Research On Topology Structure And Protection Methods Of Flexible AC/DC Hybrid Distribution System

With the development of VSC-based DC technology,AC/DC hybrid distribution system has a broad prospect of application because of its advantages including strong power supply capacity,high power supply reliability,better power quality and convenience for DG connection.Nowadays,the research for AC/DC hybrid distribution system is still in the early stage,as its basic topics such as system structure and fault characteristics,are waiting to be studied urgently.The paper researched on the topology structure,fault characteristics of typical faults and coordination method of protection of AC/DC hybrid distribution system.First,the current domestic and foreign research on the voltage level sequence and system topology are researched and analyzed in this paper.On this basis,analyses of AC/DC hybrid distribution system topology and two typical application scenarios,urban distribution network and industrial park distribution network,are proposed.Second,the fault characteristics of AC/DC hybrid distribution system are researched,from the aspects of DC faults and AC faults.Considering DC faults,two typical type of DC fault,pole-to-ground fault and pole-topole fault are chosen as research object.The fault response mechanism,circuit and characteristics of AC/DC hybrid system are researched and verified by PSCAD simulation.The simulation results prove that during DC faults,DC voltage drops significantly and great fault current flow through both DC system and AC system.Considering AC faults,because of the similarity of fault response of AC system fault and AC bus fault in VSC,AC bus fault in VSC is chosen as research object.Considering ground and phase faults,in the case of latched or latched-failed converter station,the response mechanism,circuit and characteristics of converter station and DC line are researched.The response process is presented by PSCAD simulation results,which proves the validity of the proposed response mechanism.The research proves that energy interaction within and between AC/DC system is an important part of fault response.In the latched converter station cases,the energy interaction between AC and DC system and the energy interaction within DC system happens alternately;in the latched-failed cases,besides energy interaction,besides energy interaction,the control of converter is another important factor which dominates the conduction state of the converter,fault response circuit and fault response characteristics.Third,the method for calculating the quantitative relation between the 100 Hz DC component and AC negative sequence component and the 100 Hz DC protection scheme are studied in this paper.The mathematical model for calculation is established firstly,including the low-frequency model of twolevel VSC described by duty cycle,model of direct current control system and digital realization model of SVPWM.Based on the mathematical model,the function,which reveals the quantitative relationship between DC second harmonic current and AC negative sequence component during asymmetrical fault,is derived by decomposing the AC fault current into positive sequence component and negative sequence component.The research is verified by PSCAD simulation and the result shows that the negative sequence component on the AC side is proportional to the second harmonic component on the DC side.Based on the quantitative function,a protection scheme using DC system 100 Hz protection is proposed and verified by PSCAD simulation.The first section of 100 Hz DC protection is configured as the main protection scheme of converter station,while the second section as the near backup protection of converter station and the third section as the far backup protection of the neighboring AC power line.In conclusion,this paper completes the research of the topology,the fault characteristics of typical faults,and the cooperation of AC and DC protection of flexible AC/DC hybrid distribution system.The research is innovative and has high practical value,which lays the foundation for further study of protection schemes of flexible AC/DC hybrid system.