By DB Watson

A REPORT from National Grid (NG) in a November report raised serious concerns as to the ability of the new billions of pounds investment in high-voltage, direct current (HVDC) interlinks that we are installing across the UK to transfer power in all circumstances. NG identifies that this results from our progressive weakening of large tracts of the Grid as we increase the amount of connected renewables, mostly wind turbines.

NG has now, arguably several years after they should, done some system modelling of these interconnectors and its report concludes “system strength will decrease in our transmission network over the next decade” and “we have found an increasing risk of converter instability” (this is where the new direct current lines connect to our existing alternating current Grid).

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In its highly technical report entitled Performance of Phase-Locked Loop Based Converters, NG goes on to demonstrate that as the available system strength (called fault level) falls due to increased reliance on wind turbine generation then when network faults occur there will be many scenarios where the DC/AC converters will become unstable and our voltage will start to surge and oscillate at a different frequency to the 50Hertz that we receive in our homes, which means shutdown. NG notes that this problem would not exist had we been retaining large-scale synchronous generators (such as at Longannet, Torness and Hunterston.)

NG concludes by expressing a wish “to work with manufacturers, developers and any other interested parties to further explore the risk of converter instability”.

Key Scottish HVDC links being commissioned, installed and under consideration respectively are the £1 billion “Western Link” from North Wales to near Hunterston, the £1.1bn Spittal near Thurso to Keith and the potentially circa £1bn Peterhead to Redcar.

Whilst Hunterston B is generating the Western Link can theoretically operate in both directions. When Hunterston closes in 2023, there will be a big loss of system strength in west Scotland implying the new HVDC link converters will encounter fault conditions where they become unstable, meaning Scotland could not import. This further indicates that the link will not be capable of “Black Starting” Scotland in the event of a system failure.

Torness too is scheduled for closure in 2030, meaning by then a total of 2000MW of highly stabilising generation will be lost to Scotland and it is likely that the Scottish network is so weakened, as all we may have is Peterhead (gas), some small pumped storage and hydro plus intermittent renewables, that we will have to depend upon our existing HV AC overhead transmission lines from England to keep the lights on when renewables routinely collapse.They may not always be able to meet demand.

NG also recently forecast that with further anticipated supplanting of large power stations with renewables, control of the Rate of Change of Frequency risk will be “too high” over 25 per cent of the year by 2021/22 (currently two per cent and the increasing threat of voltage collapse as we lose the ability to locally control “wattless” power to keep it afloat may necessitate countrywide construction of large scale synchronous compensators to offset.

These reports about future renewables development constraints were all issued before the Scottish Government issued its The Future of Energy in Scotland document shortly before Christmas, but it fails to address them directly or how they will be overcome in the four and 10-year time frames NG stipulates. Can we build sufficient new gas generation as The Scottish Government now finally concedes is necessary on Page 54, in time?

Our politicians will have to open their eyes, take their fingers out of their ears and act. And soon.

* The author is a retired chartered electrical engineer and the former Manager of Projects for the Scottish office of multinational energy engineers Foster Wheeler Energy.