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Application of polymer post insulators (PI) in high-voltage electric-power industry in Russia has the history of more than 30 years. For this time the industry managed to accumulate significant experience of usage allowing choosing successfully functioning designs and technological processes and determining the scope of polymers suitable for long-term usage under various, sometimes quite frigid climatic conditions. Nonetheless, the search for new materials, technologies and design of polymer insulators still continues. The accumulated experience allows making a set of observations and conclusions as for the peculiarity of the PI and the necessity for taking such peculiarities into account in process of choosing the right option of out-door installation type.

Deterioration factor

One of the main differences between PI and traditional porcelain and glass insulators is deterioration (ageing) – i.e. the possibility of deterioration of characteristics under long-term impact of multiple operational factors. Deterioration of polymer insulation was observed many times in laboratories. There are several reasons behind this phenomenon:
  1. Tracking and erosive ageing of protective coating due to the impact of thermal-oxidative and hydrologic processes;
  2. Solar irradiation impact;
  3. Decay of a glass-fiber and contact areas due to internal discharges existing for a long time;
  4. Mechanical disintegration of the insulator’s core as a result of micro cracks formation (in particular, in combination with acid environment and electrostatic field – the so-called “brittle fracture” phenomenon);
  5. Multiple changes of temperature in combination with a significant mechanical load.
The choice of a group of materials based on the silicon rubber resulted in a partial, but significant, solution of the PI outer surface deterioration problem because of the silicon rubber’s high resistance to impact of acids, alkali, UV irradiation and excellent hydrophobic properties of the protective coating’s surface. And such solution results in a significant change of a breakdown mechanism along the contaminated moistened surface.

Peculiarities of a discharge formation along the PI surface

The specific properties of the PI surface (mostly, its hydrophobic properties) are well manifested after accumulation of dust and moisture at the contaminated surface. The experience of usage demonstrates that dirt retention for PI is significantly lower than for the similar porcelain and glass insulators. The hydrophobic properties of the surface result in significantly lesser possibility of discharge formation. Single drops of water on the surface have no ohmic contact between them thus preventing usage of notions of “creeping current” and “insulation distance”. Let us examine (as an example) the results of testing of the post insulators IPRP-10-110/480 manufactured by the “Alfa-Energo”. Those insulators remained in operation for a year at the Krasnoyarsk smelter. The description of environmental peculiarities: existence of acid-based emissions and fluorides as well as thermal differential in the range from -40◦C to +40◦C. Testing has demonstrated absolute preservation of hydrophobic properties, absence of corona discharge and internal discharges formation (not more than 20 Pc). Leakage impedance in the moistened state corresponds to the cross-coiled ohmmeter indication “infinity” (with 1500 V voltage applied). The natural consequence of the year of operation of the insulator is occurrence of gray residue of contamination without any indications for deterioration of properties of the protective coating. The contamination has no adverse impact on hydrophobic properties of the surface, does not influence decrease of surface resistance and may be removed by slow technical solvents (i.e., ethanol). This is a manifestation of specific properties of the PI with silicon protective coating indicating at the necessity of development of special procedures of testing:
  1. The hydrophobic property is being passed from the surface of silicon rubber protective coating to contaminants with a certain delay, which has no definitive value under conditions of natural contaminations formation due to the slow speed of their build-up. This process shall not be intensified in course of testing with artificial contaminants, because in this case one of the most important advantages of the protective silicon rubber coating will be lost.
  2. The hydrophobic capability of the coating results in formation of single, not chain-linked drops of moisture at the surface. In course of testing one shall not intensify the humidification process up to the level when constant water run-off from the surface creates an uninterrupted film. Under natural conditions such level of humidification occurs quite rarely, and the forceful formation of the uninterrupted insulation distance distorts the process of natural phenomena formation at the surface in course of low or average intensity of humidification.

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