The production, laying, three-end manufacturing process, accessory materials, and operating conditions of power cables are closely related to the operation of the cables. Any omission in the above links will become a hidden danger of cable failure. The causes and characteristics of cable failure are as follows. 1. Mechanical damage Mechanical damage failures are relatively common, accounting for the largest failure rate (about 57%). The failure form is relatively easy to identify, and most of them cause power outages. The general causes of mechanical damage are as follows: (1) Direct damage by external forces: such as accidental damage to cables during urban construction, transportation, underground pipeline construction, piling, lifting, and transportation. (2) Construction damage: such as excessive traction, excessive bending, rough construction at ultra-low temperatures, cable stripping damage, etc. (3) Natural damage: such as the cable sheath at the cable mouth and bracket being scratched due to free travel; damage caused by land subsidence, landslides, etc.; cable damage caused by severe weather; damage caused by frequent vibration of the cable line, etc. 2. Insulation dampness Insulation dampness is another major factor in cable failure, accounting for about 13% of the failure rate. When the insulation is damp, it can be found in the insulation resistance and DC withstand voltage test, which manifests as reduced insulation resistance and increased leakage current. The following are the general reasons for insulation dampness:
(1) The sealing process of the cable middle head or terminal head is poor or the sealing fails.
(2) The cable is poorly manufactured, and the cable outer sheath has holes or cracks.
(3) The cable sheath is pierced by foreign objects or corroded.
3. Insulation aging
When the cable insulation is operated under the action of electricity and heat for a long time, its physical properties will change, resulting in reduced insulation strength or increased dielectric loss, which eventually causes insulation collapse. This is insulation aging, and the insulation aging failure rate is about 19%. The main reasons for premature insulation aging are:
(1) Improper cable selection, causing the cable to work under overvoltage for a long time.
(2) The cable line is close to a heat source, causing the cable to be heated for a long time and prematurely aged.
(3) The cable ages prematurely when operating in an environment that may cause adverse chemical reactions with the cable insulation. 4. Overvoltage Generally speaking, power cables should not be damaged by lightning strikes or other impulse overvoltages. This is because the electrical stress that the cable insulation withstands under normal operating voltage is about 1/10 of the electrical stress that a new cable can withstand during a breakdown test. Therefore, under normal circumstances, 3 to 4 times the atmospheric overvoltage or operating overvoltage will not have much impact on cables with good insulation. However, in fact, it is not uncommon for cable lines to be broken down when struck by lightning. From the dissection and analysis of the actual fault objects on site, it can be confirmed that these breakdown points often have some serious defects. Lightning strikes only stimulate the defects earlier. The defects that are easily stimulated by overvoltage and cause cable insulation breakdown are mainly: (1) The insulation layer contains bubbles, impurities or the insulating oil is dry. (2) There are knots or leaks on the inner shielding layer of the cable. (3) The cable insulation has aged seriously. 5. Overheating There are many factors that may cause cable overheating. According to the statistical analysis of the operation conditions in various places in recent years, the main reasons are as follows: (1) The cable is overloaded for a long time. This mainly includes inadequate cross-section selection and the long-term allowable current carrying capacity not being adjusted according to the actual operating conditions. (2) Burns caused by fire or adjacent cable failure. (3) Proximity to other heat sources and long-term exposure to heat radiation. Overload is an important cause of cable overheating. After long-term overheating of rubber-plastic insulated cables, the insulation material undergoes physical changes such as hardening, discoloration, loss of elasticity, and cracking; after long-term overheating of oil-paper cables, the insulation dries up, the insulation becomes coked, and even breaks when touched. 6. Product quality defects Cables and cable accessories are two important materials that are indispensable in cable lines. Their quality directly affects the safe operation of cable lines. In addition, due to the lack of necessary professional and technical training in some construction units, there are major quality problems in the production quality of cable three-way connectors. These product quality defects can be summarized as follows:
(1) Cable body quality defects: impurities and sand particles, mechanical damage and seams in the lead sheath of oil-paper cables; rubber-plastic insulated cables with eccentric cores, bubbles and impurities in the main insulation layer, knots and omissions in the inner semi-conductive layer, and water ingress in the core due to failure to seal the cable ends during storage and transportation.
(2) Cable accessories quality defects: traditional three-head quality defects include: sand holes in cast iron parts, insufficient strength of porcelain parts, rough processing of assembly parts, non-conformity or aging of waterproof rubber rings; heat shrink and cold shrink cable three-head quality defects include: bubbles, impurities or uneven thickness in the insulation tube, omissions in the sealing glue coating, etc.
(3) Quality defects of three-head production: The main quality defects of traditional three-head production are: the insulation layer is not tightly wrapped (large gaps), unclean, not sealed, and the insulation glue ratio is not equal; the main quality defects of heat shrink three-head production are: the semi-conductive layer is not clean, the stress tube is not installed in the right position, the heat shrink tube shrinks unevenly, the ground wire is not installed firmly, etc.; the main quality defects of prefabricated cable three-head installation are: the stripping size is not accurate, the residual stress is too large when the insulation parts are installed, etc.
7. Poor design
The structure and form of power cables have been basically stable to this day, but the various cable accessories of the cable middle head and terminal head have been continuously improved. These new cable accessories often do not have sufficient operating experience in new equipment, new materials, and new processes. Therefore, they should be selected with great caution. It is best to promote their use gradually based on the maturity of their operating experience to avoid large-scale quality accidents. The main disadvantages of poor design are:
(1) Waterproofing is not tight.
(2) Improper material selection.
(3) Unreasonable process procedures.
(4) Insufficient mechanical strength.
