Research On Local Defect Location Method Of Cable Based On Frequency Modulated Continuous Wave(FMCW)

The gradual development of cable defects will lead to faults.It is of great significance to detect and locate cable defects in time.At present,time-domain reflection method and time-frequency domain reflection method are effective for fault location,but they are not sensitive enough to local defects of cables.Recently,the frequency domain reflection method represented by Broadband Impedance Spectroscopy(BIS)has made great progress in the field of cable defect location due to its high sensitivity.BIS is faced with the problem:the effective detection distance of the cable is not enough and the detection efficiency is not high,the positioning error is large,and the sensitivity of the diagnosis of the severity of defects is not enough.To this end,the following studies are carried out in this paper:Firstly,a waveform modulation method based on stepped frequency continuous wave(SFCW)is proposed,which improves the multiple impedance measurement of BIS to a single voltage amplitude measurement,and improves the measurement efficiency and measurement distance.On the basis of SFCW,a waveform modulation method based on frequency modulated continuous wave(FMCW)is proposed,which solves the problem of limited modulation steps of SFCW and converts the amplitude of BIS The measurement is converted to frequency measurement,which improves the anti-noise ability and further increases the measurement distance.The test results of cable defect location show that the measurement time of FMCW is reduced from 30min of BIS to 10s;under the equivalent bandwidth of 20MHz,the measurement distance of 600m of BIS is extended to more than 800m,and the effective measurement distance is increased by about 33%.Secondly,aiming at the fast attenuation of the high-frequency part of the FMCW signal in the cable and the decrease of the equivalent bandwidth,a FMCW-based segment measurement and time-domain frequency band synthesis method is proposed.Segmented measurement solves the problem that system noise submerges high-frequency signals and reduces the equivalent bandwidth;frequency band synthesis solves the problem of segmented signal splicing and improves the equivalent bandwidth.This method takes the 0V of the time-domain mixing signal itself as a reference(the Hilbert algorithm takes the mean value of the envelope)to remove the low-frequency offset to complete the amplitude alignment of the segmented signal,realizes the frequency band synthesis of the time-domain signal,and clarifies the strategy of avoiding phase matching by rationally designing bandwidth segmentation parameters.By testing a 300m defective cable,two 25MHz bandwidths are used to synthesize a positioning spectrum equivalent to a 50MHz bandwidth.which doubles the equivalent bandwidth,reduces the measurement noise in the range of 0-200m,and improves the equivalent resolution.Then,aiming at the problem of positioning errors caused by discrete spectral line intervals when the time domain signal measured by the FDR method is transformed into a positioning spectrum by Fast Fourier transform(FFT),this paper uses the vernier caliper main scale and the vernier double scale to achieve high precision Based on the idea of ranging,combined with the corresponding relationship between FFT algorithm calculation points and spectrum scale.a variable sampling point FFT peak-finding analysis method and calculation efficiency optimization strategy are proposed.The test results show that the positioning error of this method is smaller than that of the FFT method,and the positioning error generated by the FFT method in data transformation can be reduced by more than 50%.Compared with direct interpolation methods in the frequency domain such as the energy center of gravity method,this method has higher positioning accuracy:it is at the same level as the Chirp Z transform method and has a slight advantage,but the method will not affect the shape of the phase spectrum.The difference between the results of 12 calculations under the calculation efficiency optimization strategy and 15,000 calculations without the optimization strategy is less than 0.001%.Finally,four kinds of cable defect location diagnostic functions,namely the amplitude spectrum,phase spectrum,real part spectrum and imaginary part spectrum of the FMCW mixed frequency signal after FFT transformation,are studied,and the advantages of the amplitude spectrum(power spectrum)as a positioning function are determined.The characteristics of defect location analyzed the feasibility of phase spectrum to improve the diagnostic sensitivity of cable defect severity,explored the relationship between phase spectrum shape and fault type,and proposed a high-sensitivity defect severity diagnosis and fault identification method based on FMCW phase spectrum.The test results show that relative to the amplitude spectrum,when the capacitance value of the local capacitive defect changes by 2%.the phase spectrum can increase the diagnostic sensitivity of the defect severity from 36%to 71%;Shielding damage defects.the sensitivity of the phase sensitivity spectrum is better than that of the amplitude spectrum,which can increase the sensitivity by 7%～62%.The fault type criterion can also be judged according to the change trend of the phase spectrum.The FMCW phase spectrum can improve the detection sensitivity of the BIS method for high-resistance faults from 2 kΩ(40Z0)to 10 kΩ(200Z0);the 40%shadow area of the BIS method is reduced to 18.2%within.