Power cable fault classification

Underground power cable faults are complex and changeable, and the classification of causes of power cable faults can be roughly classified into the following categories.

Mechanical damage

Cable faults caused by mechanical damage account for a large proportion of cable accidents. Some of the mechanical damage was very slight. At that time, it did not cause a malfunction. It took months or even years for the damage to develop into a failure. The main causes of mechanical damage to the cable are:

(1) Damaged during installation. The cable was accidentally bumped during installation; the mechanical traction force was too large to pull the cable; excessive bending caused the cable to be broken.

(2) Directly damaged by external forces. Civil engineering construction on or near the installed cable path will cause the cable to be directly damaged by external forces.

(3) The vibration or impact load of the moving vehicle can also lead to cracking of the lead (aluminum) package of the underground cable.

(4) Damage due to natural phenomena. For example, the inner insulation of the middle connector or the terminal head expands to burst the shell or the cable jacket; the outer sheath of the cable installed on the nozzle or the bracket is scratched; the excessive tension caused by the land settling breaks the middle joint or the conductor.

2. Insulation and moisture

When the insulation is wet, it will cause the breakdown of the cable and cause failure. The main reasons for cable moisture are:

(1) Intruded water due to unsealed or improperly installed joint box or terminal box.

(2) The cable is poorly manufactured and the metal sheath has holes or cracks.

(3) The metal sheath is perforated due to foreign objects being stabbed or corroded.

3. Insulation aging deterioration

Insulation aging can cause the cable to drop in pressure and cause a failure. The main causes of cable aging are:

(1) The slag or air gap inside the cable medium is released and hydrolyzed by the electric field.

(2) The cable is overloaded or the cable communicates badly, causing local overheating.

(3) The loss of insulation from oil-impregnated paper-insulated cables.

(4) Power cable overtime limit is used.

4. Overvoltage

Overvoltage can cause electrical breakdown of the defective cable insulation, causing cable failure. The main reasons are: atmospheric over-voltage (such as lightning); internal over-voltage (such as operating over-voltage).

5. Bad design and manufacturing process

Poor cable heads and intermediate designs and manufacturing processes can also cause cable failures. The main reasons are: the design of the electric field distribution is not careful; the material is improperly selected; the technology is not good and is not produced according to the specification.

The nature and classification of cable faults

1. Classification by fault material characteristics

Can be divided into series faults, parallel faults and compound faults.

(1) Series fault

Tandem faults (metal material defects) are faults in which one or more conductors of the cable (including lead, aluminum sheath) are disconnected. It is a generalized open circuit fault. Due to the destruction of the continuity of the core, a broken wire or incomplete disconnection was formed. Incomplete disconnection is especially difficult to detect. Series faults can be divided into: one point breaking, multiple breaking, one phase breaking, and polyphase breaking.

(2) Parallel fault

Parallel faults (insulation material defects) are short-circuit faults that occur when the insulation level between the guide body and the outer skin or conductor falls, and cannot withstand the normal operation voltage. It is a generalized cable short circuit fault. This type of fault causes short circuit, grounding, flashover breakdown, etc. due to the insulation damage between the cores or between the outer cores of the cable cores. This phenomenon occurs frequently at the site. Parallel faults can be divided into: one phase grounding, two phase grounding, two-phase short circuit, and three-phase short circuit.

(3) Compound failure

Composite faults (insufficient in insulation materials and metal materials) mean that the insulation between the cable core and the cable core has failed. It includes one phase disconnected and grounded, two phase disconnected and grounded, two phases shorted and grounded.

2. Classification by fault point insulation characteristics

According to the insulation fault Rf of the cable and the breakdown gap G, the cable fault can be divided into four categories: open fault, low resistance fault, high resistance fault, and flashover fault. This classification method is the most basic classification method for field cable faults, which is particularly advantageous for the selection of detection methods.

Among them, the size of the gap breakdown voltage UG depends on the distance G of the fault point discharge channel (ie, the breakdown gap). The size of the insulation resistance Rf depends on the degree of carbonization of the cable at the fault point. The size of the 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 destroyed and a broken wire is formed, and the insulating material at the fault point is also damaged to varying degrees. Measured in-situ with a megohmmeter, the insulation resistance Rf is infinite (∞), but in the DC withstand voltage test, there will be electrical breakdown; check the conduction of the core wire, there is a breakpoint. The scene generally appears in the form of one-phase or two-phase disconnection and grounding.

(2) Low resistance fault

The cable insulation is damaged and a ground fault occurs. Measured at the scene with a megohmmeter, its insulation resistance Rf is less than 10Z0 (Z0 is the wave impedance of the cable, generally between 10 and 40Ω). The general low-voltage power cables and control cables in the site have a high probability of low resistance faults.

(3) High resistance fault

The cable insulation is damaged and a ground fault occurs. Measured in-situ with a megohmmeter, the insulation resistance Rf is greater than 10Z0. Electrical breakdown occurs during the DC high-voltage pulse test. A high-impedance fault is the highest probability of a cable failure in a high-voltage power cable (6KV or 10KV power cable), which is more than 80% of the total fault.

In the field measurement, the author generally takes Rf = 3KΩ as the dividing line between high resistance and low resistance faults. Because Rf =3KΩ, the measurement current of 10-50mA necessary for accurate measurement of the return line bridge can be obtained.

(4) Flashover fault

Cable insulation material is damaged and flashover failure occurs. Measured in-situ with a megohmmeter, the insulation resistance Rf is infinite (∞), but flashover electrical breakdown occurs during DC withstand voltage or high voltage pulse tests. Flashover faults are more difficult to measure, especially when newly installed cables are subjected to flashover faults in preventive tests. The spot is generally detected using a direct current flash method.

3. Classification based on fault triggering reason and fault point characteristics

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

(1) Shooting failure

In the industrial and mining enterprises, due to various reasons, the power cables in operation have serious damage to the insulation and cause tripping accidents. Called a cable shot. The characteristics of this type of fault are: most of the cable fault points have lead package or copper skin cracked, and the external has different degrees of deformation; the nature of the cable fault usually shows two-phase short-circuit grounding or two-phase breakage and grounding, and its grounding resistance is generally small. , Anatomical points of failure can be found in the carbonization point of arc breakdown or tree discharge carbon channels and cracks. The failure of the cable to shoot fire has obvious fault features. In most cases, the on-duty personnel can provide the approximate position of the shot. Therefore, except for a few more complex situations that need to be measured, such faults generally need only use a multimeter to determine the specific nature of the fault (single-phase grounding, short-circuit grounding, broken wire grounding, etc.), and sound detection can be used to directly set points, which is simple and straightforward.

(2) Breakdown failure

In actual work, the cable insulation destruction event triggered by the preventive test is customarily called cable breakdown. This type of fault occurs under the direct current test voltage, and its insulation breakdown is electrical breakdown. The grounding point is generally lead-free or copper skin intact, and there is no obvious external deformation (except for mechanical trauma). Breakdown faults of cables are mostly simple earth faults with high ground faults, anatomical points of failure, and no carbonization point of insulating materials. However, carbon holes and water tree aging structures can be found through the instrument. For cable breakdown faults, especially for some high-impedance grounded cable breakdown faults, the difficulty in testing is in distance measurement. Because this kind of breakdown is more concealed, the test parameter is complicated and changeable, lack regularity, so can discover the fault point of the cable quickly, the distance is the key. "High-voltage loop method" and "electric hammer method" all have the most effective method for detecting such faults.

(3) Operation failure

It refers to the operation of the plant's power system, the cable leads of the cable feeders, motors, and transformers. The high-voltage secondary circuit is subject to voltage fluctuations or the discovery of a ground signal (earth-protected power components have a ground trip), eliminating other electrical component failures. The possibility of determining the cable fault. The biggest feature of this kind of failure is not clear. The extreme form of cable operation failure is the cable blasting (such as phase-to-phase short circuit caused by two-point grounding); the other part of the operating faults, when doing stop-point inspection, develops into cable breakdown fault (such as cable aging and insulation defect) due to pressure resistance. Etc.) Some of the cable operation failures are due to improper installation of cable outlets (such as insufficient cable spacing or distance to the ground, dirty cable heads, or flooding of the motor base, etc.). What is not clear is the faulty operation of cables that are momentarily grounded and cause unstable flashovers. After this kind of fault, after the power failure of the cable, a considerable part of the insulation resistance measurement and DC withstand voltage test can pass. After the cable is put into the system, it can also run normally for a period of time; the rest is the single-phase ground cable fault, which occupies the cable. With 40% of the operational faults, this type of ground fault is generally not significantly deformed externally and the ground resistance is not too high (usually tens to hundreds of Euro). Anatomical points of failure have subtle carbonization points.

There are two causes of ground faults in cable operation: First, due to the long running time of the cable, the natural aging of the insulating layer occurs; Second, the cable is quickly destroyed in the corrosive environment, and the corrosive gas invades the insulating layer to deteriorate. . In spite of deterioration or deterioration of the cable insulation layer, the breakdown voltage will drop, resulting in electrical breakdown at the rated power frequency voltage, resulting in a cable ground fault. Such failures can be detected by "low-voltage loop method"; detection by "ring hammer method" is also effective.