Leakage current on polluted insulators’ surface is a major cause of insulation failure in high voltage power lines. Maintenance of those lines thus necessitates the periodic cleaning the insulators’ surfaces, which is known to be a costly operation. The magnitude of leakage current on a polluted insulator depends on pollution severity and the contamination salinity, which subsequently affects the conductivity of the contamination layer. With thousands of kilometers of transmission and sub-transmission lines in Sinai, rather than relying on the costly insulator washing, composite insulators are nominated to be used instead of ceramic insulators. Composite insulators are now widely used worldwide because of their lower weight, higher mechanical strength, higher design flexibility, and their reduced maintenance. They display lower leakage current due to their higher surface resistance [1] and [2]. Silicone rubber – used to fabricate insulators – can provide long-term and satisfactory service even under polluted and wet conditions. This is due to its long-term hydrophobic surface properties. The hydrophobic surface inhibits the formation of a continuous water film and the flow of leakage current along the surface. This blocks the initiation of dry band arcing that leads to flashover. In a study by Zhang and Hackam, the strong relation between hydrophobicity and high surface was established when high temperature vulcanized (HTV) silicone rubber rods were subjected – under high voltage – to accelerated wetting in salt-fog and immersion in a saline solution [3]. The surface resistance was measured and found to depend on the duration of the exposure to the salt-fog without electric stress, the duration of the exposure to combined salt-fog and electric stress, and the specimen length.
The pollution layer accumulated on the insulator surface during normal desert atmospheric weather has a thickness that depends on the type of soil in this region and on the polluting sand grain sizes. When sand is deposited on insulator surface and in the presence of a major source of wetting, such as dew in the early morning, leakage current would flow on the surface. Conductive sand areas are then heated, and dry bands are formed leading to possible surface flashover [4].
Relevant previous work in this area included estimating the current density distributions along polluted insulator surface, using surface charges simulation method [5]. Other studies simulated the leakage current while accounting for amount of salt in the contamination layer [6]. Other experimental studies were made on the effect of desert pollution on polymeric insulator [7] and [8]. In another study, leakage current was estimated using the FEMLAB software with different conductivities of contamination layer [9].
This paper aims to investigate the prime factor responsible for initiating insulator failure under power-frequency voltage, namely leakage current flowing through surface pollution.
Insulator simulation was carried out using an accurate 3-D ANSYS software program, which is based on the Finite Elements method. The program required higher performance computing and gave results with high accuracy. The ratings of transmission lines in Sinai are mainly 500 kV, 220 kV, and 66 kV. A typical two-shed insulator, which may be used on 220 kV power lines is used as a case study. Such leakage current distributions are determined with different sand grain thickness and with different sand conductivities. Realistic data are used, which are based on sand samples collected from Sinai desert near present and future transmission lines’ corridors and were reported by an earlier study [10]. In that study, the statistical distributions of sand grains size in the desert soil were acquired from random samples, where their salinity and subsequent conductivity were measured. Based on the calculated influence of sand grain size and salinity on the resulting leakage current, statistical distribution mapping was carried out to produce the overall probability density distribution of leakage current. The cumulative statistical distribution of leakage current was then employed to assess the risk of insulator failure.