I HAVE previously questioned whether power companies in Scotland had solved the known instability risks, including to HVDC links operation, introduced by progressively increasing renewables whilst closing synchronous generation such as Hunterston etc, thereby weakening our grid.

It has transpired that National Grid Electricity System Operator’s (ESO) chief engineer with others, via the Energy Systems Integration Group in England, identified on December 1 that Scotland’s grid suffered two heart attacks (my metaphor) during the night of August 24. If there was a public statement on the crisis event from Scottish and Southern Energy, ScottishPower or EDF, the owners of Torness, I missed it.

Showing event graphical records they detailed “severe voltage disturbances” lasting 20-25 seconds, which is a long time in power system engineering, on two occasions 30 minutes apart. Oscillations shown for the 400kV system ranged between approximately 355kV to 435kV and the biggest oscillations were close to the northern end of the £1.1 billion HVDC link at Spittal, south of Thurso, with a frequency of around 8Hz (cycles/second).

Power oscillations would also have been similarly huge at various locations.

“Some users tripped off during the disturbances” including apparently some wind farms and we came close to losing most of the grid north of the central belt. Coincidently the ageing gas station at Peterhead, which would have provided much stabilising synchronous generation, was reportedly offline.

A suspected potential trigger apparently is that the newly commissioned Moray East offshore wind farm controls may have interacted with the HVDC Spittal DC/AC converter nearly 50 miles away within the very weak grid. In addition the disturbances reached Torness, near Dunbar, considerably farther.

It is presently unclear what saved us but it is a concern in the profession that Torness was adversely affected and may have come close to tripping with serious consequences in the central belt.

Detailed further investigative work is under way with ESO involvement.

It is possible Ofgem might decide to take a close look at the incident. This especially since a progressively weakening grid in Scotland through increasing renewables may be seen as risking periodic unpredictable effects on the rest of the UK grid.

My question has been demonstrably answered, however, and until the event is understood it can happen again

DB Watson, Cumbernauld.