Research On Stability Analysis And Improved Control Strategy Of The Bipolar DC System With Constant Power Loads

The DC system,which has the advantages of high conversion efficiency,large power supply capacity,and high-power quality,has become an important development direction in the future power field.The proportion of DC distributed power sources and loads is also increasing in recent years.However,in the DC system,there are a large number of constant power loads,whose negative impedance characteristic is an important cause of voltage oscillation and system instability.At present,the research on voltage oscillation and stability problems caused by constant power load is mainly aimed at unipolar DC system.Due to the unbalanced working state and inter-polar coupling,the stability analysis of bipolar system is obviously different from that of unipolar system,which needs further discussion.Therefore,the bipolar DC system with constant power load is taken as the research object in this paper.The following research work is carried out:The paper analyzes the impedance characteristics of the DC converter,and derives the transfer function of the cascaded converter system and its equivalent loop gain expression.On this basis,according to the small signal and large signal analysis of DC system,the discrimination methods of DC system stability are summarized: For small signal analysis,the characteristics and applicable scenarios of different impedance matching stability criteria are studied.For the problem of large signal stability,the direct and indirect methods of Lyapunov stability analysis of DC system based on state space theory are compared.In this paper,the voltage stability of bipolar DC system based on half bridge voltage balancer is studied.The active damping method is applied to the control of half bridge voltage balancer,which effectively suppresses the voltage oscillation caused by constant power loads.Firstly,the expression of Middlebrook stability criterion is extended to bipolar DC system.After that,the small-signal model of half bridge voltage balancer is deduced,then the model of constant power load is substituted for stability analysis.According to the structure of the voltage balancer,two damping control methods based on inductor current and capacitor current are used in closed-loop control to damp the voltage oscillation caused by constant power load.In addition,Lyapunov indirect method is used to analyze the influence of unbalanced load on the stability of bipolar system.Finally,simulation and experiment verify the validity of the theoretical analysis results.In this paper,the coupling problem between the positive and negative poles of bipolar DC system is studied.The symmetrical component method is used to analyze the bipolar DC systems which are equipped with dual input converter.The positive and negative pole components of the system are transformed into common mode component and differential mode component.This transform technique lays the foundation for the independent design of active damping in the common-mode and differential-mode network,and simplify the stability analysis and design process of bipolar system.Firstly,based on the transformation matrix of symmetrical component method in bipolar DC system,the symmetrical component circuit model of bipolar DC system is deduced.Furthermore,aiming at the stability problem caused by constant power load,active damping is combined with symmetrical component control to improve the voltage stability of the system.The corresponding simulation and experiments verify the effectiveness of symmetrical component control.