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Cause classification of power cable faults

The faults of underground power cable are complicated and changeable, and the causes of power cable faults can be classified into the following categories.


1. Mechanical damage


The cable fault caused by mechanical damage accounted for a large proportion of cable accidents. Some of the mechanical damage was minor and did not cause failure at the time, and it took months or even years for the damage to develop. The main causes of cable mechanical damage are:

(1) damage during installation. The installation of accidentally hit the cable, mechanical traction too large cable;

(2) Direct acceptance of external damage. After the installation of the cable path or near the civil construction, so that the cable directly by external damage.

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

(4) Damage caused by natural phenomena. If the inner insulating glue of the middle joint or the terminal head expands and the casing or cable sheath is swollen, the sheath of the cable which is installed on the pipe mouth or the bracket is bruised, and the middle joint or conductor is pulled off due to the land subsidence.


2. Insulation damp


The insulation will cause the cable to withstand the pressure drop and cause trouble. The main reasons for the damp of cables are:

(1) Because the connector box or terminal box structure is not sealed or poor installation leads to water intake.

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

(3) The metal sheath is stabbed by the external material or the corrosion perforation.


3. Insulation Aging and deterioration


The insulation aging will cause the cable to withstand the pressure drop and produce the malfunction. The main reasons for cable aging are:

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

(2) Cable over load or cable communication bad wind, resulting in local overheating.

(3) Oil-impregnated paper insulation cable insulation loss.

(4) Power cable exceeding the time limit use.


4. Overvoltage

Overvoltage will cause the defective cable insulation layer to be electric shock wear, causing cable failure. The main reasons are: the atmospheric overvoltage (such as Lightning), internal overvoltage (such as operating overvoltage).


5. Poor design and production process

The cable head and the intermediate design and production process is poor, also can cause cable failure. The main reasons are: poorly designed electric field distribution, improper selection of materials, poor workmanship and not in accordance with the specifications.


The nature and classification of cable faults


1. Classification of fault material features


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

(1) Series fault

In-line failure (defect of metal material) is the failure of one or more conductors (including lead and aluminum skins) to disconnect from the cable. It is a generalized cable open-circuit fault. Because the continuity of the cable core is broken, the formation of disconnection or incomplete disconnection. Incomplete disconnection is especially difficult to find. The series fault can be divided into: a little break, multiple points open, one phase disconnection, multiphase disconnection and so on.

(2) Parallel fault

Parallel fault (insulation material defect) is the guide body external skin or conductor of the insulation level down, can not withstand the normal operating voltage and the short-circuit fault occurred. It is a generalized cable short circuit fault. This type of failure due to cable core or cable core between the external skin insulation damage and the formation of short circuit, grounding, flashover and other phenomena, the occurrence of high frequency at the scene. Parallel fault can be divided into: one-phase, two-phase, two-phase short-circuit, three-phase short-circuit.

(3) Composite failure

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


2. The breakdown point insulation characteristic classification


According to the cable fault point insulation resistance RF and breakdown gap G, the cable fault can be divided into open circuit fault, low resistance fault, high resistance fault, flashover fault four categories. This classification is the most basic classification method for cable faults in the field, especially for the selection of detection methods.

Among them, the gap breakdown voltage ug size depends on the fault point discharge channel (that is, breakdown gap) distance g, insulation resistance RF size depends on the fault point cable media carbonation degree, the size of the distribution capacitance Cf depends on the point of damp degree of failure.

(1) Open circuit failure

The continuity of the metal part of the cable is damaged, the disconnection is formed, and the insulating material of the fault point is damaged by different degrees. The field uses the trillion-European meter to measure its insulation resistance RF is infinite (∞), but in the DC withstand voltage test, will appear the electric breakdown; The scene is generally in the form of a phase or two-phase disconnection and grounding.

(2) Low resistance failure

The cable insulation material is damaged and the grounding fault occurs. The field with a trillion-European meter to measure its insulation resistance RF less than 10z0 (Z0 for cable impedance, generally take 10~40ω between). There is a high probability of low resistance failure in general Low-voltage power cable and control cable.

(3) High resistance failure

The cable insulation material is damaged and the grounding fault occurs. The field uses the trillion European meter to measure its insulation resistance RF is bigger than 10z0, in the direct current high voltage pulse test, will appear the electric breakdown. High-impedance failure is a high-voltage power cable (6KV or 10KV power cable) the highest probability of cable failure, up to 80% of the total failure.

In situ measurement, the author generally takes RF =3kω as the dividing line between high impedance and low resistance fault. Because the RF =3kω, just can get back line method bridge accurate measurement of the necessary 10~50ma measurement current.

(4) Flashover failure

The cable insulation material is damaged and flashover failure occurs. The field with a trillion-European meter to test the insulation resistance of the RF is infinite (∞), but in the DC voltage or high-voltage pulse test, there will be flashover electrical breakdown. Flashover faults are more difficult to measure, especially when a new cable is in a preventive test. The field generally uses the DC Flashover method to detect.


3. Fault triggering causes and fault point characteristics classification


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


(1) Blasting fault

In the industrial and mining enterprises, the operation of power cables, for various reasons, insulation has been seriously damaged, resulting in tripping accidents. Known as cable blasting. This type of failure is characterized by: most of the cable failure point is lead bag or copper skin rupture, there are different degrees of deformation outside; the nature of cable fault is usually characterized by two-phase short circuit grounding or two-phase disconnection and grounding, its grounding resistance is generally small, anatomical fault point, can be found in the arc breakdown of carbonation point or tree-like discharge carbon and cracks. Cable blasting fault, its fault characteristics are obvious, in most cases, the operation on duty personnel can provide the approximate location of the shooting. Therefore, this kind of fault, in addition to a few more complex circumstances need to distance, generally as long as the use of multimeter to determine the specific nature of the fault (single-phase, short circuit grounding, disconnection grounding, etc.), can be used to direct the sound measurement method fixed point, simple and clear.


(2) Breakdown failure


In practice, the failure of cable insulation triggered by preventive test is used to be called cable breakdown. This kind of fault occurred in the DC experimental voltage, the insulation damage for electric shock wear, the site generally lead bag or copper skin intact, external no obvious deformation (except mechanical trauma). Most of the cable breakdown fault is simple grounding fault, its grounding fault is higher, the anatomical fault point, the insulating material does not have the carbonization point, but through the instrument can discover the carbon pore and the water branch aging structure. It is difficult to measure the fault of cable breakdown, especially the breakdown of some high resistance grounding cable. Because this kind of fault is more concealed, the test parameter is complex and changeable, lacks the regularity, so whether the cable fault point can be discovered quickly, ranging is the key. "High Voltage back line method" and "Electric Hammer Method" have the most effective methods to detect this kind of fault.


(3) Operation failure


It refers to the factory power system in operation, cable feed line, motor, transformer cable leads, its high-voltage two times circuit voltage fluctuations or found ground signal (there is ground protection of the power components of the grounding trip), to eliminate the possibility of other power components failure to determine the cable fault. The biggest feature of this type of failure is uncertainty. The extreme form of cable operation failure is cable blasting (such as two-point grounding caused by interphase short circuit); another part of the operation failure in the stop-point inspection, due to pressure through the failure to develop into a cable breakdown (such as cable aging, insulation defects, etc.) A part of the cable operation failure is due to improper installation of cable leads (such as cable or ground distance is not enough, the cable head is dirty or the Electric Machine Foundation influent and so on), these faults mainly carries on some simple processing can; the most ambiguous is those instantaneous ground, produces the unstable flashover the cable operation breakdown. This type of failure in the cable after the blackout, insulation resistance measurement and DC pressure test has a considerable part to pass, and then put the cable into the system, can also normal operation for a period of time; the rest is the single phase grounding cable fault, which accounts for about 40% of the cable operation failure, this kind of grounding fault is not obvious outside deformation, The grounding resistance is also not too high (generally dozens of to hundreds of euro). There are microscopic points of carbonation in the anatomical fault point.

There are two reasons for the grounding fault of cable operation: firstly, because the cable is running for a long time, the insulating layer appears natural aging; Secondly, the cable in the corrosive environment, the cable sheath is rapidly destroyed, corrosive gas intrusion into the insulating layer to make it worse. Regardless of the aging or deterioration of the cable insulation, its breakdown voltage will drop, resulting in the rated power frequency voltage under the electric shock, resulting in cable grounding fault. This type of failure can be detected by the "low Voltage back line method", and the "Electric Hammer Method" is used to detect the effect.


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