Processability and reliability of suspended composite insulators
As the country has increased the construction and renovation of power lines in the past decade, the use of various insulators has increased. In particular, the use of suspension composite insulators has grown rapidly. There are about 3 million composite insulators operating on the grid in my country. Compared with porcelain and glass insulators, suspension composite insulators have the advantages of compact structure, high strength, light weight, good elasticity, not easily broken, not susceptible to human damage, excellent hydrophobicity, no need for cleaning, long leakage distance, no zero value, less maintenance, and convenient installation and transportation. It has been widely used all over the world. However, if there is no guarantee through scientific design and perfect production technology, the insulators put into operation are prone to quality problems and even lead to serious accidents.
1. Umbrella skirt and sheath molding process
At present, there are three main processes for producing suspension composite insulators at home and abroad: integral extrusion sheathing, integral molding and integral injection molding.
The production cost of the overall extruded sheathed umbrella is low, the organization and production are convenient, and the cold shrinkable cable terminal can easily meet the customer requirements of multiple specifications and small quantities. However, the shed and the sheath are artificially bonded with room temperature vulcanized rubber, which is prone to interface bubbles, weak bonding, breakdown and other mechanical and electrical performance quality problems. In addition, room temperature vulcanized rubber is prone to aging compared to high temperature vulcanized rubber. This is a relatively backward production process. Integral molding is put into production with simple equipment and low price. However, due to the poor flow of solid silicone rubber, especially at room temperature, the flow performance is very poor, and it is difficult to fill mold cavities with larger sheds or more complex structures. This can easily cause more surface defects, as well as inevitable core rod eccentricity and damage, which affects the product quality after the net-hanging operation.
Integral injection molding is recognized as having a high degree of automation, high efficiency, and stable quality. Domestic and foreign experts consider it the most advanced vulcanization process and is the third generation of composite insulators. The umbrella skirt and sheath are molded from the same material at one time, without the problem of different material interfaces in the umbrella extrusion process; the silicone rubber is plasticized at a higher temperature before being injected into the mold cavity, which improves the fluidity of the silicone rubber during injection. The injection mold is injected into the mold cavity under a relatively large injection pressure, thus solving the problem of local lack of glue in molding. However, if there is no guarantee of perfect technology and effective management, the products produced are prone to quality problems.
Common problems and solutions when using the injection molding process:
Problems with mandrel eccentricity and damage. High-temperature and high-pressure silicone rubber is injected into the mold cavity from one side of the mandrel, which can easily cause eccentricity of the mandrel and uneven thickness on both sides of the sheath. In serious cases, the mandrel is not protected, which may result in brittle fracture of the mandrel after long-term operation, which may cause serious accidents. This problem can be solved in two ways. One is to improve the fluidity of silicone rubber and use liquid silicone rubber instead of solid glue, which can be easily solved. Secondly, the mold design adopts multiple positioning and reasonable methods to improve the fluidity of the solid glue to ensure that the deviation is within 0.2mm and does not damage the core rod. Its performance can fully meet or exceed the products produced by other processes.
The problem of difficult demoulding is due to the multi-shedding characteristics of high-voltage suspension composite insulators and the European-style cable joints. The contact surface between the silicone rubber and the mold is very large during the overall molding, which is prone to difficult demoulding problems. If it is difficult to demould, the mandrel will be used to adhere to the mold and crack, the lack of glue will affect the appearance, the yield will be low, and the reliability will be significantly reduced. Solutions to this problem: First, adjust the silicone rubber formula and appropriately increase the proportion of release material. The second is to choose imported steel as the mold cavity material, improve the polishing finish, improve the mold processing technology, or electroplating the inner cavity with chrome.
The problem of temperature difference damaging the core rod during segmented molding. At present, several domestic manufacturers have achieved one-time injection molding of products of 220kV and below, without the need for segmented molding. Insulators above 500kV all use segmented molding processes. Since the vulcanization temperature of silicone rubber is generally around 170°C, the temperature of the core rod in the mold cavity and the temperature in the room reach about 150°C. Different power heaters are used at the joints for segmented heating to eliminate the problem of excessive concentration of temperature stress caused by drastic changes in temperature. After improvement through this method, the problem of damage to the core rod due to temperature stress has never been found.
2. Connection process between mandrel and hardware
There are currently three main methods for connecting the mandrel and the hardware at home and abroad: bonding, wedge bonding and crimping. The bonding type is rarely used due to the problems of adhesive aging, low temperature brittleness and low efficiency, and the bonding effect is greatly affected by human factors. The wedge type is divided into inner wedge and outer wedge. The inner wedge will destroy the integrity of the core rod and reduce the core rod and mechanical strength. A small number of manufacturers use it, and the outer wedge is used by a small number of manufacturers. The connection strength is unstable, the process is complex, and the disconnection rate is high, so it is rarely used. If the crimping method adopts perfect process control, the production efficiency and product quality will be greatly improved. Currently, most manufacturers at home and abroad use the crimping method. Let’s discuss the relationship between the crimping process and quality.
Through crimping, the insulator fittings are plastically deformed and the core rod is compressed to generate strong friction to obtain the required tensile strength. The best crimping result should be that under the action of the crimping force, the fitting is fully deformed and in close contact with the mandrel without excessive deformation, which will crush the brittle mandrel and maximize the deformation resistance of the fitting. It is extremely difficult to achieve this effect. It not only requires complicated elastic-plastic performance calculations but also takes into account various external factors. It is generally solved by using experience and a large number of tests.
There are currently three main control methods for crimping:
One is the unidirectional movement of the mold. The crimping mold only has two pieces, the upper and lower parts. Through the hydraulic device, the mold is pushed up or down in one direction to squeeze and plastically deform the hardware to achieve the crimping effect. Since the fittings, mandrels, machines and molds themselves have certain processing tolerances, the plastic deformation of the fittings is uneven, the crimping speed is too fast, and it is impossible to determine whether there is overvoltage, and the crimping quality is unstable, the method is only used by a small number of manufacturers and is used on products with low voltage levels and low mechanical strength.
The second method is to use eight radially evenly distributed molds to generate constant pressure under the action of oil cylinders to control the crimping force of the hardware, and use an acoustic emission monitoring system to monitor the quality of the mandrel during the crimping process. This method has a qualitative improvement in the crimping effect compared to the first method, but there are also some fly in the ointment: First, the concentricity and synchronization cannot be very precise after the mold and the cylinder are matched. Second, the materials of each batch or the same batch of fittings are not exactly the same. The materials of the ball heads and sockets are also different. There are also certain differences in the heat treatment process of the arrester manufacturers, resulting in different hardness of each fitting and different required crimping force. The existence of these two problems ultimately leads to the disadvantage of unstable crimping effect.
The third method is to evenly distribute eight radial molds, and control the crimping amount of flexible crimping through one-time processing of force-increasing taper sleeves. The crimping process also cooperates with the acoustic emission monitoring system to monitor the quality of the mandrel. This solution solves the following problems: First, the plastic fittings are crimped with the brittle mandrel and the deformation amount is only 0.2 to 0.5m. m, if the crimping speed is too fast, it will damage the mandrel, increase the rebound of the hardware, make the hardware and the mandrel unable to be tightly combined, and reduce the crimping strength. This solution uses a flexible crimping method of slow and continuous crimping for 3 to 5 seconds and pressure holding for 0 to 9 seconds, which not only greatly reduces the chance of crimping damage to the mandrel but also improves the crimping strength of the hardware and the mandrel. The second is that there are processing deviations between the fittings and the mandrel. Each batch or the parts in the same batch have different dimensions and tightness. The pre-tightening pressure can be set during crimping. The gap between the fittings and the mandrel can be eliminated before measuring the crimping amount to ensure that each product has the same crimping effect. The third is the problem of synchronization and concentricity of crimping. When processing the modules, first process the inner and outer circles, length, mounting holes, etc., and then cut them into eight modules on average. The power-increasing taper sleeve is also a cone surface processed at one time. There is not much cumulative tolerance problem in the cooperation between the two, ensuring the overall concentricity and synchronization.
3. End sealing process
The brittle fracture of the insulator core rod will cause the most serious accident of the conductor falling to the ground. If the mandrel is not effectively protected, the intrusion of moisture and corrosive gases is one of the most important factors causing brittle fracture of the mandrel. The electric field intensity at the end is complex and is the place that needs the most protection. At present, various manufacturers of end seals mainly use room temperature vulcanized rubber for sealing; sealing with room temperature vulcanized rubber after inserting the formed sealing ring; and using high temperature vulcanized rubber for one-time overall sealing. The performance of room temperature vulcanized silicone rubber is different from that of high temperature injected silicone rubber. Its mechanical properties, resistance to electrical corrosion, and aging resistance are far worse than those of high temperature vulcanized silicone rubber. Under high-voltage outdoor operation, the product is often impacted by electrical, mechanical, outdoor sunlight, and rain. After long-term operation, end seal deterioration and moisture intrusion may occur, making it difficult for the mandrel to receive due protection. The sealing on high-temperature silicone rubber is to inject the silicone rubber at high temperature and high pressure by an injection machine, and vulcanize it integrally with the sheath in the mold. The sealant fully fills all the gaps between the hardware and the mandrel; it is tightly bonded with the hardware, completely eradicating the interface bubbles sealed with room temperature glue, weak bonding, human defects and hidden dangers, greatly improving the reliability of the end seal.
Based on the above, the suspension composite insulator jacket adopts a one-time integral injection molding, the hardware connection adopts a flexible crimping and sonic abrasion instrument, and the end seal uses a high-temperature silicone rubber one-time integral sealing production process. This is the best and most effective way to ensure the long-term operation of the product.
Yichang Hengyuan Technology Co., Ltd. specializes in the production of European-style cable joints, plug-and-pull heads, cable branch boxes, American-style elbow joints, high-voltage vacuum circuit breakers, high-voltage load switches, arresters, zinc oxide arresters, tank arresters, over-voltage protectors, counters, monitors, heat-shrinkable cable accessories, Manufacturer of cold shrink cable accessories, T-heads, wall bushings, European cable accessories, American cable joints, transformer neutral point protection devices, fuses, isolating switches, PT cable heads, elbow cable joints, 10kV outdoor terminals, switch cabinets, cable intermediate joints, cable accessories and other products
