Over nearly 30 years, INMR has travelled the globe visiting power lines and substations and has also been invited into hundreds of factories for electrical equipment as well as into test laboratories worldwide. During these numerous site visits, tens of thousands of photos were taken to support the resulting technical articles. In this upcoming 3-part series, these photos themselves have become the story.
Selecting favorite photos among such a vast number of possible choices was a challenge. In the end, photos were selected based on their meeting at least two 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 content to support the topic of interest; and/or
3. Offer special insight into some interesting aspect of power transmission and distribution.
In Part 1 of this upcoming three-part series, INMR presents the first set of these most memorable images and reviews the ‘back story’ of each.
1998 Ice Storm Battered Power Network in Québec
The Back Story
In early January 1998, a huge mass of moisture from the Gulf of Mexico met a stationary system of cold air from Labrador, resulting in sustained freezing rain and ice pellets over a vast area of southeastern Ontario, southwestern Québec and the northeast U.S. The onslaught lasted five days producing an accumulation of up to 80 mm of freezing rain and leading to what then became the largest power disruption ever recorded in North America. Millions were affected, some for weeks. Equally significant, the outage occurred during the harsh regional winter and in areas where up to 80 per cent of inhabitants rely on electricity for heating.
Most transmission towers used in Québec until that time were designed to bear up to 45 mm of radial ice – a load considered sufficiently conservative inasmuch as meteorologists claim that a situation of excess freezing rain occurs only once every 50 to 150 years. However, the combination of ice loading from the storm on these structures, which in some cases tripled their weight, and from ice-laden conductors brought down hundreds of transmission towers and tens of thousands of wood poles. In one unusual case, a 735 kV line built after 1973 survived the ice storm while a similar adjacent line built earlier was completely toppled. Although vertical capacity of the newer towers was probably less than for the older collapsed towers, balancing between components combined with anti-cascading structures managed to keep this line intact.
Most analysts of the Great Ice Storm of ‘98 agree that utilities must draw from the experience of such calamities to better prepare. For example, Hydro-Québec officials acknowledged that the utility did not have any computer simulation on how to respond to such a catastrophic power failure. While the severe freezing rain that lashed the region for days may have been without precedent in duration and intensity, these events could well become more common as the Earth experiences greater and greater climatic change.
±400 kV Line to Tibet Broke Records
The Back Story
The 750 kV/±400 kV Qinghai-Tibet Intertie has been one of China’s most ambitious network projects. Commissioned late in 2011, it connected with 750 kV AC lines running about 1500 km between the cities of Xining and Geermu, in central Qinghai Province. From the converter substation in Geermu, it continued as a ±400 kV DC line to Lhasa, more than 1000 km away. What made this portion of the project so noteworthy was that it ranked among the world’s highest overhead transmission lines and also the longest HVDC line yet built at such altitudes. The line traverses terrain that averages 4500 m with the highest point reached at the 5300 m mountain pass at Tanggula – gateway to Tibet. Apart from the obvious construction challenges across the vast, rugged plateau under extreme cold, permafrost, high UV and fragile local ecosystems, altitude impacted factors such as corona as well as electro-magnetic field. Designing the most suitable line insulators for this environment and meeting performance requirements of external insulation on HV equipment also proved demanding.
Overcoming risk of reduced hydrophobicity of silicone rubber under persistent cold temperatures required developing the most suitable shed geometry given the line’s pollution environment. The insulators ultimately selected have three different shed diameters: small, mid-size and large, each separated by a distance of 40 mm along the shank and in a configuration where there is a small shed on either side of both the mid-size and the large sheds. As such, the distance between any two consecutive large sheds amounts to 160 mm, less than the 180 mm spacing normally required in China for UHV applications.
Minimalist Towers for Dutch Power Grid
The Back Story
The Netherlands is a place where expanses of unused land are rare. In such an environment overhead transmission lines are not only unwelcome from a visual perspective but also subject to close scrutiny in terms of how they might affect the health of those nearby.
It was with these constraints in mind that engineers at TenneT, the country’s grid operator, embarked on an ambitious project to apply a new concept for 380 kV lines based on steel poles and braced line post insulators in place of conventional lattice towers. Key considerations from the outset were low visual impact as well as reduced magnetic field so as to significantly reduce the line’s required corridor.
Finding a pylon that would meet architectural considerations of being aesthetic and minimalistic and at the same time not too costly proved a challenge. The goal was therefore to find an optimal balance between keeping the footing as small as possible to satisfy the criterion of minimalism while also respecting the fact that material and construction costs would be going up. At the same time, bending under wind loading, especially for angle structures, had to be held within acceptable limits to keep them pleasing to the eye.
Another key element of the Wintrack design revolved around selection of insulators, which are all silicone rubber type. A key consideration when it came to these configurations was effective grading to ensure that maximum field stress along the insulator would always be less than 2.5 kV/cm, even lower than the 4.1 mm/kV recommended at the time by EPRI. This meant a great deal of shielding and required a double grading ring. Such a design criterion was intended to ensure the longest possible service life with minimal risk of erosion of the housing due to corona. In the end, Wintrack’s cost per km was about 1½ times higher than would be required for traditional lines. But in this case classical design was not an alternative.
Insulator Pollution Monitoring in Kalahari Desert
The Back Story
Few factors more influence performance and life expectancy of insulators than the service conditions where they are installed. Knowing the levels of pollutants that deposit onto insulators during the course of a year is the best way for engineers to quantify and plan for the stresses that will affect reliability of overhead lines and substations. One utility that has suffered from recurring pollution flashovers over the years is South Africa’s Eskom. With a vast network operating under the effects of coastal and industrial pollution as well as periodic brush fires, Eskom has for decades invested in monitoring its service environment using a far-flung network of substations and other sites where directional dust deposit gauges (DDDGs) as well as strings of reference glass insulators are installed.
The standard DDDG used by Eskom is comprised of four vertical tubes, each with a large slot of standard dimension cut into its sides and arranged to face all four directions on the compass. A removable cylindrical receptacle at the bottom of each tube collects all wind blown deposits that enter these slots. The volume of material collected in each receptacle depends on wind direction and is greatest when it blows directly into the slot and lowest when it comes from a perpendicular angle. This means that the DDDG also indicates from where contamination most impacts a particular substation and helps identify pollution sources of special concern. Readings are also taken of pollution accumulating on the surfaces of reference insulator strings composed of 7 standard glass shells and installed next to all DDDG locations. Each disc in the string is analyzed at different time intervals with the top and bottom units functioning as ‘dummies’.
Constructing ±500 kV Line Under Extreme Cold
The Back Story
Construction at Alberta-based utility, AltaLink, is done year round even during the short days of winter when wind chills send temperatures plummeting and powerful winds can ground helicopters. To facilitate site access under such conditions, special mats made of rows of linked wooden poles allow passage of incoming heavy equipment to clear snow or to erect towers and mount insulators as well as line hardware. Conductor stringing is assisted by helicopter.
The ±500 kV WATL line provides a good example of construction proceeding even under difficult wintery conditions. This project replaced the alternative of a double circuit 500 kV AC line and is insulated with 34 standard profile glass bells per string for each pole and 7 on the neutral.
Insulators arriving and stored at the site are also factory-packed in closed boxes to prevent contaminants from being deposited during transport. The key is that when the insulator string is put up it is clean and free of road salt that may have fallen onto it and which might lead to tracking from the start.
Every structure on WATL has been equipped with corona rings up to 3 m in diameter, depending on type, and which have been specially designed and tested. Moreover, all line hardware, including conductor saddles and spacers, were required to be certified corona free in terms of smooth surfaces with no rough edges. The standard is that all hardware, including the armor rods that help damping and prevent fatigue failures, must be tested for cold weather shock loading. This is done to ensure that the steel always remains ductile, even at service temperatures down to -50°C.
Surging Production of Porcelain Insulators
The Back Story
The past three decades witnessed dramatic expansion of the Chinese power grid, with many large projects completed or underway across the country. For example, China’s average annual incremental generation capacity between 2000 and 2004 was 27 GW while the annual increment in installed capacity from 2005 to 2009 averaged 88.8 GW – about the same as the then total capacity of the United Kingdom.
Needless to say, this huge annual increase in electricity generation required a great deal of new transmission infrastructure This sustained investment helped fuel rapid growth within the domestic insulator industry, which supplied the large majority of the country’s needs, both on lines and at substations. In fact, such has been the growth in demand that many new insulator factories had to be built while existing ones underwent significant expansion.
In the case of porcelain insulators, while a great deal of productivity enhancement has been achieved by developments such as isostatic pressing of pugs and computer-controlled cutting tools, the human factor remains an essential element. The skills and dedication of each factory worker is key to maintaining high product quality as well as maximum yield.
Reducing Height of 400 kV Towers
The Back Story
Research among the German public has shown that tower height is the decisive factor in perceived adverse environmental impact of proposed new power lines. The main technical characteristics of a new line design concept developed by TSO, 50 Hertz Transmission therefore aimed for significant reduction in tower height based on reducing conductor sag on 400 kV lines without compromising required minimum ground clearance of 12.5 m. Reduction of sag in spans was achieved through use of two additional steel support ropes in every conductor bundle. As a result, sag in each span is no longer defined by conductors but rather by the steel ropes with conductors attached.
Due to the highly tensioned steel ropes and increased weight, resulting mechanical forces on tension and suspension towers are significantly higher than on conventional structures. To comply with these added forces, a complete re-design of towers proved necessary. Moreover, an essential additional goal for the new tower design was reduction of dimensions at the bottom. The suspension tower was eventually designed as a single tubular, conical steel pole with ring flange connections. The dead-end tower, by contrast, was designed as portal type, consisting of two tubular, conical steel poles supporting the cross-arm. Cross-arms consist of steel profiles connected to the tower top by steel tie rods. The resulting standard suspension tower has overall width of 38 m and height of 30 m. The standard tension tower has overall width of 36 m and height of 36 m.
Transition to Polymer-Housed Cutouts
The Back Story
It is not unusual to photograph workers in the process of replacing line components. What is unusual however is capturing body language showing appreciation that borders on admiration. To understand why these line workers might feel this way, it is necessary to look at the component being replaced – porcelain cutouts.
FortisBC, a utility serving large areas of British Columbia, Canada, has experienced a history of failures of fuse cutouts made with porcelain housings. While pollution is rarely a concern in the interior of the province, wide temperature swings over the seasons impact how certain line components age. Summers are usually hot and dry while winters can be cold, often with substantial precipitation. This type of weather pattern has historically caused problems with fuse cutouts due to moisture penetration and subsequent cracking of the porcelain.
Investigation into the problem showed that among the many thousands of cutouts installed on the distribution network, there were hundreds of documented cases of failure. Interestingly, this problem was found to be independent of age since porcelain cutouts that had been in service only a few years were failing at about the same rate as much older cutouts. The conclusion was that these failures were due not only to ageing under local service conditions but also related to poor design and quality control problems during manufacture. Polymeric-housed cutouts are perceived as an effective remedy to these issues and accordingly much appreciated by line maintenance staff.
Structures Mark End of Lattice Towers
The Back Story
The meadows of Jutland are home to populations of deer stags, whose antlers might well have inspired the unique 400 kV lattice towers that connect this region with the rest of Denmark. But not long ago, this Scandinavian country became the first place where the lattice transmission tower is no longer accepted for any new construction. What made this development especially noteworthy is that Denmark is frontrunner in a trend that is already impacting a growing list of countries – overcoming public resistance to the building of new overhead lines. Rightly or wrongly, the lattice tower has become a symbol of the blemish that overhead lines sometimes impose on natural landscapes. This suggests that if power companies can find more aesthetically pleasing structures, there is reason to expect that the public will object far less.
The new double circuit 400 kV Kassø-Tejle line that runs northward along the Jutland Peninsula contributes to the goal of strengthening the interconnection between Germany and Denmark and from there to Norway and Sweden. However, realizing this goal by means of another overhead line required overcoming strong objections by those who lived in this flat region of farms and historic towns. The firm of industrial designers who developed the stylized structures eventually selected for this line – dubbed Eagle Pylons – aimed to convey “calm elegance” and allow a major 400 kV transmission line to blend into a scenic rural landscape.
Hydrophobicity At Work Enhancing Insulation Performance
It is fair to state that 25 years ago very few in the field of power transmission and distribution had ever heard the term ‘hydrophobicity’. But this changed quickly with the advent and rapid acceptance of polymeric insulation. Hydrophobicity is the single most recognized property of polymeric materials used for outdoor insulation since it provides added value in terms of enhanced pollution performance. Up to now, only the family of silicone rubber polymers has demonstrated that this hydrophobicity can endure over the full service life expected of an electrical insulator.
From a service performance point of view, polymeric housing materials used on composite insulators and similar components should provide long-term hydrophobicity, rapid transfer of hydrophobic properties into the pollution layer and, for service situations where there are stabilized surface discharges, good resistance to tracking and erosion.
Retention of hydrophobicity can be assessed using the Dynamic Drop Test, which employs the same principle as the Inclined Plane Test in IEC 60587 however has different electrode geometries and an electrolyte, without wetting agent, applied to the top surface of the plate-shaped specimen. What is evaluated is how much time is needed to achieve a continuous path of water between the electrodes.
Investigations have also aimed to determine whether this test can be used to evaluate recovery of hydrophobicity after stress treatments such as corona or UV. Additional investigations have been carried out to learn whether the principle of the Dynamic Drop Test could also be used to quantify hydrophobicity transfer.
Bushings Fight Monumental Battle Against Pollution
The Back Story
In the world of transmission & distribution there are a number of places where pollution severity is of such a scale as to represent the pinnacle of challenges for electrical insulation. One of these is in pastoral New Zealand. Almost from the instant it was built decades ago, the cable termination substation at a quiet inlet west of Wellington has preoccupied engineers and maintenance staff at Transpower, the country’s TSO. The importance of the line that passes through this transition point only added to the urgency to win this battle.
The ±350 kV Inter-Island tie links the 220 kV grids of the North and South Islands. Beginning at the Benmore Hydroelectric Plant, the line travels 535 km to Fighting Bay on the shores of windy Cook Strait. It then crosses 40 km via undersea cables to Oteranga Bay, where it resumes overhead transmission for the remaining 35 km to the Haywards Converter Station.
The site of the cable termination station at Oteranga Bay was selected because it offers the most preferential cable access but sits facing a stretch of open sea subject to steady north-south winds. These constant winds mean that salt crystals build up rapidly on the surface of insulation and, given the high humidity, can soon overwhelm even the best insulation. For years after construction of the original cable link during the 1960s, the station was open-air and the porcelain housings on the cable terminations had to be washed for up to 15 minutes every hour. In the late 1980s, it was decided to enclose the terminations in a structure. While this effectively protected the porcelain terminations, the pollution battle was basically transferred to the two bushings that protrude vertically through the structure’s roof. The Pole 1 and Pole 2 bushings have each had their own history that helps explain why, though both are silicone-housed, the two have had different designs and dimensions as well as different service experience. For both, great importance was attached to selecting the optimal dimension and location of grading rings to maintain proper distribution of electric field, minimize corona and avoid ‘hoop stress’ rings that can form on the insulator surface.
Editor’s Note: Part 2 will appear in the INMR WEEKLY TECHNICAL REVIEW starting week of June 27. Part 3 starting week of July 4.
