This Paper Briefly Introduces the Fault Detection Method of High Voltage Cable

The discovery and fixed-point work of cable faults has always been a big problem in the electrical measurement industry, mainly because most cables are deeply buried underground, the laying distance ranges from a few meters to a few kilometers, and the on-site environment is complex. In addition to the high quality requirements for cable fault test equipment, the comprehensive quality of technicians is also very strict. There are three common types of cable faults: low resistance fault, high resistance short circuit grounding fault and flashover fault. For different types of cable faults, the fault point detection methods are also very different. The following is a brief introduction to the fault detection method of high-voltage cable. There are two kinds of traditional fault detection methods for high voltage cables: bridge method and pulse method. 1、 Electric bridge method is a classical method for ranging power cables. This method is relatively simple, but it needs to know the data such as cable length in advance, and it is only suitable for low resistance and short circuit faults. However, in actual operation, faults are often high resistance and flashover faults. Due to the high fault resistance, the bridge current is very small, so it is difficult for general sensitivity instruments to detect. 2、 Pulse method 1. Pulse current method this method is safe, reliable and simple. The method is to break down the cable fault point with high voltage, collect and record the current traveling wave signal generated by the fault click through with the instrument, and calculate the fault distance according to the round trip time between the current traveling wave signal at the measuring end and the fault point. In this method, the pulse current is coupled with a transformer, and the waveform is simple and safe. This method also includes direct flash method and impulse flash method. Different from the pulse voltage method, which uses resistance and capacitor voltage dividers for voltage sampling, the pulse current method uses linear current couplers placed parallel to the low-voltage geodesic and has no direct electrical connection with the high-voltage circuit. It is particularly safe and convenient for recording instruments and operators. So people usually use this method. 2. Pulse voltage method this method can be used to measure high resistance and flashover faults. Firstly, the cable fault is broken down under the DC or pulse high voltage signal, and then the distance is measured by recording the time required for the discharge pulse to go back and forth between the measuring point and the fault point. An important advantage of pulse voltage method is that it does not have to burn through high resistance and flashover faults. It directly uses the instantaneous pulse signal generated by fault breakdown. The test speed is fast and the measurement process is simplified. 3. For low resistance and open circuit faults, the pulse echo method uses the low-voltage pulse reflection method to measure cable faults. Compared with the above bridge method, it is simple and direct. It only needs to observe the time difference between the reflection and transmission pulses at the fault point to locate the fault. During the test, a low-voltage pulse is injected into the cable. When the pulse propagates to the fault point, it will be reflected, and the pulse is reflected and sent back to the measuring point. The time difference between the transmitted and reflected pulses is recorded by the instrument, and the distance of the fault point can be calculated only by knowing the pulse propagation speed. This method is simple and intuitive. It does not need to know the original data such as cable length. It can also identify the position of cable joint and branch point according to the reflected waveform.

This Paper Briefly Introduces the Fault Detection Method of High Voltage Cable 1

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Pulse Flashover Method for Fault Detection of High Voltage Cables
Cable has a very important application scenario in our daily life. It can be said that we can't live without cable. In order to ensure the normal use of the cable, it is necessary to regularly detect and maintain the cable. In case of cable failure, the fault point shall be found out in time for maintenance. There are many cable fault maintenance methods. Today, I mainly introduce one of the high-voltage cable fault detection methods, pulse flashover method. For high resistance fault, due to the large resistance at the fault point, the reflection coefficient at this point is very small or almost equal to zero. When measured by low-voltage pulse method, the reflected pulse amplitude at the fault point is very small or there is no reflection, so the instrument can not distinguish it. There are two common high voltage flashover measurement methods, namely DC high voltage flashover measurement method (direct flashover method for short) and impulse high voltage flashover method (impulse flashover method for short). 1、 DC high voltage flashover method (referred to as direct flashover method) when the fault resistance is very high and a stable resistance channel has not been formed, the gradually increased DC voltage can be applied to the tested cable. After reaching a certain voltage value, the fault point is preferably broken down to form flashover. The flashover arc is used to form a short-circuit reflection on the added voltage, and the reflected echo is formed by a high resistance source at the input end to form an open circuit reflection. In this way, the voltage will be reflected many times between the input and the fault point until the energy is exhausted. 2、 Impulse high voltage flashover method (impulse flashover method for short) when the fault resistance is reduced and a stable resistance channel is formed, the DC high voltage cannot be added due to the limitation of equipment capacity. At this time, it is necessary to use impulse voltage test. The DC high voltage charges the cable through the ball gap until breakdown, and still generates short-circuit reflection with the flashover arc formed by it. The measuring inductance L shall be added at the cable input end to read the echo. The radio wave is reflected by short circuit at the fault point and l at the input end, and multiple reflections will be formed during this period. Due to the self induction phenomenon of inductance L, it begins to show open circuit reflection due to the current blocking effect of L, and shows short-circuit reflection after a certain time with the increase of current. The whole circuit consists of capacitor C and inductance L, which constitutes a large process of L-C discharge. Therefore, the wave process at the line input is a nearly attenuated cosine curve superimposed with fast pulse multiple reflection wave. 3、 High voltage cable fault detector is generally used. High voltage cable fault is high resistance fault, and high voltage flashover method is used to measure the distance of fault point. This method uses st-330b cable fault locator as experimental equipment. 1. Wiring mode: firstly, complete the wiring of high-voltage flashover as required; Then, connect the sampler with the ranging host with a double clamp test line, and then place the sampler next to the grounding wire of the high-voltage capacitor. 2. Equipment operation (1) start up, select the sampling method as flashover, the pulse width is the default, the cable type is the physical type, and the reading accuracy is the default. (2) Use the high-voltage equipment to discharge the cable, and then click the orange sampling button on the right to start sampling until the waveform is collected. Press the sampling key again to stop sampling. (3) Study the waveform, move the vernier ruler to the starting point and ending point of a waveform, read the waveform, and the distance of the fault point will be automatically measured in the upper left corner.
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