Stability Analysis Of Flexible Dc Railway Electrification System With New Energy

The flexible DC transmission and distribution technology is regarded as the key technology for construction of future intelligent networks.Recently,typical flexible DC systems,such as flexible HVDC transmission systems,DC microgrids,and DC ships are hot research topics.The thesis conducts research on a new flexible DC railway electrification system.Compared with the traditional AC railway electrification system,the flexible DC railway electrification system has lower losses,higher efficiency and higher energy efficiency.Meanwhile,it can provide reactive power compensation for the AC grid.Evenmore,the new DC system has the advantages of easy access to new energy,energy storage equipment and DC load.However,there are also some difficulties in the large-scale promotion and application of flexible DC railway electrification systems,such as stray current and the economics of system construction.Moreover,how to maintain the system stable is an urgent issue that requires in-depth analysis.This thesis focuses on analyzing the stability of flexible DC railway electrification system under different topologies and operating conditions,meanwhile,conducts research with the research ideology of establishing mathematical models,revealing stability mechanisms,proposing novel control schemes and experimental verification.The thesis forms the linear and nonlinear modeling methods of different converters in flexible DC railway electrification system,adopts stability criteria applicable to varying operating conditions,and reveals the mechanism and influence factors of potential instability problems in flexible DC railway electrification system.A damping compensation scheme to improve the system stability is proposed.The thesis uses hardware-in-loop experiments to support the system modeling and theoretical analysis.The feasibility of the flexible DC railway electrification system has been verified by the experimental results.The contributions of this thesis are summarized as follows:(1)The small signal average model of typical flexible DC railway electrification system is established.By classifying the converters in the system and simplifying the system topology,typical flexible DC railway electrification system include three-phase AC/DC converters(substations),boost DC/DC converters(photovoltaic,PV),constant power load(locomotive)and other components.The system adopts the droop control without communication,which helps achive the power allocation among converters and fast dynamic responses.Based on this,the DC-side small-signal impedance models of boost DC/DC converter,dqcontrolled three-phase AC/DC converter and DC locomotive are established.Particularly,in order to consider the influence of MPPT control in PV when modeling,a complete system model including power loop in PV is obtained based on the description function method.On the basis of the established mathematical model,it has laid a theoretical foundation for the in-depth development of the stability analysis and feasibility study of the flexible DC railway electrification system.(2)The stability of flexible DC railway electrification system is analyzed when no vehicle is connected to the system.When there is no vehicle connected to the system,the system is simplified as a "PV-flexible DC traction substation" interconnection system.This thesis uses the description function method to model the nonlinear part of the MPPT control in PV.At the same time,the model of the linear part in the system is modeled.Thereby,the influence factors including the power loop,system control parameters,and the number of PV are revealed.The research results show that the flexible DC railway electrification system can achieve a relatively ideal operating state even without vehicle load.(3)The stability of flexible DC railway electrification system is analyzed when vehicles are connected to the system.When the vehicle is interacted without PV access,the system can be regarded as the "flexible DC traction substation-DC vehicle" interconnection system.The Bode criterion applied in a single-input single-output system is taken to analyze the interaction between the DC traction substation and the vehicle load.The results show that the flexible DC railway electrification system can achieve stable operation when the vehicles are connected to the system.(4)The stability of flexible DC railway electrification system is analyzed when vehicles and PVs are connected to the system.On the basis of above two operations,the operation with access of PV and vehicles is taken into consideration.The system is simplified into a "flexible DC traction substation-PV-DC vehicle" cascaded system.Since the direction of power flow in the system is uncertain,this thesis applies the passive stability criterion that need not consider the direction of power flow and can convert the DC impedance of multiple converters to the total DC bus impedance.The Nyquist diagram further reveals the influence of control parameters,number of PVs,vehicle power and DC line impedance on the system stability.The results show that the flexible DC railway electrification system can still operate ideally when PVs and vehicles are connected to the system.(5)The stability enhancement strategy of flexible DC traction power supply system based on active damping is proposed.As mentioned above,when the circuit or control parameters of the system is unreasonable,the small-signal instability will occur in the flexible DC railway electrification system.Hence,the research has been carried out on the stability enhancement for the flexible DC railway electrification system with different topologies.For the "PV-flexible DC traction substation" interconnection system considering the MPPT control of PV,an virtual impedance controlstrategy based on the current feedback is proposed,which effectively improves the system stability.For the "flexible DC traction substation-DC vehicle" interconnection system,the frequency range of the source impedance falling into the forbidden domain is reduced by reshaping the source impedance.For the "flexible DC traction substation-PV-DC vehicle",aiming to ensuring that the dynamic characteristics of the vehicle are not affected by the cascaded system,the virtual impedance control strategies are designed in the control system of the three-phase AC/DC converter or boost DC/DC converter,respectively.The virtual impedance control strategy can effectively improve the system stability.The experimental results of the semi-physical simulation experiment platform have verified the proposed stability improvement scheme,and further expanded the feasibility application of the flexible DC railway electrification system in the practical engineering.This thesis reveals the interaction mechanism between PV,flexible DC traction substation and vehicle in the flexible DC railway electrification system,which can maintain stable operation with PV or vehicle or both of them.The research results have certain scientific value for promoting the theoretical innovation of flexible DC railway electrification technology and establishing a small-scale flexible DC railway electrification system.