Brit energy regulator Ofgem has opened an investigation into National Grid and Scottish Power's operations on the Western Link subsea cable.
The £1.3bn project, which transmits renewable energy from Highland wind farms via Hunterston in Western Scotland to North Wales, became operational in October 2018, three years behind schedule.
The cable increased interconnection capacity by over 2,200MW – enough to power more than 4 million homes in Scotland, Wales and England every year, according to Iberdrola, which owns Scottish Power.
As the first subsea link to use a 600kV-rated interconnector, the cable was once described by UK ministers as the perfect symbol of the country's single electricity market.
However the project has been plagued with outages as a result of faults along the 850km (528-mile) cable, 770km (478 miles) of which is under sea.
On 12 January, the subsea cable failed for the fifth time since its completion. It remains offline.
Italian cable contractor Prysmian has not clarified the issues behind the fault, raising fresh doubts over the infrastructure. The company said in a statement that the fault had been located on "pole 2 of the southern part of the link". National Grid clarified that the fault was on the "southern land cable".
Prysmian declined to comment on The Register's requests for clarification.
Prysmian built the Western Link in partnership with Germany's Siemens, which installed converter stations. Problems during construction hit Prysmian's 2018 financial results, forcing the company to cut its targets and recognise provisions of €95m. It is not known if the current problems are connected to repairs of the interconnector it completed in June 2019 (PDF). The project was fully handed over to National Grid and Scottish Power in November last year.
Failing cables are a costly problem. In such cases energy providers must compensate the wind farms, which continue to generate energy but cannot transmit it. In the latest case, National Grid and Scottish Power will foot the bill, reported to be up to £5m, but ultimately the cost will fall on customers.
The Ofgem probe will review the National Grid and Scottish Power's performance and consider whether the project's late delivery breached their licence conditions by delivering the cable late.
The investigation will also consider whether the companies breached the Electricity Transmission Licence, or the 1989 Electricity Act.
Ofgem cautiously added: "The opening of this investigation does not imply that we have made any findings about about non-compliance by National Grid Electricity Transmission or Scottish Power Transmission."
National Grid and Scottish Power said in a joint statement: "We will continue to work with Ofgem in an open and transparent manner and will fully co-operate with this investigation."
The companies added: "We are working hard to repair the current cable fault and are certain we will overcome these difficulties and establish a link that will continue to benefit consumers for many years to come."
Western Link's website has been offline since yesterday afternoon. ®