Research And Improvement Of Control Strategy For Cascaded Solid State Transformer

As the core equipment in the future energy system,the energy router not only provides a plug-and-play interface for distributed power,distributed energy storage and load,but also acts as an energy manager between regions.But as an important component in energy routers,the solid-state transformers have some problems such as low withstand voltage,low overall efficiency,and complicated control.So,the cascading topology and its corresponding control and startup methods which are widely used in solid-state transformer research were discussed in this paper.The small signal mathematical models of cascaded H-bridge(CHB)and high frequency isolation(HFI)are established.Compared with the traditional control strategy of modular cascaded solid-state transformer,a new control strategy is proposed to solve the problem of equalizing voltage and power,and a system start-up method suitable for this control method is given.Finally,the correctness of the method was proved by simulation and experimental verification.The specific research highlights are as follows:(1)Through the analysis of the CHB level topology,the modulation mode using phase-shifted carrier modulation is determined.Under the phase-shifted carrier modulation,the output level of H-bridge in CHB level is seven,which is closer to sinusoidal wave.Using single-phase DQ common duty cycle control method at CHB level,the active and reactive power can be independently controlled at CHB level,and the duty cycle of each individual converter is the same,which greatly reduces the control difficulty.The proposed control algorithm is modeled and simulated in MATLAB,and the correctness of the control algorithm is verified.(2)the small signal models of three different types of DAB,which are separate DAB converter,output parallel control DAB converter based on output voltage control and output parallel DAB converter based on power equalization control were analyzed then,which provides a theoretical basis for the design of HFI controller.(3)Next,a novel voltage balance control strategy is proposed,Under which the parallel structure of the HFI stage output can be equivalent to the parallel connection of the input series output.At the same time,the control strategy realizes the output voltage control and power at the HFI level.Balanced control greatly simplifies the structure of the controller and reduces the difficulty of controller design.The stability analysis of the proposed control strategy is carried out to ensure the stability and good operation of the system.Finally,the proposed control method is verified by simulation,and the correctness of the control algorithm is verified.(4)A 3kW cascade solid-state transformer experimental platform is built,which gives a detailed analysis of the hardware design and software design of the system platform,and solves the problems of system controller design theory and reality,and system startup through engineering practice.It proves the correctness of the control method proposed in this paper.Finally,a simulation on MATLAB and a cascaded modular solid-state transformer experimental platform were built to verify the effectiveness of the controller designed in this paper.