This is how silicone rubber is used for power cable accessories
At present, there are two main types of cross-linked power cable accessories of 10kV and above in China. The first type is cold shrinkable cable accessories, and the second type is prefabricated cable accessories.
More than 95% of cold shrinkable cable accessories are made of silicone rubber. About 80% of prefabricated cable accessories are made of silicone rubber.
1、 Cold shrinkable cable accessories
Cold shrinkable cable accessories use the high elasticity of silicone rubber material to process high-temperature vulcanized silicone rubber into tubular parts with different structures by injection molding. Then special machinery is used to expand tubular parts with different structures. When the inner diameter of the parts is expanded to about 3 times, special plastic support tubes are placed in the tubular parts, The silicone rubber tubular parts are kept in the expanded state, and these expanded state parts become the parts of the cold shrinkable cable accessories.
During on-site installation, these expanded parts are put into the end or joint of the treated cable, and the internal support pipe (strip) is pulled out. The silicone rubber parts automatically shrink and press tightly on the cable insulation. That is to say, it becomes a cable accessory. Because it depends on the elastic retraction force of silicone rubber at room temperature, rather than the heat shrinkable cable accessories that need to be heated by fire, it is commonly known as cold shrinkable cable accessories.
Cold shrinkable cable accessories have an important requirement for silicone rubber. After two years of storage, they can still be reset, shrink and press tightly on the cable insulation.
2、 Prefabricated cable accessories
Prefabricated cable accessories are made of high temperature vulcanized silicone rubber by injection molding and vulcanization, and processed into tubular parts with different structures (without expansion).
During on-site installation, the tubular parts which are molded and vulcanized and smaller than the cable insulation diameter are directly sheathed on the cable insulation at the end or joint of the treated cable. Because the inner diameter of the silicone rubber parts is smaller than the cable insulation, they can be directly pressed on the cable insulation to form prefabricated cable accessories.
It can also be said that: due to the continuous introduction of new varieties of silicone rubber materials, the gradual improvement of the physical properties of silicone rubber materials, and the progress in the processing technology of silicone rubber, especially liquid high temperature vulcanized silicone rubber materials, conditions have been created for the power cable accessory industry to develop new products, improve product quality, and improve the product qualification rate.
3、 Brief introduction of high voltage cable terminal
110-220kV XLPE
It is a classic product of insulated cable terminal, which is porcelain sleeve (or composite sleeve) cable terminal. The cable terminal is characterized by super insulation performance; Good weather resistance; It has a long history of operation.
3.1 structural characteristics of bushing type HV XLPE cable terminals
① The prefabricated rubber electric stress control cone with specific shape is sheathed on the insulation of the cable and lapped with the outer shield of the cable;
② The electric field distribution at the end of the cable is improved by the combination of semi conductive rubber material with specific curve and insulating rubber material in the rubber electric stress control cone;
③ The electric field intensity at the end of the cable is controlled by applying a basically constant interface pressure on the junction of the cable insulation and the cable external shielding section depending on the elasticity of the prefabricated rubber stress cone material.
3.2. Core components of HV XLPE cable terminal
Shape characteristics of prefabricated electric stress control cone:
Rubber electric stress control cone is an effective method to control the electric field distribution at the end of cable
A preformed electric stress control cone with a specific shape formed by the combination of insulating rubber and conductive rubber is arranged at the outer shielding cut-off port of the cable. It can effectively control the electric field distribution at the end of the cable and prevent the cable terminal from discharging.
4、 Brief introduction of high pressure integral prefabricated intermediate joint
There are three main structures of high voltage cable intermediate joint: combined assembly type, wrapping type and integral prefabrication type.
At present, the wrapped type has been basically eliminated, the consumption of combined assembly type has been reduced year by year, the integral prefabricated intermediate joint is the mainstream variety at present, and the State Grid has listed the integral prefabricated intermediate joint as the preferred structure for bidding in recent three years.
Low viscosity high-temperature vulcanized silicone rubber is the first choice for manufacturing high-voltage intermediate joint, which can effectively eliminate the air gap between conductive cone, conductive tube and insulating rubber, effectively avoid the defects between conductive cone, conductive tube and insulating rubber, and improve the electrical performance of products.
5、 Brief introduction of manufacturing and technical requirements of rubber electric stress control cone
① First, a product with a specific shape is processed with conductive rubber, which is generally called conductive cone.
② Then, the conductive cone is sheathed on the unrepaired steel die mandrel, injected with insulating rubber, heated and vulcanized to form a rubber electric stress control cone.
③ The electrical performance index of rubber stress control cone, taking the stress cone for 110 kV cable terminal as an example: the basic electrical parameters of stress cone for 110 kV cable terminal are: no discharge for 30 minutes under power frequency voltage of 160 kV, no partial discharge under background partial discharge of 5 PC and beyond background of 96 kV; Only the rubber stress control cone which has passed the above two tests can control the electric field at the end of the cable.
④ The difficulty of manufacturing rubber electric stress control cone lies in the quality control of the interface between insulating rubber and conductive rubber. If there are defects such as air gap, bubble and dent between the interface (line), the electrical stress control cone will not pass the electrical performance test. We have found that as long as there are defects between the stress cone insulating rubber and conductive rubber, the partial discharge under 96kv power frequency voltage will exceed 5pc.
⑤ State Grid Electric Power Research Institute
The State Key Laboratory for safety control and Simulation of power generation equipment of Tsinghua University has carried out simulation tests on silicone rubber stress cones of high-voltage cable accessories. Two kinds of defects, bubble and air gap crack, are simulated. The initial voltage of electrical tree is measured under power frequency voltage, and the shape of electrical tree is recorded by digital microscopic observation system, The effect of defects on the electrical tree initiation characteristics of silicone rubber was studied. The results show that the electrical tree initiation voltage decreases when there are bubbles and air gap cracks in the sample.
⑥ Tianjin power supply company points out that one of the main causes of cable system failure is the improper matching of accessories and cable joint surface and the formation of air gap through the analysis of several samples of XLPE insulated cable system defects in operation.