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Cable fault detection method and instrument
May 12, 2018

With the increasing proportion of cable usage in the entire power transmission line and the Internet, the occurrence of cable failures is increasing. Cable faults cause greater harm to production, lighter ones can cause a single electrical device to fail to operate, and severer ones can lead to power outages in the entire substation, so the rapid determination and precise positioning of cable fault points becomes very important.


First, the traditional method of cable fault detection


(A) The traditional method of cable fault location


The traditional methods of cable fault ranging are mainly the following four:


Bridge method: This is the classic method of distance measurement for power cables. This method is relatively simple, but it needs to know the cable length and other data in advance, and it is only suitable for low resistance and short circuit faults. However, in actual operation, faults are often high-impedance and flashover faults. Because the fault resistance is very high and the bridge current is very small, the general sensitivity meter is difficult to detect.


Pulse echo method: For low-resistance and open-circuit type faults, using a low-voltage pulse reflection method to measure the cable fault is simpler and directer than the above bridge method, and the distance can be measured only by observing the time difference between the reflection of the fault point and the transmitted pulse. A low-voltage pulse is injected into the cable during the test. When the pulse propagates to the point of failure, reflection occurs and the pulse is reflected back to the measuring point. Using the instrument to record the time difference between the transmitted and reflected pulses, the distance from the point of failure can be calculated by knowing only the pulse propagation speed. This method is simple and intuitive. It does not need to know the original data such as the cable length, but also identifies the position of the cable connector and the branch point based on the reflection waveform.


Pulse voltage method: This method can be used to measure high resistance and flashover faults. First break down the cable fault under DC or pulsed high voltage signal, and then measure the distance by recording the time required for the discharge pulse to make a round trip between the measurement point and the fault point. An important advantage of the pulse voltage method is that it is not necessary to burn through the high resistance and flashover faults, and the instantaneous pulse signals generated by the fault breakdown are directly used. The test speed is fast and the measurement process is simplified. However, the disadvantages are as follows: 1 The instrument measures the voltage pulse signal through a capacitive resistance voltage divider, and the instrument is electrically coupled with the high voltage circuit. It is very easy to generate high voltage signal and cause equipment damage. Therefore, the safety is poor. When measuring distance measurement, the high-voltage capacitor is short-circuited to the pulse signal, and a resistor or an inductor is needed to generate a voltage signal, which increases the complexity of the wiring and makes the breakdown point not easy to breakdown. 3 When the fault discharges, especially in the flashover When the voltage waveform of the voltage divider coupling is not sharp, it is difficult to distinguish.


Pulsed current method: This method is safe, reliable, and easy to wire. The method is to break down the fault point of the cable with a high voltage, use the instrument to collect and record the current traveling wave signal generated by the fault click-through, and calculate the fault distance according to the time when the current travel wave signal reciprocates between the measuring end and the fault point. The method uses a transformer to couple the pulse current, and the waveform is simpler and safer. This method also includes direct flash method and flash method. Unlike the pulse voltage method, which uses a resistor and a capacitive voltage divider for voltage sampling, the pulse current method uses a linear current coupler placed parallel to the low-voltage geodesic line and has no direct electrical connection to the high-voltage loop. This is for recording instruments and operators. It is particularly safe and convenient. So people generally use this method.

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(B) The traditional method of cable fault location


Here is a brief introduction to the sound and magnetic synchronization method. The method uses a high-voltage device to make cable faults click through and discharge, use the receiver to record the discharge sound, and use the magnetic field signal to synchronize them, and analyze the sound waveform and the tester through the headset to hear the fault fixed point. This method is a commonly used power cable fixed point method, but this method can only obtain sound signals at a distance of about 2 to 3 m from the point of failure, and has a higher requirement for the technical quality of field operators.


Second, the new method of cable fault detection

 

(I) New Method for Fault Location of Cables


Causal Web: Causal Web describes the intrinsic action relationship between faulty components, relays, and switches. It utilizes more in-depth knowledge and object-oriented technology than traditional expert systems to locate faults in the power system. It has the advantages of simplicity, clarity and versatility. Fault phase selection using wavelet transform: In the pulsed cable fault location detection, various electromagnetic interferences are inevitably present. The high-frequency oscillation caused by the pulse signal output lead, the inherent high-frequency interference of the acquisition system, and the use of space-borne electromagnetic interference at the site will enter the test system through the signal leads exposed outside the positioner. In severe cases, the starting point of the reflected pulse can be submerged. Inaccurate error location. For this reason, effective digital signal processing methods must be adopted to eliminate the influence of these interferences and improve fault location accuracy. Wavelet transform is a branch of applied mathematics that developed in the late 1980s. It is known as a mathematical microscope for signal analysis and is a leading topic in signal processing. Wavelet transform is widely used in the field of digital signal processing, such as filtering, singular signal detection, edge detection and so on. The wavelet multi-scale analysis method can decompose mixed signals composed of different frequencies that are intertwined into signals of different frequencies, and can be directly reflected in the time domain. The position, amplitude and waveform of the signals are all very intuitive. Effectively implements signal-to-noise separation. The wavelet transform has good time-frequency local characteristics and is very effective for analyzing the location of singular points on the signal. This feature is suitable for finding the starting point of reflection pulses in cable fault location.


Based on the entire transmission grid GPS traveling wave fault location: Global Positioning System GPS is the latest technology for communication systems developed in recent years. Transmission line traveling wave fault location has high accuracy, but it needs high speed A/D acquisition, large amount of data storage, complicated traveling wave head identification, and it is difficult to measure development faults and short distance faults. For example, a dedicated traveling wave head detection sensor, high-precision GPS clock, and efficient access method for storing traveling wave heads and moments, a specially designed traveling wave head recorder is installed at each substation to communicate with dispatchers to form a transmission network GPS traveling wave measurement network. Then, it can directly measure the exact time when the fault traveling wave head arrives at each substation, and locates the fault by scheduling.


Step voltage method: The pulsed stepping method is used to orient and locate low voltage cable faults. This method is simple in wiring and easy to operate. It can quickly direct and accurately determine the faults of the buried power cable. It is the use of cables in the soil along the ground or the ground to produce a step-by-step voltage pulse that gradually decreases or increases along the direction of the cable, to determine the direction and location of the fault. Because according to past experience, the low-voltage power cable fails, and the cable sheath at more than 90% of the fault points is damaged. In this way, a periodic pulse signal can be applied to one end of the cable to quickly determine the direction of the fault along the cable laying direction. Accurately determine the location of the fault. Under normal soil conditions, the fault point direction can be indicated at 20-30m from the fault point. In cement or hardened road conditions, the fault point direction can be indicated at the fault point l0m. Compared with the prior art, the advantages of using the pulse stride method to orient and locate the fault of the low voltage cable are as follows: 1 The direction of the fault point can be determined in a wide range and the time for the test fault can be saved; 2 The medium voltage applied to the faulty cable The pulse does not require the test cable to generate a renewed arc at the fault point, and the pulse width is only a few milliseconds to several tens of milliseconds, so it will not cause damage to the cable; 3 the measuring equipment used is convenient to use, simple to operate, and intuitive; 4 High positioning accuracy. The direction of the fault point and the magnitude of the voltage pulse are indicated by means of a light-emitting diode beam or pointer meter, and the fault point can be found quickly and accurately by detecting along the cable according to the direction indicated on the instrument.


(B) The new method of fixing cable faults


High-frequency induction method: high-frequency signal generator input high-frequency current into the cable, this will produce high-frequency electromagnetic waves, and then use the probe along the cable path on the ground to receive high frequency electromagnetic field around the cable, the electromagnetic field changes directly after receiving processing Displayed on the LCD screen, directly determine the location of the fault point according to the size of the displayed value.

 

The high frequency induction method has many advantages as compared with the conventional audio induction method. The high-frequency signal source itself is easier to implement than the audio signal source, and it is easy to manufacture. It can reduce the size and weight of the fixed-point detection device, and create favorable conditions for miniaturization and portability of the device. High-frequency signal spectrum anti-jamming performance is strong. This method can display the results directly, more reliable and more convenient than relying on human ears. The high-frequency induction method is much superior to the audio induction method, and it can perform on-line fault detection with a coupler wiring without power failure.


Infrared thermal imaging technology: based on the cable once overloaded, the temperature of the core will rise sharply. People can monitor the temperature of the cable core to determine the fault location. The steps are as follows: First, the surface of the cable is scanned with an infrared thermal imager to take a picture of the distribution of the temperature field on the surface of the cable. Further processing can give a specific numerical distribution of the temperature field, and then according to the established mathematical model of heat transfer, according to the cable structure parameters. , physical parameters, ambient temperature and surface temperature inversion of the cable core temperature calculations, in order to achieve non-contact cable core temperature fault detection. It is the infrared technology that does not need to touch the equipment, does not require the equipment to stop operation, and has the advantages of easy operation, high detection speed and high work efficiency. In the future cable fault detection, the infrared thermal image technology will certainly play a greater role.


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