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.
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.
This method can be used in areas characterized as having instantaneous pollution or high or low levels of NSDD with a long life expectancy. A financial analysis showed that this method was the optimal choice, and it is economical.
Convenient application is a major advantage that significantly affects the cost. The insulator surface should be cleaned before coating; a surface wash by high-pressure water and a sweep with isopropyl alcohol is recommended. In the event the insulator has been greased previously, it should be removed with proper solvent, such as naphtha. All surfaces must be completely dry prior to application.
With proper evaluation using the NRI pollution map, the reinsulation of many critical transmission lines has affected a long-term solution to pollution-related system outages. Moreover, when compared to insulator washing, considerable cost benefits have been realized.
Based on the contamination levels measured throughout the Khuzestan province, all ANSI class 56-3 pin insulators were replaced with ANSI class 56-5 pin insulators on a 33-kV distribution overhead line close to the Persian Gulf in Mahshahr. The insulators that were replaced showed considerable signs of pollution. To date, no failures have been reported during the past five years, and additionally, the initial cost of this replacement insulator exercise was recovered within four years.
Composite insulators installed in Hormozgan province have solved the pollution problem on transmission and distribution lines, and since their installation, no circuit failures have been reported in three years (Fig. 5).
The ESDD value alone may not necessarily reflect the real pollution situation; the new NSDD/ESDD criterion is required. It is suggested that a new category is required for the classification of pollution severity for exceptional areas such as Geshm Islan, which have an ESDD greater than 1 mg/sq cm, as well as a minimum specific creepage distance equal to 39 mm/kV.
The effect of the insulator profile on the measured ESDD level shows how important it is to use the correct type of insulator when taking these measurements.
The pollution level of a given site, when assessed with the ESDD measurement method, represents how the particular insulator is used and its expected performance for the type and magnitude of the pollution at the location. In contrast, the DDG method yields a value that is almost always a more direct measurement of the type and magnitude of the locations level of pollution.
The pollution-index criteria obtained, based on field tests, are in good agreement with the ESDD/NSDD method. Therefore, use of the simple and inexpensive DDG method is recommended for the measurement of the severity of site pollution. RTV and replacement methods are presented as useful maintenance strategies for substations and transmission lines in polluted areas.
FUTURE RESEARCH PROGRAM
Phase 3 of the project, being supported by Yazd Regional Electrical Co., has begun. This phase is evaluating pollution levels at various sites in Yazd State, a desert-like area with high UV radiation. ESDD, NSDD and leakage measurements will be performed on six different types of insulators, including a standard cap and pin, a semiconductive glaze disc, an anti-fog disc, a composite post and two types of composite insulators.
The authors wish to acknowledge: Mr. Masoudi, deputy of Development & Economic Affairs of Tavanir, and Mr. Shirani, vice president of Niroo Research Institute, for their help and support; Mr. Oskouee, Dr. Arabani, Mr. Asadian and Mr. Omidvari Nia for their invaluable assistance and advice; the managing directors of Iran's utilities and staff for their great support of this research program; all the Iranian Utilities engineers and specialists who contributed to this project; as well as Mr. Malekpour, maintenance planning manager of HREC, and Dr. C. de. Tourreil and Dr. W.L. Vosloo for their great support and useful advice.
Mohammad Reza Shariati received the BSEE degree from Iran University of Science & Technology (Tehran, Iran). He is currently engaged in the establishment of the Iran Pollution Map and the evaluation of insulators for the High Voltage Department of Niroo Research Institute. His field of interest includes insulator contamination, composite insulators and maintenance of insulator devices. He is a member of CIGRÉ. [email protected]
Ali Reza Moradian earned a BSEE degree from Tehran University (Tehran, Iran) and is the director of the Transmission and Distribution Research Center at Niroo Research Institute. His research interests are in the field of transmission line and insulation coordination and distribution systems. Moradian is a member of the CIGRÉ Iran Study Committee on Insulators and Transmission Lines. [email protected]
Seyed Jamal Addin Vaseai received a BS degree from Tabriz University (Tabriz, Iran) and a MSEE degree from the University of Mazandaran (Babol, Iran). He is currently working in the T&D Research Center at Niroo Research Institute, where he is a member of the pollution team. His research interests include T&D systems. [email protected]
Seyed Ali Kamali received a BSEE degree from the Power & Water University Technology (Tehran, Iran). He is studying for a MSEE degree at the Iran University of Science & Technology, with research interests that include T&D systems. [email protected]
Godratollah Alizadeh received a BSEE degree from Power & Water University Technology (Tehran, Iran). He currently works in the planning department of Hormozgan Regional Electrical Co., where he specializes in the maintenance of high-voltage transmission lines and substations. [email protected]
|Transmission line||Number of washings per annum|