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

In order to save space,beautify the city,cross the strait and connect offshore wind power,overhead-cable hybrid transmission lines have been widely used.Differential protection is mainly used for the main protection of hybrid lines.However,for the transmission lines with high voltage and long distance,the distributed capacitance is large,which leads to the decrease of the sensitivity of the differential protection,and the misoperation may occur when the fault is outside the zone.Due to the complex structure and the uneven parameter distribution of the hybrid lines,the existing capacitive current compensation methods are all based on a single line,which are not suitable for the hybrid lines.For a hybrid line which contains both overhead line and cable line,it is also a difficult problem whether to open the reclosing after the line fault leads to the protection action.In view of the above problems,this paper has carried out the research of hybrid lines protection and adaptive reclosing.Firstly,in order to improve the sensitivity and reliability of differential protection for overhead-cable hybrid line,a piecewise capacitive current compensation method is proposed.Aiming at the situation that there is no voltage and current measuring device at the overhead-cable connection of the hybrid line,the method in this paper calculates the voltage value at the connection between the overhead line and the cable line by using the voltage and current values at both ends of the hybrid line,Then,the time domain capacitor current compensation is carried out for the overhead line section and the cable line section respectively.and the differential protection function is realized by using the compensated current.Secondly,aiming at the problem that the overhead-cable hybrid line can’ t determine whether to open the reclosing or not,an adaptive reclosing method is proposed by distinguishing fault sections.A line trap is installed at the junction of the overhead line section and the cable section.When the hybrid line faults,the protection device at both ends of the line collects the three-phase voltage value.The frequency spectrum of line mode voltage is analyzed by discrete wavelet transformation and Fourier transformation.Then,the fault section is distinguished using the difference of high frequency components of line mode voltage at both ends of the line respectively.The distinguished results are used to decide whether to open reclosing or not.The reclosing is opened when the overhead line faults,and the reclosing is blocked when the cable line faults.Finally,a simulation model for the hybrid line fault is built by PSCAD.The results show that the methods in this paper are effective,and it is free of the fault type,fault location and transition resistance.