Fault diagnosis and detection technology and preve

2022-09-22
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Power cable fault diagnosis and detection technology and preventive technical measures

this method has hardly changed for decades. For short-circuit fault and low resistance fault, this method is very convenient. The bridge method is based on the principle that when the bridge is balanced, the product of the corresponding bridge arm resistance is equal, and the length of the cable is proportional to the resistance

1.2 low voltage pulse reflection method

low voltage pulse method is also known as time domain reflection method (TDR), which means that the pulse reflector independently measures the low resistance and open circuit faults of the cable without passing through the high voltage impactor

1.3 pulse voltage sampling method

pulse voltage sampling method, also known as impulse high voltage flashover method, is a test method for measuring high resistance leakage 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 measurement point and the fault point. Pulse voltage method mainly includes DC high voltage flashover (direct flashover method) and impulse high voltage flashover (impulse flashover method)

1.4 traditional method of cable fault location

① acoustic measurement method

this method uses the breakdown and discharge sound of the fault point during high voltage impact to accurately locate

② acoustic magnetic synchronization method

1 traditional cable fault detection method

1.1 measuring resistance bridge method

this method has hardly changed for decades. For short-circuit fault and low resistance fault, this method is very convenient. The bridge method is based on the principle that when the bridge is balanced, the product of the corresponding bridge arm resistance is equal, and the length of the cable is proportional to the resistance

1.2 low voltage pulse reflection method

low voltage pulse method is also known as time domain reflection method (TDR), which means that the pulse reflector independently measures the low resistance and open circuit faults of the cable without passing through the high voltage impactor

1.3 pulse voltage sampling method

pulse voltage sampling method, also known as impulse high voltage flashover method, is a test method for measuring high resistance leakage 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 measurement point and the fault point. Pulse voltage method mainly includes DC high voltage flashover (direct flashover method) and impulse high voltage flashover (impulse flashover method)

1.4 traditional method of cable fault location

① acoustic measurement method

this method uses the breakdown and discharge sound of the fault point during high voltage impact to accurately locate

② acoustic magnetic synchronization method

when applying impulse DC high voltage to the cable to discharge the cable fault point, a pulsed magnetic field will be generated around the cable. When receiving the pulse magnetic field signal at the sound measurement point, it can be considered that the discharge sound is emitted by the cable fault point

③ audio induction method

this method is generally used to detect low resistance faults. Its principle is: use a 1kHz audio signal generator to inject audio current into the cable to be tested, so that the cable sends electromagnetic waves, receives electromagnetic field signals on the ground, amplifies them, and then sends them to headphones or indicating instruments, and determines the location of the fault point according to the sound intensity or the value of the indicating instrument

when an impulse DC high voltage is applied to the cable to discharge the cable fault point, a pulsed magnetic field will be generated around the cable. When receiving the pulse magnetic field signal at the sound measurement point, it can be considered that the discharge sound is emitted by the cable fault point

③ audio induction method

this method is generally used to detect low resistance faults. Its principle is: use a 1kHz audio signal generator to inject audio current into the cable to be tested, so that the cable sends electromagnetic waves, receives electromagnetic field signals on the ground, amplifies them, and then sends them to headphones or indicating instruments, and determines the location of the fault point according to the sound intensity or the value of the indicating instrument

2 the new method of cable fault detection at present

2.1 the method of cable fault location

① real time expert system

expert system is a computer program with intelligent characteristics. Its intelligence is mainly manifested in that it can imitate human expert thinking to solve complex problems in a specific field. Therefore, the expert system must contain a lot of knowledge of domain experts, have the reasoning ability similar to human expert thinking, and be able to use this knowledge to solve practical problems

② use cause and effect to locate power system faults, but the possibility of further sharp decline in the non-ferrous metal industry is small. We are willing to work with Malaysia on the basis of the five principles of peaceful coexistence. There are four types of node states, symptoms, assumptions and initial causes in the causal network. The state node is used to express the state of a part or function in the field, such as circuit breaker tripping; The symptom node is the symptom that expresses the state node, for example, the symptom of circuit breaker tripping is the protection action; the hypothetical node is the diagnostic hypothesis that expresses the research system, such as the hypothesis that line fault occurs; The initial cause node is used to express the initial cause of the failure. Corresponding basic relationships can be formed between various nodes

③ application of wavelet transform in cable fault location

wavelet analysis is the crystallization of the common development of several disciplines, including mathematics, signal processing and computer vision. Wavelet analysis is mathematically realized by the prototype function of wavelet, in which the prototype function can be regarded as a band-pass filter, so wavelet analysis can also be realized by filters. The key is to find a filter bank with constant relative bandwidth, which is the core content of filter bank theory in signal processing

2.2 new method of cable fault location

① artificial neural network

artificial neural network (ANN) is an intelligent computing system that simulates biological neural network with computer network system. Each node on the network is equivalent to a neuron, which can memorize (store), process certain information, and work in parallel with other nodes. To solve a problem is to input information to some nodes of the artificial neural network, each node is processed and output to other nodes, and other nodes accept and process and then output until the whole neural network is completed and the final result is output

② GPS (Global Positioning System) traveling wave fault location

the traditional fault location of high-voltage transmission lines is mainly based on impedance algorithm, which is obviously unsuitable for high resistance grounding, multi terminal power lines, DC transmission lines, etc., and its fault location accuracy is usually 100km) in practice, which is difficult to meet the requirements of line finding

③ distributed optical fiber temperature sensor (FODT)

the basic principle of optical fiber sensing is that when light is transmitted in the optical fiber, the characteristics of light (such as amplitude, phase, polarization, etc.) will change with the change of the detection object

therefore, when light is emitted from the optical fiber, the characteristics of light have been modulated. Through the detection of modulated light, we can perceive the external information

3 cable fault detection example

3.1 on site switch tripping and grounding short-circuit fault treatment by measuring the insulation resistance of the cable to the ground is 5K Ω. The waveform collected by the cable fault detector is shown in Figure 1

it can be seen from Figure 1 that the total length of the power incoming cable here is 1138m, and the high-voltage pulse test waveform shows that the fault point is 323m away from the distribution end. After roughly measuring the distance, use a pointing instrument to accurately locate it, and sure enough, the fault point is found nearby. The reason is that there is a sewage well 5m away from here, and the insulation at the fault point is damaged, causing short circuit and disconnection

3.2 treatment of high resistance fault

there is a cable with a total length of 120m, which is judged to be a high resistance fault. The high-voltage pulse acquisition waveform shows that the fault point is at 80m. After treatment and analysis, it turns out that the cable was damaged by the bracket, the trench was wet, and the insulation was damaged after a long time, and the short-circuit grounding discharge was caused

3.3 troubleshooting of two fault points in the cable

the power incoming line of a building is a low-voltage PVC cable, with a total length of more than 130m, and the insulation resistance of the fault to the ground is 80 Ω. After collecting the waveform with low-voltage pulse, analyze that the waveform is close to the beginning. When tested with high-voltage pulse, the waveform is not ideal, and its specific position cannot be determined

after finding out the path, use the spotter to detect along the path. At 30m from the beginning, the sound measurement method hears a strong sound of discharge. After digging, the cable sheath and steel armor are rusted and broken here, but the main insulation is not damaged. After the steel armor is treated, the cable insulation is still very low. Then continue to search. Use the spotter to hear a weak sound of discharge at 5m from the steel armor fracture. Dig the cable, and find that the three phases have been disconnected and short circuited to earth. It is troublesome to find cables with more than two fault points

3.4 treatment of insufficient discharge accident at the fault point

some cables can't hear the sound of discharge around the fault point by using the high-voltage pulse method after using the low-voltage pulse to fix the point, such as the power cable of a branch, the insulation resistance between phases is zero, the phase to ground is 47 Ω, which belongs to three-phase short circuit and grounding. The low-voltage pulse method is used to measure the fault at 276m, but the acoustic method can't hear it. This situation should use the audio current induction method, that is, the audio oscillation signal is generated by the Pathfinder at the other end, and the audio signal is received by the receiver earphone. The audio signal is interrupted at 57M away from the transmitting end, which is basically consistent with the 276m distance measured at the other end by the low-voltage pulse method

after digging the ground, it was found that the fault point was here. The insulation of the cable was damaged by the long-term high-temperature fumigation of the leaked steam from a steam pipe well. The reason why the acoustic method cannot be used in this case is that the fault point is damped in a large area or discharged in a large area. Due to the long discharge creepage distance, the energy is not concentrated, and the arc is not enough to form a transient short circuit at the fault point, which is a manifestation of insufficient discharge at the fault point

4 technical measures to prevent cable failure

4.1 select the appropriate cable type

the manufacturing technology of oil paper insulated cable is relatively mature, with low cost, long service life, simple structure and convenient manufacturing, but the insulating oil is easy to flow. When laying at high drop, the insulating oil in the insulation flows from high to low, drying the cable insulation at high places, resulting in reduced insulation strength, and the lead package cracks at low places due to the increase of oil pressure, resulting in failure

cross linked polyethylene cable not only allows higher temperature rise than oil paper insulated cable, but also its allowable field strength is higher than oil paper insulated cable, and its laying is not limited by high drop. For cables with the same cross-section, cross-linked polyethylene cable is one level higher than the allowable long-term carrying capacity of oil paper insulated cable. At present, our company has replaced most oil paper insulated cables with cross-linked polyethylene insulated cables, which not only solves the fault caused by high drop, It also expands the transmission capacity of cables

4.2 improve the manufacturing process of cable terminals

cast iron cable terminals are mostly used in the early oil paper cable terminals, which are often prone to oil leakage and damage, while epoxy resin cable terminals have high mechanical strength and compressive strength, low water absorption, stable chemical performance, strong adhesion with metal, and good sealing performance. Using this cable terminal can basically solve the problems of cable oil leakage and improve the insulation performance of the cable

4.3 cable channel selection should avoid cable failure caused by corrosion

the surrounding environment of the cable is poor. The nearby soil contains acid, alkali solution, chloride and other chemical substances, which will corrode the cable. Due to groundwater pollution in the adjacent chemical plant area, the cable will also produce biochemical corrosion. Therefore, when selecting cable channel, it is necessary to investigate or inquire about the relevant geological pollution in detail, and only use continuous fiber dyeing in the required place, Especially in the chemical industry zone, cable channels should be carefully selected and effective pollution prevention measures should be taken

4.4 monitor the load current to prevent insulation breakdown caused by overload

the operating procedures of power cables stipulate that in principle, cable lines are not allowed to operate under overload and overload. Due to the increase of cable temperature rise, the aging of cable insulation is accelerated, which greatly reduces the service life of the cable. During operation, the cable insulation is weak. Recently, two kinds of phenolic foam preparation are introduced: one is the preparation of phenolic foam by heating method, such as grounding

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