Classification of causes of power cable faults

The underground power cable fault is complex and changeable, the causes of power cable fault can be classified into the following categories. 1. Mechanical damage cable fault caused by mechanical damage accounts for a large proportion of cable accidents. Some of the mechanical damage was minor and did not cause a failure at the time, and it would take months or even years for the damage to develop into a failure. The main causes of cable mechanical damage are: (1) damage during installation. Accidentally damaged cable during installation; overstretched cable due to mechanical traction; overbent and broken cable. (2) direct external force injury. After the installation of the cable path or near the civil construction, so that the cable directly subject to external damage. (3) the vibration or impact load of running vehicles can also cause lead (aluminum) clad crack of underground cables. (4) damage caused by natural phenomena. Such as intermediate joint or terminal head of the expansion of the inner insulation and cracking shell or cable sheath; installed in the nozzle or bracket on the skin of the cable scratch; due to land settlement caused by excessive tension, break intermediate joint or conductor.

2. The damp insulation will cause the decline of the cable voltage and produce the fault. The main reasons for cable dampness are: (1) due to the junction box or terminal box structure is not sealed or installation of poor water. (2) the cable is not well made, and the metal sheath has small holes or cracks. (3) the metal sheath is punctured or corroded by foreign objects. 3. Insulation aging deterioration of insulation aging can cause the decline of cable voltage and fault. The main reasons of cable aging are as follows: (1) the slag or air gap in the cable medium produces dissociation and hydrolysis under the action of electric field. (2) cable overload or poor ventilation, resulting in local overheating. (3) loss of insulation material of oil-impregnated paper insulated cables. (4) use of power cables beyond the time limit. Four. Overvoltage and overvoltage can cause electrical breakdown of the defective cable insulation layer, resulting in cable fault. The main reasons are: atmospheric overvoltage (such as lightning strike) ; internal overvoltage (such as operation overvoltage) . 5. Poor design and fabrication process of cable head and intermediate design and fabrication process, can also cause cable failure. The main reasons are: the electric field distribution is not well designed, the material is not properly selected, the technology is not good, not according to the requirements of the production procedures. Nature and classification of cable faults

1. Classification based on the characteristics of faulty materials

It can be divided into three categories: series fault, parallel fault, and composite fault.
(1) Series fault
Series fault (metal material defect) refers to a fault where one or more conductors (including lead and aluminum sheaths) of a cable are disconnected. It is a generalized cable open circuit fault. Due to the disruption of the continuity of the cable core, a broken or incomplete wire is formed. Incomplete disconnection is particularly difficult to detect. Series faults can be specifically divided into: one point disconnection, multiple point disconnection, one phase disconnection, multi-phase disconnection, etc.
(2) Parallel fault
Parallel fault (insulation material defect) refers to a short circuit fault that occurs when the insulation level between conductors decreases and cannot withstand normal operating voltage. It is a generalized cable short circuit fault. This type of fault occurs frequently on site due to insulation damage between cable cores or between cable cores, resulting in short circuits, grounding, flashover and breakdown. Parallel faults can be specifically divided into: one phase to ground, two phase to ground, two phase short circuit, three phase short circuit, etc.
(3) Composite fault
Composite failure (defects in both insulation and metal materials) refers to the failure of the insulation between the cable core and the cable core. It includes one phase disconnection and grounding, two phase disconnection and grounding, two phase short circuit and grounding, etc.

2. Classification based on insulation characteristics of fault points

According to the insulation resistance Rf and breakdown gap G at the cable fault point, cable faults can be further divided into four categories: open circuit faults, low resistance faults, high resistance faults, and flashover faults. This classification method is the most basic classification method for on-site cable faults, which is particularly conducive to the selection of detection methods.
Among them, the size of gap breakdown voltage UG depends on the distance G of the discharge channel (i.e. breakdown gap) at the fault point, the size of insulation resistance Rf depends on the degree of carbonization of the cable medium at the fault point, and the size of distributed capacitance Cf depends on the degree of moisture at the fault point.
(1) Open circuit fault
The continuity of the metal part of the cable is disrupted, resulting in wire breakage, and the insulation material at the fault point is also damaged to varying degrees. On site, the insulation resistance Rf measured with a megohmmeter is infinite (∞), but electrical breakdown may occur during the DC withstand voltage test; Check the continuity of the core wire and find any broken points. The site generally appears in the form of one or two phase disconnection and grounding.
(2) Low resistance fault
The insulation material of the cable has been damaged and a grounding fault has occurred. On site, measure the insulation resistance Rf with a megohmmeter to be less than 10Z0 (Z0 is the wave impedance of the cable, usually between 10 and 40 Ω). The probability of low resistance faults in low-voltage power cables and control cables on site is relatively high.
(3) High resistance fault
The insulation material of the cable has been damaged and a grounding fault has occurred. On site, the insulation resistance Rf measured with a megohmmeter is greater than 10Z0, and electrical breakdown may occur during the DC high-voltage pulse test. High resistance faults are the most common cable faults in high-voltage power cables (6KV or 10KV power cables), accounting for over 80% of the total faults.
When conducting on-site measurements, the author generally takes Rf=3K Ω as the boundary between high resistance and low resistance faults. Because when Rf=3K Ω, it is precisely possible to obtain the necessary 10 to 50mA measurement current for precise measurement of the loop method bridge.
(4) Flashover fault
Cable insulation