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Selection of Composite Insulator Core Rod Materials

Pulished on Jul. 24, 2020

Composite Insulator

Composite insulators are insulating equipment for hanging wires in transmission lines. Its position and function make its product performance very important. The long-term operation has proved that the safety and reliability of the mechanical properties of composite insulators have a considerable impact on the economic benefits of the power system and must be paid attention to. The mechanical failures that occur during the operation of composite insulators, in addition to the normal damage caused by the uneven stress caused by the process quality of the connection between the fittings and the core rod, the exposed corrosion and brittleness phenomenon of the non-connected parts of the product core rod occupy a considerable proportion.

1. In order to avoid the occurrence of mechanical failure of composite insulators, on the basis of ensuring the stability of the connection between the core rod and the fittings and effectively preventing the product core rod from being exposed, the product should use a core rod with good acid resistance. This can also effectively reduce the occurrence of product brittleness. To ensure that the mandrel has good acid corrosion resistance, it must be based on the characteristics of the material composition of the mandrel, taking into account the specific conditions of the mandrel extrusion process, and on the basis of not reducing the overall electromechanical properties of the mandrel, the composition of the mandrel The selection of raw materials and the process parameters of each process of mandrel pultrusion should be determined by optimization methods.

2 Reasonable determination of the material of the acid-resistant mandrel

The composite insulator core rod is composed of a glass fiber body and resin. It is formed by glass fiber impregnated resin, pultruded, and heated through a mold to form a whole. The core rod formed by this molding process makes the fiber around the fiber-filled with resin and bonded, and its overall performance depends first on the fiber material composition and the performance of each component of the resin.

2.1 Selection of glass fiber

Glass fiber is the main material in the core rod and plays a role in reinforcing the skeleton. As far as the core rods used in domestic composite insulators are concerned, the content of glass fiber accounts for about 80% of the total weight of the core rod, and its volume content is about 65%. It can be seen that the performance of glass fiber plays a leading role in the core rod.

The composite insulator core rod adopts alkali-free glass fiber. Alkali-free glass fibers are basically composed of silicon dioxide, aluminum oxide, and boron oxide. The molecular structure and properties of these oxides are relatively stable, and they all appear in the material in the state of crystal phase structure with excellent insulating properties. The small amounts of potassium oxide and sodium oxide contained in glass materials that are difficult to remove belong to alkali metal oxides. It is very easy to exist in the ionic state in the material, which is detrimental to the insulation performance. However, the "neutralization effect" with a certain ratio of potassium to sodium ion concentration can be used to reduce its influence on the insulation performance of the material. At the same time, the oxygen contained in the glass material

Calcium oxide and magnesium oxide belong to alkali metal oxides. Their existence can promote the composition of the oxide structure of the crystal phase in the material to be closer, and play a "depressive effect" that hinders the passage of alkali metal ions, which can also significantly improve the material Insulation performance. The "neutralization and suppression effect" of the structural components of glass materials can be fully considered and implemented in the production of alkali-free glass fibers. Therefore, the alkali-free glass fibers used in the core rods have high resistivity and excellent insulation properties.

If the mandrel is made of acid-resistant glass fiber, it is based on the mechanism of acid and water corrosion of the fiber

Only by reducing the content of alkaline earth metal oxides in the fiber can the acid corrosion resistance of the fiber be improved. But this will inevitably destroy the "neutralization and suppression effect" in the alkali-free glass fiberglass, promote the nonlinear conductivity of the fiber to increase sharply, and reduce the fiber's insulation performance.

It can be seen that, compared with the alkali-free fiber, although the acid-resistant fiber has good acid corrosion resistance, it has the disadvantage of large electric current. This is for the composite insulator core rod that always bears high voltage and mechanical tension. It must have a certain degree of impact on its performance. Especially when the core rod of acid-resistant fiber is used on the DC composite insulator, the ion migration phenomenon under the action of the DC electric field has a greater impact on its performance. Therefore, composite insulators should use alkali-free fiber core rods.

2.2 The choice of resin

Resin is the matrix material in the mandrel and plays the role of wrapping the bonding fibers. Although the resin only occupies about 20% of the weight of the mandrel, the space volume does occupy more than 35%. Its structural performance plays a key role in the core rod, especially the protective performance of the acid-resistant core rod with alkali-free glass fiber, which is directly related to the stability of the core rod against acid corrosion.

The resin used in the core rod is mainly composed of epoxy resin, curing agent, accelerator and release agent. The curing agent that participates in the composition of the resin body-shaped network structure, after being fully mixed with the epoxy resin matrix, the presence of the hydroxyl group, ether group, and extremely active epoxy group in the structure makes it highly binding. Therefore, after the core rod is cured and formed, the resin not only has good mechanical insulation properties but also has high bonding strength with the fiber. At the same time, the resin body-shaped network structure also contains stable benzene rings and ether bonds, and its structure is dense and closed, which makes it also have good acid corrosion resistance. Therefore, the determination of a considerable proportion of the curing agent in the resin is very important. In the process of selecting the curing agent, the first thing to consider after the resin is cured is to avoid the microscopic destruction of the resin network structure caused by the formation of acid by the curing residue in the resin. Prevent external acid from using these parts to penetrate into the interior through adsorption, penetration and diffusion to destroy the overall performance. Secondly, attention should be paid to the cured tensile strength and compression elastic deformation of the resin. These technical parameters will directly affect the stability of the overall performance of the core rod and the structural connection performance of the product end crimp interface. Then in the curing process of the resin, volume expansion measures should be taken to eliminate the residual stress in the resin network structure, so that the protective performance of the resin will not be affected by the elastic deformation of the core rod fiber during the long-term load-bearing process of the core rod. Finally, in the selection of curing agents, it should be noted that due to the change of the operating environment temperature of the core rod and the change of the external acid concentration, the acid resistance of the resin should not be greatly affected.

The release agent in the resin used in the core rod is to improve the mandrel extrusion process and the release performance and add an auxiliary agent. The choice of release agent in the resin requires that it does not react with the resin and has a lubricating effect on the surface of the metal mold cavity. It should also have good compatibility with the resin at room temperature, and it can be quickly removed from the resin at a certain curing temperature. The surface of the pre-gel migrates to the surface of the protrusion mandrel, which plays a good role in demoulding. Then, after passing through the mandrel, the rubber mold agent attached to the surface of the mandrel is removed at the curing temperature.