News Details

Keywords: Power cable fault solutions,　

　　• Introduction: With the development of the power and energy industries, various cables are increasingly used in all areas of production and daily life, and they are generally buried underground. When a cable fault occurs, how to quickly and accurately locate the fault point and restore power as soon as possible is a long-standing challenge for us. For a long time, people's understanding of cable faults has often been to mix high-voltage and low-voltage cables together, thinking that they are all cables. As long as a cable fault occurs, whether it is a high-voltage or low-voltage cable, the traditional high-voltage impulse flashover method is used to test the fault. This method (commonly called the impulse flash method) has been used up to now and has been accepted by everyone. To date, products both domestically and internationally are produced according to this principle. • Keywords: Power cable faults, solutions, safety and disaster reduction. In China, power cables became widely used after the 1960s, with limited voltage levels and narrow application ranges. At that time, to solve cable faults, researchers developed and produced cable fault testers based on the impulse flash method. The method for testing cable faults with this instrument has three steps: The first step is to use a range finder to measure the distance. In fact, it is necessary to first determine whether the cable fault is high-resistance, low-resistance, or grounding, and adopt different testing methods based on this condition. If it is a grounding fault, directly use the low-voltage pulse method of the range finder to measure the distance; if it is a high-resistance fault, use the high-voltage impulse discharge method to measure the distance, which requires many auxiliary devices: such as high-voltage pulse capacitors, discharge balls, current-limiting resistors, inductor coils, and signal samplers, etc. The operation is cumbersome and unsafe, with certain dangers. What is more troublesome is that the sampled waveform must be analyzed, requiring high knowledge from the tester. The second step is to find the path (if the path is clear, this step can be omitted). When finding the path, a signal (path signal generator) is added to the cable, and then a receiver is used to receive this signal. Walking along the path with the signal determines the cable path. However, the range of this path is roughly between 1-2 meters, not particularly accurate. The third step is to precisely locate based on the measured distance. The basis is the sound generated by spark discharge. When the loudest sound is heard through the earphones of the locator, the fault point is found. However, because it relies on listening to sound, it is affected by environmental noise, and finding the fault can be time-consuming; sometimes it is necessary to wait until night. When encountering cross-linked cables, it takes even more time because cross-linked cables generally discharge internally, and the sound is very faint, almost inaudible, so in the end, only measurement is possible. Therefore, this method can solve most power cable faults with oil-impregnated paper insulation. For cable faults with cross-linked materials and polyethylene materials as insulation that have appeared in recent years, the testing effect is not ideal because the sound produced by spark discharge is often very small (the cable sheath is not damaged, only internal discharge). In such cases, other methods must be used. Although it has these shortcomings, cable fault testers designed on the impulse flash principle have solved many cable faults for enterprises over a long period, and they are generally recognized; their contribution is well-known. They have been widely used in various industries. With the rapid development of various industries, the uses of cables are becoming more extensive, and the types of cables are increasing, so cable faults are inevitable. We know that different industries have different requirements for cable voltage levels, usage environments, wiring and distribution methods, and insulation requirements, and the fault characteristics of different cables also differ greatly because there are many factors that cause cable faults. However, due to past habits, people always want to solve all cable faults with one method, so currently, cable fault testers based on the impulse flash principle dominate the market. However, in some industries, using the impulse flash method to solve cable faults cannot detect faults at all, and it may produce serious consequences. For example, cables for street lights and underground cables in mines cannot be tested directly with the impulse flash method. Similarly, cables used in other industries have their own characteristics, which we cannot introduce in detail here. However, with the continuous development of science and technology, we should be able to find simpler testing methods, classify cable faults, and prescribe the right remedy, analyzing specific problems. In this way, we will find that some cable faults do not require the impulse flash principle and can be solved very conveniently and quickly. In many years of practical work, we have found that faults in high-voltage cables and low-voltage cables have many differences. High-voltage cable faults are mostly operational faults, and most are high-resistance faults, which are further divided into leakage and flashover types; low-voltage cable faults only have three situations: open circuit, short circuit, and disconnection (of course, high-voltage cables also include these three situations). ⒉ When laying, unlike high-voltage cables which are buried deep with sand and bricks, low-voltage cables are buried shallowly and are susceptible to external damage, leading to faults. ⒊ Cables are generally short, ranging from tens to hundreds of meters, unlike high-voltage cables which are often hundreds of meters to several kilometers. ⒌ In the vast majority of cables, there is a very obvious burnt damage phenomenon at the fault point. It is very rare for the fault point to leave no trace on the cable sheath. ⒍ The load changes greatly, and often the phases are unbalanced, making them prone to heating, which commonly causes faults. Considering the above characteristics of low-voltage cables, suggestions from users, and our actual usage in various places, our Keyu company's researchers have successfully developed the DW-type low-voltage cable fault testing and locating system. This system includes a range finder and a locator. The range finder of the DW-type system is fully intelligent and user-friendly. It automatically completes the testing of cable fault points, directly reporting the fault distance and nature without manual analysis of fault waveforms. It is battery-powered, convenient for field work, small in size, light in weight, and easy to carry, requiring no auxiliary equipment. The cable fault locator of the DW-type system is an instrument for synchronous positioning and testing of the burial path, depth, and fault point of directly buried low-voltage cables. Because it is a low-voltage cable fault locating system designed on the principles of electromagnetic induction and step voltage, it basically meets all the conditions for low-voltage cable fault testing. Compared with cable fault testers based on the impulse flash principle, this testing system has many advantages: ⒋ The cable path finding (can be determined within 30 centimeters), depth detection, and fault point location are completed synchronously, with high efficiency. ⒍ It is not affected by underground conditions (such as cable branching, bundling, joint twisting, etc.). Like detecting landmines, it finds the fault point point-to-point, with positioning error within a dozen centimeters, which is quite accurate. We know that low-voltage cables have lower insulation requirements, and during operation, the current is relatively large. After a fault occurs, there are obvious characteristics, which are specifically classified as follows: The first type of fault: The entire cable is burned out or a certain phase is burned out. This type of fault causes the current relay on the distribution cabinet to operate, and the cable is severely damaged at the fault location. The second type of fault: All phases of the cable are short-circuited. Similarly, this type of fault causes both the current relay and voltage relay on the distribution cabinet to operate, and the cable is also severely damaged at the fault point (possibly caused by external force). The third type of fault: Only one phase of the cable is open-circuited, the current relay operates, and the damage at the fault point is light but obvious. It may be caused by excessive current in that phase or by cable quality. The fourth type of fault: Internal short circuit in the cable, with no visible trace on the outside. This type of fault is generally caused by cable quality and is relatively rare. The range finder and locator in the DW-type low-voltage cable fault locating system can be used together to complete testing very conveniently. At the same time, they can also be used independently to complete testing for different fault characteristics and cable lengths. The details are as follows: For the first and second types of faults, if the cable is short (less than 500 meters), the fault locator can be used directly for fault location without the range finder. Simply hold the receiver and walk along the path (the path can be measured while walking) to determine the fault point. For the third type of fault: Because the cable is lightly damaged at the fault point, the signal leaked from the transmitter is small, and when using the locator for fault location, the indication range is narrow. At this time, first use the range finder to measure the approximate distance to the fault point, and then use the locator for precise location, which is also convenient. For the fourth type of fault: This type of fault is currently the most difficult to test among all cable faults. At this time, use the range finder to test the cable from both ends, and then compare the test results with the actual length to narrow the fault point to a very small range (1-3 meters). Then dig up the cable at this section to find the suspicious point, or simply cut off this section of cable (because low-voltage cables are cheap, have low insulation requirements, and are easy to splice), or use the locator with audio positioning in this section to determine the fault point. Currently, users of power cable fault testers based on the impulse flash method, when solving low-resistance faults and dead grounding faults in low-voltage cables, can generally use the range finder to roughly measure the distance to the fault point (the distance test for such faults does not require high-voltage discharge equipment; it uses the low-voltage pulse method), but fault point location still requires the method of sparking, discharging, and listening to sound. At the same time, the path finder and locator of such instruments are separate, which makes it impossible to locate the fault while accurately finding the path, and when locating, it is easy to deviate from the path. Moreover, due to the principle limitations of the path finder of such instruments, it is difficult to find the accurate path of the cable, generally within a width of 1-2 meters. The DW-type cable fault locator, starting from practicality, just makes up for the above defects. It can test the fault point and path synchronously. The instrument's indication of fault, path, and burial depth is very intuitive, requiring no technical analysis and completely independent of the operator's experience. This turns the originally cumbersome fault testing work into an easy and interesting task. Therefore, if users of impulse flash cable testers also own a DW-type cable fault locator, combined with the original range finder, they can form a relatively perfect low-voltage cable fault tester. At the same time, it can also quickly locate low-resistance and open-circuit faults in high-voltage cables, improving work efficiency several times. In fact, most power-consuming enterprises and units rarely come into contact with high-voltage cable maintenance in daily production, because the maintenance of high-voltage cables is generally the responsibility of municipal power departments. The number of low-voltage cables is far greater than that of high-voltage cables. For enterprises, factories, mines, residential communities, research institutes, developed towns and villages, colleges and universities, some small and medium-sized cities, and county-level power supply bureaus, solving low-voltage cable faults is their main concern. In fact, low-voltage cable users need a tool-type tester that is simple to operate, practical, cheap, and suitable for field work. Moreover, because the insulation strength of low-voltage cables is low, if the spark discharge method is used to test low-voltage cable faults, we find that this method sometimes causes secondary faults. More seriously, after spark discharge, the service life of the cable is reduced, increasing the fault rate, which seriously affects normal power supply and production.