Protection And Fault Location Scheme For High Voltage Capacitor Of DC Filter In HVDC System

High Voltage Direct Current(HVDC)has the advantages of large transmission capacity,long transmission distance,low loss and no need to consider synchronization,etc.It is widely used in the fields of long-distance transmission of electric energy and regional interconnection of large power grids.The converter generates characteristic harmonics on the DC side,which will interfere with the communication system and reduce the power supply quality.At the same time,harmonics could induce resonance,which will cause the DC equipment to be damaged due to over-voltage and over-current.Therefore,a DC filter is installed at the DC bus to suppress the harm of harmonics.Based on the analysis of the protection adaptability of the DC filter,it is found that there are blind areas,maloperation and rejection,and the fault cannot be accurately located.Therefore,studying two aspects protection and fault location for DC filter high voltage capacitors.A grounding fault protection method for high voltage capacitor of DC filter based on characteristic harmonic impedance ratio is proposed to solve the problem that DC filter differential protection maloperation due to the inconsistent transient characteristics of the transformer.Firstly,the change characteristics of the equivalent circuit of the DC filter,and the deviation of the tuning point and resonance point after the grounding fault of the high-voltage capacitor are analyzed.Secondly,the variation trend of harmonic impedance corresponding to the tuning frequency and resonant frequency is derived theoretically,and the ratio of the two is defined as the characteristic harmonic impedance ratio.According to the analysis results,the characteristic harmonic impedance ratio increases significantly after the grounding fault of the highvoltage capacitor,and is not affected by the difference in the transient characteristics of the transformer.Based on this,the ground fault identification criterion of the DC filter is constructed.Finally,the reliability and adaptability of the proposed protection method under different working conditions are verified by simulation.The proposed protection is not affected by the difference in transient characteristics of the transformer.An open circuit fault protection method for DC filter based on capacitance parameter identification is proposed to solve the problem of blind protection in the case of symmetrical fault and continuous fault.Firstly,the structure and parameter variation characteristics of high-voltage capacitor after open-circuit fault are analyzed.The parameter changes of high-voltage capacitors can be characterized by the changes of electrical such as terminal voltage and current.Therefore,in this paper,the voltage and current of the high-voltage capacitor are calculated by the parameters of the DC filter and the measurable electric volume.The capacitance differential equation is established,the capacitance parameters are identified by the differential equation,and the relative capacitance error is defined.According to the analysis,it can be found that the capacitance relative error is only related to the number of capacitor components with open fault.The reliability of the proposed protection method is verified by simulation,and the problem of the protection blind zone for traditional unbalanced protection is effectively solved.There is very little research on fault location of high-voltage capacitors in DC filters.In actual engineering,after high voltage capacitor failures,manual measurement of capacitance values is often used to troubleshoot and locate faults.There are problems of low efficiency and threatening the maintenance staff personal safety.Therefore,a fault location method for high-voltage capacitor of DC filter based on current characteristics is proposed.In this paper,the current direction of unbalanced bridge branch and the relationship between the current of bridge branch and the current of filter are analyzed in detail.The fault location for high-voltage capacitor of DC filter is realized based on the characteristics of the current variation.Finally,the reliability and effectiveness of the fault location method under different operating conditions are verified by simulation.This paper analyzes the adaptability of unbalanced protection and differential protection of DC filter.The proposed grounding fault protection method for high voltage capacitor based on characteristic harmonic impedance ratio can effectively avoid the influence of the difference in transient effect of the transformer for differential protection.The proposed open circuit fault protection method for high-voltage capacitors based on capacitance parameter identification can effectively solve the problem of protection blind zone for continuous faults and symmetrical faults.The proposed fault location method based on current characteristics can improve the maintenance efficiency to a certain extent,and guarantee the personal safety of operation and maintenance personnel.The proposed method is expected to provide theoretical and technical support in the research of the protection and fault location of high voltage capacitor for DC filter.