Effect on corrosion and service life of insulators

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Azerbaijan State Oil and Industry University

Effect on corrosion and service life of insulators

Safiyev E.S., Ass. professor

Huseynov K.M., Master student

Baku, Azerbaijan

Annotation

Porcelain insulators must be replaced periodically, but their service life is not precisely defined. One factor affecting service life is corrosion on the pin and cap - each made of zinc-plated iron. A number of porcelain insulators used in different lengths were collected at different locations, and the corrosion mechanisms of the cap and pin were investigated. The corrosion mechanism of the cover is mainly galvanic corrosion, and that of the pin is electrolytic and crevice corrosion, as well as galvanic corrosion.

Keywords: corrosion, insulators, porcelain, lifetime, service life, surface area.

Аннотация

Влияние на коррозию и срок службы изоляторов

Сафиев Э.С., доцент, Гусейнов К.М., магистрант, Азербайджанский государственный университет нефти и промышленности, Баку, Азербайджан

Фарфоровые изоляторы необходимо периодически заменять, но срок их службы точно не определен. Одним из факторов, влияющих на срок службы, является коррозия штифта и колпачка, каждый из которых изготовлен из оцинкованного железа. Было собрано несколько фарфоровых изоляторов разной длины в разных местах, и были исследованы механизмы коррозии колпачка и штифта. Механизм коррозии крышки в основном представляет собой гальваническую коррозию, а штифта - электролитическую и щелевую коррозию, а также гальваническую коррозию.

Ключевые слова: коррозия, изоляторы, фарфор, срок службы, срок службы, площадь поверхности.

Insulators are used to mechanically support transmission lines and provide electrical isolation from the grounded tower structure. There are three main types of insulators: glass, polymer and porcelain. The insulator occupies a small part (5-8%) of the installation costs of overhead transmission lines, but accounts for more than 50% of the repair costs in case of failure and plays an important role in determining the reliability of the line. Therefore, it is important to replace insulators in time. Porcelain insulators are mainly composed of three parts: porcelain, metal (cap and pin) and cement (bonding the porcelain and metal parts). Factors affecting the service life of porcelain insulators are ceramic deterioration, cement degradation and corrosion of metal parts. In this paper, corrosion mechanisms in the cap and pin of porcelain insulators are described and the effect of corrosion on the service life of insulators is discussed. Porcelain insulator samples are collected from various 25,000-pound transmission lines operating at 154 kV in South Korea. These are discarded from utility systems to access the state of similar samples. It has been classified into different pollution levels. Pollution degrees in South Korea are divided into A, B, C and D in ascending order of pollution level. It is classified as distance from the coast and class D is the most polluted. Table 1 describes the relevant specifications of the samples collected. The dependence of corrosion on 154 kV porcelain insulators on geographical features is analyzed.

The corrosion mechanism of the cover is mainly galvanic corrosion, and that of the pin is electrolytic and crevice corrosion, as well as galvanic corrosion. Figure 2 shows examples of insulators used at different times. For an insulator that has been used for 40 years in Gangneung, the pin showed little rust, although there were numerous scratches on the cap. For the insulator used in Dae-gu for 48 years, there was significant rust on one side of the cap and a lesser amount of rust on the pin. For the insulator used in Namyangju for 51 years, the corrosion of the cap and pin was not very serious. Based on observation, it was determined that the corrosion rate and corrosion rate depend more on where the insulators are installed than on the service life of the insulator (Figure 1). The factors required to calculate the corrosion rate are the weight of the metal, the density of the metal, the total initial surface area of the metal and the reduction in the length of the service life during the reference period [4-6].

Table 1

Figure 1. Appearance of pins and caps of porcelain insulators used for (a) 38 years (b) 40 years i (c) 48 years, and (d) 51 years

The role of zinc is galvanic protection and oxide film formation. Galvanic corrosion occurs when two dissimilar metals in an electrolyte come into contact, causing current to flow between them. As a result, it prevents corrosion of highly -corrosion resistant metals (cathode) and promotes corrosion of highly active metals (anode). When iron and zinc are in contact, if the iron is exposed due to a flaw or scratch, it gradually becomes cathodic. This zinc hydroxide film is stable in the pH range of 6-12 and acts as a barrier against the diffusion of oxygen, which suppresses the corrosion of zinc. Figure 2 shows the dependence of zinc corrosion rate on PH. In a real freshwater environment, a complex surface layer consisting of zinc oxide (ZnO) and basic carbon zinc (2ZnCO3*3Zn (OH)2) and zinc hydroxide is formed under the influence of various dissolved ions. The corrosion rate of zinc in distilled water shows a relatively high value of about 0.25 mm/y at pH 7. However, the surface coating formed in actual fresh water serves as a protective layer to reduce the corrosion rate of zinc to a lower level.

Figure 2. Zinc corrosion rate dependence on pH

service life porcelain insulator corrosion iron pin cap

Finally, the main material of iron is exposed. At this time, the surrounding galvanized sheet cannot serve as a sacrificial anode, because the polarity is reversed relative to the iron, and the valuable potential of the zinc coating surface accelerates the corrosion of the exposed iron matrix. As the pH changes from neutral to acidic, the corrosion rate of zinc increases. This is because basic materials such as zinc oxide, zinc hydroxide and basic carbonate salts, which form a protective coating on the zinc surface, are neutralized and destroyed in the acidic environment. Figure 3 shows the corrosion rates of zinc at different thicknesses and environments. A metal surface with 5% rust is reported to require repair or replacement. For example, a thickness of 75 pm zinc will last about 56 years in industrial areas and more than 100 years in rural areas.

Figure 3. Corrosion rate of zinc in different environments

Conclusion

This article focuses on corrosion in porcelain insulator and its dependence on geographical conditions of the region. Various types of corrosion occurring on the pin and cap have been reported. The cap and pin of the porcelain insulator are made of iron. Everyone knows that iron rusts over time. Corrosion prevention methods have been investigated. A study of porcelain insulators collected from different regions of Korea and used for different periods of time shows that the geographical environment has a greater influence on corrosion than the service life. The amount of acid rain and sea salt that causes rapid corrosion depends on the geographic environment.

Список литературы / References

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