Research On Energy Storage Type Solid State Transformer And Its Control Strategies

Solid state transformer(SST)is a new type of smart transformer.When multiple identical modules are connected in series,it can not only improve its voltage level and capacity,meet the needs of different power applications,but also reduce the electrical stress of each module switch.However,since it does not have the ability to store energy,it cannot perform energy compensation when the voltage is interrupted or dropped deeply,which will affect the normal work of the subsequent circuit.Therefore,a kind of energy storage solid state transformer is proposed and studied in this thesis.The main research work of this thesis is as follows:1.The working principle of the input stage of the energy storage solid state transformer is analyzed and its mathematical model is established in the dq coordinate system.Under the same circuit parameters,in order to achieve output voltage balancing,a common duty cycle control strategy based on single phase DQ vector transformation is proposed.The feasibility of the control strategy is verified by the simulation experiment.2.Aiming at the unbalanced output voltage on DC side of input stage,the voltage balance principle is analyzed.Based on the control of the common duty ratio,a voltage balance control strategy based on the sliding mode controller is proposed.The simulation results show that the system can achieve voltage balance under the action of the control,and has good response speed and steady state performance,which verifies the effectiveness of the control strategy and achieves the design goals.3.The isolation stage of energy storage solid state transformer,the dual active bridge(DAB)DC/DC converter,is analyzed in detail,and its small signal model is derived.For DAB stage power balance problem,in order to avoid sampling high-frequency currents,an indirect power equalization control strategy is proposed.That is,a common duty cycle control based on a sliding mode controller is adopted in the input stage to adjust power balance,a voltage follower control is adopted in the DAB stage to adjust the voltage balance.The simulation results show that the system can achieve the balance of voltage and power under this control,and the feasibility of the control strategy is verified.4.The working mode and performance of bidirectional DC/DC converter applied to energy storage system are analyzed and introduced,the small signal model under two modes is established and the corresponding PI controller is designed.The simulation results show that the output current ripple amplitude of the system is reduced and the output voltage can be stabilized at a given value,which verifies the feasibility of the interleaved parallel topology and the rationality of the PI controller design.5.The parameters of the energy storage solid state transformer were analyzed and designed,and the simulation experiments are carried out in MATLAB.The simulation results show that the energy storage type solid state transformer has a good energy support and can guarantee the normal operation of the post-stage circuit.The effectiveness and feasibility of the energy storage solid state transformer are verified.