Research On Protection Setting And Adaptive Reclosing Of Overhead-cable Hybrid Lines

In order to improve the layout of urban suburban transmission lines and to transmit power across rivers or channels,overhead-cable hybrid lines are becoming more and more widely used in power systems.The structure of the overhead-cable hybrid line is complex and its parameters are not uniformly distributed.However,the grounding distance protection setting method takes it as a whole,resulting in insufficient protection sensitivity.On the other hand,overhead line faults are mostly transient faults,which will return to normal operation after reclosing action,while cable line faults are mostly permanent faults,which will fail again or even cause explosion after reclosing.Therefore,whether the overhead-cable hybrid line is put into reclosing is also one of the problems to be solved urgently.In this paper,the following research work is carried out on the grounding distance protection and reclosing of overhead-cable hybrid lines.Firstly,a self-adaptive fault section reclosure method based on free oscillation frequency is proposed for overhead-cable hybrid transmission lines.In this method,after the fault of the mixed line,the impedance element is used to determine whether the fault is near the bus,if it is near the fault,the fault section is directly determined;On the contrary,matrix bundles are used to extract the free oscillation frequency main frequency of the fault voltage,and the difference in voltage main frequency between overhead line section faults and cable line section faults is used to form a criterion for fault section discrimination.Finally,the decision on whether to open reclosing is made based on the discrimination results.When the overhead line section malfunctions,open the reclosing;When there is a fault in the cable line section,the reclosing is locked.Secondly,aiming at the problem of insufficient sensitivity of ground distance protection,a setting calculation method of ground distance protection for overhead-cable hybrid transmission lines is proposed.The influence of zero sequence current compensation coefficient on measured impedance was analyzed.Based on the fault phase voltage characteristics of the overhead line side and the cable side at different fault locations,a setting method was proposed for the overhead line side protection to use a positive sequence impedance value of 70%of the total length of the line from the busbar as the setting impedance,and for the cable side protection to use 70%of the positive sequence impedance of the entire line as the setting impedance.Finally,an adaptive fault section reclosure method based on fault indicator(FCI)is proposed for overhead-cable hybrid distribution lines.Firstly,the alarm indications of FCI in the event of faults in different types of hybrid lines such as overhead line cable and overhead line cable overhead line were analyzed,and based on this,the identification of fault sections was achieved.If the fault is located on the overhead line,reclosing will be opened,and if the fault is located on the cable,reclosing will be locked.Secondly,the installation principles of FCI were provided,and the performance of FCI based on two principles:overcurrent action method and adaptive load current overcurrent sudden change method was analyzed.The overcurrent action method FCI was selected.The overhead-cable hybrid transmission and distribution network model was constructed using PSCAD/EMTDC,and various fault simulations were conducted.The results showed that the method proposed in this paper can improve the sensitivity of grounding distance protection,achieve fault section identification of hybrid lines,and be unaffected by fault location.It can achieve adaptive fault section reclosing.