Research On Key Issues Of DC Distribution Systems

To meet the challenge of energy crisis,the renewable energy has to be widely applied to human society.The distribution system is extremely important to integrate and take full advantage of the renewable energy sources.The nowadays distribution systems suffer severe problems in power transmission capacity,power quality,integration ability of renewable energy and new load.With the rapid development and technical support of power electronics technology,the DC distribution systems have become a promising way to solve the problems.However,the research on the application of DC distribution systems is far from enough.The key issues of the DC power systems technology will be analyzed in detail in this dissertation,including the model and control of power electronic devices,system-level flexible power flow control,the operation mode of DC distribution systems and the nonlinear analysis of DC distribution systems.The research in this dissertation will contribution to the complete structure of the technologies in DC distribution systems.Following researches will be finished in this dissertation:1.Based on the characters of the DC distribution technology,the typical structure of future DC distribution system is proposed.According to the characters of the bipolar DC distribution systems,the topology and control strategy of the VSC are studied to achieve the effective bus voltage control.Besides,the cooperative control and energy management strategy are also focused.The proposed method can suit to the DC distribution system scenario and the power quality is also improved.2.The line power control problem in DC distribution systems is studied.A DC/DC converter is used in series with the line to control the line power actively.The topology,operation mode and control strategy of the DC/DC converter are analyzed in detail.The influence of the line power control on the system-level power flow distribution is also presented.3.Based on the system analysis model of bus voltage control and line power control,a cooperative control method to organize the two techniques is proposed to achieve the high flexibility on the system-level power flow control.4.To satisfy the requirements of both power flow control and the friendly integration of renewable energy sources simultaneously,the bus types in DC distributions are expanded.With the expanded bus types,the flexibility and performance of DC power system operation are largely improved.The proposed bus type selection method can assure the feasibility and maximized high-level requirements.5.A complete new power system analysis theory is derived and proposed.The linear and nonlinear properties of power systems are deeply analyzed and the LRBNP(Linear Relationship Based Nonlinear Problem)is defined.The strict mathematical derivation of the proposed theory is performed in detail.According to the derivation results,some relationships can be clearly seen which are hard to tell in traditional power system analysis methods.The topology analysis and bus types analysis are successfully separated from the nonlinear power system equations and they are all abstracted to constant matrices.The mathematical and physical essence of some abstract problems in power systems can be clearly presented by the proposed theory.It constructs strong foundation for the nonlinear problems analysis in DC power systems.6.The correctness methods for the proposed power system analysis theory are discussed.With the correctness methods,the LRBNPs in power systems can be perfectly solved.Combined with the specific correctness methods,a new power system calculation method is proposed.The mathematical and physical essence of the voltage collapse phenomenon are also discussed.The correctness method and a new direct method based on the understanding of the physical essence of the voltage collapse are also proposed to analyze steady state stability analysis problem.The speed of the above methods are all largely improved without any sacrifice of accuracy.