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.


Introduction

At the start of 2018, the MEW network comprised some 600 substations that connect nearly 10,000 circuit-km of overhead lines operating at 400 kV, 300 kV, 132 kV and 33 kV. These run along some 500 km of shoreline as well as across flat sandy desert.

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This power system serves a typical load profile dedicated mainly to extensive air conditioning and water desalination with the former alone typically accounting for nearly 70% of peak load and over 45% of total electricity consumption. As annual demand continues to grow rapidly, MEW has faced challenges in aligning its strategic decisions to assure consistent high quality service. For example, starting in 2015, management and technical staff implemented an action plan to improve key performance indicators of the transmission and distribution system. The goal was to keep the Kuwaiti network among the most reliable and with one of the lowest outage rates in the region – in spite of being exposed to severe pollution. Among the various initiatives was one focused on predictive and preventive maintenance as well as remedial measures to improve performance of line insulators. One of the cornerstones of this has been a program to apply RTV silicone coatings to porcelain long rods installed on overhead lines.

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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Outdoor Insulation on MEW O/H Network

MEW’s overhead system is insulated mainly using anti-fog type porcelain long rods. This applies to the entire 400 and 300 kV network as well as to some 98% of the 132 and 33 kV systems. The balance is served by porcelain cap & pin strings.

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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To mitigate impact from deposition of contaminants as well as heavy wind loads, 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 kV and 300 kV tension and suspension insulators are 24 mm and 20 mm respectively. Tables 1 and 2 summarize circuit lengths as well as insulation and technical characteristics of the long rod insulators being used on each system.

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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

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Maintenance Strategy

Pollution has not been the only factor affecting transmission lines in Kuwait. For example, the network near the Al-Zour Power Station in the south and indeed the southern network in general have also recently suffered from dense fog, sandstorms and heavy winds. In fact, a combination of these led to a blackout in February 2015 that provided important lessons and offered an impetus to finally overcome perceived weaknesses in insulation on overhead lines. 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 the 2015 event, at least 24,000 towers were live washed annually while, in 2017, some 900 towers were inspected using infrared cameras as part of a targeted preventive and predictive maintenance program.

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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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

There has been growing interest among utilities worldwide to apply RTV silicone coatings to mitigate pollution-related flashovers of ceramic insulators due to inappropriate initial selection or insufficient self-cleaning. This 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 and surface damage due to discharges. In fact, studies have demonstrated that there can be significant differences between different RTV coating materials in terms of longterm performance, especially after ageing in a harsh desert environment. Given this, MEW engineers devoted a great deal of effort to identifying the most suitable type of coating formulation for application in Kuwait, including the most reliable primer as bonding agent. Testing confirmed that the RTV formulation eventually selected showed the desired adhesion properties and indeed was virtually impossible to remove from coated surfaces once cured.

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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. Moreover, taking down and coating existing long rod insulators was deemed more cost effective than the alternative of changing insulator technology. Such a solution can be mastered and implemented inside a coating plant that includes an area devoted to curing and where critical parameters such as temperature and humidity can be closely controlled.

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Cleaning porcelain long rods removed from lines is first step in coating procedure.
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Construction of this type of localized unit for RTV coating in Kuwait was the result of collaboration between all stakeholders. Indeed, the air-conditioned workshop proved a model of what could be achieved since end quality was judged comparable to what could be expected from a largescale coating factory. The space offered separate areas that together allowed a finished coated product meeting all MEW specifications and requirements. For example, one area is reserved for insulator cleaning and preparation of the silicone material. A second is dedicated to application, which is accomplished using conventional air spray guns and an original machine with rotary axes modified to hold up to three long rod insulators at a time. This set-up allowed optimizing application time and costs such that at least 40 long rod insulators could be coated each day. A third area is devoted to curing the coated insulators while the fourth focuses on quality control and packing such that the insulators are well-protected during transport and also fully ready for installation at site.

 

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Application of RTV silicone coating by spray gun.
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Since dust has great adverse impact on adhesion of an RTV coating to porcelain, cleaning and preparing insulators in advance of coating application is of paramount importance. All incoming long rod insulators are therefore thoroughly washed using 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. Final coating quality requires a consistent primer layer of less than 10 μm, without trapped air.

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Measuring coating thickness.
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insulator Coating Porcelain Line Insulators to Prevent Pollution Flashover Screen Shot 2018 04 23 at 12
Great care required in packing coated insulators for delivery to installation site.
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This is followed by drying time of between one to two hours at ambient temperature. Then, not 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 to 300 microns, as specified for MEW’s power 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 material and solvent as well as on ambient temperature and humidity inside the workshop and curing must always take place under these consistent, controlled conditions.

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Quality engineers use pre-calibrated ultrasonic thickness gauges to conduct daily inspections of coating thickness on each cured long rod insulator. Reports of results from at least five different measurements taken at different locations on the top as well as from under-rib shed surfaces can show thickness variations of between 125 and 322 microns.

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

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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 Time for Live Replacement of HV String Insulators
Table 4: Time for Live Replacement of HV String Insulators.
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