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Deciding the best route and technology for new high voltage electricity transmission lines means balancing conflicting technical, environmental, socio-economic and cost considerations, writes Penny Hitchin, who examines how these issues are being addressed in the UK

Shifting to low-carbon generation requires changes to transmissions networks as countries move away from fossil fuelled generation built near centres of demand to accommodate outlying sources of intermittent renewable energy. Large new modern nuclear power stations also require new transmission capacity.

In the UK, much of the new electricity generation will be sited on the coast or offshore in areas where there is currently very little existing transmission. As well as new electricity transmission capacity, upgrades and reinforcements to existing areas of the network are required.

National Grid is the operator of the high voltage transmission system for Great Britain and its offshore waters, and owns the high voltage transmission system in England and Wales. Much of National Grid’s transmission system of 7200 km of overhead lines and 700 km of underground cabling operating at 400 kV and 275 kV was constructed in the 1960s.

National Grid’s UK North West Coastal Connection (NWCC) is a multi-billion pound project to build new transmission infrastructure to provide grid connection to link the proposed new 3.2 GW nuclear power station at Moorside in West Cumbria into the transmission network over 50 km away.

Currently the area is served by a 132 kV distribution network, but two new 400 kV circuits are needed to accommodate 3.4 GW additional generation capacity. The West of Duddon Sands and Walney Extension offshore windfarm projects, with a combined capacity of 1126 MW, will feed into the connection at Heysham. Plans for the 1 GW Celtic Array, another offshore windfarm project due to connect into the transmission infrastructure, were scrapped in summer 2014.

Agreeing the route and technology for new connections requires comprehensive consultation. West Cumbria’s location means that any NWCC route will be challenging. To the west lies the Irish Sea. An offshore cable route would have to avoid a live firing range and cope with limited seabed availability due to existing offshore wind farms, oil and gas pipelines and cables.

To the east lies the scenic upland area of the Lake District, one of England’s most iconic national parks. The Holford Rules on overhead line routing, first set out in 1959, state that routing through major areas of highest amenity value should be avoided even if this increases total mileage. Reconciling technological, economic and environmental factors to establish a route for the connection is going to be tricky.

New nuclear needs connection

The UK’s 2008 Energy Act kicked off a new race for suitable land on which to build a new generation of nuclear power stations. While West Cumbria had disadvantages – absence of grid connection, distance from centres of demand, questionable suitability of land and availability of cooling water – its economic dependence on Sellafield means it possesses a skilled and experienced nuclear workforce and supply chain, and is arguably the most pro-nuclear community in the UK.

As competition to obtain land for new nuclear build heated up, RWE bought rights on farmland adjacent to Sellafield and applied for a grid connection for 1200 MW for the end of October 2021, with a further 1200 MW to be available a year later. In 2009 rights to a tranche of Sellafield land were sold to a consortium including GDF Suez, which proposed to build a new nuclear power station on the site. The following year RWE dropped its grid connection agreements, clearing the way for the new consortium to acquire agreements for its project, now renamed Moorside.

A final investment decision on the nuclear station is currently expected around 2018 and, if the project goes ahead, the new transmission connection is due to be available in three tranches of 1.13 GW at yearly intervals from November 2024 to November 2026.

National Grid’s work on identifying the route for the Cumbrian transmission network started in 2009. It engaged with local authorities and environmental, business and community organisations and came up with half a dozen strategic options which were the subject of a consultation in 2012. In broad terms, the options included running the two double circuits required either to the north or to the south of Moorside or constructing two to the north and two to the south, to link into the transmission infrastructure

National Grid decided the way forward was to look at developing a connection made up of a double circuit connection going north from Moorside to join the existing national electricity network near Carlisle, and a double circuit connection going south to connect into the existing network in Lancashire. Two strategic options – one entirely onshore and one involving offshore cable to the south – were taken forward. Work then focused on studying different locations, technologies and designs and appraising them against a range of environmental, socio-economic, technical and cost factors to identify route corridors in which the new connection could be built.

The southern arc of the circuit proved most contentious. Alternative routes (less favoured by the TSO) which were included in the consultation were an offshore south option and an onshore south route. The offshore south route is the most expensive and technologically challenging. The onshore south route is the most economical and straightforward, but would encounter substantial environmental challenges as it would cross many areas of environmental and historical importance.

A 12-week, wide-ranging public consultation started in September 2014, by which time the options on the table had been refined into four route corridors. National Grid’s emerging preference was for an onshore route following the path of existing 132 kV distribution pylon lines (which could be removed and replaced with higher voltage equipment), combined with an onshore tunnel under Morecambe Bay.

Robert Powell National Grid’s NWWWC project manager told PEi: “Public engagement is very important in determining the route and we have spent a lot of time listening and learning from what we hear. We seek to develop the project in a way which achieves a balance between meeting our country’s future energy needs and protecting the very special landscapes it touches upon.”

Transmission developments

Overhead lines made up of the conductor, pylon and insulator are the easiest way to move electricity from the source of generation into the network. Pylons need to be tall enough to ensure that the clearance between each conductor, and the space between the lowest conductor and the ground, is sufficient to prevent electricity jumping across. The distance between pylons depends on factors including pylon height, conductor capacity, landscape topography and changes in direction. The distance between pylons is typically 360 metres.

A standard steel lattice electricity pylon has been used in the UK since 1928, but a change of design is in the pipeline. In 2011 a Danish-designed T-Pylon won a Pylon Design Competition jointly run by DECC, RIBA and National Grid. Over 250 entries were received from around the world with the single pole T-Pylon announced as the unanimous winner.

The T-pylon uses an innovative diamond ‘earring’ arrangement to carry the cables off one arm in a much smaller space than the three-arm arrangement of the lattice pylon. The T-shaped cross arms enable the height of the pylon to be reduced from the 50 metres used by the lattice pylon to 36 metres for the T-pylon. The modern design will also enable overhead cables to be better routed along the contours of the land instead of the more angular changes in direction of lattice pylons. The T-pylon is being offered for the first time for the new transmission line for EDF Energy’s proposed nuclear station at Hinkley Point C in Somerset.

Cost estimates for an overhead cable on the NWCC project are between £2.52 million ($3.87 million) and £3.02 million per km.

Going underground

Routing transmission lines across environmentally sensitive or built-up areas can be problematic. One potential solution is to use cable and go underground. Underground cable currently makes up about 10 per cent of the existing transmission in England and Wales, a figure similar to that for other transmission systems worldwide.

Installing underground cable is more expensive than overhead lines. Capital build costs vary greatly depending on terrain, route length and power capacity. The cost of cable is a major element of the capital cost differential. An underground conductor has to be bigger than its overhead counterpart to reduce its electrical resistance and the heat produced. Special insulation is needed to maintain the cable’s rating. However, a study of the costs of new electricity transmission infrastructure by the Institute of Engineering and Technology (IET) found that the cost of operation, maintenance and energy losses over the 40+-year life of the connection are broadly the same for underground and overhead lines.

There are two types of underground cabling: direct burial and tunnelling. The traditional cable installation technique for high voltage cables in urban and rural areas is by direct burial. This involves excavating a trench around 1.5 metres wide and 1.2 metres deep for each single cable circuit. The cables are installed on a bed of selected sand and backfilled with cement-bound sand to ensure a known thermal conductivity around the cables (to maintain the cable rating). Sheet piling or timber is used to support the sides of the trenches. Reinstatement of the excavated trench is then carried out using approved backfill material placed directly around the cables, with protection covers placed above them in the excavation. Bays at intervals of approximately 500–1000 metres allow for the jointing of the individual sections of cable.

Proposed subsea tunnel

Tunnel installation usually costs more than direct burial. Tunnelling is generally only used in urban areas where direct burial would cause unacceptable disruption. Tunnelling has the advantage that underground services such as water and sewerage are unaffected and river or railway crossings can be made.

Environmental and technical challenges in the NWCC project mean that National Grid’s preferred option for part of the southern route is to drive a 22 km tunnel 25 to 35 metres beneath the shifting sands of Morecambe Bay. Building a tunnel is a major undertaking which requires detailed understanding of the geology of the route. Tunnelling needs plenty of land at the primary construction site. Substantial head house buildings (16 metres in diameter and seven metres high) to provide access for maintenance and for installation of the cables must be built at each end.

A 12 metre diameter shaft takes the cable down to a five metre diameter tunnel needed to accommodate a 400 kV double circuit connection including 12 cable cores and joint bays. A tunnel typically maintains a slope of 1:1000 to provide free drainage. Cable cooling is provided by forced air from electrically driven fans, and additional cooling can be provided by a water cooling system.

Tunnels need inspection, emergency access and exit points along the route. A feasibility study suggests that three emergency exits and ventilation shafts are needed. These could be made into ‘islands’ in Morecambe Bay. Within the tunnel, rail-mounted access vehicles can be used to facilitate inspection, maintenance and repair.

Detailed ground condition surveys are required to determine the most appropriate design for any tunnel. While historic borehole data exists from the last century when proposals to build a barrier across Morecambe Bay were developed, more specific detailed borehole information is needed. Borehole surveys precede design work which in turn determines the design and manufacture of specialist tunnel boring machines.

For the NWCCC project, National Grid says that the unit cost for underground cable is between £13–32 million per km based on a double circuit. Detailed tunnel costs and the construction programme will only be available once a final design is agreed.

High voltage offshore transmission

Electric power is normally generated, transmitted and distributed as alternating current (AC). AC offshore cable voltages are limited to 220 kV, transmission distances are restricted and it is difficult and expensive to joint AC cable offshore. For long-distance transmission or where long subsea crossings are involved, High Voltage Direct Current (HVDC) transmission may be more appropriate.

The capital costs of HVDC installations vary depending on technology used and route length. Bulky and expensive DC converter stations must be installed at each end of the HVDC system where it links into AC infrastructure. Generally HVDC is more cost-effective when installed over long distances.

An offshore HVDC link for the southern route was considered for the NWCC project, but was deemed the least preferred option. To date no nuclear power station has been connected by HVDC circuits, and although 2 GW converters could probably be developed within the project timescale, they are not currently available, thus the offshore south route carries a technology risk. The preferred route for an HVDC cable would be a 190 km circuit which would avoid both the Ministry of Defence Eskmeals firing range and the existing Round 1 and Round 2 offshore wind farms. However, the Round 3 offshore wind farm cables would cross the HVDC cable.

Although the cable would be trenched, shifting sands at sea could expose and damage it. Problems in locating and repairing damage to offshore cables are exacerbated by the presence of multiple pipeline and cable crossings. A fault on an offshore HVDC system can mean it could be out of action for six months while it is located and replaced.

HVDC circuits proposed for offshore south route would be limited to 2000 MVA per circuit, with no power capacity above that for the Moorside power station. AC circuits in the other southern options would provide more power export capability. National Grid estimates unit cost for this option at £2 million per km, plus up to £33 million per pair of converter stations.

Finding routes for new transmission infrastructure is a complex and time-consuming process which involves balancing technical, technological, environmental, socio-economic and cost considerations.

Work on the North West Coastal Connection project started once an agreement to provide a grid connection to the proposed nuclear power station was granted in 2009. Six years on, proposals for the preferred route corridors and technology have been established, but much work remains to be done before a detailed design is agreed.

With the investment decision for Moorside unlikely to be taken until 2018 at the earliest, construction of the multi-billion pound new grid connection may not get underway until the next decade.

Penny Hitchin is a freelance journalist specializing in energy matters.

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