A giant industrial shed will be built in the Kergord Valley to handle electricity generated by Shetland’s community wind farm, if plans by Scottish and Southern Energy (SSE) get the green light.
The building will cover an area more than twice the size of a football pitch to house large, noisy, converter transformers weighing up to 150 tonnes each.
They will change the wind energy from alternating current (AC) to direct current (DC) to prevent excessive power loss from seabed cables running from Weisdale Voe to a landing point at Portgordon, near Buckie, on the Scottish mainland.
At this stage there is uncertainty over how many sets of high voltage DC (HVDC) cables will be required for the £200-300 million project.
The electricity regulator Ofgem requires that SSE puts forward the cheapest scheme through its power lines network company SHETL (Scottish Hydro-Electric Transmission Limited), making its preferred option a single 550 megawatt circuit carrying all the wind farm’s output.
If the cable circuit was broken the wind farm would not be able to export power (and presumably none could be imported to Shetland either) but SSE considers this nightmare scenario may not be bad enough to justify the cost of a second circuit. It calculates that doing without could mean a saving of £100 million from the project costs over the course of 40 years.
At present, accepted practice does require there to be full back-up in the event of breakage, perhaps by a ship’s anchor, and technical restrictions on the power capacity of cables means that actually three cable circuits may have to be laid.
There is as yet no cable in existence that can carry over 350MW, so two circuits would be required for the load plus a third as a spare.
However, technology is expected to produce a single cable capable of carrying the full output from Viking Energy’s farm, allowing SSE to meet the requirements with just two cable circuits.
That could be cut to one if the rules change to do away with the need for back-up.
The cable circuits would be laid 500 metres apart on the seabed to minimise the chance of more than one of them being damaged at the same time.
Obviously in Weisdale Voe they will have to come together but should still be 50m apart at the landing point, to the south of the Swedish houses.
Details of the proposed Shetland high voltage electricity transmission project were discussed with members of the public at a roadshow in Scalloway on Tuesday.
The roadshow planned for the previous day in Whiteness had to be cancelled after the Scottish and Southern three-strong team’s flight from Aberdeen was delayed by a technical problem and snow at Sumburgh. It will be held on Tuesday, 8th April instead.
Around 100 people visited the presentation, including members of the anti-wind farm group Sustainable Shetland.
People have until 25th April to submit comments. All the official bodies were consulted some time ago.
Once the plan is finalised it will go forward for planning permission.
SHETL has nothing to do with the network of cables or poles that will snake from the wind turbines to sub-stations and onwards to the Kergord converter station. Plans for those will come later from Viking Energy.
The converter station will go on a site at Upper Kergord, further north than the end of the existing road.
Project manager Greg Taylor said on Tuesday that despite the size of the building it should not be very visible in the scenic valley and the intention is to try to screen it using trees.
Twelve sites around the Central and South Mainland were considered before it was chosen.
One advantage in running the cables through Kergord to the sea is the relative lack of undisturbed peat that would have to be dug up, since most of the valley down to the voe is already “improved” land.
This summer a marine survey is to be carried out to check the proposed cable route, identifying hazards on the seabed including rocky outcrops and steep slopes.
The cables will be buried where it is technically possible and covered by protection in other places.
It will have to cross existing seabed cables and pipelines.
At the other end of the marine cable, the power will go into the up-graded Beauly-Denny overhead tower, if that project eventually gets the go-ahead.
In contrast, Orkney’s renewable energy exports will go into the Dounreay power transmission line and will not need to be converted from AC to DC and back again to make the leap across the Pentland Firth. The distance is short enough to avoid too much power being lost in transit.
Comment on the plans can be sent in writing to: Dr Keith MacLean, SHETL, Inveralmond House, 200 Dunkeld Road, Perth, PH1 3AQ or email email@example.com.
28 March 2008
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