Study On Time-Frequency Domain Analysis Method For Fault Location Signal Of XLPE Power Cables

With the development of power grid construction in China,the land resources required for the construction of power transmission and distribution projects become more and more limited.Power cable has the advantages of short insulation distance between lines.In addition,it can be laid under the ground to reduce the space.It has become an important part of China's power transmission and distribution system,and its reliability is related to the normal operation of the power system.Since the power cable is generally laid in a tunnel,a pipe,or buried in the ground,the operating environment is rather harsh.Thus,the lifespan of cable itself is greatly reduced.Moreover,local defects such as insulating air gaps and protrusions existing in the cable that are affected by external unfavorable factors such as electric,thermal and mechanical effects,resulting in insulation aging of the cable insulation layer.If the cable fault is not checked in time,a large-scale power outage would occur,which will not only bring serious losses to social economy,but also seriously affect people's daily production and.life.Therefore,how to timely and accurately locate the fault spot is of great significance to the safe and reliable operation of the power system.Therefore,this paper takes the most widely used cross-linked Polyethylene(XLPE)power cable as the research object,and has carried out the following researches:First,the paper introduces the development of XLPE power cable,and analyzes the research methods of cable fault location by domestic and foreign scholars,including impedance measurement method,intelligent ranging algorithm and traveling wave measurement method,and analyzes their principle and scope of application.The technical limitations of these methods have been also illustrated.At the same time,the research status of time-frequency domain analysis is introduced,which lays a foundation for introducing this method into power cable fault location.Second,the structure of XLPE power cable is introduced,such as conductor core,inner and outer semi-conductive shielding layer,cable insulation layer,metal sheath and the armor part.The causes of cable faults can be summarized as external damage,insulation moisture,chemical corrosion,long-term overload operation and ambient temperature.Finally,the simulation model has made relevant research and analysis on the generation of traveling wave signals in the cable,signal deflection and several common traveling wave fault location methods,and laid the theoretical foundation of the last two chapters.Then,this paper applies the mature time-frequency domain analysis method in communication and coaxial cable to XLPE power cable fault location.This method uses Wigner-Ville Distribution(WVD)time-frequency processing technology for fault location.The reflected signals are processed and a time-frequency cross-correlation algorithm is used to construct a time-frequency joint function to measure the fault point.In the process of fault location,the coarse measurement is first performed by using the time offset of the reflected signal,and then corrected by the time compensation function defined by the frequency offset.When the traditional time-frequency domain analysis method is applied to WVD to process multi-component signals,there is a serious cross-term interference problem,which is the main reason for the large error of the time-frequency domain analysis method in power cable fault location.The short-time Fourier transform is proposed to suppress the cross terms,and the experimental results are carried out with the Choi-Williams window function method.Comparative analysis shows that the method can effectively suppress cross terms and preserve high signal resolution,which is conducive to the precision of fault location.The electromagnetic transient simulation software ATP-EMTP is used to simulate open circuit fault,low resistance fault and high resistance fault,and the fault signal is analyzed by MATLAB software.The simulation results show that the time-frequency domain analysis method has higher fault location accuracy than the frequency domain analysis method,and the improved time-frequency domain analysis method after suppressing the WVD cross term by using the short-time Fourier transform is compared with the traditional time-frequency domain analysis method.The accuracy of the fault location method has been further improved.Finally,a preliminary design scheme for a XLPE power cable fault location system is proposed.