What are the reasons for the failure of underground power cables?
With the rapid development of social economy, the development of my country's industry and agriculture has been promoted to a great extent, and the demand for electricity has also increased sharply, thus putting forward higher standards for the operation of the power grid. As an important carrier of electric energy transmission and distribution in my country, underground power cables is directly related to the construction of local economy and even the whole national economy whether its normal operation or not. In recent years, with the rapid development of underground power cables networking, the operating environment of cable lines has become more complex. Therefore, it is particularly important to strengthen the monitoring and prevention of underground power cables faults, so as to ensure normal production activities.
The reasons for the failure of underground power cables are as follows
1. Mechanical damage
Cable faults caused by mechanical damage account for the largest proportion of cable accidents. Some mechanical damage is minor and did not cause failure at the time, but the time period required for the damage to develop into destruction is also very long. The main causes of underground power cables mechanical damage are:
(1) Damage during installation. Accidental contact with the cable during installation; excessive mechanical traction may strain the cable; excessive bending may break the cable.
(2) Directly damaged by external force. Civil construction is carried out on or near the installed cable path, so that the cable is directly damaged by external forces.
(3) The vibration or impact load of running vehicles can also cause cracking of the lead (aluminum) cladding of underground cables.
(4) Damage caused by natural phenomena. For example, the expansion of the inner insulating glue of the intermediate joint or the terminal head cracks the shell or the cable sheath; the cable sheath installed on the nozzle or the bracket is worn; the excessive tension caused by the ground subsidence will break the intermediate joint or the conductor.
2. The insulating material is damp
The damp of the cable insulation mostly refers to the joint part of the cable. The main reason for the damp of the cable joint is that it is not strictly sealed during the installation process, which causes moisture to enter. At the same time, during the installation process, if the weather is dark and humid, it is easy to cause moisture to corrode the joints. In this way, under the action of the electric field, the insulation of the cable is greatly reduced, which greatly damages the cable and causes cable failure. The main reasons for the cable to be damp are:
(1) The junction box or terminal box has water ingress due to unsealed structure or poor installation.
(2) The cable is poorly manufactured, and the metal sheath has holes or cracks.
(3) The metal sheath is punctured due to being stabbed or corroded by foreign objects.
3. Insulation aging and deterioration
Under the thermal effect of current, when the load current flows through the cable, it is easy to cause the conductor to heat up. At the same time, the skin effect of the charge and the loss of the insulating medium are also likely to cause additional heat, which will increase the temperature of the cable. Operating in an environment with high temperature, coupled with the hot summer weather, can easily lead to aging of the insulation layer, resulting in insulation damage. Insulation aging will cause cable breakdown due to voltage drop. The main reasons for cable aging are:
(1) The slag or air gap inside the cable medium will dissociate and hydrolyze under the action of an electric field.
(2) The cable is overloaded or the communication between the cables is poor, resulting in local overheating.
(3) The insulation of the oil-impregnated paper insulated cable is damaged.
(4) The service life of the power cable exceeds the specified number of years.
Overvoltage will cause electrical breakdown of defective cable insulation, resulting in cable failure. The main reasons are: atmospheric overvoltage (such as lightning strike); internal overvoltage (such as operating overvoltage).
5. Materials and materials
During the production process of the cable head, there will be uneven insulation in the package, and the difference in the power saving constant. The cable is affected by the power plant and is prone to aging. The main reasons are: the design of the electric field distribution is not meticulous; the material selection is improper; the process is poor and the production is not in accordance with the specified requirements.