| Power electronic transformer(PET)is also called solid-state transformer,as a new type of power transformer,compared with the traditional power transformer,PET has the advantages of small volume,small weight,high power density and good controllability,it is expected to be widely used in railway traction system,new energy power generation and smart grid.Single-phase cascade PET is one of the topologies widely used in PET field,its topological structure belongs to a three-stage structure,namely an input stage,an isolation stage and an output stage respectively,wherein the input stage is a cascaded PWM rectifier formed by cascading a plurality of single-phase PWM rectifiers,the isolation stage is formed by connecting a plurality of double active bridge DC-DC converters in parallel,the output stage adopts a three-phase inverter circuit.Single-phase cascade PET has made some achievements in topology design,mathematical model analysis and operation characteristics,but its control strategy is seldom studied,in this paper,the control strategy of single-phase cascade PET three-stage circuit is deeply analyzed and studied.The main research contents of this paper are as follows:(1)Firstly,the structure and principle of the input stage are introduced,the modulation modes of single-phase PWM rectifier and single-phase cascaded H-bridge PWM rectifier are analyzed respectively.Then,the topology and working principle of the isolated-stage double-active bridge DC-DC converter and the output-stage three-phase inverter are analyzed,the working state of double active bridge DC-DC converter under double phase shift control is introduced.(2)The mathematical model of single-phase PWM rectifier is established,For the problem of slow response speed in traditional control,an improved deadbeat control strategy is proposed,this strategy introduces a current correction algorithm based on deadbeat control.Then the mathematical model of cascaded PWM rectifier is established,since the control of cascade PWM rectifier is similar to that of single-phase PWM rectifier,therefore,the improved deadbeat control can also be applied to cascaded PWM rectifiers.The simulation results show that this strategy can effectively eliminate the control delay,the response speed of the system is improved and the dynamic performance of the system is improved.(3)The causes of output voltage imbalance on DC side of each H-bridge unit in input stage are analyzed,aiming at the problem that the traditional voltage sequencing method has more switching times of H-bridge units due to more sequencing times,an improved capacitor voltage balance control strategy is proposed,the simulation results show that while the capacitor voltage does not fluctuate greatly,this strategy effectively reduces the switching times and switching losses of H-bridge units.(4)The power characteristics and small signal model of isolated-stage double active bridge DC-DC converter are analyzed,and the soft switching conditions and return power characteristics under double phase shift control are also analyzed,in order to reduce the reflux power under double phase shift control,an improved double phase shift control strategy is proposed,the simulation results show that the control strategy can reduce the return power generated by the double active bridge DC-DC converter,and make the return power zero within a certain transmission power range,meet the soft switching conditions,and make the switching zero voltage on.The coordinate transformation,modulation method and mathematical model of the output stage three-phase inverter are analyzed,a double closed-loop decoupling control strategy based on SVPWM is adopted,the simulation results show that the control strategy can make the output waveform have good sinusoidal characteristics,maintain the stability of the output voltage,the system has fast response speed and good dynamic and static performance. |
