Studies On Topologies And Control Strategies Of New Power Electronic Transformer

In modern power grid, more and more distributed renewable energy resource (DRER) and distributed energy storage device (DESD) need to be connected to power grid. After these renewable energy sources are connected to power grid, the interface is required to add functionalities such as reactive power compensation, voltage regulation, intelligent energy management and plug-and-play features for the distribution resources. However, traditional transformers always fail to provide those functions. Therefore, a new Power Electronic Transformer (PET) is introduced to replace traditional transformer as interface device for renewable energy resources in this thesis.PET is a new type of power electronics device in power system, it combines power electronics technology and a high-frequency transformer to achieve the function of transformation and insulation. Power electronic devices can’t be used directly under high voltage and to solve this problem, modular cascaded PET is introduced. The major problem of modular cascaded PWM converter is the voltage unbalance among each cascaded modules. As a result, some control strategies to maintain voltage balance of cascaded modules were studied as focus in this thesis.The development history and status of PET, and following the modular cascaded PWM converter, are first introduced in this thesis. Several typical topological structures of PET and its operating principles were introduced and analyzed. For a modular cascaded PWM converter, its operating principles and control strategies, as well as switching model and small signal model, are analyzed and built. Two control strategies were adopted in this thesis, common dutyfactor control method and voltage balance control method. The controllers were designed based on the small signal model bode plot.The software and hardware of the experiment were designed in this thesis. Design of power circuits and parts of control circuits are introduced respectively, and control program is wrote in DSP to control the whole system. Simulation result and hardware experiment result are given at the end of this thesis. These two control strategies were validated through PSIM simulation and hardware experiment.