The complexity of the underground power cable fault changes, causing the power cable fault classification can be summarized as the following categories.
1. mechanical damage
The cable failure caused by mechanical damage accounts for a large proportion of cable accidents. Some mechanical damage is very slight, did not cause a failure, to a few months or even years after the injury was developed into a fault. The main causes of mechanical damage to the cable are:
(1) damage during installation. The installation of careless bruises the cable; mechanical traction too large tensile cable; excessive bending damage cable.
(2) directly by external damage. In the installation of the power cable path or near the civil construction, the cable directly by external damage.
(3) The vibration or impact load of the vehicle will also cause the lead (aluminum) of the underground cable to crack.
(4) damage caused by natural phenomena. Such as the middle connector or the end of the internal insulation of the plastic expansion and expansion of the shell or cable jacket; installed in the nozzle or bracket on the cable skin abrasion; due to land settlement caused by excessive tension, pull off the middle connector or conductor.
2. Insulation damp
Insulation damp will cause the cable to withstand pressure drop and failure. The main causes of cable dampness are:
(1) Water is caused by improper sealing or poor installation of the joint box or terminal box.
(2) poor cable manufacturing, metal sheath with small holes or cracks.
(3) metal sheath due to foreign objects stabbed or corroded perforation.
3. Insulation Aging -
Insulation aging can cause cable breakdown voltage and failure. The main reasons for the aging of the cable are:
(1) cable media within the slag or air gap, under the action of the electric field to produce free and hydrolysis.
(2) cable overload or cable trench ventilation, resulting in local overheating.
(3) loss of insulation of oil-impregnated paper insulated cables.
(4) power cable over time use.
4. Overvoltage
Overvoltage causes electrical breakdown of the defective Power Cable Wires Supplier insulation, causing cable failure. The main reasons are: atmospheric over-voltage (such as lightning); internal over-voltage (such as operating over-voltage).
5. Poor design and production process
Cable head and the middle of the design and production process is bad, can cause cable failure. The main reason is: the electric field distribution design is not careful; improper selection of materials; poor process, not according to the requirements of the production process.
The nature and classification of cable failures
1. Classification of faulty material features
Can be divided into series fault, parallel fault and composite fault three categories.
(1) series fault
Series fault (metal material defect) refers to the cable one or more conductors (including lead, aluminum skin) off the fault. It is a generalized cable open circuit failure. Due to the continuity of the cable core is damaged, the formation of broken or incomplete disconnection. Not completely broken is particularly difficult to find. Series fault can be divided into specific: a little break, more points off, a broken line, multi-phase disconnection.
(2) parallel fault
Parallel fault (insulating material defect) refers to the conductor on the skin or the insulation between the conductor level down, can not withstand the normal operation of the voltage and short circuit failure. It is a generalized cable short circuit fault. This type of fault due to the cable core or between the cable core insulation between the formation of short-circuit, ground, flashover breakdown and so on, appear in the field higher frequency. Parallel fault can be divided into: a phase ground, two-phase ground, two-phase short circuit, three-phase short circuit.
(3) composite failure
Composite failure (insulation, metal materials are defective) refers to the cable core and the cable between the insulation are faulty. It consists of a disconnected and grounded, disconnected and grounded, two-phase shorted and grounded.
2. Classification by fault point insulation feature
According to the cable fault point insulation resistance Rf and breakdown gap G case, the cable fault can be divided into open circuit failure, low resistance fault, high impedance fault, flashover failure four categories. This classification is the most basic classification method for on-site cable failure, which is particularly beneficial to the selection of detection methods.
The size of the gap breakdown voltage UG depends on the distance G of the fault point discharge channel (ie, the breakdown gap). The size of the insulation resistance Rf depends on the degree of carbonization of the cable material. The size of the distributed capacitance Cf depends on the degree of damp The
(1) open circuit failure
The continuity of the cable metal parts is damaged, the formation of disconnection, and the fault point of the insulating material is also subject to varying degrees of damage. In the field with megger, the insulation resistance Rf is infinite (infinity), but in the DC voltage test, there will be electrical breakdown; check the core conduction, there are breakpoints. The scene is generally in the form of a phase or two broken and grounded.
(2) low resistance fault
The cable insulation is damaged and a ground fault occurs. The scene with megger measured insulation resistance Rf less than 10Z0 (Z0 for the cable wave impedance, generally take between 10 ~ 40&Omega ;). The general low-voltage power cables and control cables in the field of low resistance to the higher probability of failure.
(3) high resistance fault
The cable insulation is damaged and a ground fault occurs. Field with megger measured insulation resistance Rf greater than 10Z0, in the DC high voltage pulse test, there will be electrical breakdown. High-resistance failure is the highest probability of cable failure (up to 80%) of high-voltage power cables (6KV or 10KV power cables).
Field measurement, the author generally take Rf = 3KΩ for the distinction between high resistance and low resistance fault line. Because Rf = 3KΩ, it is possible to get the 10 ~ 50mA measuring current necessary for the accurate measurement of the bridge.
(4) flashover failure
The cable insulation is damaged and a flashover fault occurs. In the field with megger, the insulation resistance Rf is infinite, but in the DC voltage or high voltage pulse test, there will be flashover electric breakdown. Flashover failure more difficult to test, especially the new laying of the cable to carry out preventive tests flashover failure occurs. The field is usually used to detect the DC flashover method.
3. Classification by fault trigger and fault point feature
According to the power cable in the operation or preventive test, the cable, cable head and the middle box of different characteristics of the insulation damage, can also be divided into blasting failure, breakdown failure and operational failure three categories.
(1) blasting fault
In industrial and mining enterprises, the operation of the power cable, due to various reasons, insulation serious damage, resulting in tripping accidents. Called the cable blasting. This type of fault is characterized by the fact that most of the cable fault points have lead or copper rupture, and there are different degrees of deformation in the outside. The cable fault performance is often characterized by two-phase short-circuit grounding or two-phase disconnection and grounding, the grounding resistance is generally small, Anatomical failure point, can be found in the arc punctured carbonation point or dendritic discharge carbon and cracks. Cable blasting failure, the fault characteristics are obvious, in most cases, the operation of duty personnel can provide a rough location. Therefore, such a fault in addition to a few more complex cases need to range, the general as long as the multimeter to determine the specific nature of the fault (single-phase ground, short-circuit ground, broken ground, etc.) can be directly measured by sound measurement, simple and clear.
(2) breakdown failure
In the actual work, due to preventive testing and trigger the cable insulation damage events, used to call the cable breakdown. The failure occurred in the DC experimental voltage, the insulation damage for the electric breakdown, the grounding point of the general lead or copper intact, no external obvious deformation (except for mechanical trauma). Cable breakdown failure is simple ground fault, the ground fault is higher, anatomical failure point, the insulation material does not have carbonation point, but through the instrument can be found in carbon hole and water tree aging structure. The cable breakdown failure, especially some high resistance grounding cable breakdown failure, the test difficult in the distance measurement. As the kind of fault is more subtle, the test parameters are complex and changeable, lack of regularity, so can quickly find the cable fault point, ranging is the key.have the most effective method of detecting such failures.
(3) running fault
It refers to the factory power system in operation, the cable feed line, motor, transformer cable lead, the high voltage secondary circuit voltage fluctuations or find the ground signal (grounded power components appear ground trip), excluding other power components failure The possibility of determining the cable failure. The biggest feature of this type of failure is not clear. Cable running fault is the extreme form of cable blasting (such as two-point ground caused by the phase short circuit); another part of the operation failure to do stop check, due to pressure through the development of cable breakdown failure (such as cable aging, insulation defects (Such as cable phase or ground distance is not enough, the cable head dirty or the motor base water, etc.), these failures are mainly some simple processing can be; the most likely to deal with the cable; It is not clear that those who are instantaneous ground, resulting in unstable flashover cable failure. This type of fault after the cable power failure, insulation resistance measurement and DC voltage test can be a considerable part of the cable can be put into the system, but also normal operation for some time; the rest is the single-phase grounding cable failure, they account for about cable 40% of the operation failure, this ground fault is generally no obvious deformation of the external, grounding resistance is not too high (generally tens to hundreds of Europe). Anatomical point of failure has a slight carbonation point.
There are two reasons why the cable is running to ground fault: First, because the cable running for a long time, the insulation layer of natural aging; Second, the cable in the corrosive environment, the cable sheath was quickly destroyed, corrosive gas into the insulation layer to deteriorate The Cable insulation regardless of aging or deterioration, the breakdown voltage will drop, eventually leading to the rated power frequency voltage breakdown, resulting in cable ground fault. This type of failure can be used "low-voltage loop method" detection; with "hammer" and "detection, the effect is better. "