Classification of power cable failure causes

The faults of underground power cables are complex and changeable. The causes of power cable faults can be roughly classified into the following categories.

1. Mechanical damage

Cable faults caused by mechanical damage account for a large proportion of cable accidents. Some mechanical damage is so minor that it 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 mechanical damage to cables are:

(1) Damage during installation. Accidentally bump the cable during installation; Mechanical traction too much strain cable; Excessive bending damages the cable.

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

(3) Vibration or impact load of driving vehicles will also cause lead (aluminum) cladding damage of underground cables.

(4) Damage caused by natural phenomena. Such as the middle joint or terminal head of the internal insulation adhesive expansion and crack shell or cable sheath; Cable skin abrasion installed on the pipe mouth or bracket; Rupture of intermediate joints or conductors due to excessive tension caused by land settlement.

2. Insulation is damp

Insulation damp will cause cable voltage drop and failure. The main reasons for cable dampness are:

(1) Due to the joint box or terminal box structure is not sealed or poor installation caused by water.

(2) The cable is not well manufactured, and the metal sheath has small holes or cracks.

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

3. Insulation aging and deterioration

Insulation aging will lead to cable voltage drop and failure. The main reasons for cable aging are as follows:

(1) Slags or air gaps in cable media produce dissociation and hydrolysis under the action of electric field.

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

(3) insulation loss of oil-impregnated paper insulated cable.

(4) Use of power cable beyond time limit.

4. The overvoltage

Overvoltage will cause electrical breakdown of defective cable insulation layer, causing cable fault. The main reasons are: atmospheric overvoltage (such as lightning strike); Internal overvoltage, eg operating overvoltage.

5. Poor design and manufacturing process

Poor design and manufacturing process of cable head and middle will also cause cable failure. The main reasons are: the design of electric field distribution is not thorough; Improper selection of materials; Poor process, not according to the requirements of the production process.

The nature and classification of cable faults

1. Classification by fault material characteristics

It can be divided into series fault, parallel fault and compound fault.

(1) series failure

Series fault (metal material defect) refers to the fault that one or more conductors (including lead, aluminum sheath) of cable are disconnected. It is a generalized cable open circuit fault. Break or incomplete break due to breakage of continuity of cable core. Incomplete disconnections are particularly difficult to spot. Series fault can be divided into: one-point break, multi-point break, one-phase break, multi-phase break and so on.

(2) Parallel fault

Parallel fault (insulating material defect) is a short circuit fault that occurs when the insulation level between the outer skin or conductor of the guide body decreases and cannot withstand the normal operating voltage. It is a generalized cable short circuit fault. This kind of fault is due to the insulation damage between cable cores or between cable cores, resulting in short circuit, grounding, flashover breakdown and other phenomena, which occur frequently in the field. Parallel fault can be divided into: one-phase ground connection, two-phase ground connection, two-phase short circuit, three-phase short circuit, etc.

(3) compound failure

Compound failure (defective insulation, metal material) is a failure of the insulation between the 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. Classify according to insulation characteristics of fault points

According to the insulation resistance Rf and breakdown gap G of cable fault point, cable fault can be divided into four categories: open circuit fault, low resistance fault, high resistance fault and flasover fault. The classification method is the most basic classification method for on-site cable faults, which is especially beneficial to the selection of detection methods.

Among them, the size of gap breakdown voltage UG depends on the distance G of discharge channel at the fault point (i.e. breakdown gap), the size of insulation resistance Rf depends on the carbonization degree of cable media 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 damaged, forming a broken line, and the insulation material at the fault point is also damaged to varying degrees. The insulation resistance Rf measured by megohm meter is infinite (∞), but in the DC voltage test, there will be electric breakdown; Check the conductor conduction. there is a breakpoint. The site is generally in the form of one-phase or two-phase disconnection and grounding.

(2) low resistance failure

The cable insulation material is damaged and the grounding fault occurs. The insulation resistance Rf measured by megohmmeter is less than 10Z0 (Z0 is the wave impedance of the cable, generally between 10 and 40 ω). Low - voltage power cable and control cable have a high probability of low - resistance failure.

(3) high resistance fault

The cable insulation material is damaged and the grounding fault occurs. The insulation resistance Rf is greater than 10Z0 measured by megohmmeter in the field, and electric breakdown will occur in the dc high voltage pulse test. High resistance fault is the cable fault with the highest probability of occurrence of high voltage power cable (6KV or 10KV power cable), which can reach more than 80% of the total fault.

In field measurement, the author generally takes Rf =3K ω as the boundary between high resistance and low resistance faults. Because Rf =3K ω, just can get loop bridge accurate measurement of the necessary 10 ~ 50mA measurement current.

(4) Flashover failure

Cable insulation material is damaged and flashover failure occurs. The insulation resistance Rf measured by megohm meter is infinite (∞), but flashover electrical breakdown may occur in DC voltage withstand test or high voltage pulse test. Flashover fault is difficult to measure, especially when flashover fault occurs in preventive test of newly laid cable. Dc flashover method is generally used for detection in the field.

3. Classify faults based on trigger causes and fault points

According to the insulation damage of cable, cable head and middle box in operation or preventive test, it can be divided into three kinds of blasting failure, breakdown failure and operation failure.

(1) Blasting failure

In industrial and mining enterprises, the insulation of power cables in operation is seriously damaged due to various reasons, resulting in tripping accidents. This is called cable firing. The characteristics of this kind of fault are as follows: most of the cable fault points are broken by lead or copper, and there are different degrees of external deformation; The fault nature of cable is usually manifested as two-phase short-circuit grounding or two-phase disconnection and grounding, and its grounding resistance is generally small. Dissecting the fault point, carbonization point or tree-shaped discharge carbon channel and crack of arc breakdown can be found.

Cable shooting fault, its fault characteristics are obvious, in most cases, the operation on duty can provide the general position of shooting. Therefore, this kind of fault in addition to a few more complex situations need to distance, generally as long as the use of a multimeter to determine the specific nature of the fault (single-phase grounding, short-circuit grounding, broken grounding, etc.), can be directly fixed by acoustic measurement method, simple and clear.

(2) Breakdown failure

In practice, the cable insulation damage event triggered by preventive test is called cable breakdown. This kind of faults all occur under dc experimental voltage, and the insulation damage is electrical breakdown. The ground point is generally intact with lead or copper skin and no obvious external deformation (except mechanical trauma). Cable breakdown faults are mostly simple grounding faults, which have higher grounding faults and dissected fault points. There is no carbonization point in insulation materials, but the aging structure of carbon holes and water branches can be found by instruments. For cable breakdown fault, especially for some high resistance grounding cable breakdown fault, the difficulty of measurement is ranging. Because this kind of fault is hidden, the test parameters are complex and changeable, lack of regularity, so whether the cable fault point can be found quickly, ranging is the key. "High voltage loop method" and "electric hammer method" are the most effective methods to detect this kind of fault.

(3) Operation failure

It refers to the cable fault determined by ruling out the possibility of fault of other power components in the operation of the power system in the factory, when voltage fluctuation occurs in the high voltage secondary circuit of the cable feed line, the cable lead of the motor and the transformer.

This type of failure is characterized by uncertainty. The extreme form of cable operation fault is cable blasting (such as two points grounding caused by the phase short circuit); The other part of the operation fault in the stop point inspection, due to the voltage through but developed into cable breakdown fault (such as cable aging, insulation defects, etc.);

There is also a part of the cable operation fault is due to the improper installation position of the cable outlet line (such as the cable phase or the distance to the ground is not enough, the cable head dirty or motor foundation water, etc.), these faults can be mainly treated simply;

The least clear are operating failures of cables that are instantaneously grounded and produce unstable flashover. This kind of fault in the cable power failure, insulation resistance measurement and DC voltage test can pass a considerable part, and then put the cable into the system, can also normal operation for a period of time;

The rest is single-phase grounding cable fault, they account for about 40% of the cable operation fault, this kind of grounding fault generally has no obvious external deformation, grounding resistance is not too high (generally dozens to hundreds of ohms). Anatomical fault points have fine carbonization points.

There are two reasons for grounding fault of cable operation. First, the insulation layer appears natural aging due to the long running time of cable. Secondly, the cable sheath is rapidly destroyed in the corrosive environment, and the corrosive gas invades the insulation layer to deteriorate it. No matter the cable insulation layer aging or deterioration, its breakdown voltage will drop, and eventually lead to electrical breakdown under rated power frequency voltage, resulting in cable grounding fault. This kind of fault can be detected by "low voltage loop method"; Using "electric hammer method" detection, the effect is also better.