Abstract: This article introduces several commonly used detection methods in the fault detection work of power cables and their analysis and comparison in application effects.
With the increasingly widespread application of power cables in urban power grids, the longer the running time, the more frequent the failures. How to deal with faults in a timely and effective manner to ensure the normal operation of the city's power supply and power grid depends on whether or not faults can be identified quickly and accurately. Nature and location. In order to solve this problem, Huaibei Power Supply Company purchased a YM-type cable fault detection proposal in 2002. It began to be used in distribution laboratories and was then used in the experimental class of the repairing and testing institute to perform fault detection on the cables under the jurisdiction of the company. After active exploration and analysis, it has played an excellent role and effect in the detection of multiple cable faults, and has accumulated rich experience. Now it is the cause, nature, principle and method of detection, and practical application of cable faults. Explore.
1. The causes of faults in the cable caused by cable faults are various, and several common causes are summarized as follows:
mechanical injury. Many of the cable faults are caused directly by the mechanical damage caused by the installation or installation or the external force damage caused by the construction of the cable after laying. Although the damage is sometimes slight, the insulation of the damaged part will gradually decrease after several months or even several years, resulting in breakdown.
Poor design and manufacturing processes, not made in accordance with the requirements of the regulations, is often an important reason for the formation of cable faults.
Chemical, electric corrosion. Electrical corrosion of lead outside the cable causes moisture intrusion and insulation damage.
The manufacturing defect of the cable.
Due to the long-term overloaded operation of the cable, the temperature of the cable will increase. Especially during the hot summer months, the temperature rise of the cable often leads to the first breakdown of the weak point of the cable and the butt joint.
Loss of cable insulation.
2. Cable fault pre-positioning method The most important step in cable fault location is to identify the type of cable fault. Once a fault occurs, determine the type of fault, according to the type of fault and the equipment conditions of the unit to select the appropriate detection method, directly affect the speed of the accident treatment. In fact, any type of failure of the cable may occur at any location, and the ability to quickly resolve the problem depends on the actual experience of the field staff. Usually use a multimeter to determine the fault cable resistance, according to the size of the cable fault is divided into two groups: low resistance fault - less than 100kΩ; high resistance fault - more than 100kΩ. Each type of cable fault requires a special method for pre-positioning, and the more commonly used pre-positioning methods are as follows.
2.1 Low-voltage pulse reflection method This measurement method is to send a high-frequency low-voltage pulse to the cable. The pulse propagates along the cable until the impedance is mismatched, such as the middle connector, T connector, short circuit point, disconnection point, and terminal head. At these points, reflections of the waves are caused, and the reflected pulses are received by the test equipment when they return to the test end of the cable. Practice has proved that the vast majority of faulty cables in the field can't measure the fault location by using low-voltage pulse reflection method. The reflected waveform can only test the full length of the cable. Figure 1 shows the standard waveform of low-voltage pulse reflections.
2.2 Measurement of high-impedance faults 2.2.1 DC high-voltage flashover method The DC high-voltage flashover method is used for flashover faults, that is, no resistance channels are formed at the fault point (or the resistance value is extremely high) but the voltage rises to a certain value. Flashover occurs.
Working principle: add negative DC high voltage to the faulty cable. When the voltage rises to a certain value, flashover occurs at the fault point. The flicker meter shows the waveform at the measurement end, as shown in Figure 2. The fault distance is the distance corresponding to the actual time interval from the starting point T0 of the waveform to the inflection point T1 at the falling point.
(a) Detection of faults (b) Waveforms In practice, flashover faults of cables are common, and almost all cables that have been subjected to pre-test breakdown have a flashover process, and about half of the cable faults that run breakdown are flashovers. process. When the faulty cable is directly pressurized, the flashover process of the cable varies in length. Some faults only flash several times to form a stable channel and no flashover occurs. Therefore, in the process of discovering the flashover of the cable, you should seize the opportunity and cherish the time. This phenomenon is tested. Because the direct flash method has better accuracy than the flash method waveform, the direct flash method is used as far as possible.
2.2.2 Fault point burn-through method Fault point burn-through method is applied to high-impedance fault. The device passes the output DC negative high voltage to process the high-resistance fault point so that the fault point will generate arc discharge and carbonize the insulating medium, because the carbonization connection point is Low-resistance, high-impedance failure into a low-impedance failure, and then measured using low-voltage pulse reflection method. Fault point burn-through method is mainly used for oil-paper insulated cables. The disadvantages of using the burn-through method at the fault point are that the burn-through time is long, the manpower is consumed, and the metal short circuit is easily formed, which makes it difficult to find the fault point. The resistance at the fault point is restored and a second burn-through must be performed, so this method is generally not used.
2.3 Impact High-voltage flashover method The flashover method is divided into two types: resistor and inductor flashover. For the former, due to the voltage divider in the circuit, the voltage applied to the faulty cable is low, which is unfavorable for discharge. Especially for those faults with higher resistance, it is more difficult to discharge. Therefore, there are certain limitations. Sex, usually the latter. The advantage of the inductive flash method is that it can adapt to almost any type of fault. A large number of facts have proved that the inductive flash method is the most powerful means to deal with those that are not measured by others and is called the most stubborn failure. Therefore, the inductive flash method is the most important test method. The measurement circuit of the impulse DC high-voltage inductance measurement method (referred to as the rush L method) is shown in Fig. 3(a). When the power is turned on, the storage capacitor C is first charged by the DC high voltage. When the voltage on the capacitor is high to a certain amplitude, the ball gap Q is broken down and discharged, and the negative high voltage is instantaneously added to the cable fault phase at t0. Propagation to the point of failure, followed by flashover discharge at the point of failure. The short-circuit arc at the time of fault discharge reflects the voltage wave sent along the cable back to generate the waveform shown in Figure 3(b) between the measuring end and the fault point. The sharp pulse in the figure is due to the differential action of the inductance L. Caused. This waveform is added to the instrument by the resistor divider R1 and R2.
(a) Measuring the faults by the rush-L method (b) Measuring the waveforms by the rush-L method The rush-L method is mainly used to measure the leaky high-impedance fault and also to measure the flashover high-impedance fault. It should be pointed out that the cable fault YM cable fault tester, although it is more advanced equipment, but they are all coarse measurement instruments, when the rough point of the fault point is determined, it is necessary to try to determine the precise point, the current method is mainly used sound Measurement point method.
3. Precaution of faulty cable faults in cable faults According to the current test level, it is not difficult. If the YM cable fault location system is used, the distance from the fault point to the test end can be measured in only a few minutes when the rough determination of the cable waveform and the path are clear, and the pre-positioning error will generally not exceed 10m. However, due to the error and incompleteness of cable operation data, the different types of faults, and the complex diversity of environmental factors of cable fault points, such as the lack of accurate cable lengths and circuit diagrams, in powerful noise sources and power frequency electromagnetic fields. Nearby, cables laid in buried pipes and difficult to enter into buildings, etc., will bring many unexpected difficulties to the precise point of failure. A large number of practices have proved that the problem of precise determination has become the main contradiction in finding faults quickly. Mainly using acoustic and magnetic signals to receive the fixed point method synchronously, this method adds a surge voltage with a sufficiently high amplitude to the faulty cable, causing the flashover discharge at the fault point to generate a considerable “啪, 啪†discharge sound, and at the same time, A loop is induced in the loop formed by the cable and the earth. This loop generates a pulsed magnetic field around the cable. Use a listening device that contains a grounded microphone receiver and headphones to probe the ground. The closer the fault point is to the microphone, the greater the flashover sound. While listening to the sound signal and receiving the pulsed magnetic field signal, it can be judged that the sound is generated by the fault point discharge, and the fault point is in the vicinity, otherwise it can be considered as interference. The maximum value of the flash sound can be detected at the location of the fault point.
When encountering the resonance in the cable body in the vicinity of the flashover or near the point of failure, the sound can be heard within a large section of the cable during discharge, and if the size is the same, accurate positioning is difficult. In view of the fact that there is no better technology yet, the problem of fixed point can be improved. Only according to the pre-position distance and cable data, open the cable trench cover or dig the surface of the buried section, and perform fixed point work directly on the cable body.
4. Conclusion Power cable fault detection methods and techniques, in addition to the above-mentioned several main methods, there are high voltage bridge method, capacitance method, step voltage method and audio induction method. To use a cable fault tester to detect a fault, not only should you familiarize yourself with the use of the instrument, but also understand the nature of the fault and the test waveforms. The author thinks that it is very necessary to choose the appropriate test method when detecting the cable fault, which can greatly reduce the fault detection time, and concludes that:
Low-resistance ground fault detection (R <10kΩ), easy to use low-voltage pulse method, bridge method test;
High-impedance failure (R>100kΩ) with insulation recovery, best for direct flash test;
High-resistance grounding fault (R>100kΩ) is the most accurate with flashover test.
Since the acquisition of the YM cable fault location system in 2002, the high-voltage impulse method has been used mainly for cable fault detection. The success rate for several years has been 100. To ensure timely and effective troubleshooting and ensure the normal operation of the city's electricity and grid, it has played a very good role.
With the increasingly widespread application of power cables in urban power grids, the longer the running time, the more frequent the failures. How to deal with faults in a timely and effective manner to ensure the normal operation of the city's power supply and power grid depends on whether or not faults can be identified quickly and accurately. Nature and location. In order to solve this problem, Huaibei Power Supply Company purchased a YM-type cable fault detection proposal in 2002. It began to be used in distribution laboratories and was then used in the experimental class of the repairing and testing institute to perform fault detection on the cables under the jurisdiction of the company. After active exploration and analysis, it has played an excellent role and effect in the detection of multiple cable faults, and has accumulated rich experience. Now it is the cause, nature, principle and method of detection, and practical application of cable faults. Explore.
1. The causes of faults in the cable caused by cable faults are various, and several common causes are summarized as follows:
mechanical injury. Many of the cable faults are caused directly by the mechanical damage caused by the installation or installation or the external force damage caused by the construction of the cable after laying. Although the damage is sometimes slight, the insulation of the damaged part will gradually decrease after several months or even several years, resulting in breakdown.
Poor design and manufacturing processes, not made in accordance with the requirements of the regulations, is often an important reason for the formation of cable faults.
Chemical, electric corrosion. Electrical corrosion of lead outside the cable causes moisture intrusion and insulation damage.
The manufacturing defect of the cable.
Due to the long-term overloaded operation of the cable, the temperature of the cable will increase. Especially during the hot summer months, the temperature rise of the cable often leads to the first breakdown of the weak point of the cable and the butt joint.
Loss of cable insulation.
2. Cable fault pre-positioning method The most important step in cable fault location is to identify the type of cable fault. Once a fault occurs, determine the type of fault, according to the type of fault and the equipment conditions of the unit to select the appropriate detection method, directly affect the speed of the accident treatment. In fact, any type of failure of the cable may occur at any location, and the ability to quickly resolve the problem depends on the actual experience of the field staff. Usually use a multimeter to determine the fault cable resistance, according to the size of the cable fault is divided into two groups: low resistance fault - less than 100kΩ; high resistance fault - more than 100kΩ. Each type of cable fault requires a special method for pre-positioning, and the more commonly used pre-positioning methods are as follows.
2.1 Low-voltage pulse reflection method This measurement method is to send a high-frequency low-voltage pulse to the cable. The pulse propagates along the cable until the impedance is mismatched, such as the middle connector, T connector, short circuit point, disconnection point, and terminal head. At these points, reflections of the waves are caused, and the reflected pulses are received by the test equipment when they return to the test end of the cable. Practice has proved that the vast majority of faulty cables in the field can't measure the fault location by using low-voltage pulse reflection method. The reflected waveform can only test the full length of the cable. Figure 1 shows the standard waveform of low-voltage pulse reflections.
2.2 Measurement of high-impedance faults 2.2.1 DC high-voltage flashover method The DC high-voltage flashover method is used for flashover faults, that is, no resistance channels are formed at the fault point (or the resistance value is extremely high) but the voltage rises to a certain value. Flashover occurs.
Working principle: add negative DC high voltage to the faulty cable. When the voltage rises to a certain value, flashover occurs at the fault point. The flicker meter shows the waveform at the measurement end, as shown in Figure 2. The fault distance is the distance corresponding to the actual time interval from the starting point T0 of the waveform to the inflection point T1 at the falling point.
(a) Detection of faults (b) Waveforms In practice, flashover faults of cables are common, and almost all cables that have been subjected to pre-test breakdown have a flashover process, and about half of the cable faults that run breakdown are flashovers. process. When the faulty cable is directly pressurized, the flashover process of the cable varies in length. Some faults only flash several times to form a stable channel and no flashover occurs. Therefore, in the process of discovering the flashover of the cable, you should seize the opportunity and cherish the time. This phenomenon is tested. Because the direct flash method has better accuracy than the flash method waveform, the direct flash method is used as far as possible.
2.2.2 Fault point burn-through method Fault point burn-through method is applied to high-impedance fault. The device passes the output DC negative high voltage to process the high-resistance fault point so that the fault point will generate arc discharge and carbonize the insulating medium, because the carbonization connection point is Low-resistance, high-impedance failure into a low-impedance failure, and then measured using low-voltage pulse reflection method. Fault point burn-through method is mainly used for oil-paper insulated cables. The disadvantages of using the burn-through method at the fault point are that the burn-through time is long, the manpower is consumed, and the metal short circuit is easily formed, which makes it difficult to find the fault point. The resistance at the fault point is restored and a second burn-through must be performed, so this method is generally not used.
2.3 Impact High-voltage flashover method The flashover method is divided into two types: resistor and inductor flashover. For the former, due to the voltage divider in the circuit, the voltage applied to the faulty cable is low, which is unfavorable for discharge. Especially for those faults with higher resistance, it is more difficult to discharge. Therefore, there are certain limitations. Sex, usually the latter. The advantage of the inductive flash method is that it can adapt to almost any type of fault. A large number of facts have proved that the inductive flash method is the most powerful means to deal with those that are not measured by others and is called the most stubborn failure. Therefore, the inductive flash method is the most important test method. The measurement circuit of the impulse DC high-voltage inductance measurement method (referred to as the rush L method) is shown in Fig. 3(a). When the power is turned on, the storage capacitor C is first charged by the DC high voltage. When the voltage on the capacitor is high to a certain amplitude, the ball gap Q is broken down and discharged, and the negative high voltage is instantaneously added to the cable fault phase at t0. Propagation to the point of failure, followed by flashover discharge at the point of failure. The short-circuit arc at the time of fault discharge reflects the voltage wave sent along the cable back to generate the waveform shown in Figure 3(b) between the measuring end and the fault point. The sharp pulse in the figure is due to the differential action of the inductance L. Caused. This waveform is added to the instrument by the resistor divider R1 and R2.
(a) Measuring the faults by the rush-L method (b) Measuring the waveforms by the rush-L method The rush-L method is mainly used to measure the leaky high-impedance fault and also to measure the flashover high-impedance fault. It should be pointed out that the cable fault YM cable fault tester, although it is more advanced equipment, but they are all coarse measurement instruments, when the rough point of the fault point is determined, it is necessary to try to determine the precise point, the current method is mainly used sound Measurement point method.
3. Precaution of faulty cable faults in cable faults According to the current test level, it is not difficult. If the YM cable fault location system is used, the distance from the fault point to the test end can be measured in only a few minutes when the rough determination of the cable waveform and the path are clear, and the pre-positioning error will generally not exceed 10m. However, due to the error and incompleteness of cable operation data, the different types of faults, and the complex diversity of environmental factors of cable fault points, such as the lack of accurate cable lengths and circuit diagrams, in powerful noise sources and power frequency electromagnetic fields. Nearby, cables laid in buried pipes and difficult to enter into buildings, etc., will bring many unexpected difficulties to the precise point of failure. A large number of practices have proved that the problem of precise determination has become the main contradiction in finding faults quickly. Mainly using acoustic and magnetic signals to receive the fixed point method synchronously, this method adds a surge voltage with a sufficiently high amplitude to the faulty cable, causing the flashover discharge at the fault point to generate a considerable “啪, 啪†discharge sound, and at the same time, A loop is induced in the loop formed by the cable and the earth. This loop generates a pulsed magnetic field around the cable. Use a listening device that contains a grounded microphone receiver and headphones to probe the ground. The closer the fault point is to the microphone, the greater the flashover sound. While listening to the sound signal and receiving the pulsed magnetic field signal, it can be judged that the sound is generated by the fault point discharge, and the fault point is in the vicinity, otherwise it can be considered as interference. The maximum value of the flash sound can be detected at the location of the fault point.
When encountering the resonance in the cable body in the vicinity of the flashover or near the point of failure, the sound can be heard within a large section of the cable during discharge, and if the size is the same, accurate positioning is difficult. In view of the fact that there is no better technology yet, the problem of fixed point can be improved. Only according to the pre-position distance and cable data, open the cable trench cover or dig the surface of the buried section, and perform fixed point work directly on the cable body.
4. Conclusion Power cable fault detection methods and techniques, in addition to the above-mentioned several main methods, there are high voltage bridge method, capacitance method, step voltage method and audio induction method. To use a cable fault tester to detect a fault, not only should you familiarize yourself with the use of the instrument, but also understand the nature of the fault and the test waveforms. The author thinks that it is very necessary to choose the appropriate test method when detecting the cable fault, which can greatly reduce the fault detection time, and concludes that:
Low-resistance ground fault detection (R <10kΩ), easy to use low-voltage pulse method, bridge method test;
High-impedance failure (R>100kΩ) with insulation recovery, best for direct flash test;
High-resistance grounding fault (R>100kΩ) is the most accurate with flashover test.
Since the acquisition of the YM cable fault location system in 2002, the high-voltage impulse method has been used mainly for cable fault detection. The success rate for several years has been 100. To ensure timely and effective troubleshooting and ensure the normal operation of the city's electricity and grid, it has played a very good role.
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