1. Mechanism of self-detonation of tempered glass
The glass part of glass insulator is tempered glass, which is characterized by compressive stress on the surface and tensile stress inside, as shown in the figure.
Stress stratification of tempered glass
The reason of glass stress is the change of temperature during glass processing. When the glass has been heated to the softening temperature (760~780℃), the surface layer cooling force contraction, but the internal temperature is still high in the expansion state, resulting in the surface layer contraction blocked, leaving compressive stress in the surface layer; Then the internal temperature decreases and the contraction begins, but at this time the surface layer has hardened, resulting in the internal contraction is blocked and tensile stress is generated. These two stresses are permanent stresses that are uniformly distributed in the glass until the glass is completely cooled and the temperature gradient disappears.
Once the balance between compressive stress and tensile stress in the glass parts of glass insulators is destroyed, cracks will be generated rapidly under the action of stress, and then the glass parts will be crushed, that is, self-detonation.
2. Causes and characteristics of self-detonation
The reasons for the self-detonation of glass insulators can be divided into product quality reasons and external operating environment reasons, and there are often two reasons in actual cases.
(1) Reasons for product quality
It is mainly the glass insulators contain impurities inside the glass, the most common is NIS particles. The phase transition state of NIS is not complete in the process of melting and annealing of glass parts. After the insulator is put into operation, it is believed that the phase transition and expansion occur slowly, resulting in cracks inside the glass. When the diameter of particle impurities is less than a certain value, it may not be able to be removed through hot and cold shock, resulting in high self-detonation rate of the insulator in operation. When the impurity particles are located in the internal tensile stress layer of the glass, the probability of self-detonation is higher. Because the glass itself is a brittle material, it can withstand pressure but is not tensile, so most of the glass breakage is caused by tensile stress.
Features:
The self-detonation caused by the internal impurity particles of a is high in the first three years of operation, and will gradually decline thereafter, which is an important rule for judging the cause of self-detonation.
b insulator string has the same self-detonation probability at different positions;
(2) External causes
Mainly pollution and temperature change. Under the simultaneous action of fouling, moisture and electric field, the leakage current on the surface of the insulator is too large, resulting in part of the dry belt. When air breakdown occurs in the dry belt, the arc generated will erode the glass umbrella skirt. When the corrosion depth is deep, it will cause self-detonation. If the insulator is struck by lightning during the above process, the probability of self-detonation of glass insulator which has been eroded by arc will be greatly increased. Excessive fouling is one of the key factors. It may be that the salt density is too high, or it may be that there are too many metal particles in the pollution.
Features:
① Self-detonation may not be obvious a few years before the operation, and it may occur at a certain time after several years of operation (major changes in local pollution sources cause excessive pollution);
②The probability of self-detonation at the high voltage end and low voltage end of insulator string b is greater than that in the middle (the high voltage end and low voltage end have strong electric field, and local creepage occurs at the steel pin of insulator first when the pollution is too heavy);
③ There is damage to the steel foot of the insulator that has not self-exploded in the same tower (local arc caused by excessive pollution damage to the glass parts near the steel foot), and there are fine cracks inside the umbrella surface;
3. Countermeasures
(1) Network access control, through sampling detection of mechanical damage, steep wave impact performance to control the quality of glass insulators into the network.
(2) Composite insulators are used in heavy pollution areas. In case of concentrated self-detonation caused by excessive pollution, composite insulators can be used instead of glass insulators.
(3) Strengthen inspection, in bad weather such as lightning strikes should be timely to the transmission line special patrol.
(4) Pay attention to transportation, infrastructure, rush repair transportation toughened glass insulator, should be protected by protective materials, so as not to be damaged.