Composite arm doubles pylon capacity
Replacing the steel arms of electricity pylons with insulating composite materials could allow existing national grid lines to be up-rated, at least doubling their power capacity, according to University of Manchester spin-out Arago Technology.
The same technique, said company founder Professor Ian Cotton, could be used to reduce the height of new-build power lines.
Conventional pylons, also known as towers, are steel lattice structures which support six cables – two three-phase circuits – of 132, 275, or 400kV.
These cables hang from horizontal steel lattice cantilevers, called cross-arms, via an insulator up to 4m long.
The size of any particular tower is dictated by the need to keep the lower conductor a safe distance from earth, as well as keeping all the conductors a safe distance from the tower and each other.
“If you could get rid of the insulator, you could make the tower shorter by around 20%,” Cotton told Electronics Weekly. “And you have to allow an insulator to swing, so if you remove it you can reduce the width of the tower by around 20%.”
Arego’s way of removing the hanging insulator is to make it redundant by replace the steel cross-arm with an rigid composite insulating cross-arm.
With insulating cross-arms, a 40m high 12m wide conventional 400kV tower could be as small as 30m by 10m wide, estimates the firm.
Conversely, a conventional tower designed for 132kV could carry 275kV (a power increase of 4.3x) or one designed for 275kV could carry 400kV (2.1x).
Steel and porcelain are not expensive materials. Can composite cross-arms reduce overall cost?
Calculations are not conclusive as there are many factors in the cost equation.
“Most utilities have moved away from porcelain or glass insulators to composite insulators that are light and easy to handle, and you have to paint steel regularly,” said Cotton. And for new-builds “our cross-arm costs more than a steel cross-arm and an insulator, but if you are reducing height by 20%, you are also reducing the size of the base and the foundations because the bending moment is lower. And a lower tower has planning advantages.”
Costing is no simpler in retrofit applications.
There are three things to think about: voltage issues, mechanical issues and maintenance issues,” said Cotton. “In terms of voltage compatibility, insulating cross-arms get you all the way there, from 132 to 275kV, or from 275 to 400kV.”
Mechanically, “You do have to have a bigger conductor, or two similar conductors spaced apart to give the effect of a bigger conductor, which increases the load on the tower and you may have to strengthen it,” said Cotton.
Why a bigger conductor?
“With higher voltage, the electric field is higher so, unless the conductor is bigger to reduce the field, corona discharge makes noise bigger and radio interference higher. Noise is a legal requirement.”
Form the maintenance perspective: “There may be some issues with live line working – the smaller you build a tower, the more restrictive operations are, so companies will have to consider ease of maintenance,” said Cotton.
This would apply to voltage up-grades and compact new-builds.
“Studies suggest up-grades are possible, but you have to look at every line on a case-by-case basis – for example, in Northern Scotland there is ice in the winter, and ice loading on two conductors is higher than one,” said Cotton.
Energy firms are interested, and reliability trials are booked.
Scottish Hydro Electric Transmission (SHETL) and National Grid will deploy six cross-arms on a 132kV line this year and six cross-arms on a 400kV line in 2014.
“We are working with three other companies I cannot name: two power companies and one component supplier,” said Cotton.
Each of the four struts in the composite cross-arm are made of a solid smooth-sided fibreglass core, surrounded by a silicone rubber insulator with the classic corrugated surface to increase creepage distance – the surface distance or a 4m insulator is around 12m.
“The glass fibre gives it strength, but its electrical performance is poor when it is wet. Silicon rubber delivers electrical longevity,” said Cotton.
These materials are common in the power industry. Why haven’t insulating cross-arms been tried before?
Weight, is the answer, said Cotton. The lower two of the four struts necessary to form a cross-arm are in compression, and traditional cylindrical strut fat enough to resist crushing would be too heavy to deploy.
The firms unique intellectual property is a strut whose fibreglass core has a tri-lobular cross-section which can resist the crushing forces without weighing so much.
Why not simply make a larger diameter hollow cylinder to resist crushing?
“Hollow is really difficult as you have to fill it with oil, silicone gel, or sulphur hexafluoride to maintain insulation. Utilities have never used hollow insulators because they would have to keep checking them,” said Cotton.
Cotton’s co-founder is Professor Simon Rowland, also of the University of Manchester, and the company was in partnership with insulator specialist EPL Composite Solutions of Loughborough.