Study On Stability And Transient Performance Of The Buck-Buck Cascaded System

Compared with the previous centralized power system,distributed power system is widely used in the field of renewable energy.Among various connecting structures,the cascaded structure is the most common one.However,owing to the strong independence between the sub modules of the cascaded system,the interaction will occur after being cascaded,which further affects the stability and transient performance of the system,so it is particularly essential to make the sub modules well-designed and analyze the stability of the whole system accurately.In order to deeply analyze the stability and transient performance of the cascaded system,this thesis takes the buck-buck cascaded system as an example to study the interaction between the source converter and the load converter and the influence of different control methods on the system’s stability and transient performance.Different from the cascaded system discussed in the previous literature,this thesis studies the stability of the cascaded system when the source converter is unstable and the load converter is stable.Firstly,the small signal models of the main power and control circuit of the buck converter operating in continuous conduction mode(CCM)are established with the time average modeling method,and the corresponding closed-loop transfer functions are derived.Based on this,the voltage ripple of the intermediate voltage and its stabilizing effect on the unstable source converter are discussed.The loop gain and stability boundary of the system are derived with the virtual ramp compensation.Secondly,using the energy conservation model,this thesis establishes the main power and small signal model of control circuit when buck converter operates in discontinuous conduction mode(DCM),and deduces the corresponding closed-loop transfer functions.Then,the stability of single-stage converter when it operates in DCM is discussed.The stability boundary of the system is derived based on the analysis of the ripple of the intermediate voltage when the load converter operates in CCM.In addition,the influence of different control methods on the stability and transient performance of cascade system is studied.The influence of different control methods on the closed-loop output impedance of source converter and closed-loop output impedance of the load converter is analyzed.The closed-loop transfer functions related to the transient performance are proposed and the influence of different control methods on the transient performance of the cascade system is analyzed.Finally,the experimental results verify the effectiveness of the proposed method.