1. Common types of cable faults
1. Low resistance fault, cable insulation material is damaged, and ground fault occurs. The insulation resistance Rf is less than 10Z0 (Z0 is the wave impedance of the cable, generally between 10 and 40Ω). Generally, low-voltage power cables and control cables are more likely to have low-resistance faults on site.
2. Open circuit failure, the continuity of the metal part of the cable is destroyed, forming a disconnection, and the insulation material at the fault point is also damaged to varying degrees. The insulation resistance Rf measured with a megger on site is infinite (∞), but during the DC withstand voltage test, electrical breakdown will occur; check the continuity of the core wire, and there is a break point. The scene generally appears in the form of one-phase or two-phase disconnection and grounding.
3. High resistance fault, cable insulation material is damaged, and ground fault occurs. The insulation resistance Rf is greater than 10Z0 measured with a megohmmeter on site, and electrical breakdown will occur during the DC high-voltage pulse test. High-resistance faults are cable faults with a high probability of high-voltage power cables (6KV or 10KV power cables), which can account for more than 80% of the total faults.
4. Flashover failure, the insulation material of the cable is damaged, and a flashover failure occurs. The insulation resistance Rf measured with a megger on site is infinite (∞), but in the DC withstand voltage or high-voltage pulse test, flashover electrical breakdown will occur. Flashover faults are more difficult to detect, especially when flashover faults occur during preventive tests on newly laid cables. The DC flashover method is generally used for detection on site.
5. Breakdown failure. In actual work, a cable insulation damage event triggered by a preventive test is customarily called a cable breakdown. This kind of faults all occur under the DC test voltage, the insulation damage is electric breakdown, the grounding point is generally lead-clad or copper-clad intact, and there is no obvious external deformation (except mechanical trauma). Cable breakdown faults are mostly simple ground faults. The ground faults are relatively high. The fault points are dissected. The insulation material has no carbon spots, but the aging structure of carbon holes and water branches can be found through the instrument.
6. Operation fault refers to the fact that the power system of the factory is in operation, the cable feeder, the motor, the cable lead of the transformer, the voltage fluctuation of the high-voltage secondary circuit or the grounding signal is found (the power component with grounding protection has a grounding trip), A cable fault determined by eliminating the possibility of other power component faults. The characteristic of this type of failure is that it is not clear. The extreme form of cable operation failure is cable blasting (such as
Point-to-phase short circuit caused by grounding); another part of the operation fault developed into a cable breakdown fault (such as cable aging, insulation defect, etc.) due to the failure of the withstand voltage during the stop-point inspection.
Cable fault
2. Reasons for power cable failure
1. Mechanical damage
Cable failures caused by mechanical damage account for a large proportion of cable accidents. Some mechanical damages were minor and did not cause failures at the time, but the damaged parts did not develop into failures until months or even years later.
2. Insulation is damp
3. Insulation aging and deterioration
The internal air gap of the cable insulating medium is released under the action of the electric field, which causes the insulation to drop. When the insulating medium ionizes, chemical products such as ozone and nitric acid are generated in the air gap, which corrode the insulation; the moisture in the insulation causes the insulating fiber to hydrolyze, causing the insulation to drop.
Overheating can also cause deterioration of insulation. The air gap inside the cable generates electric dissociation, causing local overheating and carbonizing the insulation. Cable overload is a very important factor in cable overheating. Cables installed in poorly ventilated areas such as cable trenches and cable tunnels, cables passing through drying pipes, and parts close to heating pipes will cause accelerated insulation damage due to overheating.
5. Overvoltage
Atmosphere and internal overvoltage will cause the cable insulation to break down and cause faults. The breakdown point is generally defective.
Cable fault
6. Poor design and production process
The waterproof and electric field distribution design of the middle joint and the terminal head is not thorough, the material selection is improper, the workmanship is not good, and the production is not in accordance with the regulations will cause the cable head to malfunction.
7. Material defects
Material defects are mainly manifested in three aspects.
The first is the problem of cable manufacturing, the defects left by the lead (aluminum) sheath; during the wrapping insulation process, there are defects such as wrinkles, cracks, breaks and overlapping gaps on the paper insulation;
Second, defects in the manufacture of cable accessories, such as cast iron parts with blisters, insufficient mechanical strength of porcelain parts, other parts that do not meet specifications or are not sealed during assembly, etc.;
The third is the improper maintenance and management of insulation materials, causing the cable insulation to become damp, dirty and aging.
8. Corrosion of the protective layer
Due to the influence of underground acid-base corrosion and stray current, the lead sheath of the cable is corroded and pitted, cracked or perforated, causing malfunction.
9. Loss of cable insulation
When laying oil-impregnated paper-insulated cables, the trenches are uneven, or the outdoor head on the pole, due to the undulations and the high-low difference, the insulating oil in the high places flows to the low places. The insulation performance of the high-altitude cable is reduced, leading to failures.