Judging from the faulty position of the cable, the weakest part of a cable is the middle connector. The general cable has one or more intermediate connectors. Because of the environmental conditions when making the cable middle connector, and after the cable is laid, no moisture treatment is performed. Inadequate crimping of the indirect tube during production may result in moisture in the middle of the cable connector and process defects. During the operation, corona and free discharge are generated under the action of high-voltage electric field, so that the insulating body forms water trees until the insulation ages and breaks down.
From the analysis of the structural characteristics of low, medium, and high voltage cable faults that have been found this year, the single-phase ground fault of the cable is more common, mostly because of the external damage caused by the cable. Do not rule out the quality of the body, but this internal short circuit does not see traces from the outside. There are few short-circuit faults in the cable phase-to-phase. This is because phase-to-phase short circuits generally occur during operation. In the event of a fault, a strong short-circuit current causes a quick-break protection action and trips. High temperatures caused by strong currents generally blow the cable and cause open-circuit faults. The internal short circuit of the cable does not show traces on the exterior. Such failures are generally caused by the quality of the cables and are relatively rare.
From the nature of the cable fault can be divided into open, low resistance, high resistance and flashover failure of four kinds: open failure is the working voltage can not be transmitted to the terminal, or although the terminal has voltage, but the load capacity is poor.
The low resistance fault is the insulation of the cable between the phases or the ground, and its insulation resistance is reduced to less than 100KΩ.
A high-impedance fault is an insulation resistance of more than 100 kΩ between phases of the cable or ground.
Comprehensive analysis of the following points is the key to finding cable faults:
1. Determine whether a cable fault belongs to an open-circuit fault, a low-impedance fault, or a high-impedance fault.
2. Understand the length and specific direction of the cable (measurement with the instrument will have an error ranging from 0.5m to 2m);
3, determine the length of the cable and the approximate position of the middle head;
4. Whether the occurrence of a cable fault is related to the substation power-off protection action;
5. Is there any construction excavation around the cable? Are there any unclear sounds?
6, the depth of the cable and the site environment.
Flashover faults are sudden breakdowns in the process of high pressure holding, and can withstand pressure at this voltage. Different from high-impedance faults, faults that can surely break down when the high voltage reaches a certain voltage.
Fault nature Breakdown of Rf gap Open circuit ∞ Breakdown under DC or high voltage pulse Breakdown Low resistance Less than 100Z0Rf Not too low, high voltage pulse breakdown High resistance greater than 100Z0 High voltage pulse Breakdown flashover ∞ DC or high voltage pulse breakdown Note: In the table, Z0 is the wave impedance value of the cable, and the power cable impedance is generally between 10-40Ω. )
The purpose of the above classification is also to select the convenience of the test method. According to the current popular fault location technology, low-voltage pulse reflection method can be used for open-circuit and low-resistance faults, flashover method is used for high-impedance faults, and DC flash can be used for flashover faults. Network test. The above kinds of faults can all be tested with the secondary pulse method. This is currently the most advanced fault testing technology in the world, represented by Germany and Austria abroad. The fact that on-site personnel have a fault with Rf < 100KΩ is called a low-impedance fault, mainly because the traditional bridge method can measure such faults.
From the analysis of the structural characteristics of low, medium, and high voltage cable faults that have been found this year, the single-phase ground fault of the cable is more common, mostly because of the external damage caused by the cable. Do not rule out the quality of the body, but this internal short circuit does not see traces from the outside. There are few short-circuit faults in the cable phase-to-phase. This is because phase-to-phase short circuits generally occur during operation. In the event of a fault, a strong short-circuit current causes a quick-break protection action and trips. High temperatures caused by strong currents generally blow the cable and cause open-circuit faults. The internal short circuit of the cable does not show traces on the exterior. Such failures are generally caused by the quality of the cables and are relatively rare.
From the nature of the cable fault can be divided into open, low resistance, high resistance and flashover failure of four kinds: open failure is the working voltage can not be transmitted to the terminal, or although the terminal has voltage, but the load capacity is poor.
The low resistance fault is the insulation of the cable between the phases or the ground, and its insulation resistance is reduced to less than 100KΩ.
A high-impedance fault is an insulation resistance of more than 100 kΩ between phases of the cable or ground.
Comprehensive analysis of the following points is the key to finding cable faults:
1. Determine whether a cable fault belongs to an open-circuit fault, a low-impedance fault, or a high-impedance fault.
2. Understand the length and specific direction of the cable (measurement with the instrument will have an error ranging from 0.5m to 2m);
3, determine the length of the cable and the approximate position of the middle head;
4. Whether the occurrence of a cable fault is related to the substation power-off protection action;
5. Is there any construction excavation around the cable? Are there any unclear sounds?
6, the depth of the cable and the site environment.
Flashover faults are sudden breakdowns in the process of high pressure holding, and can withstand pressure at this voltage. Different from high-impedance faults, faults that can surely break down when the high voltage reaches a certain voltage.
Fault nature Breakdown of Rf gap Open circuit ∞ Breakdown under DC or high voltage pulse Breakdown Low resistance Less than 100Z0Rf Not too low, high voltage pulse breakdown High resistance greater than 100Z0 High voltage pulse Breakdown flashover ∞ DC or high voltage pulse breakdown Note: In the table, Z0 is the wave impedance value of the cable, and the power cable impedance is generally between 10-40Ω. )
The purpose of the above classification is also to select the convenience of the test method. According to the current popular fault location technology, low-voltage pulse reflection method can be used for open-circuit and low-resistance faults, flashover method is used for high-impedance faults, and DC flash can be used for flashover faults. Network test. The above kinds of faults can all be tested with the secondary pulse method. This is currently the most advanced fault testing technology in the world, represented by Germany and Austria abroad. The fact that on-site personnel have a fault with Rf < 100KΩ is called a low-impedance fault, mainly because the traditional bridge method can measure such faults.
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