Pollution Deposits Can Accumulate on an Insulator Surface and become conductive paths when the insulator surface is wet due to rain or fog. This creates an increase in the leakage currents over the insulator surface, which decreases the electrical withstand voltage of the insulator, finally resulting in a flashover.

Iran has a transmission system that comprises 90,000-circuit km (56,000-circuit miles) operating at voltages of 400 kV, 230 kV, 132 kV, 66 kV and 63 kV. The country's distribution network extends 300,000-circuit km (186,000-circuit miles) with operating voltages of 33 kV, 20 kV and 11 kV. The capacity of the transmission substations is more than 170,000 MVA, and some of these substations, located close to marine, industrial, desert and agricultural regions, are subjected to pollution.

Insulator contamination caused by the close proximity of the Persian Gulf and Oman Sea, coupled with industrial pollution, have a major impact on the reliability of the transmission system and distribution network. Climatic conditions play a significant role in the pollution of insulators with the atmospheric characteristics of relative and absolute humidity, dew, fog and rain, establishing the formation of the conductive film. The resulting insulator flashovers in the regions of Iran that experience severe pollution have an adverse effect on the lifetime of insulation and system reliability. Operating records indicate that the transmission and distribution networks in many Iranian states are not able to cope effectively with these difficult service conditions, especially the overhead lines in the southern provinces.

The performance of insulation, circuit operational history and maintenance cost in the southern provinces of Iran have prompted the need for in-depth studies of the pollution problem. One of the Iranian Ministry of Energy's priorities has been to assist utilities in fully understanding their pollution environments to optimize the selection of insulators designed to withstand these adverse service conditions. This task was recently assigned to the Niroo Research Institute (NRI), which was established in 1982. As the country's principal power research organization, NRI has played a leading role in developing new technologies for the electricity supply industry.

IRAN'S INSULATION POLLUTION PROBLEM

In accordance with DIN50019, the coastal districts of the Persian Gulf are classified as an extreme hydrothermal climatic zone. Unusually high saturation vapor pressure, up to 53 millibar, has been recorded in this region, justifying this classification. The air humidity phenomena is unique, with daily temperature variations of 30°C to 35°C (86°F to 95°F) and intense night deposits of dew for several months coupled with long periods of rainless and sultry weather. These are the contributing environmental factors for the deteriorating reliability of the transmission and distribution systems. In spite of frequent maintenance, Iran's transmission and distribution networks have experienced many insulator failures. Figure 1 shows examples of failed and polluted insulators.

Periodic cleaning of line and substation insulators has been the common approach to the problem, but this solution has to be repeated at a rate that will stay ahead of the rate of contamination in order to be effective. Records confirm that in the most heavily polluted region of Iran, the frequency of line insulator washing is five or more times per annum and as high as 20 times per annum at substations, as shown in the table.

POLLUTION STUDIES

Many methods have been devised to quantify the pollution stress of the environment on the insulators. The most commonly used are the Equivalent Salt Deposit Density (ESDD) method and the Non-Soluble Deposit Density (NSDD) method, but more recently, the new Directional Dust Gauge (DDG) method is being applied.

In 2000, NRI in cooperation with engineers from Iran's Khuzestan Distribution Electric Co. (KDEC) and Tavanir, the Iran power generation, transmission and distribution management company, embarked on a project to evaluate pollution levels at several sites along the coast near the tip of the Persian Gulf. Established for a period of about two years, this project was the first phase of what became a three-phase project.

In the first phase, 30 stations were established in the Khuzestan province to investigate insulator behavior and pollution severity. Each station contained an insulator string of five cap-and-pin discs, an ANSI class 56-3 pin insulator, an ANSI class 56-5 pin insulator, a long-rod insulator and a pedestal insulator supported on a concrete or wood pole structure (Fig. 2).

In Phase 2, which was supported by Tavanir, 75 other test stations were installed in West Azerbaijan, Guilan, Mazandaran, Bushehr, Hormozgan, and Sistan and Baluchistan provinces, each containing an insulator string of six cap-and-pin discs. The top dummy insulator disc was used to evaluate the self-cleaning properties of the insulator. A total of 105 ESDD/NSDD and DDG test stations were located in heavy industrial and marine pollution areas of Iran with recorded adverse operational histories.

The insulators and DDG instruments were exposed to contamination conditions for a year. ESDD/NSDD measurements were done based on IEC-60507 in three monthly sampling intervals; a pollution index was obtained using the maximum of 12 monthly DDG measurements.

A pollution map of Iran was established using NRI's pollution field studies and by analyzing the results of Phase 1 and Phase 2 of the project (Fig. 3). This map is used to select the number and type of insulators for a given line and to establish the effectiveness of insulator-maintenance procedures, such as greasing, washing, silicon-rubber coating and creepage extenders for substations.

HIGH CONTAMINATION AND SYSTEM RELIABILITY

Several approaches to improving reliability in contaminated environments have been developed over the years, and many are incorporated into industry standards and utility practices around the world. The many approaches to dealing with contamination can be categorized into three classes: design, cleaning and insulator-surface modification.

Washing line insulators imposes costs as well as operation and coordination problems; therefore, washing is limited by technical and economical constraints. Also, it is not effective in instantaneous pollution and high NSDD regions.

Applying silicon grease on insulators to improve performance is a costly exercise, and it is not recommended for use in an environment where a high level of NSDD pollution is present.

The third approach is to modify the surface properties of the insulator. Due to its ability to maintain and recover water repellence, or hydrophobicity, silicones of various forms are commonly applied to achieve improved performance. The hydrophobic material on the surface interrupts the flashover mechanism at an early stage, preventing the formation of a conductive water film, thereby limiting leakage currents. For new construction, many types of distribution- and transmission-class substation and line equipment are currently available with silicone-elastomer housings that offer this benefit. But for existing equipment in problem areas, silicone greases and elastomeric coatings (room temperature vulcanized [RTV]) have been developed. Figure 4 shows RTV-coated equipment.