Analysis On Stability Of DC Distributed Power System

Distributed power system is widely used in new energy,communication,aerospace and other fields due to its high reliability,high power density and high flexibility.When the subsystem is designed separately,it can meet the requirements of stability and dynamic performance,but when several subsystems are cascaded into a distributed power system,it may lead to the instability of the whole system.In this thesis,the reason of the instability of cascaded system is studied by impedance analysis,and the corresponding solutions to reduce the output impedance are put forward.In view of the instability of the two converters,this thesis first establishes a small-signal model of the converter by the state space averaging method.Through the small-signal model,the open-loop and closed-loop output impedance of the front source converter and the open-loop and closed-loop input impedance of the post-stage load converter are obtained.The impedance characteristics and the optimized impedance are analyzed.The foundation is established,and then the common source effect and load effect transformation formula of cascaded system are derived according to the established converter model.The impedance criterion method suitable for cascaded systems is proposed.Finally,the constant power load characteristics,closed-loop input and output impedance characteristics of the cascaded system are analyzed,and the corresponding results are given.In order to solve the problem of large amplitude oscillation of the bus voltage caused by the overlapping of the input and output impedance,the influence factors of the voltage oscillation amplitude of the bus bar are studied and the performance of the cascaded system is analyzed.The impedance optimization method suitable for cascaded systems is proposed.The output current feedback is used to reduce the peak amplitude of the output impedance of the pre-stage converter,and the oscillation amplitude of the bus voltage in the cascaded system is reduced,thus the stability of the cascaded system is improved.Through the simulation and analysis of two cascaded voltage feedback Buck converter cascaded models,the effectiveness of the proposed output current feedback is verified and the voltage ripple amplitude of the cascaded system is reduced.In view of the instability of cascaded systems caused by constant power load,the reasons for instability of cascaded systems are given from theperspective of the transfer function and loop gain,and the stability conditions of cascaded systems are obtained.In view of the conservatism of the existing impedance criterion,the phase angle range of the input and output impedance overlaps in five cases is analyzed.The concept of absolute stability is put forward,which provides a theoretical basis for the design of cascaded system.In view of the current impedance stability criterion,the stability of cascaded system is judged only from the amplitude characteristic,and the influence of damping filter on the input impedance characteristics of the converter is analyzed and the corresponding conclusion is given.The simulation results verify the correctness of the cascaded system stability from the phase angle characteristics.This research provides a theoretical basis for power electronic system integration,and provides corresponding engineering design reference for the design of modular power supply.