Isles of Scilly Subsea Cable
Island LinkOperationalGB7.5 MW33 kV55 km1989
Also known as:Isles of Scilly 33 kV subsea cable, Isles of Scilly power cable, St Mary's 33 kV submarine cable, Whitesands–Porthcressa 33kV cable
Status:Operational
Capacity:7.5 MW
Project Company:
Western Power Distribution
Type:island link
Expected Life:25 years
The Isles of Scilly subsea power cable is a domestic 33 kV alternating‑current (HVAC) submarine distribution circuit that links the Cornwall mainland to the electricity system on St Mary’s, the principal island in the Isles of Scilly. The asset provides the islands’ principal export/import electrical supply and is the single...
Supply Chain (5)
Project Timeline (12)
4 Apr 2017Cable Repair
Following the March 2017 anchor strike on the 33 kV, 55 km subsea cable between Land’s End and St Mary’s Power Station, Western Power Distribution initiated a full repair campaign, including fault location, deployment of the cable repair vessel CS Sovereign with an ROV to cut and lift the damaged section, subsea jointing works, and associated high-voltage testing, culminating in completion of repairs that enabled reconnection of the Isles of Scilly to mainland power in early April 2017.
4 Apr 2017System Restoration
After final high-voltage pressure testing of the repaired 33 kV subsea cable in early April 2017, Western Power Distribution re-energised the link and transferred the Isles of Scilly back from temporary generator supply to mainland grid power, with energisation occurring around 24 hours after the completion of night-time testing on or about 4 April 2017.
Apr 2017System Testing
On the first weekend of April 2017, after repair jointing of the damaged Isles of Scilly 33 kV subsea cable was completed, Wootton & Wootton returned to perform final VLF pressure testing with the HVA200 on the fully reconnected circuit from St Mary’s back to the Cornish mainland, testing both while the new joint was on the repair vessel and again after it was lowered to its final seabed position prior to re‑energisation on 4 April 2017.
Mar 2017Cable Repair
Following catastrophic anchor damage to the 55 km, 33 kV subsea cable supplying the Isles of Scilly, Wootton & Wootton used their HVA200 VLF test set from St Mary’s Power Station in early March 2017 to carry out pre‑repair pressure testing of the island‑to‑fault section, manually operating at 0.01 Hz and 32.47 kV for 30 minutes to confirm the integrity of the cable up to the break.
Mar 2017Major Outage
At the beginning of March 2017, the single 55 km, 33 kV subsea power cable supplying the Isles of Scilly from the Cornish mainland suffered catastrophic damage, believed to have been caused by the anchor of an unidentified vessel, leading to failure of the only grid connection and forcing around 2,000 residents onto diesel generator supply while repairs were organised.
3 Jun 1989Commercial Operations Full Power
The Isles of Scilly mainland subsea cable link entered operational service following completion of all project works by March 1989 and a formal Commissioning Ceremony at St Mary’s Generating Station on 3 June 1989, conducted by HRH Prince Charles, marking the energised 33 kV connection between Whitesands Bay in Cornwall and St Mary’s.
3 Jun 1989Onss Commissioned
The onshore facilities for the Isles of Scilly submarine cable link, including the new 33 kV land cable into St Mary’s Generating Station, associated 33 kV switchgear, transformer and related mainland works, were formally commissioned at a ceremony on 3 June 1989 at St Mary’s Generating Station, conducted by HRH Prince Charles and marking completion of the mainland link project.
1 Mar 1989Cable Installation Complete
Installation of the Isles of Scilly mainland link cable system was completed by March 1989, following the laying of a 33 kV, 55 km submarine cable manufactured by Pirelli General and installed by Balfour Kilpatrick’s converted cargo ship Star Hercules between Whitesands Bay in Cornwall and Porthcressa Bay on St Mary’s, as well as associated 33 kV land cable on St Mary’s and overhead line works on the Cornish mainland.
1 Aug 1988Cable Installation Start
In August 1988, installation of the Isles of Scilly 33 kV submarine cable commenced, with contractor Balfour Kilpatrick laying the approximately 55 km, three-core copper, steel-wire-armoured cable manufactured by Pirelli General between Whitesands Bay on the Land’s End peninsula and Porthcressa Bay on St Mary’s using the converted cargo ship Star Hercules. The works formed part of the Isles of Scilly Electrification Project to link the islands to the mainland network, supervised by project engineer John Heath aboard the laying vessel and supported by associated onshore 33 kV works on St Mary’s and the Cornish mainland.
1 Sept 1984Fid Financial Close
In September 1984, following government approval for the Isles of Scilly inter-island electrification scheme, SWEB proceeded with the investment for installing 11 kV submarine cables between St Mary’s and the other inhabited islands, a £3 million project co-funded by the European Regional Development Fund, SWEB, the Duchy of Cornwall, the Countryside Commission and local property owners. With the go‑ahead and funding structure in place, contracts were awarded to Balfour Kilpatrick and AEI Cables for the manufacture and laying of approximately 16 km of 3‑core, paper‑insulated, lead‑sheathed, steel‑armoured submarine power cables.
£3M