analysis! Power cable failure and pre-control measures

Analysis of the cause of insulation failure

1, insulation fault

The insulation aging of cables mainly occurs in the later stage of operation, and generally occurs in cable lines running for 15 years or more, resulting in a significant increase in cable failure rate. Insulation aging is mainly divided into dendritic aging, electric aging and insulation aging. The air gap inside the cable insulation medium is free under the action of the electric field to reduce the insulation. When the insulating medium is ionized, chemical substances such as ozone and nitrate drum are generated in the air gap, and the insulating layer is corroded, and the moisture in the insulation causes the insulating fiber to be decomposed, resulting in decomposition. The insulation strength is reduced.

Overheating accelerates the deterioration of insulation aging. The electrical detachment generated by the internal air gap of the cable insulation causes local overheating, which causes the insulating material to carbonize, causing the insulation strength to decrease. Cable overload is an important factor in cable overheating. Cables and cable paths installed in cable-contaminated areas, cable trenches, and cable tunnels that are poorly ventilated in parallel or intersect with heat pipes without effective insulation measures can overheat the cables and accelerate the damage of the insulation layer.

Cable insulation runs for a long time under the action of electricity and heat, and its physical properties will change, resulting in a decrease in dielectric strength or an increase in dielectric loss, which eventually causes insulation breakdown and aging to fail. The main causes of insulation aging are:

(1) improper cable selection, resulting in long-term operation of the cable under overvoltage;

(2) The cable line is close to the heat source, so that the cable or the entire cable line is heated for a long time and prematurely aged;

(3) The cable works prematurely in an environment with an adverse chemical reaction with insulation;

(4) When multiple cables are running in parallel, one or several of them are in poor contact, causing other cables to run in parallel with their parallel cables;

(5) When the cable accessory is manufactured, the cable connection tube is not firmly crimped, resulting in an increase in contact resistance and overheating.

2, the attachment problem

The cable intermediate joint and the terminal head are usually completed on site by the installer at the laying site, and it is prone to leakage when not paying attention. Cable accessory failures account for the main part of cable line faults, and their macroscopic performance is mainly due to composite interface discharge and accessory material aging. The failure of the cable accessories is often caused by the incomplete production process and the irritability of the personnel. During the production process, defects such as bubbles, moisture and impurities appear inside the accessories, causing partial discharge and insulation breakdown, which is mainly reflected in:

(1) The quality of the cable intermediate joint and terminal head is not high.

(a) When the outer semi-conductive layer is peeled off, there are half particles, dust and other impurities on the underlying insulation or insulating surface, or the semi-conductive layer has a short removal distance, and the creepage distance is not enough. After the test or put into operation, the impurities are strong. The electric field is free under the action of an electric field, and an electric branch is generated.

(b) During the manufacturing process, the quality of the crimping of the metal connecting pipe is poor, the joint contact resistance is too large and the heat is generated, or the heat shrinkage is excessive, which causes the insulating carbonization, thereby causing the insulating layer to aging and breakdown, resulting in cable grounding or phase-to-phase short-circuit failure. It is possible to injure other cables nearby.

(c) The cable joint process is not standard, and the seal is not standardized, so that the inside of the insulation is intruded by moisture and moisture, causing the insulation of the intermediate joint to be deteriorated by moisture. In severe cases, large areas of the main insulation of the cable enter the water, resulting in a decrease in the overall insulation of the main insulation, and eventually a cable breakdown failure.

(d) The conductor connection tube has a poor handling process. The crimping die of the connecting tube of the conductor is unreasonable, and the edge and corner are not smoothed, especially at the edge of the crimping die, and there are sharp corners, burrs and protrusions locally, which may easily cause uneven electric field in the part, and partial discharge occurs during operation to make insulation. Aging, insulation performance is reduced, and breakdown failure occurs.

(e) The installation size is wrong. The installation position of the stress tube is too low or the stress cone is not effectively overlapped with the break of the semi-channel layer, resulting in no reliable evacuation of the semi-conductive fracture part of the cable. In the test or long-term operation, the fracture part is seriously charged. Halo discharge, causing overheating to reduce insulation, eventually leading to breakdown.

(f) The grounding wire connection of the cable metal shield is unreliable and does not meet the grounding resistance requirements, resulting in excessive grounding resistance. When the cable is subjected to an overvoltage, the metal shield layer will generate a high induced overvoltage, which will cause aging breakdown of the insulating portion.

(2) During the operation of the cable, the cable expands and contracts due to changes in load and environmental factors. In particular, the heat-shrinkable accessory cannot lose its sealing effect with elastic deformation, and forms a breathing effect between the accessory and the cable insulation layer. The moisture and moisture in the atmosphere are brought into the accessory, causing a short circuit failure inside the cable accessory. The shrinkage attachment is not of high quality, the shrinkage force is reduced, or there is a defect in the seal that requires a reliable seal, which will cause external moisture to invade, eventually leading to cable failure.

(3) Due to the environment moisture and humidity, the cable head was not subjected to reliable dehumidification and moisture-proof measures. The cable insulation was partially damp and the insulation performance was degraded. It developed into a penetrating passage during operation, resulting in cable breakdown accident.

3, the outer layer problem

In the medium and high voltage power grids, cables are being used more and more widely. The outer sheath of the power cable is the first line of defense for protecting the cable. Its integrity is directly related to the safety of the internal structure and the service life of the cable. There are three main reasons for cable sheath failure:

(1) Damage to the hard object around the cable or damage to the external force. The buried cable has a hard object tip and a direct contact with the outer sheath. Especially in the vehicle passing section, the road surface vibrates for a long time, and the hard object corner may pierce the outer sheath, resulting in damage to the internal structure, plus cable load. Changes, the cable itself is inflated and contracted, and the electric field of the damaged part is unevenly distributed, which eventually leads to damage to the insulation layer; when the pipe is laid, the step of the pipe connection or the inner wall is not smooth, the outer sheath may be damaged; around the cable path Mechanical construction or pipe jacking work, resulting in damage to the outer sheath.

(2) Defects and hidden dangers left during construction. During the cable laying process, the outer sheath is strained and cracked in the pipe, and the personnel cannot find it in time; after the cable bending part of 110kV and above is running for a period of time, cracking occurs, the outer sheath insulation is reduced, and the metal sheath is grounded at multiple points. The circulation increases, eventually leading to thermal breakdown of the insulation.

(3) Termite erosion. Once a termite erosion site is found, there should be multiple sites on the cable line, and we should pay enough attention. The damage of termites is relatively rare, and the northern power grid is relatively rare.

Take measures to prevent cable failure

According to the analysis of the causes of insulation, accessories and outer sheath failure, the cable line engineering is a system engineering. Only the design, construction, operation and maintenance, etc. can be managed in the whole process to ensure its safe operation.

(1) From the beginning of the design, fully understand the grounding system used for the cable, and select the cable that meets its voltage level to avoid the cable working under long-term overvoltage conditions. The selection of the outer sheath should meet the requirements of the use environment and service life. At the same time, the selection of the cable sheath protector should meet the principle that the protector reliably passes the ground current without damage when the grounding is relatively grounded.

(2) Cable path selection should avoid the external environment such as overheating, corrosion, external damage, etc., and also avoid excessive concentration of cables, causing heat to not spread in time, and internal factors that cause overheating. In addition, the double-loop power supply cable path is not recommended to be laid in the same path of the pipeline to prevent simultaneous damage, resulting in large-scale power outages.

(3) Strengthen the selection of cable and cable accessories, factory supervision, and acceptance inspection to ensure the quality of cables and cable accessories. At the time of on-site inspection, the manufacturer, the construction party, the supervision party and the project management department shall be present at the scene, in accordance with the packing list, and shall promptly record the problems found and propose rectification suggestions, which shall be signed and approved by many parties. For parts that are easily damp, after the inspection is completed, it should be sealed in time to prevent the normal use of moisture.

(4) Strengthen the personnel training work, carry out the necessary business qualifications and technical assessments for the cable head production personnel, and hold the certificates. If there are two consecutive failures due to production reasons during the warranty period, the qualification of the cable head production shall be cancelled, and the re-training shall be resumed before being re-employed.

(5) Strengthen the concealed works and intermediate links of all aspects of cable engineering, strictly check the quality, and thoroughly correct the defects and hidden dangers found in the acceptance of civil engineering and electrical engineering, and make various records, and leave photos if necessary. , film and other information.

(6) Using on-line monitoring technology such as external sheath circulation online monitoring technology, online optical fiber temperature measurement technology, online partial discharge detection technology, etc., strengthen real-time online operation monitoring of cables, and discover hidden defects in advance to avoid power failures.

Whether the cable line can operate safely is directly related to the safe operation of the entire power grid and system stability. Only by strengthening quality control and acceptance check from cable production, transportation, laying, installation, testing, inspection, inspection, etc., can cable faults be minimized to ensure long-term safe operation of cable lines.