Protection Operation Sequences And Risks In ACDC Line Touch Fault

With the construction of HVDC engineering,AC-DC lines are mixed and operated in the power grid,and the AC-DC line cross-over phenomenon is very common.The AC-DC crossover line is prone to line failure due to the close distance between the AC line and the DC line.Due to the large difference in AC and DC electrical characteristics,the fault characteristics of AC/DC line touch faults are more complicated.This will pose a serious challenge to DC protection and AC protection,and even to the safe and stable operation of the grid system.At present,the research on the impact of AC/DC line touch faults on AC/DC relay protection at home and abroad is still in the exploration stage.Although DC protection has been configured with AC/DC line touch protection,it still lacks targeted research on its adaptability.AC protection does not have a special AC/DC line touch fault protection.There is still a lack of research on the protection coordination and potential risks between DC systems and AC systems where AC/DC line faults may occur.This dissertation mainly focuses on theoretical analysis,combined with the actual HVDC transmission engineering model simulation of electromagnetic transient simulation software,to carry out research on protection timing sequence and risk after AC/DC touch line fault.Firstly,the AC-DC cross-overlay model is built based on the actual DC project,and the fault simulation is carried out under different fault conditions.When constructing the model,since the two mainstream DC technologies in China have great differences in control and protection,models will be built separately and analyzed.After the line is touched,both the AC and DC systems have ground fault characteristics and line fault characteristics,and are affected by the fault conditions and the system s own state.Secondly,according to the fault characteristics,the adaptability of existing DC protection and AC protection to AC/DC line touch faults is analyzed.The DC protection action is discrete due to the entry of the AC amount,and the AC protection action characteristic is similar to that of the high resistance ground fault.Then,combined with the protection characteristics of AC and DC,analyze all the fault processing sequences that may occur in the whole system.After exhaustive analysis and combing all possible conditions,it is found that the existing protection settings have great hidden risks:AC line current offset,DC overvoltage,and protection refusal due to improper coordination of AC and DC protection may occur.Based on the above analysis,this paper proposes suggestions for the improvement of the protection of AC and DC line protection,protection of exports and communication protection logic.