In countries where electrical transmission and distribution networks use exposed overhead power lines, engineers face the challenge of choosing materials that provide electrical insulation and strength while being capable of withstanding lightning and overvoltages. For example, the national electricity system in England and Wales consists of around 4,500 miles of overhead lines, 900 miles of underground cable and over 300 substations. Here Patrick Loock, segment business owner for products and applications at Exel Composites, explains how insulating composites can help protect the grid.
In a recent YouTube video for the Discovery Channel, veteran motoring journalist Richard Hammond climbed to the top of the world’s tallest building, the Burj Khalifa in Dubai. Accompanied by an engineer, Hammond’s task was to replace the battery in the building’s active lightning arrester.
Considering that a single lightning bolt can carry up to a billion volts, the purpose of fitting a lightning arrester is that the building acts as a faraday cage. It conducts the electricity and provides a safe route to earth, while keeping the occupants and the surrounding infrastructure safe.
However, tall buildings aren’t the only structures at risk. Overhead power lines in electrical transmission and distribution networks are also susceptible. Although many parts of these networks are installed underground in big cities and urban areas, many networks across the world are still built overground, owing to cheaper installation and repair costs.
Here, the exposed three-phase high-voltage power lines are carried by pylons. This ranges from 400 kV and 275 kV high voltage transmission lines, to 11 kV and 33 kV medium voltage distribution lines. However, while the tower itself is naturally grounded, it’s important that the structure is insulated from the electricity flowing through live wires and from lightning strikes. Not only does this protect people, it also protects sensitive electronics like transformers and substations from being overloaded and permanently damaged.
This isolation of live wires is typically achieved by using a variety of insulators, or surge arresters. These can be small pin-type insulators made from porcelain or fiberglass that feature a groove in the top to secure the conductive wire. They can also take the form of long rods or tubes that suspend the conductive wire below the arms of the tower.
This second type of arrester is made from a metal-oxide varistor (MOV) at its core, a material that provides insulating properties under normal voltages, but becomes a conductor at extremely high voltages, such as during an overvoltage or when lightning strikes, safely conducting the electricity to ground.
The MOV is enclosed within a fiberglass tube, which is overmoulded with rubber or silicone and fitted with ceramic or porcelain rings. The rings increase the surface area that the electricity has to travel, and also prevent rainwater from pooling or creating a thin film, which could also create a short circuit.
While many of these components have been used for decades, they require regular maintenance to replace parts that have degraded or been damaged. Ceramic and porcelain in particular are costly and prone to damage, but even older fiberglass tubes made to conform around irregular profiles could experience hairline cracks, especially with larger diameters over 100 mm — which could result in a failure of the tube to maintain its insulating properties.
This is why infrastructure managers are increasingly upgrading their network assets with the latest insulators. Not only do modern composites deliver better strength-to-weight ratios, they also perform better with regards to torsion, tension and dielectric performance, thanks to advancements in manufacturing techniques like pultrusion.
Pultrusion, a method of manufacturing composites, starts with reinforcing fibers — in this case made from corrosion resistant, electronic glass fibers — which are aligned and guided from a spool into a die. Here, the fibers are impregnated with liquid epoxy resin, before being pulled into a heated die and shaped into a profile. The resulting composite is then continuously pulled through the process and cut into the required length.
Exel Composites is a global market leader in pultrusion-manufactured composites and manufactures a wide range of composite insulators, rods and tubes with diameters up to 150 mm, in spooled lengths over 2 km and custom profiles to suit customer needs.
So, whether it’s overvoltages or lightning strikes, the next generation of composites offer a step up from older materials in protecting the grid, without you having to climb to the world’s tallest building to find them.
To find out more about insulating composites for electrical applications, visit the Exel Composites website at: https://exelcomposites.com/composite-solutions/composites-in-infrastructure-applications/utility-power-transmission-distribution/