The selection of insulators is one of the most important contents of the insulation coordination of UHV transmission lines. Reasonable determination of the form of insulators is of great significance to control the manufacturing cost of equipment while ensuring the reliability of power system operation. High-voltage line insulators mainly include glass insulators, composite insulators and porcelain insulators, which have been practically used in national UHV lines. We discuss the three types of insulators in terms of life expectancy, failure rate and detection rate, and electrical performance, and give suggestions for the selection of UHV insulators.
The insulating parts of porcelain insulators are made of alumina ceramics, an inorganic material, which has excellent anti-aging ability and chemical stability. The glass insulator is made of tempered glass as the insulator, assembled with cement adhesive and other metal hanging parts, and adopts the "thermal tempering" process, which endows the glass surface with a permanent prestress of 100-250 MPa, which increases the strength of the tempered glass. The thermal stability is improved, the aging resistance is strengthened, and the service life is prolonged.
According to the electromechanical performance tracking comparison test conducted by the state on glass and porcelain insulators that have been in operation for 5 to 30 years, the service life of glass insulators depends on metal accessories, and the service life of porcelain insulators depends on insulating parts; operating experience shows that glass insulators run for 40a, the mechanical and electrical properties change little, and the average life cycle of porcelain insulators is 15 to 25a.
The external insulation of composite insulators is made of organic material silicone rubber, which is relatively easy to age under the action of electromagnetic discharge, ultraviolet radiation, humid environment, temperature changes and chemical corrosion. Research on its service life requires long-term tracking and observation. At present, composite insulators only have 20 years of operating experience, there is not enough data to support. From the foreign operating experience, as long as the composite insulator can guarantee the quality of the factory, there is no problem with the service life reaching 10a.
The failure manifestation of porcelain insulators is that after long-term operation, the material ages, and the insulation performance drops to very low or even zero. Such low-value or zero-value insulators cannot be seen from the outside and need to be detected through tests.
The failure performance of glass insulators is zero-value self-breaking, that is, when the glass insulators lose their insulating properties, the glass umbrella disks will burst and break. After the glass insulator self-breaks, the maintenance personnel can directly observe the broken glass umbrella plate with the naked eye, so the failure detection rate of the glass insulator is much higher than that of the porcelain insulator. The inspection workload is also much smaller than that of porcelain insulators. Another statistics show that the average failure rate of domestic glass insulators in its life cycle is 1 to 2 orders of magnitude lower than that of porcelain insulators.
The inner insulation distance and outer insulation distance of composite insulators are almost equal. It is a non-breakdown insulator in structure, and there is no problem of zero-value insulators, so zero-value detection is not required. However, the failure manifestations of composite insulators are corrosion of shed silicone rubber and hidden "interface breakdown", which cannot be directly observed. Instruments must be used to detect and replace them one by one, resulting in increased maintenance workload and cost.
The insulators that are widely used on national lines are mainly: double-layer umbrella insulators, three-umbrella porcelain insulators, ordinary porcelain insulators and glass insulators in mild to moderate pollution areas; composite insulators in heavily polluted areas and high altitude areas.
The composite insulator has the highest flashover voltage per unit length under different salt densities, and with the increase of salt densities, the flashover voltage decreases slowly, and the pollution flashover resistance is the best. The second is the three-umbrella type porcelain insulator. When the salt density is small, the three-umbrella ceramic insulator has a higher flashover voltage per unit length, but with the increase of the salt density, the flashover voltage decreases rapidly. Compared with the above two insulators, the ordinary glass insulators have the worst pollution flashover resistance.
The shape of the umbrella skirt has a great influence on the pollution flashover resistance of insulators of the same material: the pollution flashover resistance of umbrella insulators is obviously better than that of ordinary insulators; the pollution flashover resistance of three-umbrella insulators is better than that of double-umbrella insulators. As the altitude increases, the air pressure decreases, and the flashover voltage of the dirty insulator also decreases.
The falling slope of composite insulators is small, and the high-altitude insulation performance is good; glass insulators have better high-altitude resistance performance than three kinds of porcelain insulators in mildly polluted areas; double-umbrella porcelain insulators and three-umbrella porcelain insulators increase with altitude. The insulation performance is greatly reduced.