Research On Open-circuit Fault Diagnosis Methods Of Converter Device In Double-terminal VSC-HVDC

With demand for electric power development and new energy grid-connected generation,VSC-HVDC is being applied widely in the electric power system at home and abroad,its research of mathematical model,operation control strategy and protection are deeply concerned by domestic and foreign scholars.Converter device is the key equipment to realize AC and DC transformation in VSC-HVDC,its open-circuit fault will increase the system voltage and current harmonics,and the equipments or components of VSC-HVDC will be subjected to abnormal stresses such as overvoltage,overcurrent or overheat.Which may affect the reliability of entire VSC-HVDC system and safety of the equipments,thus the VSC-HVDC system cannot be restored quickly.So diagnosing converter device with open-circuit fault in VSC-HVDC rapidly and accurately,and adopting reasonable protection strategy are crucial to the whole VSC-HVDC system.However,for most open-circuit fault diagnosis methods,the faulty IGBT in VSC-HVDC needs to be diagnosed on two terminals at present,and cannot be identified and located just through one-terminal signals,which increases the cost and complexity of diagnosis system.Irn addition,the study shows that the percent of converter fault caused by free-wheeling diode is up to 10%.However,there are few records about free-wheeling diode open-circuit fault diagnosis.Therefore,aimed at double-terminal VSC-HVDC,the deep research about above two problems will be done in this paper,which could make diagnosis methods of converter device in VSC-HVDC more perfect,and improve the operation security and stability of VSC-HVDC.Firstly,based on three-phase static coordinate system and synchronous coordinate system,the mathematical model is established respectively in this paper by analyzing basic topology of VSC-HVDC.According to the research on respective adjustment principle of active and reactive power,the four quadrant operation characteristics and running area of VSC-HVDC are derived.Based on the analysis of control structure,station control,converter control and valve control of VSC-HVDC are studied,and control model of VSC-HVDC is established.Two-level and double-terminal VSC-HVDC simulation model is built based on Matlab/Simulink platform.The simulation results indicate that the simulation model has good static and dynamic stability under three-phase short circuit fault,power mutation and DC voltage mutation.Secondly,the equivalent model of IGBT with open-circuit fault in converter device is established,and the fault transmission characteristics is derived.And the diagnosis method of IGBT open-circuit fault based on the characteristics is proposed in this paper,which includes zero-marking method and phase method.The zero-marking method is used to identify and position faulty IGBT in the terminal with diagnosis system.And the phase method is applied to diagnose faulty IGBT in the opposite terminal.Therefrom,the proposed method overcomes the status that faulty IGBT in VSC-HVDC needs to be diagnosed on two terminals.Then,for the current situation that there are few records about open-circuit fault diagnosis of free-wheeling diode in converter device,the working principle of free-wheeling diode in process of rectification or inverter is analyzed.And THD(total harmonic distortion)difference of faulty phase current between positive and negative half wave is studied,while the free-wheeling diode open-circuit fault is occurred.Based on the characteristics of difference on phase current THD,the diagnosis method of free-wheeling diode with open-circuit fault is proposed in this paper.Therefrom,the proposed method could identify and position faulty free-wheeling diode in VSC-HVDC.Finally,with the two-level and double-terminal VSC-HVDC simulation model,the proposed diagnosis methods of open-circuit converter device are verified.For IGBT open-circuit fault diagnosis method based on fault transmission characteristics,simulation results indicate that location of faulty IGBT in the terminal with diagnosis system can be determined precisely by zero-marking method,and faulty IGBT in the opposite terminal can be identified by phase method,when the diagnosis system is installed on sending or receiving terminal.Thus,the faulty IGBT in whole VSC-HVDC system can be diagnosed on signal terminal.While the diagnosis system is installed on both two terminals,phase method in one terminal can be used as a standby diagnosis method of zero-marking method in another terminal,which could increase operation safety and reliability of VSC-HVDC.Meanwhile,the study in this paper can provide an important reference for faulty IGBT diagnosis in multi-terminal VSC-HVDC.For diagnosis method of free-wheeling diode with open-circuit fault based on the characteristics of difference on phase current THD,simulation result indicates that the proposed method can identify and position faulty free-wheeling diode in VSC-HVDC rapidly and accurately.