Cable fault positioning method for realizing wide-range measurement

Abstract

The invention discloses a method capable of realizing wide-range positioning of a cable fault, a spread spectrum step frequency wave signal is obtained by performing binary phase shift keying (BPSK) and pseudorandom sequence modulation on a step frequency signal, and the method can overcome the problem that the cable fault detection range of a spread spectrum time domain reflectometry (SSTDR) is limited by code element duration. A cable fault detection device is installed at the head end of a cable, a detection signal is injected from the head end of the cable, a high-speed ADC is used for signal collection, cable fault information is obtained, and wide-range detection of cable faults is achieved.

Description

Technical Field

[0001] The present invention relates to the field of cable fault detection, and particularly to a cable fault location method for realizing wide-range measurement. Background Art

[0002] With the proposal of the dual-carbon strategy, China has accelerated industrial upgrading and vigorously developed renewable energy. The power cable has a small insulation spacing and occupies little space during laying; the cable is installed in the underground pipe gallery, has little impact on power electronic equipment, and has high power transmission reliability and safety, so it is widely used in the power transmission and distribution network. As the service life of the cable increases, the probability of cable failure increases greatly. There are many intermediate branches in the low-voltage distribution network, significant stress damage, frequent tree and rodent damage, high leakage probability, high electricity theft risk, etc. The low-voltage cable is laid underground through pipes, and it is difficult to monitor and verify when it is damaged. The traditional method is to conduct fuzzy excavation in a large range after power outage, with low verification and diagnosis accuracy, great excavation difficulty, high consumption of manpower and material resources, and great pressure on power outage services. The overhead cable is easily affected by bad weather such as strong wind, lightning strike, icing, etc., resulting in faults such as fracture, short circuit, and loose intermediate joints, which affect the power supply stability. Moreover, the power transmission lines of the distribution network are dozens of meters to thousands of meters long. For such a wide range of cable fault detection, there is still a lack of effective countermeasures in the current routine inspections.

[0003] At present, according to different working principles, the cable fault location methods can be divided into two categories: impedance method and traveling wave method. The impedance method is only applicable to the detection of open circuit faults of cables, with limited applicable scenarios. The traveling wave method is divided into time domain reflection method and frequency domain reflection method according to different incident signals and data processing methods. The time domain reflection method detects cable faults by injecting pulses or modulated signals into the cable. This method is simple and easy to operate, but it is affected by the measurement blind area and measurement range. The frequency domain reflection method has complex data processing. Therefore, it is of great significance to study a wide-range cable fault location method and effectively monitor it regularly during cable operation to ensure the safe and reliable operation of power cables. Summary of the Invention

[0004] The purpose of the present invention is to provide a wide-range cable fault location method.

[0005] The technical solution for achieving the purpose of the present invention is as follows:

[0006] A wide-range cable fault location method, the specific steps are as follows:

[0007] The first step, incident signal acquisition:

[0008] The signal used in the present invention is the signal after BPSK modulation of a stepped frequency signal and a pseudo-random sequence. The signal expression is:

[0009] x(f i,t) = Am1(f i ,t)m2(f i ,t)*sin(2πf i t + θ i ) (1)

[0010] Where A is the amplitude of the modulation signal, the frequency f of the stepped frequency signal ranges from 1 MHz to 31 MHz, m1(f i ,t) is a 5th-order m-sequence with a fixed symbol duration, and the duration of a single symbol is 1 μs. m2(f i ,t) is a 5th-order m-sequence with a symbol duration that steps with the frequency of the sine signal.

[0011] In the second step, the obtained modulation signal is injected from the head end of the cable, and the reflected signal is acquired at the head end of the cable; the expression of the reflected signal is as follows:

[0012] y(f i ,t) = B i m1(f i ,t - τ)m2(f i ,t - τ)*sin(2πf i (t - τ) + θ i ) (2)

[0013] The attenuation degrees of signals with different frequencies in the cable are different. Therefore, the amplitudes of different frequency components of the reflected signal are different. B is used i to represent the amplitudes reflected by different frequency components, and τ is the transmission delay of the incident signal. The signal collected at the head end of the cable is the superposition of the incident signal and the reflected signal, and its expression is:

[0014] x1(f i ,t) = x(f i ,t) + y(f i ,t) (3)

[0015] In the third step, the cross-correlation processing is performed on the incident signal and the reflected signal, and its calculation formula is as follows:

[0016]

[0017] In the fourth step, the maximum peak value and the second maximum peak value of the cross-correlation result are obtained to get the transmission delay of the high-frequency signal. Combining with formula (5), the fault location of the cable is obtained.

[0018]

[0019] The SSFWR signal is modulated by multiplying three parts: a sinusoidal wave with frequency stepping, a 5th-order m-sequence with a fixed symbol duration, and a 5th-order m-sequence whose symbol duration varies with the period of the frequency-stepping sinusoidal wave. Through this modulation, the obtained SSFWR signal overcomes the drawback of the SSTDR method that the ranging range is limited by the symbol duration, retains the advantage of the frequency-stepping signal being sensitive to cable defects, and simplifies the data processing process of the frequency-stepping signal. In Table 1, the ranging ranges of m-sequences with different symbol durations are summarized. There are 31 different m-sequences with different symbol durations in this modulation method. Assuming the propagation speed of the high-frequency traveling wave in the cable is 209.4 m / μs, the cable fault detection range of this method can be obtained from the following formula. It can be calculated that the cable fault detection range of the SSFWR method is 3.4 m - 3245.7 m.

[0020]

[0021] Table 1 Maximum measurement distances for different frequency symbols

[0022]

[0023] In order to further optimize the technical solution, the present invention designs a cable fault detection system based on the SSFWR method, which is characterized by including: an SSFWR signal design module, a signal generation module, a signal sampling module, a signal processing module, and a defect location module.

[0024] The SSFWR signal design module is used to design a signal that can achieve wide-range distance measurement and transmit the signal to the signal generation module;

[0025] The signal generation module uses a DAC chip to convert the designed signal into an analog signal and inject the incident signal into the cable;

[0026] The signal sampling module is used to collect the incident signal and the reflected signal at the cable head end and transmit the data to the data processing module;

[0027] The signal processing module is used to process the data of the signal generation module and the signal acquisition module, perform cross-clock domain processing on the data, and transmit the data to the cable defect location module;

[0028] The cable defect location module locates the cable fault based on the cross-correlation location algorithm proposed in this patent.

[0029] The beneficial results of the present invention are:

[0030] The present invention fully considers the actual laying situation of power cables, solves the limitation of the code element duration on the cable ranging range in the extended spectrum time domain reflectometry method, realizes wide-range cable fault ranging. At the same time, the SSFWR signal has strong anti-interference ability, can realize the live detection of cable faults, and is of great significance for improving the stability of the power supply system. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order to more clearly illustrate the technical solution of the present invention, the accompanying drawings are briefly introduced below.

[0032] Figure 1 Schematic diagram of the SSFWR cable fault location system;

[0033] Figure 2 Schematic diagram of SSFWR signal modulation;

[0034] Figure 3 Measured diagram of SSFWR signal, (a) is the incident signal collected at the cable head end; (b) is the mixed signal collected at the cable head end;

[0035] Figure 4 Distributed parameter model of the cable

[0036] Figure 5 Results of cable open circuit fault and short circuit fault location using the method in the present invention. SPECIFIC EMBODIMENTS

[0037] The first step is the installation of the cable fault detection device.

[0038] When a fault occurs in the distribution network, it will induce the relay protection device of the distribution network to operate and disconnect the connection of the faulty line. At the same time, in order to ensure the safety of maintenance, when troubleshooting cable faults, it is necessary to further detect whether the faulty line is energized. The present invention is connected at the start end of the cable fault line. If the cable to be tested is a two-core cross-linked polyethylene cable, the transmitting end of the cable fault detection device is connected to the two cable cores. If the cable to be tested is a single-core coaxial cable, one end of the transmitting end of the cable fault detection device is connected to the cable core, and the other end is connected to the copper shielding layer. If the cable to be tested is a multi-core shielded cable, the cable fault detection device can be connected to the cable core to be tested, and the other end is connected to other cable cores or the shielding layer as a potential reference. The receiving end of the cable fault detection device is connected in the same way as the transmitting end.

[0039] The second step is to obtain cable detection data.

[0040] After connecting the cable terminal to the cable fault detection device, turn on the cable fault detection device and emit a signal every 1 s. And store the data acquisition data. Transmit the collected data to the upper computer through the serial port to obtain cable fault information.

[0041] Step 3: Obtaining the cable fault location.

[0042] The similarity between the fault signal and the reference signal collected at the cable head is used to measure the cable fault distance. Mathematically, the cross-correlation function is used to characterize the similarity between two sequences. For discrete sequences x(n) and y(n) of length N, the definition of their cross-correlation is:

[0043]

[0044] Similarly, the autocorrelation function is used to characterize the similarity between x(n) and the sequence x(n + τ) after its own sequence delay, and its expression is defined as:

[0045]

[0046] Perform cross-correlation processing on the SSFWR signal to obtain two peaks, which respectively represent the positions of the incident signal and the reflected signal. The propagation time of the signal in the cable can be calculated, and the fault point position can be obtained from the following formula:

[0047]

[0048] Among them, v is the propagation speed of the traveling wave in the cable, l is the fault distance, and t is the transmission time of the traveling wave in the cable.

[0049] Step 4: Obtaining the cable fault type.

[0050] The transmission characteristics of the cable for signals can be described by the transmission line theory. The transmission characteristics of the transmission line for high-frequency signals depend on factors such as its length and the signal wavelength. The wavelength λ of the sine signal is defined as shown in the following formula:

[0051]

[0052] Among them, v is the propagation speed of the high-frequency signal, f is the frequency, and the higher the signal frequency, the shorter the wavelength. When the length of the transmission line is long enough and the transmission line characteristics account for an important proportion in the entire input impedance, at this time, a distributed parameter model needs to be used for analysis, as Figure 4 shown.

[0053] In the figure, R, L, G, and C respectively represent the resistance, inductance, conductance, and capacitance per unit length of the cable. The characteristic impedance Z0 of the cable can be calculated from the following formula:

[0054]

[0055] When the high-frequency signal is transmitted in the cable, R << jωL, G << jωC, then the characteristic impedance can be approximated as:

[0056]

[0057] When a cable fails or has a defect, the characteristic impedance at that point changes, and the high-frequency traveling wave transmitted in the cable will be refracted and reflected at the fault point. The characteristic impedance at the fault point is Z1, ρ is the reflection coefficient, which is the ratio of the reflected voltage to the incident voltage at the fault point, γ is the transmission coefficient, which is the ratio of the transmitted voltage to the incident voltage. ρ and γ can be obtained from the following formula:

[0058]

[0059] It can be seen from Equation (13) that the polarity of the transmitted signal is the same as that of the incident signal, the signal strength is related to the degree of cable fault, and the polarity and strength of the reflected signal are related to the characteristic impedance at the fault point. When a short-circuit fault occurs in the cable, the incident signal is totally reflected, and the polarity of the reflected signal is opposite to that of the incident signal. At this time, ρ = -1. When an open-circuit fault occurs in the cable, the polarities of the incident signal and the reflected signal are the same. At this time, ρ = 1. Thus, the cable fault type can be determined.

[0060] By integrating the above data processing methods, the cable fault type and fault distance can be obtained.

Claims

1. A cable fault location method for achieving wide range measurement, characterized in that: Including: The first step is to generate a stepped frequency wave with extended spectrum (SSFWR) signal The signal used in the present invention is the signal after BPSK modulation of the stepped frequency signal and the pseudo-random sequence. The signal expression is: x(f i ,t)=Am1(f i ,t)m2(f i ,t)*sin(2πf i t+θ i ) (1) Where A is the amplitude of the modulation signal, the frequency range of the step frequency signal f is 1MHz-31MHz, m1(f i ,t) is a 5th-order m-sequence with a fixed symbol duration, and the duration of a single symbol is 1μs, m2(f i ,t) is a 5th-order m-sequence whose symbol duration steps with the frequency of the sinusoidal signal. The second step is to install the cable fault detection device The present invention is installed at the head end of the cable fault line. When the cable to be tested is a two-core cross-linked polyethylene cable, the transmitting end of the cable fault detection device is connected to the two cores. If the cable to be tested is a single-core coaxial cable, one end of the transmitting end of the cable fault detection device is connected to the core of the cable, and the other end is connected to the copper shielding layer. If the cable to be tested is a multi-core cable with a shielding layer, one end of the signal transmitting end of the cable fault detection device is connected to the core to be tested, and the other end is connected to other cores or the shielding layer. The receiving end of the cable fault detection device is connected in the same way as the transmitting end. The third step is to obtain cable fault detection data After connecting the cable terminal to the cable fault detection device, start the detection device, cache the data of the analog-to-digital conversion module and the digital-to-analog conversion module, and then transmit the data to the upper computer for processing through the serial port. The fourth step is to obtain the cable fault location Use the similarity between the fault signal collected at the head end of the cable and the reference signal to perform cable fault ranging. Denote the data of the analog-to-digital conversion module as x(n), and the data collected by the digital-to-analog conversion module as y(n). For the discrete sequences x(n) and y(n) with a length of N, the definition of their cross-correlation is: Similarly, use the autocorrelation function to characterize the similarity between x(n) and the sequence x(n+τ) after its own sequence delay. Its expression is defined as: Perform cross-correlation processing on the SSFWR signal to obtain two peaks, which respectively represent the positions of the incident signal and the reflected signal. The fault point position can be obtained by the following formula: Where, v is the propagation speed of the traveling wave in the cable, l is the fault distance, and t is the transmission time of the traveling wave in the cable. The fifth step is to obtain the cable fault type The cable fault type is determined by the impedance characteristics at the fault point. The extraction of the cable fault type is analyzed using the distributed parameter model. As shown in Figure 4, in the figure, R, L, G, and C respectively represent the resistance, inductance, conductance, and capacitance per unit length of the cable. The characteristic impedance Z0 of the cable can be calculated by the following formula: When the high-frequency signal is transmitted in the cable, R<<jωL, G<<jωC, then the characteristic impedance can be approximated as: When a fault or defect occurs in the cable, the characteristic impedance at this point changes. The characteristic impedance at the fault point is Z1, ρ is the reflection coefficient, which is the ratio of the reflected voltage to the incident voltage at the fault point, γ is the transmission coefficient, which is the ratio of the transmitted voltage to the incident voltage. ρ and γ can be obtained by the following formula: It can be seen from Equation (8) that when a short-circuit fault occurs in the cable, the incident signal is completely reflected, and the reflected signal has the opposite polarity to the incident signal. At this time, ρ=-1. When an open-circuit fault occurs in the cable, the incident signal and the reflected signal have the same polarity. At this time, ρ=1. Thus, the cable fault type can be determined.

2. A cable fault location method for realizing wide range measurement as described in claim 1 can change the size of the traveling wave velocity v according to the differences in cable manufacturers and insulation material properties, and flexibly set the range of the step frequency signal bandwidth according to the actual measured cable length, so as to make it more suitable for wide range fault location work of cables of different types and lengths.

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