Composite insulator, also known as synthetic insulator, was first born in Germany. It was successfully developed by Hearst Company of Germany in 1967 and put into operation on 123kV line. It has more than 30 years of operation experience. Composite insulators have been rapidly developed in the United States, Britain, France, Italy, Russia, Japan and other countries, and have been widely promoted worldwide. The production of composite insulators in China started relatively late. In 1974, epoxy resin insulators were developed by the National Railway Research Institute. In 1978, silicone rubber composite insulators were developed, and thousands of products were put into trial operation, which gained valuable operational experience. In the past 10 years, the use of composite insulators in China has grown rapidly. It has ranked second in the world in 1998. Since 2000, it has been used extensively on ±500kV DC transmission lines. There are currently about 6 million composite insulators operating in China's power system.

Silicone rubber composite insulators are generally synthesized from two or more organic materials. They are light in weight, high in strength (core rod), high in impact resistance, high in flashover resistance, easy to install, no need to measure zero, lightning strike, and no flashing. The advantages of insulation breakdown, composite insulator consists of mandrel, sheath, shed skirt, end fittings and accessories, which is the first choice for heavy pollution.

The original rubber of silicone rubber has low mechanical strength. The "peroxide" is used as a vulcanizing agent. The vulcanization temperature is 150 ° C ~ 180 ° C. The organic oxide is decomposed and crosslinked with the rubber organic side groups to form a vulcanized silicone rubber. (ie high temperature silicone rubber). Silicone rubber is added to the meteorological method of white carbon black (the cost is reduced by the precipitation method of white carbon black, its strength is greatly reduced) to ensure the tensile and tear strength of silicone rubber.

Adding a certain proportion of "aluminum hydroxide fine powder" to the silicone rubber can significantly improve the arc burning resistance of the silicone rubber.

Liquid silicone rubber refers to a vinyl-containing polydimethylsiloxane having a "functionality" of two, which is divided into two components, which are uniformly mixed to produce an "addition reaction" under the action of a platinum compound. A silicone rubber in which chain growth and chain crosslinking are simultaneously performed at a temperature of from 90 ° C to 110 ° C.

Composite insulators are "all-resistance" products, which are different from ceramic, glass and other disc-shaped insulators, spacer steel caps, steel stud insulators with stray capacitance distributed in the middle (disk-shaped insulators have larger longitudinal characteristics due to structural characteristics). A capacitor (approximately 70-100 PF) that distorts the electric field of the wire to adjust the axial voltage distribution of the insulator. In the ultra-high voltage line, the composite insulator is in a very uneven electric field. Only a relatively short distance is in the high field strength position near the wire. The relatively long insulation string in the middle is at a relatively low electric field position. The metal equalizing ring is installed at the end to improve the distribution voltage of the insulator string, protect the over-voltage short circuit, the arc is burned to the silicone rubber shed when the flashover occurs, and the core rod and the metal fitting at both ends are protected from leakage and damage due to electric leakage. Performance, the interface between the metal fittings and the materials such as the mandrel and the sheath, which is the most dangerous interface of the composite insulator. The interface has many kinds of materials, the electric field strength is high, and the air at the interface is prone to ionization, resulting in dendritic discharge of the silicone rubber. The silicon rubber sheath is electrically etched. Causes the interface to be in direct contact with the atmosphere. Under the action of electric field, corona is generated, and nitrogen in the air reacts with oxygen to produce nitrous acid anhydride, and then reacts with water vapor to form nitrous acid, which corrodes the mandrel, causing the composite insulator to be brittle and causing a significant drop in the line. In the accident, corona discharge is easily generated near the port of the high-strength gold fittings, so that the sheath with silicone rubber as the main material can be subjected to corona ablation for a long time, and the water repellency is weakened until it disappears.

The composite insulator is equipped with a equalizing ring at both ends, and the main capacitor C between the rings is proportional to the equivalent area of the equalizing ring and inversely proportional to the distance between the equalizing rings. Since the equivalent area of the equalizing ring (750px) is small and the distance of the insulator is (10000px) long, the main capacitor C is small, and the stray capacitance of the composite insulator to the ground, the tower and the wire still plays an important role, and the electric field is based on the stray capacitance. The distribution is extremely uneven.

There is a voltage equalizing ring at both ends of the composite insulator, which increases the main capacitance at both ends, reduces the influence of stray capacitance, and achieves the purpose of voltage equalization. Taking 500kV as an example, before the grading ring is installed, 15% of the mandrel bears 100% of the working voltage; after installing the grading ring at both ends, the 55% of the mandrel bears 100% of the working voltage, which is visible. Pressure equalization. Although the normal composite insulator is equipped with a voltage equalizing ring at both ends, the distribution voltage of the high voltage end is still about 30kV∕cm. Although the electric field strength does not penetrate the air at this time, under the high electric field, the silicone rubber sheath of the composite insulator , mandrel, etc. is still prone to excessive leakage current, causing local overheating, causing partial discharge and accelerated aging or erosion.

The experimental conclusions: the lightning impulse discharge characteristics of porcelain, glass insulator string and silicone rubber composite insulator string are determined by the distance between the two ends of the metal. Under the same environmental conditions, the dry arc between the two ends of the insulator string The same distance, the impact discharge voltage value is basically the same, if the insulator string is installed with a voltage equalizing ring, its dry arc distance is the distance between the rings, the impact discharge characteristics occur at this time between the voltage equalization rings, and whether the insulator string is porcelain No matter the material of glass or silicone rubber.

Although the grading ring can improve the overall distribution voltage of the composite insulator, the effect is that there is no uniform distribution of the disk insulator strings, but the field strength at the junction of the mandrel and the metal fitting can be significantly reduced. While improving the field strength and selecting the appropriate outer diameter of the equalizing ring and the diameter of the pipe diameter (experimental evidence: the larger the pipe diameter r of the high pressure end equalizing ring, the lower the field strength and the average field strength of the ringing side, when r >10mm, the end field strength can be reduced to below the air breakdown field strength (3.0kV/cm); the radius R of the equalizing ring is too small, which will increase the field strength at the insulation distance of 10% from the high voltage side. The larger the radius R of the grading ring, the lower the average field strength and the more even the electric field distribution. Therefore, it is recommended that the radius R of the 500 kV high pressure end grading ring is generally 250 mm to 300 mm. It also has a great relationship with the distance of the equalizing ring deep (lifting) the shed. When the grading ring is deeper than Δh≈0, the core rod and the fitting at the opening plane of the grading ring will withstand the maximum field strength. If the maximum field strength exceeds the air breakdown field strength (3.0kV/cm), a more severe corona discharge will occur, which will cause microporosity, material defects or different interfaces in the silicone rubber material. Leakage tracking, that is, the operating voltage of 25% to 30% on the insulator string of about 6% length, thereby accelerating the aging of the mandrel and the sealing performance of the silicone rubber sheath due to electro-corrosion perforation, causing the composite insulator core The rod broke off the string accident. The test proves that the test voltage of 190kV is applied to the 330kV composite insulator, and the field strength of the crimping ring of the mandrel and the steel foot is more than 5.5kV/cm to 6.5kV/cm when the pressure equalizing ring is deeper than Δh=0. The distribution voltage is 28kV ~ 34kV (accounting for 20% ~ 26% of the operating voltage). When the pressure equalizing ring is covered and shielded from 2 to 4 sheds (ie, raised by 120mm to 150mm), the field strength at the end of the mandrel is reduced to 0.4kV/cm to 1.6kV/cm, and the maximum distribution voltage of the shed is only It is 10% of the operating voltage. However, if the pressure equalization ring is too deep, the dry arc distance between the cross arm and the wire will be shortened, and the lightning impulse discharge voltage value of the insulator string will be reduced. In addition, when the line construction and project completion acceptance, most of the units did not accept the in-depth size of the composite insulator equalizing ring. It is generally used to visually observe whether the upper and lower equalizing ring openings of the composite insulator are aligned on the ground to verify its entireity. Installation quality. Because some employees do not know the principle of the pressure equalizing ring, the reverse mounting phenomenon is found in the project. When the high-pressure side pressure equalizing ring of the composite insulator is correctly installed, the end field strength of the fitting is 14.12kV/cm, and when flipping, it is 30.79 kV/ Cm, when the pressure equalizing device is reverse mounted, the field strength at the end of the high pressure side of the gold fitting is increased after the flipping.

Pressure equalization ring reverse loading leads to discharge

Due to the high voltage side equalizing ring flipping, the initial corona voltage is only 276kV, which is lower than the normal operating voltage. When the line is at the maximum operating voltage of 318kV, there is a strong visible corona at the high pressure side mandrel sheath and the end of the fitting. Discharges and forms a brush-like discharge whose brush-like discharge front reaches the first large shed on the high pressure side of the insulator. If it is used for a long period of time, the electrical corrosion and cracking of the end interface sealant will be accelerated.

Explain that the correct installation of the equalizing ring has a great influence on the electrical performance of the composite insulator. Secondly, some units also replaced the original double-string disc-shaped anti-fouling glass insulator string with a composite insulator. The FJP-500XDL (waist-shaped) equalizing ring of the double-string type was not replaced at the same time, and the original was incorrectly replaced. The waist-shaped equalizing ring of the disc insulator replaces the circular equalizing ring brought by the new composite insulator, so that after replacement for less than one year, the newly replaced composite insulator core rod brittle fracture occurs again.