the problems of the traditional centralized grid well with advantages of low pollution, high efficiency and flexible installation. As wind energy and solar energy have the characteristics of random fluctuations and intermittent, their large-scale connections to grid will bring some problems which jeopardize the steadiness and safety of grid. The applications of storage provide a buffer between users and grid which is a good way to solve the problems mentioned above. Power storage is a common way of energy storage, which contains the advantages of good characteristic, high energy density, environmental-friendly and so on. There is a good application potential of power storage for the merits it owns. As a interface between battery and grid, the good performance of power conversion systems is so crucial.Three-level topology has significant advantages over two-level topology in the situation where the voltage is high and the capacity is large,such as connecting the battery to grid. Compared to NPC three-level topology, T-type three-level topology is smaller in size, easier to be protected and has lower conduction losses. In some given power level and switch frequency, T-type three-level topology works with higher energy density and efficiency than NPC three-level topology. This paper researches on a T-type three-level power conversion system for power storage, The main research work are as followed:(1) Reviewed the research status of the three-level topologies and the modulation algorithms for those typologies. Introduce the structure of the T-type three-level topology and its working principle. Research on the losses of T-type three level circuits, and give the calculation methods for the conduction losses and the switching losses.(2) Build up the math model of T-type three-level converter. Research on the control algorithm based on the math model. The SVPWM modulation algorithm and the factors which influence the neutral-point voltage are thoroughly studied. Propose methods both in software way and hardware way to solve the unbalance of the neutral-point voltage.(3) Construct a full digital prototype of the T-type three-level on-grid converter, and introduce the construction of the main circuits. The designs of the digital control circuits and the control software are deeply studied.(4) Build a simulation model in Matlab\Simulink to simulate the working conditions of the T-type three-level on-grid converter, and validate the theory and methods studied in this paper. The experiments of battery charging, discharging and switching between the two conditions are done in the prototype. The theory and methods are validated in a more comprehensive way.