Research Of Power Cable Fault On-line Detection Based On Spread-spectrum Time-domain Reflectometry Method

As an important carrier of power grid to achieve power transmission,the reliability of power cable relates to the operation safety of entire power grid system.In recent years,the probability of cable failure increases because of external damage,rough production process and aging insulation.The traditional off-line detection methods need to cut off the power to maintain the cable that not only can not complete the detection and location of intermittent fault but also cause additional economic loss inevitably. In this paper,the spread spectrum time domain reflectometry(SSTDR) on-line detection method are used for the continuous cable detection,this low level and high noise immunity method is well suited for intermittent fault detection of cable.Firstly,this paper analyzes the merits and drawbacks as well as the difficulties and facilities of various existing cable fault detection method.Combined with the basics of power cable just as its internal structure,common fault types and causes,the basic theory and ranging principle as well as traveling-wave transmission characteristics of SSTDR are studied and explored. And a enhanced SSTDR is proposed to improve the performance of detection of far fault.Secondly,Around the overall structure of SSTDR system,the optimal launch sequence is found from numbers of linear and nonlinear pseudo-random sequence through simulation and comparison by MATLAB.After the sequence selection,the type and frequency of carrier modulation and parameter of m sequence are researched and simulated from the perspective of SSTDR system performance to find out the optical test signal.Since the original data collected by SSTDR detection system with lots of noise,this paper analyzes the time domain and frequency domain characteristics of the cable failure transient process to design a rational high-pass filter.Thirdly,To improve the accuracy of fault location in low SNR,the fourth-order cumulant1 D slice algorithm is used to estimate the time delay instead of generated cross correlation and secondary cross correlation in SSTDR system because the fourth-order of Gaussian random process is zero. In order to solve the shortcomings in the calculation of cumulant,a corresponding optimization is put forward.To meet the needs of real-time on-line faultdetection,the fast FFT algorithm is used to achieve time domain cross correlation operation.The improved algorithm of fault location is proved to be more feasible and effective than others through simulation in white noise and colored nose environment.Finally,combining with the theoretical research and practical simulation,the paper establishes mathematical modeling of SSTDR by MATLAB.The signal-phase ground fault and signal-phase short circuit fault are simulated to demonstrate the above study.And the result of simulation show that:SSTDR still achieve precise location of the cable fault even at low SNR.