① Electrical dendritic aging
Under the action of power frequency, lightning and switching overvoltage, the insulation layer of XLPE power cable, due to the defects of the insulation itself, such as impurities, water droplets, bubbles, etc., makes the local electric field concentrated, impurities, water droplets, bubbles, etc. dissociate under the action of the electric field, producing free electrons and positive ions, and free electrons continue to collide with other neutral particles under the action of the electric field, when the electric field is large enough, When the free electrons are accelerated and the energy reaches the free energy of the neutral particle with which they collide, collision dissociation will occur, and new free electrons and positive ions will be generated. This will increase the number of free electrons, and the free electrons generated by collision dissociation will participate in the collision, forming new collision dissociation. After many collisions, a large number of electrons and positive ions will be generated. On the one hand, the number of electrons is increasing, on the other hand, The free area continues to develop outward. Due to the randomness of the free electron movement, the development path of free discharge is random, rather than uniformly developing in each direction. The physical model of the discharge channel or discharge trace is as shown in Figure 1.2. Long-term occurrence of such discharge will cause small current in the insulation to pass through and generate heat in the insulation layer, resulting in cable insulation aging, namely dendriform aging.
② Water tree aging
Due to the limitation of the construction environment and the low construction technology, the cable head will be left with moisture during the production process: during operation, it will also be affected by moisture due to the humidity of the environment, that is, the cable contains moisture inside. Water exists in the cable insulation layer in the form of water droplets or dissolution. When water exists in the medium in the form of water droplets, because water is not a good insulating medium and has a certain conductivity, the voltage shared by the water droplets is very small, which increases the strength of the electric field borne by the insulating medium, leading to dielectric discharge, and the long-term discharge of the medium forms aging. Water trees can be divided into three types: internal water trees generated from the inner semi-conducting layer of the conductor, external water trees generated from the outer semi-conducting layer of the insulation, and butterfly node-shaped water trees generated from the gap in the insulation layer. In particular, the inner water conducting tree generated from the inner semi-conducting layer will greatly reduce the insulation strength of the cable.
③ Chemical aging
Chemical aging is caused by the operating environment of power cable, such as the expansion of polyethylene sheath caused by laying the cable underground containing petroleum chemicals, one of which is called vulcanization aging, which has the greatest impact on the cable insulation, because the sulfide (hydrogen sulfide, etc.) through the sheath and insulation layer and the copper conductor of the cable produce chemical reaction to produce copper sulfide, copper oxide and other substances, These substances develop in a dendritic form from the inner guide side to the sheath side in the insulation layer, just like water trees. This aging is collectively referred to as chemical trees. Different types of oil and chemical substances cause different degrees of aging of cable insulation, but all of them will cause expansion of various components of the cable, decrease in physical properties and electrical properties.