Underground power cable faults are complex and changeable, and 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 of the mechanical damage was so minor that it didn't break down at the time, and it took months or even years for the damage to develop into a failure.The main reasons for mechanical damage of cables are as follows:

(1) damage during installation.Accidentally bump the cable during installation;Too much mechanical traction to pull the cable;Excessive bending hurts cables.

(2) direct external damage.Civil construction shall be 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 damage of underground cables.

(4) damage caused by natural phenomena.If the inner insulating adhesive of the middle joint or terminal head expands and cracks the shell or cable sheath;Abrasion of cable skin mounted on pipe mouth or bracket;The intermediate joint or conductor is broken due to excessive tension caused by land subsidence.

Damp insulation

Damp insulation will cause cable voltage drop and cause failure.The main causes of cable moisture are:

(1) water inlet is caused by unsealed structure or poor installation of joint box or terminal box.

(2) poor cable manufacturing, metal sheath with holes or cracks.

(3) the metal sheath is punctured or corroded by external objects.

3. Aging and deterioration of insulation

Aging insulation will cause cable voltage drop and failure.The main reasons for cable aging are:

(1) the slag or air gap inside the cable medium will generate dissociation and hydrolysis under the action of electric field.

(2) the cable is overloaded or poorly ventilated in the cable trench, resulting in local overheating.

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

(4) over time use of power cables.

4. The overvoltage

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

5. Poor design and production technology

Poor cable head and intermediate design and manufacturing technology can also cause cable faults.The main reasons are: the electric field distribution design is not thorough;Improper material selection;Poor workmanship, not according to the requirements of the production procedures.

The nature and classification of cable faults

1. Classification by fault material characteristics

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

(1) series fault

Series fault (metal material defect) refers to a fault in which one or more conductors (including lead and aluminum) of a cable are disconnected.It is a generalized cable open circuit fault.The continuity of the cable core is broken, resulting in a break or incomplete break.Incomplete breaks are especially difficult to detect.The series faults can be divided into one-point open fault, multi-point open fault, one-phase open fault and multi-phase open fault.

(2) parallel fault

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

(3) composite faults

Composite faults (defects in insulation materials and metal materials) refer to the failures in 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 cable fault point insulation resistance Rf and breakdown gap G, 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 cable faults in the field, especially for the selection of detection methods.

Among them, the size of the gap breakdown voltage UG depends on the distance G of the discharge channel at the fault point (that is, the breakdown gap), the size of the insulation resistance Rf depends on the degree of carbonization of the cable medium at the fault point, and the size of the distributed capacitor Cf depends on the degree of moisture exposure 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.Field megohm meter is used to measure its insulation resistance Rf is infinite (infinity), but in dc withstand voltage test, electric breakdown will occur;Check the core wire conduction, there is a break point.The site is usually in the form of one - or two-phase disconnection and grounding.

(2) low resistance fault

Cable insulation material is damaged and ground fault occurs.Field by measuring the insulation resistance by megger are Rf less than 10 Z0 (Z0 for the wave impedance of the cable, generally take between 10 ~ 40 Ω).Generally, low-voltage power cables and control cables in the field have a high probability of low resistance faults.

(3) high resistance fault

Cable insulation material is damaged and ground fault occurs.Field megohm meter is used to measure its insulation resistance Rf is greater than 10Z0, and electric breakdown will occur in dc high-voltage pulse test.High resistance fault is the cable fault with the highest probability of high voltage power cable (6KV or 10KV power cable), up to more than 80% of the total fault.

On a field measurement, the author take Ω Rf = 3 k for high resistance and low resistance fault line.Because when Ω Rf = 3 k, just get back to the line method bridge necessary for accurate measurement of 10 ~ 50 ma measuring current.

(4) flashover fault

Cable insulation material is damaged and flashover fault occurs.Field megohm meter is used to measure its insulation resistance Rf is infinite (infinity), but flashover electric breakdown will occur in dc withstand voltage or high voltage pulse test.Flashover fault is difficult to detect, especially when flashover fault occurs in preventive test of newly installed cable.Dc flashover method is generally used for detection in the field.

3. Classification of fault trigger causes and characteristics of fault points

According to the power cable in operation or preventive test, the cable, cable head and intermediate box have different characteristics of insulation damage, but also can be divided into blasting fault, breakdown fault and operation fault three.

(1) shooting fault

In industrial and mining enterprises, power cables in operation, due to a variety of reasons, insulation has been seriously damaged, resulting in tripping accidents.This is called cable firing.The characteristics of this kind of fault are: most cable fault points have lead or copper rupture, external deformation of varying degrees;The characteristics of cable faults are usually two-phase short-circuit grounding or two-phase disconnection and grounding, and the grounding resistance is generally small. The fault points can be dissected, and carbonized points or tree-shaped discharge carbon channels and cracks of arc breakdown can be found.Cable firing fault, its fault characteristics are obvious, in most cases, the operation on duty can provide the approximate location of firing.Therefore, this kind of fault in addition to a few more complex cases need to be ranging, 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 point sound measurement method, simple and clear.

(2) breakdown failure

In actual work, the cable insulation damage event triggered by preventive test is commonly called cable breakdown.This kind of fault occurs under dc experimental voltage, and its insulation damage is electric breakdown. Generally, the connection point is in good condition with lead or copper sheath, and no obvious external deformation (except mechanical trauma).The breakdown faults of cables are mostly simple grounding faults with high grounding faults, anatomical fault points, no carbonization points of insulation materials, but the aging structure of carbon holes and water branches can be found through the instrument.For cable breakdown fault, especially some high resistance grounding cable breakdown fault, the test difficulty lies in ranging.Because this kind of fault is relatively hidden, the test parameters are complex and variable, the lack of regularity, so whether the cable fault can be found quickly, ranging is the key.The high-voltage loop method and the electric hammer method have the most effective methods to detect such faults.

(3) operation failure

It refers to the power system in the factory in the operation, cable feeder, motor, transformer cable lead, its high voltage secondary circuit voltage fluctuations or found grounding signal (grounding trip of power components with grounding protection), excluding the possibility of other power components fault and cable fault.The great characteristic of this kind of fault is that it is not clear.The extreme form of cable operation fault is the cable blasting (such as the interphase short circuit caused by two-point grounding);Another part of the operation fault in the stop-point inspection, due to the pressure can not pass and develop 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 cable lead out line installation position is improper (such as cable phase or insufficient distance to the ground, cable head dirt or motor foundation water, etc.), these faults can be mainly some simple treatment;What is less clear are the transient grounding and unstable flashover of the cable operating failures.After the power failure of the cable, a considerable part of insulation resistance measurement and dc voltage withstand test can be passed. After the cable is put into the system, it can run normally for a period of time.The rest is the single-phase grounding cable fault, which accounts for about 40% of the cable operating fault, such a grounding fault generally has no obvious external deformation, grounding resistance is not too high (generally dozens to hundreds of euros).Anatomical failure points have fine carbonization points.

There are two reasons for grounding fault of cable operation: first, due to the long running time of cable, the insulation layer appears natural aging;Secondly, in the corrosive environment, the cable sheath is rapidly damaged, and corrosive gases invade the insulation layer to degrade it.No matter the cable insulation layer appears aging or deterioration, its breakdown voltage will decline, and eventually lead to the rated power frequency voltage of electric breakdown, resulting in cable grounding fault.Such faults can be detected by "low-voltage loop method";With "electric hammer method" detection, the effect is also better.