1. Insulation aging of insulation fault cables mainly occurs in the later stage of operation, generally in cable lines that have operated for 15 years or more, resulting in a significant increase in cable failure rate. Insulation aging is mainly divided into dendritic aging, electrothermal aging and insulation material aging. The air gap inside the cable insulation medium is free under the action of electric field, which reduces the insulation. When the insulation medium is ionized, ozone, nitrate drum and other chemicals are generated in the air gap, which corrodes the insulation layer. At the same time, the moisture in the insulation decomposes the insulation fiber, resulting in the decline of insulation strength. Overheating will accelerate the aging and deterioration of insulation. The electric dissociation generated by the air gap inside the cable insulation will cause local overheating, carbonize the insulating material and reduce the insulation strength. Cable overload is an important factor of cable overheating. Cables installed in poorly ventilated places such as cable dense areas, cable trenches and cable tunnels, cable paths parallel or cross with thermal pipelines without effective thermal insulation measures will overheat the cables and accelerate the damage of the insulation layer. When the cable insulation operates under the action of electricity and heat for a long time, its physical properties will change, resulting in the reduction of insulation strength or the increase of dielectric loss, which will eventually lead to insulation collapse and aging and failure. The main causes of insulation aging are: (1) improper cable selection, resulting in long-term over-voltage operation of the cable; (2) The surrounding of the cable line is close to the heat source, so that part of the cable or the whole cable line is heated for a long time and ages prematurely; (3) The cable is aged prematurely when working in an environment with adverse chemical reaction with insulation; (4) When multiple cables are running in parallel, one or more of them are in poor contact, resulting in overload operation of other cables in parallel; (5) When making cable accessories, the cable connecting pipe is not firmly crimped, resulting in increased contact resistance and overheating. 2. Accessories: the cable intermediate joint and terminal head are usually completed by the installation personnel at the laying site, and mistakes are easy to occur without paying attention. Cable accessory failure accounts for the main part of cable line failure, which is mainly manifested in composite interface discharge and accessory material aging. The failure of cable accessories is often due to the poor manufacturing process and the carelessness of personnel. During the manufacturing process, bubbles, moisture, impurities and other defects appear in the accessories, resulting in partial discharge and insulation breakdown, which is mainly reflected in: (1) the manufacturing quality of cable intermediate joints and terminals is not high; (a) when stripping the outer semiconducting layer, Damage the lower insulation or there is half a track of particles, dust and other impurities on the insulation surface, or the removal distance of the semi conductive layer is short and the creepage distance is not enough. After the test or operation, the impurities are free under the action of strong electric field and produce electric branches. (b) During the manufacturing process, the crimping quality of the metal connecting pipe is poor, resulting in excessive contact resistance and heating of the joint, or excessive thermal shrinkage, resulting in insulation carbonization, aging and breakdown of the insulating layer, resulting in cable grounding or phase to phase short circuit fault, and may damage other nearby cables. (c) The cable joint process is not standard and the sealing is not standard, so that the insulation is invaded by moisture and moisture, resulting in the deterioration of the insulation of the intermediate joint. In serious cases, water enters into a large area inside the main insulation of the cable, resulting in the overall moisture of the main insulation, the insulation is reduced, and finally the cable breakdown fault occurs. (d) The treatment process of conductor connecting pipe is poor. The selection of crimping die for conductor connecting pipe is unreasonable, and the edges and corners are not polished smoothly. Especially at the edge of crimping die, there are sharp corners, burrs and protrusions, which is very easy to cause uneven electric field at this part, partial discharge during operation, aging insulation, degradation of insulation performance and breakdown fault. (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 half layer fracture, resulting in the failure of reliable stress evacuation at the semi conductive fracture of the cable. During the test or long-term operation, serious corona discharge occurs at the fracture, resulting in overheating, reducing the insulation, and finally leading to breakdown. (f) The connection of the grounding wire of the cable metal shielding layer is unreliable and does not meet the requirements of grounding resistance, resulting in excessive grounding resistance. When the cable is subjected to overvoltage, the metal shielding layer will produce high induced overvoltage, which will lead to aging and breakdown of the insulating part. (2) During operation, the cable expands and contracts due to the change of load and environmental factors, especially the heat shrinkable accessories can not lose the sealing effect with elastic deformation, forming a respiratory effect between the accessories and the cable insulation layer, bringing moisture and moisture in the atmosphere into the accessories, causing the internal short circuit fault of the cable accessories. Poor quality of cold shrinkable accessories, reduced shrinkage or defective sealing at the parts requiring reliable sealing will lead to external moisture intrusion and eventually cable failure. (3) When making the cable head, due to the high environmental moisture and humidity, reliable dehumidification and moisture drive measures were not taken, the cable insulation was partially affected by moisture, the insulation performance decreased, and developed into a penetrating channel during operation, resulting in cable breakdown accident. 3. The problem of outer sheath is more and more widely used in medium and high voltage power grid. The outer sheath of power cable is the first line of defense to protect the cable. Its integrity is directly related to the safety of internal structure and the service life of the cable. There are three main causes of cable outer sheath failure: (1) hard object damage or external force damage around the cable. There are sharp corners of hard objects at the top and bottom of the directly buried cable that directly contact the outer protective layer. Especially in the road section with vehicles, the sharp corners of hard objects may pierce the outer protective layer for a long time, resulting in damage to the internal structure. In addition, with the change of cable load, the thermal expansion and contraction of the cable itself and the uneven distribution of electric field at the damaged parts, resulting in damage to the insulating layer; During pipe laying, the outer protective layer may be damaged if the steps or inner wall at the pipe connection are not smooth; The outer sheath is damaged due to mechanical construction or pipe jacking around the cable path. (2) Defects and hidden dangers left during construction. During the cable laying construction, the damaged and cracked parts of the outer sheath are in the row pipe, and the personnel cannot find them in time; After running for a period of time, the bending part of 110kV and above cables will crack, the insulation of the outer sheath will be reduced, the metal sheath will be grounded at multiple points, and the circulating current will increase, resulting in thermal aging and breakdown of the insulation. (3) Termite decay. Once a termite erosion site is found, there are often multiple erosion sites on the cable line, which should be paid enough attention. Termite erosion is relatively rare in the northern power grid.