What are the detection methods of buried cable faults?

In China's rural network reconstruction work, many areas have adopted buried cables. Although polyvinyl chloride insulated plastic cables have good chemical stability and have many advantages such as not occupying farmland, operating safety, and reducing line losses, after a long period of use, due to a variety of reasons, the phenomenon of failure of buried wires gradually increased. . It is understood that the laying of buried lines for more than one year has caused failures in many power supply jurisdictions. Therefore, how to quickly and accurately find fault points and eliminate faults in a timely manner has become a top priority for all agricultural power departments to fulfill their service commitments, gain trust with the people, and advance the progress of rural power grids.
I. Analysis of Causes of Faults Occurring on the Earth's Buried Lines Understanding the cause of faults in buried lines can effectively prevent faults from occurring, reduce the probability of faults, and help eliminate faults in time. Generally speaking. The breakdown of buried wires is due to the following reasons:
1, external damage: is the main reason for failure. For example, repeated construction above the buried lines, laying of water pipes, construction of aqueducts, tree planting, and construction of buildings, etc., can easily damage the buried insulating layer, resulting in disconnection faults or ground faults.
2, joint failure: due to the joint process is not standardized in construction, can easily cause joint damage at the machinery. Or there are problems such as large contact resistance at the joint and poor insulation. After the cable runs with the load for a period of time, hidden dangers at the joint often cause disconnection or ground faults.
3, laying is not standardized: When laying the buried line, it must have strict process requirements. However, due to the large amount of work in the rural grid reconstruction and the scarcity of personnel, in some areas in the construction of buried lines, there are often unconventional construction practices. For example, when the trench is backfilled, no gauze is used, and the soil block containing the gravel is directly backfilled, which can easily cause damage to the insulating layer. Or the depth of the trench is too shallow. For example, if the depth is less than 0.6m, it is very easy to cause disconnection or ground fault under the ground surface pressure. In high-cold areas, if the laying depth is less than the thickness of the frozen soil layer, it is easier to cause line breakage due to thermal expansion and contraction.
4, other reasons: For example, there are quality defects in the buried wire itself; chemical erosion; termite or rodent bite damage, etc., will also cause landline failure.
Second, the buried line fault detection methods and characteristics
1. Analysis and Judgment Method For the occurrence of buried faults, it is generally necessary to first understand the related conditions of the fault, and then conduct a comprehensive analysis to find out the cause of the fault, and then targeted to find out the exclusion. For example, looking for informed parties such as construction workers, electric wire users, and other related personnel, to understand the situation in detail, it is often possible to eliminate the fault within a short time at a small cost.
The advantage of analysis and judgment is that it is simple and easy, it does not require complicated equipment, and it can eliminate some faults in time. The disadvantage is that most faults are difficult to find the exact cause of the fault, so it is difficult to quickly eliminate with this method. For a buried wire with a buried length of 100 meters, in the absence of equipment, it is determined by human analysis to find faults, sometimes excavation of more than a dozen places, it takes more than ten days to find fault points, and sometimes it is difficult to find the same point Damage to other wires inside and new failures. This method is generally not used alone, but with other instrument detection methods.
2. Testing faults with a simple ground fault detector Many brands of ground fault testers currently on the market have similar operating principles and test methods. The working principle is as follows: a fixed voltage signal is applied to the grounding wire, and an electric field is formed on the ground around the grounding point. The closer to the fault point, the greater the potential difference between the same distance, and vice versa. According to this principle, the ground fault point can be found. The market for such test instruments is priced at a few hundred dollars to several thousand dollars, and it is possible to achieve the purpose of testing a buried fault with a small grounding resistance. It is understood that. Due to the low cost, many rural power departments purchased such instruments.
However, the use of this instrument to detect failures in landfills has very limited limitations. Because of the failure of the buried wire, the grounding resistance at the fault point is very high, usually the resistance is tens of thousands of ohms, and the resistance is as high as several megahertz or even several hundred megaohms. Therefore, the use of such instruments to eliminate the failure of the lower efficiency, high false positive rate, it is difficult to achieve rapid, accurate troubleshooting purposes.
3. It is a matter of recent years that a large number of faults are buried in the ground with an intelligent cable fault tester. Therefore, the test methods for buried faults are not very familiar to many rural electricians and repair personnel. The failure of the buried line, if there is no advanced equipment and better test methods, the fault can not be ruled out within the time limit of the power company's service commitment. This will not only delay the time, waste manpower, and the electricity farmers have opinions and influence. The rural power supply company image has dampened the enthusiasm of farmers for the rural power grid transformation work. Therefore, the rural landfill urgently needs to be more professional. More effective fault detection equipment.
Compared with buried lines, buried power cables have been used for decades in China, and buried cable fault detection equipment has been developed from flash resistance bridge, capacitor bridge test, standing wave test, and later developed into flash Testing. The development of the instrument has gone through the ordinary oscillograph display flash detector, storage oscilloscope display flash detector, and has now developed and produced intelligent instruments. The flash detector uses a large-scale integrated circuit, computer processing technology, large-screen liquid crystal display technology and other new technologies and new processes. After using the power cable test equipment to improve and test the failure of the buried wire, the effect is very good and the efficiency is high. After a great deal of practice, a buried fault line is generally tested. Within 30 minutes from the start to the end, the accuracy rate is extremely high.
3.1. The instrument consists of an intelligent cable fault detector, such as the CD series cable fault detector of Zibo Xinyi Electric Co., Ltd. It is generally composed of four major components: First, the cable fault distance meter (flash detector) can Measure the approximate location of the fault within a few minutes. The positive and negative errors are within three meters. The second is the cable path detector, which can accurately and accurately find out the exact laying position of the buried wire and find the error of ±0.1 meters. The third is the cable fault fixed point instrument, which can cooperate with the high-voltage pulse power supply to accurately find the location of the fault point after the fault is measured, and the fixed-point error is generally within ±0.2 meters. The fourth is an integrated high-voltage signal generator that can apply several thousand volts up to 10,000 volts of high-voltage pulses to faulty wires, forcing momentary breakdowns between faults and generating shock waves to precisely determine points on the ground.
3.2 Working principle of each part Introduction 1. Fault point distance meter (flash detector): According to the principle of pulse reflection method, a pulse signal is applied to the faulty wire. After the signal wave is transmitted to the fault point, it returns due to impedance mismatch. Test side. Based on the time difference between the transmitted wave and the transmitted wave, the approximate location of the fault point can be quickly determined. For different faults, pulse waves are applied, and there are points of low-voltage pulse waves and high-voltage pulse waves. The instrument is suitable for measuring various faults such as breakage lines, short circuits, and low resistance grounding.
2 Pathmeter: A fixed-frequency AC signal is applied to the faulty wire, which is received by a dedicated receiver. The use of different magnetic field receiving effects in different areas above the wire can quickly find the faulty wire buried location.
3 Fixed-point instrument: When a high-voltage pulse is applied to the faulty wire, the high-energy high-voltage pulse is instantaneously released, and the flashover discharges at the fault point to form a current impact, generating seismic waves and electromagnetic waves. The fixed point instrument uses a special sound and magnetic sensor to receive and can accurately find the fault point.
4 Integrated high-voltage pulse power supply: It is generally composed of a voltage regulator, a high-voltage transformer, a high-voltage silicon stack, a high-voltage pulse capacitor, a discharge gap, and a high-voltage discharge rod. Can output 10,000 volts DC negative high voltage, after the discharge gap to the fault wire to apply a fixed-frequency transient high-voltage pulse. After a lot of practice, low-voltage cables or low-voltage wires are subjected to high-voltage pulses of 10,000 volts or less in the event of a fault point discharge path. Because the pulse duration is extremely short, wire insulation is generally not damaged, or damage to wire insulation is minimal. .
3.3. Advantages and Disadvantages Analysis The advantages of the intelligent cable tester with its fast test speed and high accuracy are clear. The disadvantage is that due to its technological content, manufacturing costs are low. Therefore, the selling price is also higher and it requires trained professionals to operate.
Third, the buried line fault test should pay attention to the buried line fault test, is a comprehensive system of various aspects of experience and technical engineering, the test method is not a single, the actual work of flexible use of various testing methods. In general, intelligent cable fault test equipment is gradually being accepted and used by agricultural power supply companies. It is believed that these types of instruments will become the main instruments for landfill fault testing in the next few years. Because smart devices are expensive, pay attention to the following points when purchasing and using:
1. When purchasing equipment, we must carefully examine the situation of the manufacturers. The products of general regular manufacturers are not only of reliable quality, but have a low market share. Their technical training and after-sales services are guaranteed. This point is particularly important for agricultural power supply companies with tight funding and lack of experience.
2. The instrument user must be trained to operate the instrument, especially for the high-voltage test technology must have sufficient understanding and attention.
3. The high voltage DC pulse power used in the test should generally use a power supply with an output voltage lower than 15,000 volts. The output voltage of the power supply is generally controlled within 10,000 volts, and in the test loop, there must be a faulty wire, that is, an instantaneous release loop of high-voltage pulse energy can be formed. It is absolutely forbidden to apply high voltage pulses between good wires or between good wires and ground.
4. During the high voltage test, other wires in the same wire trench shall be temporarily powered off. The test terminal and the user end of the tested wire shall be specially guarded. The user end wire and the indoor power supply line shall be completely separated to ensure personal safety and the safety of electrical appliances.
5, after the test is completed, discharge the high voltage side completely.

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