About the mechanical strength of insulators

[Admin]

About the mechanical strength of insulators

One of the reasons leading to wire breakage of overhead power lines is the destruction of the insulators with which the wire is attached to the support. An important property of the insulator is its tracking resistance. Usually, tracks on the insulator surface appear due to its contamination with a conductive composition (dust, salts, minerals, etc.) and deterioration of the hydrophobic properties of the outer coating. A similar problem was encountered on the 35-500 kV overhead power lines running within the city of St. Petersburg and the October Road power lines (27.5 kV). The way out of this situation was the use of polymer insulators (Fig. 1) in combination with the use of insulated wire.

Currently, more than 250 thousand polymer insulators are in operation in the Russian energy systems, which include the electricity supply industry. Their implementation has significantly reduced the cost of construction and operation of the lines. Global practice shows that the use of advanced polymer insulation on overhead lines will continue to increase steadily.

Non-ceramic insulators are divided into composite ones, consisting of several types of polymers, and solid ones (made of one material). Composite products are most widely used. The IEC 1109 (1992) standard applies only to linear composite insulators (suspended and tensioned insulators, overhead line spacers). It was developed first, and based on it, the bulk of the polymer composite suspension elements were subsequently created. The structure of the considered insulators is shown in Fig. 2.

Some of the first types of composite suspension insulators from various manufacturers proved to be mechanically unstable even after a very short service life. They collapsed under significantly lower mechanical loads on the insulators than the nominal ones. The fracture surface of fiberglass was noticeably different from that observed in laboratory tests. This type of fracture is called a brittle fracture.

It most often occurs inside the metal reinforcement of insulators, where the distribution of mechanical stresses along the cross-section of the rod is particularly uneven, or at a distance of 5-10 cm above the lower end (here, in the absence of screens, the greatest electric field strength is observed). The crack, which begins brittle fracture under the influence of a tensile load, slowly spreads until, due to a gradual decrease in the cross-section of the rod, the stress increases to a sufficiently high level, producing a rupture of the fibers. Examining the surface of brittle fracture with a microscope allows you to see the "stop lines" where cracks begin.


Evaluation of many brittle fractures shows that they are associated with high mechanical stress, slow crack propagation, and their initiation on the surface of the fiberglass rod. An obligatory fact accompanying such damage is the presence of active chemicals in contact with fiberglass, especially an acid solution. Thus, brittle fracture is associated with the erosion of the fiberglass material in combination with mechanical stress. When the acid comes into contact with the glass fibers, an ion exchange occurs between the acid and the glass lattice. This increases the loads on the glass fiber surface, causing spiral cracks on the glass surface.

As is known, fiberglass rods of composite insulators are made of fiberglass embedded in a polymer resin. They determine the high mechanical strength of the rods. Cracks start in the resin and usually stop spreading near the fiberglass. If acid reaches the glass fiber (usually near or on the surface of the rod), the fiber breaks in the plane of crack propagation. The breaks occur gradually, fiber by fiber. Acid can also migrate longitudinally, contributing to the spread of brittle fracture along the rod. At the same time, the mechanical stress in front of the crack increases, and it develops even faster. At the final stage, when the rate of crack formation reaches the speed of sound in fiberglass, the fracture changes from brittle to normal.

The phenomenon under consideration is observed on composite insulators exposed to normal atmospheric conditions, since some acids in different concentrations may be contained in the air. Thus, nitric acid can form on the surface of the elements during electrical discharges in humid air. The risk of brittle fracture increases dramatically if the ribs of the insulator shell are damaged and expose the core. A particularly sensitive area is the transition point from the insulator shell to its end fittings. Materials with different coefficients of thermal expansion are used here. They must be interconnected in such a way as to avoid moisture entering the internal cavity of the reinforcement.

Brittle fracture is inevitable when using unidirectional fiberglass rods with a longitudinal orientation of glass fibers. The destruction of one fiber directed along the applied mechanical load increases the stresses of the remaining fibers in this direction.

Another objective negative factor affecting the fault tolerance of composite insulators is the presence of several separation surfaces (boundaries) between different materials. In most composite insulators, the following interface surfaces are present: fiberglass — impregnating resin; filler particles — polymer; skirt — skirt; intermediate layer — skirt; shell — rod and metal reinforcement. The most dangerous of them are fiberglass — impregnating resin, core — sublayer, sublayer — shell. These interface surfaces have a direction that coincides with the direction of the electric field strength. In the presence of microdefects in them, irreversible phenomena occur under the influence of an electric field, reducing electrical strength.

There are cases when a microbubble of air at the rod—shell interface caused the entire insulator to fail. The mechanism of destruction is as follows. The air cavity is gradually filled with moist air, and under the influence of electrical voltage, electrolytic processes occur with the formation of gases, including acid. The cavity increases under gas pressure, and the shell swells and detaches from the rod. The acids in the gas, when interacting with the rod, initiate the process of brittle fracture. The direction of separation along the electric field strength contributes to the development of these processes.

One of the ways out of this situation is to use a monolithic polymer insulator with chaotic glass fiber reinforcement. It can be made of epoxy resins and reinforced with short glass fibers. The body of the insulator, along with the skirts, is solid. In this case, the surface of the glass fiber—impregnating resin interface is oriented chaotically, and only a small part, up to a fraction of a percent, is oriented according to the electric field strength. Such reinforcement reduces the strength of plastic.
As calculations and foreign experience show, an increase in the diameter of the insulator by 1.5... 2 times makes it possible to achieve the same strength characteristics as those of highly directional fiberglass. Reliability indicators have become higher. It should be borne in mind that increasing the strength of oriented fiberglass to 800 MPa is not an end in itself. The large coefficients and safety margins of fiberglass rods are caused, among other requirements, by the statistical distributions of these indicators.

In order to increase reliability and reduce the likelihood of failure, more and more durable oriented fiberglass is used in construction, while the reason for failures lies in the oriented materials themselves. The mechanical and electrical characteristics of polymers with chaotic reinforcement are isotropic, and the curves of the destructive load distribution on the samples do not have the negative trends characteristic of highly oriented plastics.

We can assume that such materials behave like a monolithic homogeneous body. Their use significantly reduces the possibility of a brittle fracture (theoretically it is absent). The protection of such an insulator can also be carried out by an organosilicon rubber shell in the form of a film up to 2 mm thick. The experience of using such coatings has been accumulated in North American countries on substation insulators and in Asian countries on insulators of all types.

The use of such a coating does not completely exclude the insulator body — shell interface, but the interface repeats the leakage path, the configuration of the insulator and does not lie along the direction of the electric field strength. Micro Cavities that may be on the edge of the insulator skirt do not lead to irreversible consequences. Even partial exposure of the insulator skirt does not lead to brittle fracture, since the skirt does not carry a mechanical load, the electric field acts at a different angle and is weaker, and the plastic does not have highly loaded long fibers.

Let us briefly discuss the experience of using coatings for insulators made of various resins, as well as for insulators made of porcelain and glass. Currently, organosilicon shells (silicones), which have proven themselves in operation, are applied not only to fiberglass in composite insulators (the widest range of applications), but also to porcelain substation insulators. They are much less often used on glass and porcelain linear insulators. Such resins are used to improve the reliability of parts in heavily polluted areas. In the literature, these coatings are most often referred to as RTV ("room temperature vulcanization").

[СЦБот]

Эта тема была перенесена из раздела Комната совещаний.

Перенес: Admin. Держитесь и всего вам доброго.