Coating Porcelain Line Insulators to Prevent Pollution Flashover

The Gulf State of Kuwait is nestled in one of the driest places on earth. A highly urbanized country, it is also one of the highest per capita consumers of energy, mostly for the oil and gas sector as well as for air conditioning and water desalination plants. Electricity demand peaks during the long dry summer when daily highs can reach 55ºC. There are also temperature fluctuations of as much as 25°C between day and night, which raise relative humidity and lead to frequent morning dew and even fog. Among the catalysts for flashover are frequent seasonal dust storms that result in rapid accumulation of pollution on porcelain insulators – especially along the country’s southern grid. T&D insulation coordination specialist, Raouf Znaidi reports on how maintenance and technical staff at Kuwait’s Ministry of Electricity & Water (MEW) implemented countermeasures to combat pollution flashover and enhance system reliability in an environment dominated by combinations of desert, industrial and marine pollution.


As of January 2018, the MEW network comprised 600 substations that connect 9931 circuit-km of overhead lines operating at 400 kV, 300 kV, 132 kV and 33 kV. This network runs along some 500 km of shoreline and also crosses a flat sandy desert to supply regional desalination plants as well as petroleum refineries and related petrochemical industries. Eng. Jassem M. Al-Nouri – Assistant Under Secretary in charge of the transmission network – explains that MEW produces, transmits and distributes electricity and water to serve the needs of an expanding urban population with one of the world’s highest per capita energy demands while also benefiting from among the lowest electricity prices. The power system in Kuwait serves a typical load profile dedicated mainly to extensive air conditioning (A/C) and water desalination. A/C systems alone account for nearly 70% of peak load and over 45% of total electricity consumption.

insulator Coating Porcelain Line Insulators to Prevent Pollution Flashover Pollution from Al Zoor Power Station combines with contaminants from nearby dry salt lake
Pollution from Al Zoor Power Station combines with contaminants from nearby dry salt lake.
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As annual demand grows rapidly, reports Al-Nouri, MEW faces challenges in aligning its strategic decisions to assure more consistent high quality service to citizens and other electricity users. Indeed, starting 2015, MEW management and technical staff in the electrical sector implemented an action plan to improve key performance indicators of the transmission and distribution system. The end goal is to keep the Kuwaiti network among the most reliable and with one of the lowest power outage rates – in spite of being exposed to severe pollution. Among the initiatives is one focused on predictive and preventive maintenance as well as remedial measures, the cornerstone of which is an ambitious program to apply RTV silicone coatings to improve pollution performance of installed porcelain long rods. This program is especially notable since it is the first in the GCC Region to feature live replacement using the latest generation of modified heavy Zoom line along with high-performance techniques in terms of lifting capacity and work speed.

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Outdoor Insulation on MEW O/H Network

Eng. Fawaz Al Lanqawi – Director Under-Ground Cable and OHL Maintenance – states that MEW’s overhead system is insulated mainly using anti-fog type porcelain long rods. This applies to the entire 400 and 300 kV network and some 98% of the 132 and 33 kV systems, the balance being served by porcelain cap & pin strings.

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To mitigate impact from deposition of contaminants as well as heavy wind loads, he explains that almost all lines in Kuwait are over-dimensioned, leading to unified specific creepage distances of as high as 65 and 63 mm/kV (according to the latest IEC 60815/TS) and to 320 kN and 230 kN minimum failing loads. Coupling size for 400 and 300 kV tension & suspension insulators are 24 mm and 20 mm respectively. Tables 1 and 2 summarize circuit lengths as well as insulation and technical characteristics of long rod insulators being used on each system.

insulator Coating Porcelain Line Insulators to Prevent Pollution Flashover Example of pollution deposition on anti fog profile porcelain long rod in Kuwait
Example of pollution deposition on anti-fog profile porcelain long rod in Kuwait.
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Table 1: Length & Insulation of Kuwaiti T&D System
* Type A* insulated with double 3 long rod insulators
** Type B** insulated with double 4 long rod insulators
*** with extension
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insulator Coating Porcelain Line Insulators to Prevent Pollution Flashover Technical Characteristics of Anti Fog Type Porcelain Long Rods
Table 2: Technical Characteristics of Anti-Fog Type Porcelain Long Rods. CLICK TO ENLARGE

 

 

insulator Coating Porcelain Line Insulators to Prevent Pollution Flashover Severe service conditions in north and south Kuwait
Fig.1: Severe service conditions in north and south Kuwait.
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Maintenance Strategy

Pollution has not been the only factor affecting MEW transmission lines. For example, the network near the Al-Zour Power Station in the south and the southern network in general have also suffered from dense fog, sandstorms and heavy winds in recent years. In fact, a combination of these led to the February 2015 blackout, which Al Lanqawi remarks provided important lessons and offered an impetus to finally resolve perceived weaknesses in insulation on overhead lines.

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Al Lanqawi adds that events such as this have influenced subsequent decisions in regard to the most effective global maintenance strategy that includes pollution countermeasures, reinforcement of lines, periodic inspection, live washing and even dead line cleaning. For example, to reduce incidence of network outages following this event, at least 24,000 towers have been live washed annually while last year some 900 towers were inspected using infrared cameras as part of the targeted preventive and predictive maintenance program.

insulator Coating Porcelain Line Insulators to Prevent Pollution Flashover MEW Maintenance Program 1
Table 3: MEW Maintenance Program: 2015 & 2017.
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RTV Coating Program

The past decade has seen a sharp growth in interest among utilities worldwide to apply RTV silicone coatings to mitigate pollution-related flashovers of ceramic insulators that suffer poor pollution performance due to inappropriate selection or insufficient self-cleaning. Such a solution is usually based on a range of criteria, including effectiveness in suppressing leakage current, maintaining hydrophobicity as well as satisfactory adhesion and minimizing arc degradation or surface damage due to discharges. In fact, published studies have demonstrated that there can be significant differences between different RTV coating materials available on the market in terms of long-term performance, especially after ageing in a harsh desert environment.

Given all this, Al Lanqawi explains that much effort was devoted to selecting the most appropriate coating type and formulation to be applied in Kuwait, including the most reliable primer as bonding agent. MEW’s implementation team now looks to this solution to minimize pollution related leakage current while optimizing the costly and time-consuming remedial measures used in the past. Maintenance engineer Al-Habib, says that testing has confirmed that the RTV formulation finally selected by MEW showed exceptional adhesion properties and was virtually impossible to remove from coated insulator surfaces after curing.

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Cleaning porcelain long rods removed from lines is first step in coating procedure.
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Salam Mohamed Mobasher, a Consultant and former Lines Maintenance Manager at MEW, reports that the solution of applying RTV coatings combines the reliable mechanical properties of porcelain with the excellent pollution withstand and leakage current suppression of silicone rubber. He also claims that taking down and coating existing long rod insulators with RTV silicone is more cost effective than alternatives such as changing insulator technology. This solution, he states, is easy to master and implement inside a coating plant, which also consists of a curing area with controlled temperature and humidity.

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Aymen El Deeb, Project Manager at IMCO Engineering, says that construction of the localized unit for RTV coating in Kuwait was the result of close collaboration between all stakeholders. He points to the air-conditioned workshop as a model that has achieved similar end quality to what one might expect from a dedicated large-scale coating factory. The space consists of separate areas that together allow for a finished coated product that meets all MEW technical specifications and requirements. One area is reserved for insulator cleaning & silicone preparation. A second is dedicated to coating application, which is accomplished using conventional air spray gun equipment and an original machine with rotary axes modified to hold up to three long rod insulators. This set-up has allowed optimizing application time and costs such that at least 40 long rod insulators can be coated each day by the team of qualified workers. A third area sees ambient curing of coated insulators while the fourth focuses on quality control and packing of insulators such that they are protected during transport and ready for installation on site.

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Application of RTV silicone coating by spray gun.
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Al-Lanqawi and Mobashar explain that since dust has great impact on adhesion of RTV coatings, cleaning and preparation of insulators is of paramount importance. All incoming long rod insulators are therefore washed carefully by pressurized water and then cleaned by hand and dried. A first thin layer of special primer is then applied, consisting of a bonding solution of reactive siloxanes and silianes that adhere firmly to porcelain under the controlled ambient temperature and humidity. El Deeb explains that final coating quality requires a consistent primer layer of less than 10 µm, without any trapped air. This is followed by a drying time of between one to two hours at ambient temperature. Then, no more than 5 hours later, two or more successive RTV silicone layers are sprayed on so as to achieve average coating thickness in the range of 250/300 microns, as specified for the MEW electrical network. As the solvent mixed with the RTV material evaporates, moisture in the air triggers vulcanization that forms a solid rubber coating. The speed at which this process takes place depends on type of RTV silicone and solvent as well as on ambient temperature and humidity inside the workshop. According to Mobasher, the curing process for coated insulators must always take place under consistent, well-controlled environmental conditions.

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Measuring coating thickness.
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Great care required in packing coated insulators for delivery to installation site.
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During the visit by INMR, IMCO quality engineers and MEW supervisors were carrying out daily quality control inspections of coating thickness on each cured long rod insulator. This is done using pre-calibrated ultrasonic thickness gauges. Reports of results from at least five different measurements taken at different locations on top and from under-rib shed surfaces show thickness variations of between 125 and 322 microns.

Special attention is devoted to protecting the RTV silicone coating with bubble sheet protectors and wood packing before storage and transport to the job site by truck with an adapted control crane. The entire process – from insulator preparation to coating to curing to inspection to packing and transport for live replacement – is closely supervised by MEW supervisors or representatives.

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Delivery to site of coated insulators. 
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Live Insulator Replacement

MEW devoted much time and effort to mobilizing the human and material resources as well as equipment specially adapted for live line work. The goal was to complete live replacement of existing 300 kV and 400 kV porcelain long rod insulators by the same type but having locally-applied RTV coatings. Since all HV insulator strings in Kuwait are standardized and interchangeable, the option taken by Technical and Maintenance staff to optimize installation time was to remove existing long rod insulators and simply replace these by the locally coated insulators from the workshop. To accomplish this, two specially modified zoom lines were brought in. One crane was dedicated to horizontally (i.e. for tension towers) or vertically (i.e. for suspension towers) holding from 2 to 4 long rod insulators strings through an insulated stick extension. The second zoom line, equipped with an insulated basket for live work, served to hold workers.

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Equipment capabilities in terms of up to an 80-ton lifting capacity and up to 70 m height, along with insulated telescopic length for up to 500 kV, combined with high performance work techniques. Together, these have typically allowed live replacement of four existing long rod strings by four locally coated strings in less than one hour and a half for suspension towers and less than 3 hours in the case of tension towers.

insulator Coating Porcelain Line Insulators to Prevent Pollution Flashover Live Insulator Replacement 3
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insulator Coating Porcelain Line Insulators to Prevent Pollution Flashover Live Insulator Replacement 2
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insulator Coating Porcelain Line Insulators to Prevent Pollution Flashover Screen Shot 2018 04 16 at 12
Table 4: Time for Live Replacement of HV String Insulators.
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