Ofgem Rules on Thanet OWF 2015 Cable Fault

UK energy market regulator Ofgem has accepted Thanet OFTO Limited’s claim on a partial outage in electricity transfer in 2015 due to an export cable fault. As a result, Thanet OFTO Limited’s reported system incentive performance shall be adjusted to offset the full duration of the outage.

The reported system incentive performance for incentive year 2 (beginning 1 January 2015) was 482,192.30 MWh.

The Thanet offshore transmission system encountered issues on 23 February 2015 and identified an export cable fault to be the root cause of the problem. Inspection and repair started immediately, with the transmission brought back to full operation on 7 July that year.

Since then, Thanet Offshore Wind Farm (TOWL) has also confirmed the reduction in system availability as attributable to a Root Cause as defined in the Cable Indemnity Agreement, while Thanet OFTO Limited has been working on pinpointing what led to the export cable fault.

The offshore transmission owner has submitted multiple updates to Ofgem, with the last two reports providing the results of investigations into the cause of the reduction in system availability.

According to the reports, the fault on the export cable was caused either by the failure to earth the copper tubing surrounding the fibre optic cables at the offshore substation – which occurred during the subsea cable installation, or the outer plastic sheath of the fibre optic cables not uniformly semiconducting due to a defect in the manufacturing process.

Ofgem concluded that Thanet OFTO Limited acted in accordance with Good Industry Practice by having in place operational and maintenance procedures that are consistent with Good Industry Practice.

The UK energy market regulator also added that after the initial failure event, the transmission system owner “took prudent and timely steps to identify the root cause of the fault, restore transmission services and mobilised appropriate resources to repair the fault in a timely manner.” Here, Ofgem pointed out that Thanet OFTO Limited arranged the deployment of a temporary vessel that allowed the works to begin while the preferred vessel was unavailable immediately.

In May 2018, Ofgem brought a similar decision for a similar claim for an outage that happened in 2016.

Namely, Thanet offshore transmission system suffered a cable failure on 5 March 2016, with a temporary repair undertaken and services restored on 28 April 2016. A permanent repair was carried out between 5 September and 10 October 2016.

Thanet OFTO Limited submitted the full facts of the claim for assessment on 27 January 2017, along with independent technical analysis, commissioned by the insurance assessor ERA. The report concluded the likely cause of the cable failure was combination of two factors: “Jointer error in the preparation of the copper to aluminium weld which led to a raised weld and hence a reduction of insulation thickness over the weld” and “poor amalgamation of the conductor screen tapes allowing insulation material to be forced under the tapes leaving the edge of the conductor screen tapes forming a stress raiser.”

The error has resulted in a reduction of the thickness over the weld and leading to a region of high electrical stress and degradation of the insulation. This lead to the failure of the weld (i.e. the joint) and subsequently the failure of the cable, according to documents available on Ofgem’s website.

Ofgem accepted the cable failure as an exceptional event and directed that Thanet OFTO Limited’s reported system incentive performance be adjusted to offset the full duration of service reductions. The reported system incentive performance for incentive year 3 (beginning 1 January 2016) was 198,765 MWh – 195,165 MWh fo the period from 5 March to 28 April, and 3,600 MWh for the period from 9 October to 10 October 2016.

Thanet OFTO Limited is owned by Equitix (80%) and Balfour Beatty (20%).

Photo: Vattenfall/ Archive

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