Over its nearly 30 years, INMR has travelled the globe covering power line and substation projects on all continents. We have also been invited into hundreds of factories for electrical equipment worldwide as well as into most major high voltage and high power laboratories. During these numerous site visits, we have taken tens of thousands of photographs to support the resulting technical articles. In this second of a 3-part series, the photos themselves are the primary focus.
Photos were selected based on their meeting all or one of the following criteria:
1. Unique in the sense that they capture a special situation that cannot easily be replicated;
2. Provide unusually high technical or visual overview into the topic of interest; and/or
3. Offer special insight into some aspect of power transmission and distribution.
RTV Coatings Combat Cold Fog Pollution
The Back Story
Most engineers in the power supply business can quickly identify locations in their networks that have been persistent trouble spots for outdoor insulation. In the case of Canadian utility, Hydro One, foremost among these used to be Hamilton Beach – a 230 kV station serving a concentrated industrial area. Located alongside steel and other plants, it also sits directly below a highway overpass that is salted during winter. Due to the combination of cold winter and moisture coming off nearby Lake Ontario, Hamilton Beach is also exposed to freezing rain such that salt contamination adheres to insulators in ice formations.
Over a 10-year period, measures to combat this pollution environment included high creepage insulation and washing up to 30 times per year. Yet problems of flashover affecting insulators and equipment continued. One day in Dec 1989, for example, over a million dollars worth of porcelain insulators were destroyed during a series of pollution flashovers. In 1991, there were 20 different phase-to-ground flashovers – all over a one-night period. By 1994, the situation with flashovers was so bad that all 230 kV beakers were put out of operation, effectively shutting down the station.
As a first step to resolving these problems, research using a special cold fog chamber was undertaken to study the particular flashover mechanism applicable to this location and to evaluate the relative performance of alternative remedial solutions. Among the countermeasures found to be most effective for this environment was application of resistive-graded porcelain insulators as well as coating insulators with RTV silicone. The RG porcelains offered good performance since heat generated from the continuous small leakage through the glaze prevents formation of the ice accretions that mixed with pollution. By contrast, use of hollow core composite insulators to combat pollution-related flashovers was found not to be as effective a solution.
The second major remedial measure involved applying RTV silicone to apparatus such as bushings and cable terminations. These coatings were then pressure washed each year at 800-100 psi from a distance of about 4 to 5 m, typically just before winter. The unexpected finding was that 10 years later most of these bushings still had their original coatings and that these had maintained their hydrophobicity in spite of being exposed to local contaminants such as carbon black.
Hanging by a Thread
The Back Story
Pole failures in desert areas of Oman are triggered by excessive leakage currents on polluted insulators that have become wetted. The main pollution in this case is windblown calcium and quartz found in sand. Dampness comes from an increase in overnight humidity – especially during summer – leading to formation of dew. Around dawn, as surface moisture starts to evaporate, insulators are left with dry and wet patches along their surfaces. Given the service voltage, wet patches can be at different potential such that surface currents start to flow between them, visible at night as blue flashes.
Since pollution levels are not uniform on each insulator, the cross-arm itself soon sees a rise in voltage as the current tries to reach earth by going down either the pole or the stay wire. Given the dry atmosphere, the pole’s cross-arm bolts tend to loosen due to pole shrinkage and thereby allow arcing to occur between bolt and cross-arm strut. This can then ignite the heartwood and a pole-top fire ensues.
Sometimes these die out, in which case the cross-arm bolt is usually left exposed at the center of a hollow burnt-out space. However, as the day heats up, the pole can smolder all the way to its base and leave the cross-arm hanging from the conductor.
Birds and Power Structures
The Back Story
Service experience from various countries suggests that a high proportion of outages on overhead lines can be directly attributed to birds. Even though these types of outages are almost always characterized by successful automatic re-closure, utilities are still concerned. In the past, concern was due mainly to old oil circuit breakers that required maintenance after each cycle of about 15 operations. More recently, the worry is due to requirements for power quality and availability since many utilities set internal rules on average outages permitted each year per 100 km of line.
Any discussion of birds and power lines must first distinguish the situation for distribution voltages from that in the case of transmission. The two are vastly different problems requiring much different solutions. In the case of distribution systems, the central problem is electrocution of birds. Estimates of the numbers of birds killed each year are shocking. Especially dangerous are lines with vertically installed pin-type insulators because the conductor sits on the insulator, not below as in a suspension string.
What makes the situation the more tragic is that the threat posed to birds by distribution lines has been documented for years. Moreover, there is adequate knowledge on how to solve or least largely reduce the problem by insulating the conductors close to the tower using protective devices of different shape and design. The principal requirement for such devices, usually made of polymeric materials, is that they have a service life similar to that of the components they are intended to protect. In this regard, they must be resistant to deterioration from weathering and UV.
Fortunately, this image from the Kalahari Desert bordering Namibia captures a rare case where birds have built their massive nest with no obvious danger to them or to the line. Actually, these sociable weavers benefit from the power structure since it is one of the few elevated places in a flat expanse of desert that gives them protection from ground-based predators.
Not All Rod Fractures are Same
The Back Story
Each insulator technology has its own unique failure mode. In the case of toughened glass and porcelain insulators, these are spontaneous shattering and dielectric puncture, respectively. For composite line insulators, the most serious failure mechanism involves mechanical fracture of its core rod, which can result in sudden drop of conductor.
There is a tendency to automatically label all such failures as ‘brittle fracture’ since this specific failure mechanism has been widely reported and studied now for over 35 years. Typically, insulators experiencing this problem have failed at very low mechanical loads, after service for only months or years, and the fracture surface is characteristically flat and smooth. Fortunately, as a result of design improvements such as use of acid proof, boron-free rods and better field control to prevent corona damage to the housing, incidence of brittle facture has become very low in relation to the population of latest generation composite insulators now in service.
The incidence of another rod failure mode – decay-like fracture – is even lower than for brittle fracture but the consequences can be equally severe. While insulators experiencing this problem have also typically failed at low mechanical loads, the fracture surface in this case is not planar but ‘crisp’, like dead wood. Also, chalking has been found on surfaces of nearby sheds and on the glass fibers in the rod, separate from the resin matrix. In China, for example, this mode of failure has occurred mainly at 500 kV and it is important to note that the rate seems to have increased.
The mechanisms behind decay-like fracture are still not fully clear although it is known that these are often preceded by localized temperature rise and as such detectable by IR imaging. But this only helps in detection and offers no solution on how to prevent the problem.
Finally, it has been found that some composite insulators can fail in-service by another rod fracture mechanism that could easily be mislabelled ‘brittle fracture’. Research at the University of Denver showed that composite insulators can fail in-service even under low mechanical load if crimping deformations applied to the fittings are either excessive or improperly distributed. This type of failure is purely mechanical in nature and with no acid involved, even though the macro-failure characteristics closely resemble brittle fracture.
Squeezing In Another Line Required New Design & Insulators
The Back Story
Pollution and vandalism have been the main factors behind application of composite insulators on transmission lines in Brazil. However, in 2002 engineers at Furnas in Rio de Janeiro were looking to add another transmission line in a narrow corridor between two existing 345 kV lines. Environmental restrictions did not allow this corridor to be expanded and therefore a compact design was deemed the only option.
The 50 km São José-Mage Line thus became the first transmission line at this utility to be equipped with composite insulators along its entire route. Line design engineers at Furnas had in the past been reluctant to ‘experiment’ with a relatively new insulator technology, especially since they recalled problems with original designs of composite insulators dating back to the early 1980s. But in this case, composite line posts were the most logical choice, allowing the new 138 kV line to be built within a much narrower corridor.
Porcelain Long Rods Stressed by Pollution
The Back Story
Insulators have traditionally been one of the major problem areas affecting reliability of the MV and HV overhead networks in Israel. For example, network management once estimated that about one in every four permanent faults on these networks were directly traceable to insulator failures with this proportion rising to nearly one in three if unexplained faults were also considered. Climatic conditions associated with the country’s sub-tropical location have been an important factor since the dry season lasts 8 months during which marine pollution, desert dust and industrial and agricultural contaminants accumulate on insulators. This pollution layer tends to be adhesive and is only partially cleaned away during the months of rain. To compound the problem, a large number of inferior porcelain insulators used to be installed on the network.
Over the years, Israel Electric tried to cope through over-insulation using insulators having as high as 38 mm/kV specific creepage as well as with special shed profiles offering extended creepage. However, neither changed the basic dynamics of the situation. Given the importance of these problems, a dedicated team of specialists was set up to systematically research insulator failures and to evaluate alternative solutions. In addition, an energized test station was set-up in the desert to monitor the comparative performance of different designs of insulators, arresters and other components.
Initial use of composite insulators in Israel began in 1987 with the original motivation for their selective installation being resistance to vandalism on distribution lines. Given this narrowly defined selection criterion, composite insulators were specified even though their life expectancy at the time was estimated at only about 15 years and in spite of a then acquisition cost which was higher than comparable ceramic insulators.
Subsequent years were marked by a growing understanding of the characteristics and features of composite insulators and this was accompanied by a consistent increase in their numbers installed on the network. The transition toward specification of composite insulators for all new line projects in Israel starting in the 1990s was therefore the outcome of a prolonged process of investigating the performance and financial benefits offered. Being a developed country located in a challenging sub-tropical climate only made these benefits that much more attractive.
Handling Hollow Core Composite Insulators
The Back Story
While composite hollow core insulators are now a mature option for electrical insulation applications, it is easy to forget that not long ago few knew how to handle or store these correctly. In fact, due to exaggerated initial claims about resilience of composite line insulators, there was the incorrect belief that these were virtually indestructible. Today, users have come to learn that composite hollow types must be lifted so as not to place undue mechanical stress directly on sheds. Neither should they be stacked such that sheds have to absorb excessive weight placed on them.
While proper handling in the field is always a concern, one of the factors that promoted immediate OEM interest in these insulators is their ease of handling in the factory.
Evaluating Composite UHV Line Insulators
The Back Story
Korea Electric Power Corp. was one of the first electricity supply companies in the world to use composite insulator technology at UHV when it selected silicone-housed bushings for its 765 kV Shin Ansung GIS substation. Still, in spite of the success of this early installation, application of silicone insulators on transmission lines in Korea has lagged. Then, in 2010, looking to take advantage of lower procurement costs and to eliminate the need for periodic washing of lines, KEPCO decided to specify composite insulators for suspension applications on both its 154 kV and 345 kV networks. When it came to the critical 765 kV network, however, changeover in insulator technology required greater investigation. KEPCO is highly conservative and prides itself on operating among the most reliable electrical systems in the world, with an average outage time per customer of barely a few minutes per year. Given this, any new transmission technology tends to be studied extensively to ensure that it will not ultimately have a negative impact on reliability. Indeed, KEPCO evaluated composite insulators for more than two decades before reaching a decision to apply this technology at lower to middle level transmission voltages. Similarly, the design of Korea’s 765 kV grid was evaluated for 15 years before its first practical implementation in 2003.
That is why in 2007 a program was started to evaluate the comparative performance and life expectancy of different designs of silicone insulators on UHV lines. While economic and technical considerations were considered important in final line design, the top priority was meeting the country’s strict environmental guidelines, which in the case of the UHV power highway included ground noise of less than 50 db and ground electric field of only 3.5 kV/m. Achieving such performance came about due to extensive research and testing of different tower and conductor configurations to identify those that optimize corona effects as well as electromagnetic fields. While 765 kV grids now exist in at least nine countries, the network in Korea is perhaps unique in terms of its application of double circuit lines at such a voltage level. The country is relatively small, leaving little choice but to rely on vertical array designs which double the energy carrying capability of a single right-of-way.
Virtually all of KEPCO’s development and testing of key line components such as towers, conductors, spacers and insulator assemblies has been conducted at a special test facility in Gochang, on the country’s western coast. Located only about a hundred meters from the seashore, the site is classified as medium pollution but, depending on winds, can also be exposed to heavier contamination. The original test program for these was carried out in co-operation with EPRI in the U.S. and concluded in 2010. During this period, insulators were inspected regularly for signs of corona, erosion and whitening while infrared inspection was conducted from the tower twice a year.
Among the reasons KEPCO first considered changeover in insulator technology on its 765 kV network was contamination flashover accidents. A program of annual live line washing of the porcelain cap & pin strings on these lines was started but this had to be done by helicopter since the height of towers did not permit sufficient water pressure needed for efficient cleaning if done from the ground.
Testing Performance of Composite Poles
The Back Story
Wood pole framing is common on transmission networks throughout North America and indeed across the globe. However, while the typical service life of such poles is normally between 50 and 60 years, damage caused by various species of woodpeckers has, in extreme cases, required these structures to be replaced in only a fraction that time. Moreover, poles in affected line corridors may also need more frequent inspection since incidence of hole pecking occurs over a narrower time frame than most other problems. All this has made the activity of these birds a growing asset management challenge.
Maintenance practice at Canadian utility, FortisBC, has been that line inspection personnel assess the extent of any woodpecker damage to poles and decide whether an affected structure must be replaced or if more detailed inspection is required. Moreover, data on number of woodpecker holes is collected to determine whether the problem is stable or increasing, possibly due to factors related to climate change.
Moving to steel or concrete poles presents an obvious solution but comes with the challenge of transporting and erecting much heavier structures, often at remote sites that are not accessible by road. Both materials also offer less BIL than wood and this can impact an existing line’s electrical characteristics. Another option is composite fiberglass poles that are light and offer lower installation costs than steel or concrete alternatives. But such structures are generally much more costly than wood poles and their service life in this service area under high UV and periodic wetting still needs to be fully assessed. Yet another concern is that such structures will not prove as easy to climb as wood poles and therefore require a bucket truck for maintenance.
Editor’s Note: Part 3 of this Series will appear next week.