Decoupling Control And Efficiency Optimization Of ISOP Cascaded DC Transformer

The DC transformer is the core equipment of DC distribution system.It requires high voltage gain,high breakdown voltage,high power,bidirectional flow of energy,electrical isolation,and fault isolation and other characteristics.Limited by the power rating of current switching devices,DC transformer based on the input series output parallel(ISOP)connected dual active bridge(DAB)DC/DC converter is an ideal solution.In the ISOP system,it`s necessary to solve the multi-objective control problem,that is,efficiency optimization and input voltage sharing of each DAB converter while the output voltage is realized.However,these control objectives are severely coupled together.Therefore,this paper focuses on the problem of decoupling control for input voltage sharing and output voltage control and efficiency optimization of DC transformers based on ISOP connected DAB converters.In this paper,the effects of the soft switching characteristics,current stress characteristics and reactive power characteristics of DAB converter are analyzed,and the efficiency optimization problem is transformed into current stress minimization and reactive power minimization respectively.The comparison of two optimal control shows that the current stress minimization without soft switching constraints can get higher efficiency than reactive power minimization.On this basis,this paper analyzes the coupling relationship between the optimal control and the output voltage control with dual-phase-shift(DPS)modulation.A large signal model with transmission power as input is established,and a closed-loop control method based on direct power control is designed to realize decoupling control.This method can realize the precise control of output voltage and decouple it from efficiency optimization control.Based on the direct power model of DAB,a mathematical model of the DAB input series output paralleled system is established,and the coupling relationship between input voltage sharing and output voltage control and efficiency optimization control of the system is analyzed,and the corresponding decoupling control method is proposed.On the basis of precise control of output voltage,this method not only realizes the input voltage sharing of ISOP system,but also realizes the overall efficiency optimization,and the control of several objectives is decoupled.Finally,a 6 kW DC transformer composed of three DAB converters with input series output and parallel connection experimental platform is built to complete the experimental verification.Experimental results show that the decoupling control strategy proposed in this paper is feasible and effective.