SHETL are seeking to reinforce the transmission system to the Isle of Lewis to accommodate large renewable generation developments. The transmission link will require a HVDC (high voltage direct current) link which is intended to operate entirely on cable circuits, underground and subsea. The cables will cross areas of moorland including extensive areas of peat. Peat is known to be a very difficult substance to work with as it has a poor thermal resistivity when dried out and can die when disturbed leading to long term changes in the vegetation. The HVDC cables will run at temperatures of up to 70•C which may exacerbate the drying out process, increasing the thermal resistivity and changing the flora in the vicinity of the cable route. It is also recognised that the installation style, depth and backfill materials could also impact on the hydrology of the peat, again increasing the tendency to dry out.

The test installation involves the installation of sections of heating cable, designed to emit heat to match the anticipated losses on the HVDC cable circuit. There are four sections of cable, each installed using different installation methods to allow comparisons to be made. In addition, there are two control sections where the ground has been disturbed but no cable installed.

The Scottish Agricultural College have been employed to monitor the effect on the soil and flora. ABB Power Technologies have been employed to provide the necessary heating cable, temperature sensors and expertise in calculating the thermal resistivity.

Objectives

Using a test cable installation, this project aims to assess thermal resistivity and environmental impact in peat moorlands.

The project aims are:

• To establish a suitable thermal resistivity for peat moorlands to facilitate cable design

• To establish the environmental impact of a cable installation across peat moorlands

• To assist in the preparation of method statements to minimise the overall environmental impact.