Classification Of Causes Of Power Cable Faults

The failure of underground power cable is complex and changeable, and the causes of power cable failure can be roughly summarized into the following categories.

1. Mechanical damage

Cable failures caused by mechanical damage account for a large proportion of cable accidents. Some mechanical damage was minor and did not fail at the time, but took only months or even years later. The main causes of the mechanical damage of the cable are:

(1) Injury during installation. Accidentally touch cable during installation; excessive mechanical traction pulls cable; excessive bending break cable.

(2) Direct external damage. Civil construction is carried out on or near the cable path after installation, so that the cable is directly damaged by external forces.

(3) The vibration or impact load of the driving vehicle will also cause the lead (aluminum) package crack damage of the underground cable.

(4) Damage caused by natural phenomena. If the internal insulating glue of the intermediate joint or the end expands and crack the shell or the cable sheath; scratch the cable skin on the pipe port or support; pull the intermediate joint or conductor.

2. Insulation is kept to damp

The insulation after damp will cause the cable withstand voltage drop and produce failure. The main reasons for cable moisture are:

(1) Water intake is caused due to the unsealing or poor installation of the joint box or terminal box structure.

(2) the cable manufacturing is bad, and the metal sheath has small holes or cracks.

(3) The metal sheath is stabbed or corroded by external objects.

3. Insulation is aging and deterioration

Insulation aging will cause cable voltage drop and failure. The main reasons of cable aging are:

(1) The slag or air gap inside the cable medium is free and hydrolysis under the action of the electric field.

(2) Cable overload or cable trench poor ventilation, resulting in local overheating.

(3) Loss of insulation of oil-soaked paper insulated cables.

(4) Power cable is used beyond the time limit.

4. Overvoltage

Overvoltage will breakdown the defective cable insulation layer and cause cable failure. The main reasons are: atmospheric overvoltage (such as lightning strike); internal overvoltage (such as operating overvoltage).

5. Poor design and production technology

Poor cable head and intermediate design and production technology can also cause cable failure. The main reasons are: inadequate field distribution design; improper material selection; poor process and not production according to the regulations.

Nature and classification of cable faults

1. Classification of fault material features

It can be divided into three categories: series fault, parallel fault and composite fault.

(1) Series fault

Series fault (metal material defect) is the fault that one or more cable conductors (including lead and aluminum skin) are disconnected. It is a broad cable open circuit fault. Because the continuity of the cable core is broken, broken lines or incomplete broken lines are formed. Incomplete disconnection is especially difficult to find. Series faults can be divided into: one point break, multiple point break, one phase line, multiphase line, etc.

(2) Parallel connection fault

Parallel fault (insulation material defect) is a short circuit fault where the insulation level between the guide body and the conductor drops and it cannot withstand the normal operating voltage. It is a generalized cable short circuit fault. This kind of failure forms short circuit, grounding, flashover breakdown and other phenomena due to the insulation damage between the core or the core between the skin, which has a high frequency in the field. Parallel faults can be divided into: one phase ground, two phase ground, two-phase short circuit, three-phase short circuit, etc.

(3) Composite fault

Composite failure (insulation and metal materials are defective) refers to the insulation between the cable core and the cable core. It includes one-phase broken line and grounding, two-phase broken line and grounding, two-phase short circuit and grounding, etc.

2. Classification by the insulation features of the fault points

According to the situation of the insulation resistance Rf and the breakdown gap G, the cable fault can be divided into four categories: open circuit fault, low resistance fault, high resistance fault and flashover fault. This classification method is the most basic classification method for field cable failure, which is especially beneficial to the choice of detection method.

Among them, the size of the gap breakdown voltage UG depends on the distance G of the discharge channel (i. e., the breakdown gap), the size of the insulation resistance Rf depends on the carbonization degree of carbonization of the cable medium at the fault point, and the size of the distribution capacitor 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 damaged, forming a broken line, and the insulation material of the fault point is also damaged to varying degrees. The insulation resistance Rf is infinite (), but during DC withstand test; check the conduction of core wire with breakpoint. The site generally appears in the form of one-phase or two-phase broken line and grounding.

(2) Low-resistance fault

Cable insulation material is damaged, and the grounding fault occurs. Use megohms to measure the insulation resistance Rf is less than 10Z0 (Z0 is the wave impedance of the cable, generally between 10 and 40 Ω). On site, low voltage power cables and control cables have a higher chance of low resistance failure.

(3) High-resistance failure

Cable insulation material is damaged, and the grounding fault occurs. If the insulation resistance Rf is greater than 10Z0, electric breakdown will occur in the DC high-voltage pulse test. High resistance failure is the cable failure with the highest probability of high voltage power cable (6KV or 10KV power cable), reaching more than 80% of the total failure.

In the field measurement, the author generally takes Rf = 3 KΩ as the boundary between high resistance and low resistance faults. Because at R f = 3 KΩ, exactly the measured current of 10 ~ 50 mA necessary for the accurate measurement of the return method bridge can be obtained.

(4) Flashover failure

The cable insulation material is damaged, and the flashover failure occurs. The insulation resistance Rf of the meter is infinite (), but in the DC withstand voltage or high voltage pulse test, there will be will occur. Flashover failure is difficult to measure, especially when flashover failure occurs in the preventive test of newly laid cables. The Dc flashover method is generally used for detection in the field.

3. Classification by fault trigger causes and fault point characteristics

According to the power cable in the operation or preventive test, cable, cable head and middle box appear different characteristics of insulation damage, can also be divided into three types: firing failure, breakdown fault and operation failure.

(1) Shooting gun failure

In industrial and mining enterprises, the operation of power cables, due to various reasons, the insulation is seriously damaged, resulting in tripping accidents. It is called a cable firing. The characteristics of this kind of fault are: most of the cable fault points have lead pack or copper skin rupture, the external deformation; the cable fault nature is often manifested as two-phase short circuit grounding or two-phase broken line and grounding, the grounding resistance is generally small, the arc failure point or tree discharge carbon channel and crack can be found. Cable firing fault, its fault characteristics are obvious, in most cases, the operation on duty personnel can provide the approximate location of firing. Therefore, this kind of fault in addition to a few more complex situations need to distance, generally as long as the multimeter to determine the specific nature of the fault (single-phase ground, short circuit grounding, broken grounding, etc.), can be directly fixed point, simple and clear.

(2) The breakdown fault

In practice, the cable insulation failure event caused by the preventive test is habitually called the cable breakdown. These kinds of faults occur under the DC experimental voltage, the insulation failure is electric breakdown, the contact site is generally lead bag or copper skin intact, and there is no obvious external deformation (except mechanical trauma). Cable breakdown faults are mostly simple grounding faults, with high grounding faults, anatomical fault point, and no carbonization point of insulation materials, but the aging structure of carbon holes and water branches can be found through the instrument. For the cable breakdown faults, especially some high-resistance grounding cable breakdown faults, the test difficulty is in the ranging. Because this kind of fault is relatively hidden, the test parameters are complex and changeable, and the lack of regularity, so whether the distance finding can quickly find the cable fault point is the key."High voltage return method" and "electric hammer method" both have the most effective method to detect this kind of fault.

(3) Operation failure

It refers to the cable fault determined in the operation of the cable feeding line, motor, transformer cable lead, the voltage fluctuation or the grounding signal (the power element with grounding protection has ground tripping) and the possibility of other power element faults. The biggest characteristic of this kind of failure is the ambiguity. The extreme form of cable operation fault is cable firing (such as alternate short circuit caused by two grounding); the other part develops into cable breakdown fault (such as cable aging, insulation defects, etc.); Some cable operation fault is due to improper installation position of cable lead line (such as cable alternate or insufficient ground distance, dirty cable head or water into the motor foundation, etc.). These faults can be mainly handled simply; the most obvious are those cable operation faults with instantaneous grounding and unstable flashover. After the cable power failure, a considerable part of the insulation resistance measurement and DC voltage resistance experiment can be passed, and then the cable can operate normally for a period of time; the rest is the single-phase ground cable fault, which accounts for about 40% of the cable operation failure. The grounding fault generally has no obvious external deformation, and the grounding resistance is not too high (generally tens to hundreds of euros). Anatomical failure points have subtle carbonization points.

There are two reasons for the grounding failure of the cable operation. First, the cable insulation layer is naturally aging; second, the cable sheath is rapidly damaged in the corrosion environment, and the corrosive gas invades the insulation layer. Whether the cable insulation layer is aging or deterioration, its breakdown voltage will drop, and eventually lead to the electric breakdown under the rated power frequency voltage, resulting in the cable grounding failure. This kind of fault can be detected by "low voltage return method".