Dealing with the energy bottleneck with minimal disruption

Paul Boughton
Mark Gledhill and Allan Russell outline the benefits of aluminium conductor technology which is less prone to corrosion and is an effective power conductor.

The UK is just one of many countries investing in wind power as a future source of energy - and building offshore wind farms has to be an essential part of that mission. However, we believe that there needs to be as much focus on looking at how 'fit for purpose' the rest of the network is, in order to ensure that energy can be easily transported to the point of need.

The reality is that many of these offshore wind farms are going to depend on the onshore distribution network which, unless modernised in the right places, could lead to transmission and distribution network bottlenecks. Given that new network building onshore is in many cases extremely limited due to environmental or legislative factors, lateral thinking is required, as has been the case in other countries around the world. Operators in mainland Europe, the Americas, Asia and Africa are using innovative methods that increase the ampacity of existing lines, with minimal impact on the existing network infrastructure. The key to this is the installation of high temperature low sag conductor technology, which is designed as a 'drop in' replacement to increase ampacity by double or more.

In the UK, Ofgem has previously cited three main barriers to renewable energy deployment: planning objections, a shortage of wind turbines and access to the high voltage transmission system. While the first two are widely reported - particularly in relation to supply chain requirements - the third is less so. Yet upgrading the existing network infrastructure is vital to achieving the UK's mission to have 18GW of viable wind energy by 2020. While we may have had close to 2GW by January 2011, according to the British Wind Energy Association, that's just part of the bigger picture.

It's obvious really: the plan is for offshore wind farms to connect to substations along the UK's shorelines, but these substations were - in the main - originally built to be the 'end of the line'. We often use the comparison of trying to use a B road as a motorway. It simply is not feasible to use them to deliver large volumes of wind power in their current state, but nor is replacement typically an option. Land acquisition, planning permission, environmental concerns, cost, time ... all these add up to a pretty prohibitive set of obstacles, even when considering underground installation as an alternative to overhead cables.

Nor is replacing existing overhead lines with more of the existing or traditional power conductors really an option. Traditional conductors - typically ACSR or ACSS - use a combination of steel and aluminium. To date, they've done an excellent job: steel is strong, while annealed aluminium is less prone to corrosion and is an effective power conductor. The problem is that many of these circuits are already thermally constrained and if ampacity was to be increased then sagging would become a problem. This cannot always be remedied by raising the towers, as the conductors are typically only designed to operate at 75°C.

The UK is far from alone in facing these problems, which is why so much R&D work has gone into finding solutions around the world. There has been particular emphasis on aluminium: given its inherent benefits of ampacity, flexibility and corrosion-resistance, how can its limitations be overcome? There have been several breakthroughs in the past few years, such as ACCR (Aluminium Conductor Composite Reinforced) which has been installed in more than 100 networks worldwide and developed with the support of the US Department of Energy.

No two installations are the same, but typically, capacity gains are two or threefold while ensuring mechanical loads, sag and tension requirements are met and any major construction work is avoided. ACCR can simply be inserted as a replacement to existing conductors. The US Department of Energy's Oak Ridge National Laboratory (ORNL) in Tennessee tested ACCR extensively and found that it retains its integrity after exposure to temperatures higher than the rated continuous operating temperature of 210°C and the emergency operating temperature of 240°C.

Given that manufacturers have been experimenting with aluminium for decades, what's the big difference this time?

The breakthrough is in the aluminium's structure: the outer wires are hardened aluminium zirconium, stranded in either round or trapezoidal wire (which is similar to existing conductors). The aluminium zirconium enables operation at higher temperatures without any impairment to performance, plus its resistance to corrosion makes it ideal for the salt-laden air of the UK's shorelines.

The core is a multi-strand design of 7, 19 or more wires, each of which is composed of tens of thousands of very fine alumina fibres, infused with additional high purity aluminium. The combination of the core and the outer wires provides the strength-to-weight ratio required to ensure that high ampacity levels can be achieved without causing significant sagging. As a result, operators can maintain mechanical load, tension and clearances, even over wide distances or high wind conditions. For example, ACCR has been used to span wide river crossings.

Of course, operators need to consider investment costs. As customers' own business models have shown, ACCR offers a viable alternative to building new structures, both in terms of speed and financial cost. In fact, customers' studies have demonstrated that ACCR can actually lead to overall cost savings.

ACCR is not just being used to connect remote power sites; indeed, it is often installed in dense urban areas where major construction projects are not viable. Companies who have adopted ACCR so far include Tata Power Company in India, Shanghai Electric in China, CPFL Energia in Brazil, and Xcel Energy in the US. More recently, European operators have started to install or trial ACCR, including Réseau de transport d'électricité (RTE), which operates one of the largest networks in Europe. RTE has installed ACCR to give it the flexibility to increase overhead line transmission capacity in the case of any short term emergencies. Using ACCR has made this possible without needing to create a new transmission infrastructure.

While ACCR is of course specific to 3M, it is part of a worldwide movement towards finding new ways to improve ampacity while limiting the impact on existing power network infrastructures. The industry has no choice: we need to increase power, but there will a need to minimise disruption to the surrounding environment. This isn't a problem that is going to go away, so the more we can look at 'non-invasive' techniques to meet the world's energy shortage, the better. Technologies such as ACCR could play a pivotal role in meeting the UK's wind energy ambitions.

Mark Gledhill and Allan Russell are with 3M.

Recent Issues