| Direct current transmission has the technical advantages of large transmission capacity,long transmission distance,small energy loss,and easy power adjustment.It has been widely used in China in recent years,and the traditional AC power grid has gradually transformed into a hybrid power grid.Due to the deep coupling of AC-DC system operation,the interaction of fault characteristics has brought many problems and challenges to the protection of AC-DC systems.This paper combines the research needs of major national special projects,and focuses on the in-depth research work on the impact of AC-DC hybrid power grid coupling fault characteristics on relay protection and the improvement of protection principles.Based on the basic structure and protection configuration of the AC-DC hybrid power grid,the thesis analyzes the impact of the AC-DC complex coupling fault characteristics on the DC system protection and the AC power grid protection when the power grid fails,and summarizes the main needs to be further deepened in the current protection system Problems,to provide guidance for the improvement and perfection of protection principles.The short circuit of the AC power grid is one of the main reasons that cause the commutation failure of the inverter on the inverter side of the DC transmission system.In view of the shortcomings of the existing commutation failure detection methods,according to the generation mechanism of commutation failure and the change trend of DC current and valve side current during commutation failure,a commutation based on the comparison of DC current and valve side current waveform characteristics Failure detection method.This method can directly use DC current and valve side current for detection and judgment,without the need to extract the cut-off angle information in the control system,which is easy for engineering application.Digital simulation results verify the good performance of the proposed method.The converter transformer is the key electrical equipment connecting the AC-DC systems.When a short-circuit fault occurs inside or outside the converter,a drop in AC voltage may cause the DC system to fail commutation.The DC current fed by it may easily saturate the converter core and affect the performance of the converter differential protection.This paper analyzes the main causes and influencing factors of the saturation of the converter core when the valve side and grid side of the converter are faulty.According to the basic principle of the converter differential protection,combined with digital simulation,the action behavior and possible problems of the converter differential protection are evaluated and summarized.The conclusion of the analysis is helpful to make up for the deficiencies of the existing research and further deepen the understanding of the performance of the converter differential protection.The short circuit fault of the AC power grid will cause the discharge of the DC line capacitance,which will increase the differential current of the DC line current longitudinal differential protection.In order to prevent the protection from malfunctioning,it is necessary to extend the operation time of the differential protection in engineering applications,which seriously affects the rapidity of protection.In this paper,a novel pilot protection based on open-close filtering and multi-resolution morphological gradient is proposed based on the characteristics of current waveform changes on both sides of the line during internal and external faults in the DC line.The protection principle has a fast action speed,no capacitor current compensation is needed,and it reflects strong grounding resistance and has good anti-noise ability.At the same time,it only needs to transmit the logical quantity signal through the longitudinal connection channel,which helps to improve the reliability of the protection action and is easy to implement in engineering.At the end of the thesis,the work done and the research results obtained are summarized,and the future research work to be carried out is prospected. |
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