With apologies to Jules Verne – high voltage direct current (HVDC) cables convey rather more than 20,000 volts. According to Wikipediai most typically use voltages between 100 kV and 800 kV (although they are not limited to this range). Generally HVDC cables are used for wind farms some distance off shore (or to circumvent issues associated with overland transmission), but increasingly the UK’s wind generation is planned to come from islands, with a plan to “export” surplus electricity long distances. HVDC cables are seen as the means by which the electricity is to be transferred from the islands to the mainland.

As we saw in For Peat’s Sakeii and Utilities Futility,iii the Viking Energy wind farm on Shetland is one such development. The plan by SSE is to connect to the National Grid on the mainland via a new HVDC cable. More accurately, the power from Shetland has to reach the mainland via two long subsea cables and a switching station. All elements have to work or the whole system fails. There will obviously be some loss of energy because of the length of transmission required. Not surprisingly, in the face of considerable local opposition, SSE is expressing great confidence in the reliability of such a cable, as reported by Shetland Newsiv:

THE PROJECT director in charge of building the Kergord converter station and 600MW interconnector infrastructure linking Shetland to the national grid expects the subsea link to be available for 99.8 per cent of the time.

Curiously, however, the report in the Shetland News also goes on to say:

Lerwick Power Station will remain on standby, should the cable fail, until at least 2035.

A sensible precaution, or perhaps a sign of less confidence than the official line suggests? Also, given that the plan is that electricity is to be exported from Shetland to the mainland (which implies electricity surplus to Shetland’s needs), why the need for Lerwick Power Station to remain on standby? Oh I see – because wind power is unreliable, and if Shetlanders were forced to rely simply on the electricity generated by the wind farm, even in such a windy place, they would be left without power for significant periods of time.

SSE/Viking Energy may also have run into a new problem. According to the Energy Voice websitev:

Suspected unexploded ordnance has been found along the route of the planned subsea cable that will link Shetland to the mainland.

However, significant though that development might (or might not) prove to be, that’s a side issue. My purpose here is to take a look at the performance of HVDC cables to date, and to ask questions regarding the reliability of offshore and island wind farms as a source of steady power for the UK mainland National Grid.

Caithness – Moray HVDC Cable

SSE have been keen to cite the Caithness – Moray HVDC cable as an example of an earlier cable that hasn’t caused any problems.

In our experience with the Caithness to Moray cable – the most directly relevant one, which we installed in 2018 – we had no issues at all with the cable being damaged or being out of service.”

But is this so? Laid (in fact) in 2017, it appears that by 2018 problems had already become apparent. Because of the nature of the necessary remedial works, and the sensitive nature of the site, it was necessary for an Environmental Appraisal Report to be commissioned. Although elements of it have been redacted, it is available onlinevi.

From this we learn:

Scottish Hydro Electric Transmission Plc (SHET) have developed a High Voltage Direct Current (HVDC) electricity transmission link between Caithness (Noss Head) and Moray (Portgordon)… In early 2018, information became available that suggested there was a fault in the (offshore) installed cable. The location was identified as being at KP 13.158 (approximately 13km from Port Gordon landfall). A cable repair and associated activities were undertaken by NKT in early 2018.

The report was required in connection with additional works, identified as including the following:

Cable remediation and backfill at KP1.6‐3.6, including rock placement if required; Installation of 2 x new lengths of DC cables & 1 x FO of cable between KP11‐16 and potentially KP83‐86 (dependent on cable inspection results);

Cable de‐burial and inspection at KP 83‐86;

Burial of newly installed cable, use of rock placement if required; and

Removal of old cables at KP11‐16 and KP83‐86 (dependent on cable inspection results).

Maybe the claim that they had had no problems with the cable laid in 2018 involved a careful choice of words, if the problems all related to the cable laid in 2017, and not to a replacement cable laid in the following year?

Western Link HVDC cable

This cable has experienced numerous problems, as the Energyst websitevii makes clear:

The failure of the high voltage undersea cable between west Scotland and north Wales last month resulted in National Grid ESO paying almost £31m for wind farm operators to curtail output.

Consultancy Cornwall Insight calculated the figure based on Balancing Mechanism data and prices during what was an exceptionally windy month.

The Western Link HVDC cable went down on 10 January and remained offline until 8 February. It was the cable’s third complete trip in as many years.

So problematic has this cable proved to be, that:

Following the latest outage,Ofgem has opened a probe into the £1.3 billion Western HVDC connector, which links Highland wind farms via Hunterston to North Wales.

So far as I am aware, we still await the outcome of the Ofgem probe. Since the above article appeared (on 25th February 2020), there have been additional problems, with a further outage lasting from 15th February to 13th March 2021. Whenever a problem of this nature occurs, there are inevitably costs. As an online article from S&P Globalviii tells us:

Since the link went offline on Feb. 15, GB metered wind output has been sharply constrained, with output up to 2.1 GW lower than National Grid’s final forecasts, failing to outturn above 12 GW,” S&P Global Platts Analytics said March 3.

Within the Balancing Mechanism, the volume of accepted bids in Scotland reached 430 GWh in February, up from 46 GWh in January when the link was fully available, it said.

Between Jan. 1 and Feb. 15 National Grid reported daily average constraint costs of GBP0.5 million/day. This rose to GBP6.1 million/day from Feb. 16 to Feb. 25, before wind generation dropped.

“Balancing Services Use of System (BSUoS) charges – the cost of balancing the transmission system of which both generators and suppliers are liable – reached an intra-day high of GBP15.3/MWh on Feb. 25, the highest level since Dec. 27, 2020,” Platts Analytics said.

BritNed HVDC cable

As the UK increasingly phases out fossil fuels from its electricity generation programme, the need to import electricity from elsewhere becomes increasingly important. At the very moment I type this, a late summer’s evening in 2021, 17.4% of the UK’s electricity is being supplied via interconnectors, and (by chance – the number fluctuates) 3.5% of the whole of the UK’s electricity is at this moment being supplied via the BritNed HVDC cable, that is the interconnector between Britain and the Netherlands. It can readily be appreciated that this is an important cable. And yet it has an unhappy history. I can do no better than tell its story by quoting from an article which appeared on Watt-Logic’s websiteix on 17th March 2021:

The 1 GW BritNed interconnector linking GB with the Netherlands has suffered another fault and is expected to be out of service until late April, meaning it has spent much of this winter out of action. In addition to contributing to a tight GB market, these outages illustrate some of the challenges facing offshore electricity infrastructure, particularly in sub-sea environments.

BritNed tripped on 8 December and did not return to service until 9 February due to a cable fault. The damaged cable section was found approximately 100 km off the Dutch coast at a water depth of 40-50 meters. Repairs were successfully carried out within 29 days despite severe weather. But now the interconnector has developed another fault, probably with the cable again, and is expected to remain out of service until 22 April.

These outages mean that BritNed will have been offline for a significant portion of Winter 2020, a risky position given it holds a capacity market contract for the current delivery year. There have been two Capacity Market Notices (“CMNs”) issued this year, the second of which came just days before BritNed’s first outage. While a failure to deliver following a CWN [sic] does not attract penalties unless a Capacity Market Stress Event occurs, the issuance of both CWNs [sic] and Electricity Margin Notices this winter, combined with high price volatility, indicate the markets have been much tighter than usual.

By the way, please see “Counting the Cost”x for an explanation of Capacity Markets.

Orkney – Pentland East cable

This cable was replaced in late 2020 and the announcement of the completion of the work was welcomed with much publicity from SSE and others. However about 2 months later it was very quietly announced that the new £30 million cable had unexpectedly failed.

There is as yet no word on whether or not it is to be repaired, as the old one was reconnected, at considerable expense. We can only assume that the cable failure was catastrophic.

As I understand it, Ofgem have conditionally approved a larger capacity cable to Orkney, with final approval depending on enough wind farm capacity being approved to justify the cable. Perhaps if the larger cable is fully approved, SSE will simply write off the failed cable. Presumably they are hoping that the old, reinstated cable lasts until the larger cable is approved.

Whether or not this speculation is correct, this is all on the back of pre-existing problems that were reported by the Orcadianxi in 2019.

Western Isles

This is a particularly dismal story. On 16th October 2020 the cable linking the Outer Hebrides to Skye was cut, causing a blackout affecting 18,000 Hebridean homes. The Guardian reported on it at the timexii:

The 20-mile (32km) cable, which runs on the sea bed from Skye to Harris, failed without warning six days ago.

That forced Scottish and Southern Energy Networks to call on a diesel-fuelled power station in Stornoway to provide back-up electricity, and airlift extra diesel generators from the mainland in case Stornoway’s turbines fail.

SSEN announced on Thursday that the 33,000V cable was unrepairable, adding that it could take between six months and a year before a new cable could be commissioned and laid.

It believes the breach happened at a depth of over 100 metres about 15km offshore from Skye, and is now investigating whether it was cut by a trawler or another vessel. SSEN has asked for local shipping records so that it can pinpoint which vessels were in the area at the time the cable failed.

When the cable failed, a larger replacement cable was requested by many interested in the issue. However SSE do not have much wind farm interest in the Western Isles, and whether or not that influenced them, the larger cable request was rejected. It is worth noting that, when the cable failed, there was an adverse impact on some smaller community based wind farm projects: they could not market the electricity that they were producing, without the cable, and they lost income as a result. These smaller projects were not eligible for constraint payments.

I haven’t yet been able to find confirmation that the replacement cable is operational, though I believe it arrived in May, and the plans were that it should be in place by the end of August 2021.

Gwynt y Môr wind farm

This story confirms the problems, but from a slightly different point of view. As reported on the New Power websitexiii the company that owns the link to Gwynt y Môr wind farm argues that a series of repair outages required following a cable failure in October 2020 should be underwritten by consumers because insurers are leaving the market.

The company notes that the cost of insurance has risen 40 per cent in the past two years and many insurers have declined to provide cover. Offshore wind farm cables have been under the spotlight for outages to repair cable problems, due to manufacturing faults, accidents and other issues.

In April this year, Danish wind farm operator Orsted said it had put aside £350 million to repair or replace cables within its wind farms that had been damaged due to interaction with the sea floor.


Subsea cables fail for many different reasons, including the activities of fishing trawlers, damage from anchors, manufacturing errors, and environmental factors (e.g. seabed roughness and tidal flows) leading to corrosion and abrasion of the cables.xiv Only a few days ago it was reportedxv that the Australian authorities have arrested the Ukrainian master of a Maersk container ship for allegedly damaging a telecommunications cable connecting Singapore and Western Australia, with the ship’s anchor. In that case the cable affected related to telecommunications, and was not an HVDC cable, but the principle remains the same. These cables are vulnerable, and not just in theory – the brief case history above illustrates that the problems are real in practice too.

It’s bad enough blighting beautiful island landscapes with industrial scale wind turbine developments. It compounds the issue when the benefits claimed for them hang by such a fragile thread.

i https://en.wikipedia.org/wiki/High-voltage_direct_current

ii https://cliscep.com/2021/06/29/for-peats-sake/

iii https://cliscep.com/2021/07/06/utilities-futility/

iv https://www.shetnews.co.uk/2021/08/19/subsea-cable-very-reliable-says-project-director/

v https://www.energyvoice.com/renewables-energy-transition/344908/suspected-unexploded-ordnance-found-along-route-of-planned-shetland-hvdc-link/

vi http://marine.gov.scot/sites/default/files/environmental_appraisal_-_redacted_0.pdf

vii https://theenergyst.com/western-link-failure-sees-national-grid-pay-31m-to-turn-off-wind-farms/

viii https://www.spglobal.com/platts/en/market-insights/latest-news/electric-power/030521-uks-western-hvdc-link-due-back-in-service-week-starting-march-15-nat-grid

ix http://watt-logic.com/2021/03/17/another-britned-outage-illustrates-the-risks-of-the-current-offshore-infrastructure-regime/

x https://cliscep.com/2021/08/15/counting-the-cost/

xi https://www.orcadian.co.uk/power-cable-fault-results-in-rackwick-work/

xii https://www.theguardian.com/business/2020/oct/22/scottish-energy-company-ssen-investigates-blackout-caused-by-subsea-cable-failure

xiii https://www.newpower.info/2021/08/gwynt-y-mor-reveals-battle-to-insure-offshore-wind-cables-40-premium-hike-and-scores-or-providers-decline/

xiv https://www.prosperoevents.com/why-subsea-power-cables-fail/

xv https://www.poandpo.com/news/maersk-ship-captain-charged-over-underwater-cable-damage-off-perth/

via Climate Scepticism


August 30, 2021