Study On Leakage Current Characteristics Of Typical Defects Of DC Cables

Compared with traditional AC transmission,DC transmission has the advantages of low line cost,small active loss,fast adjustment speed and high system stability.It is suitable for long-distance high-power transmission,submarine power transmission,and transmission of underground cables to large cities.The Voltage Source Converter based High Voltage Direct Current Transmission(VSC-HVDC)system is more flexible,and the system is more controllable.It is suitable for island transmission,distributed generation grid connection,urban power supply and many other fields.The current flowing from the core through the main insulation layer to the metal sheath in the DC cable is the leakage current,and the leakage current can reflect the deterioration of the medium in the cable.Leakage current waveforms detected in cables with different defects are different.By analyzing these differences,leakage current can be used as a basis for diagnosing fault defects in DC cables.In this paper,based on the leakage current,the cable defect model is made according to the basic structure of DC cable and the leakage current generation mechanism.The experimental platform for collecting current signals is designed.The collected data is analyzed by wavelet packet multi-scale decomposition method to extract the leakage current waveform characteristics of cable defect models under different voltage levels,temperatures,heat aging days.The main research work of this paper includes the following parts:Firstly,based on the basic structure of XLPE cable and the factors that cause faults in DC cable during operation or manufacturing,this paper designs and manufactures five common DC cable fault models: air gap,suspension,surface,scratch and tip.Then an experimental platform for measuring the leakage current in the DC cable was built.The source of the test interference is analyzed and the corresponding measures are taken to shield it.The experiment with the voltage level as the variable uses the step-up method to test the voltage of the cable under test at a voltage level of 5/10/15/20/25 kV.The experiment with temperature as a variable sets the voltage level to 20 kV.Leakage current collection was collected at the temperature of 20/30/40/50/60 °C.The cable was heated at 70 °C for 12 hours per day in the experiment with heat aging days as a variable.The leakage current of each defect was collected by using step-up method at 10/20/30 days of heat aging.Next,the mechanism of parameters such as conductivity and electric field strength in XLPE cable under various conditions is analyzed.Then the principle of wavelet packet analysis method is explained.The leakage current waveforms collected in these fault models are firstly smoothed and denoised by the wavelet packet multi-scale decomposition method,and then decomposed into smooth and detailed components for comparative analysis.Finally,according to the difference of current waveforms of different fault defects under the same voltage level and the difference of the same kind of defects at various temperatures or different heat aging days,the characteristics of leakage current waveforms of different defect models under different operating conditions are summarized.The experimental results show that the wavelet packet decomposition can effectively decompose the leakage current waveform in DC cable into two parts: smooth component and detailed component.The comparison of these two components can obtain the leakage current characteristics of DC cable under different operating conditions.Various voltage levels,temperature and heat aging days have different effects on different defects.The DC cable fault mode is diagnosed based on the difference between the leakage current waveforms,and the leakage current can be used as a basis for DC cable fault detection.