Power cable fault types and diagnostics

Large and complex cable networks often fail due to many factors such as production quality, improper construction and poor operation and maintenance, and it is very important
to diagnose faults in time and solve problems.

The production, laying, three-head process, accessory materials, operating conditions, etc.
of power cables are closely
related to the normal operation of cables.
Among these links, the omission of a certain link will bury the hidden danger
of cable failure.
In terms of the causes and characteristics of cable failure, the following points
are mainly summarized.

1.
1 Mechanical damage type

Mechanical damage faults are the most common, accounting for the largest proportion of failures (about 57%)
.
The form of mechanical damage failure is also relatively easy to identify, and it is also easy to cause power outage accidents
.

The causes of mechanical damage are: 1) Direct damage
by external forces.
For example, pipeline construction, transportation, on-site collisions, etc.
, will accidentally damage the cable
.
2) Construction damage
.
For example, the mechanical traction force is too large and the cable is strained, and the insulation layer or shielding layer is damaged by excessive bending (such as the cable peeling size is too large, the knife mark is too deep, and other damage).

3) Natural damage
.
The insulating compound of the middle head or end head expands to the point of swelling the shell or nearby cable sheath.

Excessive tension can also easily pull off the middle joint or cable body (damage to the cable due to frequent vibration of large equipment or vehicles, etc.
).

1.
2 Cable moisture type

Insulation moisture is also a major factor in cable failure, accounting for about 13%
of failures.
Insulation moisture can be detected by insulation resistance and DC withstand voltage test, which is mainly manifested as the decrease of insulation resistance and the increase
of leakage current.
The main causes of insulation moisture are as follows: 1) Poor sealing process or sealing failure of the cable intermediate head or terminal head
.
2) The cable is poorly manufactured, and the outer sheath of the cable has holes or cracks
.
3) The cable sheath is pierced by foreign objects or perforated
by corrosion.

1.
3 Overvoltage breakdown type

In theory, the power cable will not be damaged by lightning strikes or other impact overvoltage, and the cable line is easy to be excited by overvoltage when there is one of the following serious defects, resulting in cable insulation breakdown
.
The main ones are: 1) the insulation layer contains bubbles, impurities or dry insulating oil; 2) There are knots or omissions on the inner shield of the cable; 3) The cable insulation has been seriously aged
.

1.
4 Overheating damage type

The main reasons for cable overheating damage are: 1) long-term overload work of the cable; 2) burns from fire or adjacent cable failure; 3) Close to other heat sources, long-term reception of heat radiation
.

1.
5 insulation aging type

Cable insulation operates under the action of electricity and heat for a long time, and its physical properties will change, resulting in an increase in its dielectric strength or dielectric loss, so that the insulation collapses as the aging
of the insulation.
The failure rate is about 19%.

The main reasons for premature aging of insulation are: 1) improper cable selection, resulting in long-term cable work under overvoltage; 2) The surrounding area of the cable line is close to the heat source, so that the local or entire cable line of the cable is heated for a long time and premature aging; 3) The cable works in an environment with adverse chemical reactions with cable insulation and premature aging
.

1.
6 Quality defect type

Cables and cable accessories are two important materials indispensable in cable routes, and their quality directly affects the safe operation
of cable routes.
Due to the lack of necessary professional and technical training of some constructors, there are large quality problems
in the production of the three ends of the cable.

Product quality defects can be summarized into the following aspects: 1) Cable body quality defects
.
There are impurities, sand particles, mechanical damage and seams in the lead sheath of oil-paper cable; The main insulation layer of rubber and plastic insulated cable is eccentric, contains bubbles, impurities, and the cable is not terminated during storage and transportation, resulting in a large amount of water ingress of the core
.
These defects are generally not easy to find, often in the maintenance or test only found that its insulation resistance is low, leakage current is large, and even the withstand voltage test breakdown
.
2) Cable accessories quality defects
.
The quality defects of the three ends of heat shrinkable and cold shrinkable cables are: bubbles, impurities or uneven thickness in the insulated pipe, and omissions in the sealing glue
.
3) Three-head production quality defects
.
The heat shrinkable tube shrinks unevenly, and the ground wire is not firmly installed
.
The quality defects of the three-end installation of prefabricated cables mainly include inaccurate stripping and cutting dimensions, and too large residual stress when insulating parts are set
.
4) There are also some cases of dismantling old cables and accessories in cable routes, although it is beneficial
in using materials and saving money.
However, it has a great
impact on equipment uptime.

1.
7 Poorly designed type

The structure and type of power cable have been basically stable, but the various cable accessories of the cable intermediate head and terminal head have been continuously improved
.
These new cable accessories often do not have enough operating experience
in new equipment, new materials, and new processes.

1 The main disadvantages of poor design are: 1) waterproof is not strict; 2) The selection of materials is inappropriate; 3) Insufficient mechanical strength
.

2.
Common faults of cable lines

According to the nature of cable faults, common cable faults mainly include short circuit (grounding) type, broken wire type, flashover type, composite type
.

3.
Methods and steps of line fault diagnosis

In the long-term coal production practice, people have also explored and summarized many test methods
for cable failures.
This "classic method" has certain limitations, that is, only low-impedance faults can be tested, in fact, most of the cable faults are high-impedance faults
.
When encountering high-resistance faults, it must go through a time-consuming and laborious "burn-through" resistance reduction process, and the fault resistance of new insulating material cables is extremely difficult to burn through and reduce resistance, so the "classical method" can not meet the requirements
of various types of cable fault tests.

The current pulse reflection test technology can directly measure the ranging of high-resistance faults without going through the "burn-through" resistance reduction process
.
It includes low-voltage pulse method, pulse voltage method and pulse current method, especially suitable for various types of cable fault testing, and its applicability and accuracy are becoming more mature and improved
.
Compared with the "classic method", it has many advantages
such as simple testing, high test efficiency, more accurate testing, wide range of application, and suitable for development.

The diagnosis of power cable line faults must be carried out in accordance with certain procedures and steps, and cannot be blindly tried
.
First of all, the diagnostic object
should be determined according to the cause and characteristics of the cable fault.
Once the diagnosis is determined, the fault is easy to eliminate
.
At the same time, the types and characteristics of some common faults should also be mastered in order to facilitate rapid diagnosis and elimination
.