Research On On-line Partial Discharge Monitoring Of XLPE Cable

As the main equipment in power transportation,the insulation performance of XLPE cable determines the safety and reliability of power system,and the decline of its insulation performance will directly threaten the stability of the whole system.Traditional cable fault monitoring is mostly off-line monitoring,which cannot evaluate the insulation condition of cables in real time and find potential insulation faults in time.Therefore,it is of great significance to monitor and identify the partial discharge on-line of the running cable and find out the insulation hidden danger in time to ensure the stable operation of the power system.Based on the analysis and comparison of several common on-line monitoring methods,this paper selects electromagnetic coupling method to build on-line monitoring system,and studies on on-line monitoring of XLPE cable partial discharge and its anti-interference technology.The main work is as follows:(1)This paper expounds the classification and causes of partial discharge,explains the phenomenon of partial discharge and analyzes the mechanism of partial discharge based on the three-capacitance model of cable insulation,and gives four mathematical models according to the characteristics of partial discharge signals.Taking double exponential decay model as partial discharge pulse signal at fault point,the propagation of partial discharge signal in XLPE cable is simulated and analyzed by Simulink software,and from the analysis,it can be inferred that the different frequencies and different discharge positions of partial discharge signals produced by cable faults will have an impact on the monitoring results of the sensor,which provides theoretical support for the research on online monitoring of cable partial discharge.(2)The hardware design of partial discharge on-line monitoring system is completed.A broadband sensor based on self-integration loop is designed,and the relationship between response sensitivity and bandwidth parameters of the Rogowski Coil is simulated and analyzed.After determining its shape,size and magnetic material,the best integral resistance and coil turns are selected,so that the current sensor can reach a wide working frequency band and maintain a certain response sensitivity,and then its performance is verified by simulation in MATLAB.At last,Finally,the design of the following conditioning circuit,data acquisition circuit and power supply circuit are completed.(3)The problem of white noise interference in the on-line monitoring of partial discharge is studied in this paper.In view of the complexity of traditional noise reduction methods for partial discharge signals,the wavelet transform method is analyzed,and spectral subtraction with low computational complexity is introduced based on the original noise reduction algorithm.An improved noise power spectral density estimation scheme is adopted,which is combined with wavelet analysis.After signal acquisition,the partial discharge signal is decomposed in the first layer by wavelet analysis to obtain approximate subband and detail subband,and then the estimated value of noise power spectral density is obtained and subtracted from the noise power spectrum of the captured partial discharge signal.This greatly improves the noise reduction efficiency,and can simultaneously solve the problems of complex wavelet transform calculation and fixed spectral subtraction resolution.(4)The improved noise reduction algorithm is simulated and verified.The noise reduction effect of db3 wavelet on partial discharge signal is simulated and analyzed.Four thresholds are used to denoise the signal.It is concluded that rigrsure threshold is the best way to remove white noise,so this noise reduction method is compared with the improved noise reduction method.Using MATLAB to test the noise reduction performance of modified algorithm and rigrsure threshold wavelet at 5d B,0d B and-5d B noise levels in turn,and the signal-to-noise ratio is used for evaluation.The obtained figures and data show that compared with traditional methods,the improved denoising method has better denoising effect and has certain practical value.