Studies On Three-Phase Cascaded Multilevel Converter Based Solid State Transformer And Its Control Strategies

In recent years,energy and environment issues are increasingly serious,how to solve these two problems is related to the sustainable development of human beings.In this context,the distributed power generation system has been developed rapidly.The construction of smart grid has become the inevitable trend.Smart grid can flexibly manage energy,improve power quality,and is more secure and reliable.Solid state transformer is one of the core devices of smart grid.Its main functions are:energy management system,which makes the energy distribution more reasonable,the input AC side power factor correction,which improves the quality of power grid,isolation between power grid and users,which improves the safety,providing DC interface for distributed generation system and energy storage equipment.In this paper,the topology of the main circuit of the three-phase solid state transformer is a three stage module cascade structure,which is composed of the three parts:the input rectifier stage,the intermediate isolation stage and the output inverter stage.The rectifier stage converts the input frequency AC voltage into DC voltages.In order to improve the input voltage level,the rectifier stage adopts a cascaded structure,and each phase of the rectifier stage consists of three cascaded H-bridges.The intermediate isolation stage implements the conversion of the voltage level of the rectified output voltages and isolation.Each rectifier module is followed by a dual active bridge(DAB).DAB converter consists of two symmetric H-bridges and a high-frequency transformer.Paralleling the output voltage of each DAB module can get low-voltage busbar which provides a network interface for distributed generation system and the energy storage system,as well as DC power for users.The inverter converts the DC voltage of the DAB output into three-phase AC voltages.By analyzing the topological structure of the three-phase solid-state transformer,it is determined that the rectifier stage adopts carrier phase shift modulation and DAB stage adopts single phase shift control.Under the control of carrier phase shift,tthe input voltage of the rectifier cascade module is seven levels,which is closer to the sine wave.There are six kind of working state of the leakage current of DAB high frequency transformer in one switching cycle under the single phase shift control.The relationship between the transmission power and the electric quantity can be obtained by analyzing the working state.The small signal models are established,and the transfer function are deduced.It's proposed that three-phase rectifier of SST adopts common-duty-ratio control based three-phase dq transformation and DAB stage adopts the voltage following control strategy.Under common-duty-ratio control,the average output voltage of the rectifier is stable on the set value.DAB output voltage follows the input voltage in a certain proportion under voltage following control.For cascaded multilevel converter based SST,due to unbalanced issue of DC output voltage of each rectifier module or transmitted power in each DAB module,SST often suffers instability problems.This paper presents a voltage balance control strategy in multilevel rectifier stage and a sensorless power balance control strategy for the modular DAB converter to solve voltage unbalance issue and the power unbalance issue caused by mismatch of the parameters of each DAB module.Furthermore,there is no current sensor used in this power balance control algorithm,which reduces the costs.In order to verify the correctness of the theory,the whole system is simulated in the PSIM software,and the hardware platform is built.Finally,the validity of the proposed control strategies in the thesis are confirmed by the results of simulation and experiment.