Study On Operation State Assessment And Maintenance Method Of HVDC Grounding Electrode

Grounding electrode is an important part of high voltage direct current transmission(HVDC).Some of China's DC transmission projects have been built early,and the early DC transmission grounding electrode have been in operation for nearly two or three decades.The long-term operation of the grounding electrode will cause problems such as Coke aging and feeding rod corrosion,resulting in the decline of the electrical performance of the grounding electrode.The grounding pole can not guarantee the normal operation,which may affect the safe and stable operation of HVDC transmission system.In this paper,the influence of coke aging and feed rod corrosion on the electrical performance of concentric double-ring grounding electrode is studied.The main research work and achievements of this paper are as follows:1)By introducing surface charge into the interface between coke bed and soil of grounding electrode,this paper proposes an algorithm of grounding electrode current field based on boundary element method considering the influence of coke bed.Verify the accuracy and effectiveness of the algorithm by using the design parameters and the measured data of the Xintian grounding electrode.2)The influence of increasing coke resistivity due to coke aging on electrical properties of grounding electrode is calculated and analyzed in this paper.According to the change of grounding resistance and the distribution ratio of inner and outer rings,an evaluation method for the aging state of coke of double ring grounding is proposed.And according to the aging of coke,the operation and maintenance methods are put forward.3)This paper uses the CDEGS simulation software to bulid the corrosion morphology model of double-ring grounding electrode feeder rod,analyze the changes of electrical properties of grounding electrode under different corrosion patterns.Put forward a method of judging whether there is corrosion or not and delimiting the scope of corrosion break line by means of analyzing the cable diversion and a method to find the corrosion break position by measuring step voltage.With the increasing scale of UHVDC and EHVDC transmission systems in China,the service time of HVDC grounding electrode in early commissioning is getting longer and longer.The evaluation of electrical performance index and the methods of maintenance of grounding electrode for UHVDC and EHVDC transmission are important subjects to be studied.The research and conclusions of this paper are of great significance to the safe operation of DC transmission system.