Research On Voltage And Current Balance Control Of Cascaded Power Electronic Transformer

Power transformer is one of the most important equip me nt in power transmission system and power distribution system. It has a direct impact on the reliability of power supply and power quality. Tradit ional transformer has many disadvantages, such as poor controllability, no DC interface and no energy storage capacity. It is difficult to meet the needs of modern power systems well. Therefore, a kind of three stage ISOP(input series and output parallel) power electronic transformer is studied in this paper. By adopting an improved control strategy, it overcomes some disadvantages of the traditional control strategy, and achieves the purpose of adjusting the power factor, improving the power quality and realizing the system voltage balance and power balance.The basic principle of the cascaded H bridge rectifier is analyzed. And the small signal model is established in the d-q coordinate system by using the state space averaging method. For the common duty cycle control strategy based on the single phase d-q vector space, the closed- loop parameters are designed, a nd the control strategy is realized by simulation.The traditional control strategy of the input stage is complex, and the dynamic response is slow, so a common duty cycle control strategy based on the quasi proportional resonant controller is designed. And in order to solve the problem of input current distortion, a 100 Hz notch filter is designed in the closed loop control of voltage. Simulation results show that the proposed control strategy can effective ly improve the control performance of the system input stage. At the same time, a three loop control strategy is designed to solve the problem of voltage unbalance between the two modules.In the traditional phase shift control mode, the work process of the Dual active bridge(DAB) DC- DC converter structure is analyzed in detail. And the small signal model of the DAB structure is established. Aiming at the power balance control of the DAB paralle l structure, a power balance control strategy is designed. The voltage balance is achieved in the isolation stage, and the power balance is regulated in the rectifier stage. The sampling of the high frequency current component in the DAB structure is avoided, the current sensorless power balance control is realized. The control strategy is verified by simulation. The simulation results show that the voltage and power balance are realized by this control strategy, and the secondary ripple in DAB output voltage is very small.The experimental platform of the power electronic transformer is built, and the software design and hardware design of the system is completed. For the system control strategy: the input stage adopts a quasi PR controller control strategy with 100 Hz notch filter, and the isolation stage adopts an improved current sensorless indirect power balance control strategy, preliminary experimental verification is carried out.