The Control Strategy Research For Multi-module Distributed Serial-Parallel System

Nowadays,power electronics technology is booming,and higher and higher power and voltage level requirements have emerged.The requirements for the withstand voltage of power electronic devices are also increasing.Rail transit DC power supply systems have such problems.A bidirectional DC/DC converter system consisting of series-input and parallel-output(ISOP)is a solution to this problem.It is ideal for high input voltage and output current and high power levels.In order to ensure stable operation of the system,it is very necessary to realize the control of the input voltage sharing and output current sharing.Not only that,but the topology of a single module also affects the normal operation of the system.In this paper,from the parameter requirements of a single module,in order to adapt to the input and output of a wide voltage range,and considering the device selection,cost and loss,a bidirectional isolated DC/DC converter composed of two-stage topology is adopted.The first stage is a three-level bidirectional Buck/Boost circuit that stabilizes the intermediate DC voltage;the second stage is a bidirectional isolated DC/DC that enables isolation conversion.The working principle,modulation and control strategy are studied.It is verified that the overall topology can realize a wide range of voltage input and stabilize the output.Aiming at the problem of the input side voltage sharing and the output side current sharing in multi-module ISOP system,considering the application background of the system in the auxiliary power supply layer of rail transit DC bus,a new module-level droop equalization control strategy is proposed to input voltage.Controlling the output inductor current allows control mode changes without switching control strategies.Due to the decentralized control,each module only collects its own voltage and current,which is more reliable.On this basis,the secondary regulation is also applied to realize the control of the input voltage equalization and the output current sharing from the forward direction and the reverse direction respectively,so as to ensure the normal operation of the system when the power is lost.By establishing the average switching model of the sub-module topology and using the small-signal analysis method to design the parameters,the stability of the first-level and second-level topologies of the single module under the given parameters is analyzed according to the root trajectory distribution map;In the whole system,a small signal model is established from the input side of the series and the output side of the parallel,and the equivalent impedance model and the transfer function characteristic equation of the voltage and current disturbance are derived.The number of different modules on the series side and the parallel side is analyzed under given parameters.The effect on the equivalent impedance.It is concluded that the number of sub-modules does not affect the stability of the system and the current disturbance on the parallel side has a greater influence on the distribution of the root trajectory.Aiming at the above control strategy,the simulation model is built in Matlab/Simulink platform to verify its feasibility.Then a single-module experimental platform is designed and verified.Finally,the control strategy experimental verification of ISOP system is realized by RT-LAB semi-physical platform.