Scottish and Southern Electricity Networks owns and operates the Orkney electricity distribution network, which is joined to the Scottish mainland by 2 33 kV submarine cables. The subsea cables possess a capacity of around 40 MW. The demand for electricity on Orkney fluctuates between approximately 8 MW and 32 MW. Implementing traditional approaches to network planning resulted in a maximum protected hosting capacity for distributed generation (DG) of 28 MW.
A quick protection system was implemented to allow a further 20 MW of DG to be linked to enhance the capacity of those islands. This creation would instantly be actuated off the system in the event of an outage. The grid connection rules in the UK demand that connecting a single voltage greater and customers must finance reinforcements to the grid in their voltage. Additional DG connections beyond the 48 MW (28MW plus additional 20MW) could trigger the need for a further subsea cable as well as multiple inner island network updates.
A subsea cable’s minimum price has been calculated as #30m. Timescales to implement the subsea cable and the cost meant that their relationship software would not be progressed by any individual DG developer. Together with 48 MW of generation already joined, the grid has been considered ‘full’.