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NETWORK / 9 / FEBRUARY 2020 demand and the system's frequency fell so fast it caused further DG to trip due to ROCOF. 'Frequency containment reserve' is scheduled by the ESO, mostly from generators but also from batteries, to respond automati- cally to an imbalance between generation and demand. However, there was not enough response to prevent the system's frequency from falling so far be- low statutory limits that, within 80 seconds of the lightning strike, it triggered the discon- nection of 1.1 million customers by automatic 'low frequency demand disconnection' (LFDD) equipment. The E3C report notes that some 'essential services' were affected. Those disconnected by the action of LFDD included supplies to Newcastle airport, two hospitals, railway signalling at two sites, railway traction supplies at one site and a water treatment works. The E3C report also notes further losses during the incident: traction supplies at two locations, railway signalling supplies at six sites, and sup - plies at two hospitals, one water treatment works and an airport. These were all for as-yet unex- plained reasons, although, for the most part, back-ups (such as standby generation) operated successfully. However, supplies to an oil refinery and chemicals manufacturing plant were also reported by E3C to have been disconnected with full opera - tions taking several weeks to be restored. Once the incident had started, was the impact inevitable? The Grid Code, with which all parties connected to the electric- ity transmission system are obliged to comply, requires that generators should 'ride through' temporary faults on the net - work, such as that which initi- ated the incident on August 9th. Ørsted's plant at Hornsea and RWE's at Little Barford failed to do so. Although individual infeeds can and do trip with at least one going pretty much every day, the Grid Code's 'low voltage ride through' require- ments are essential for prevent- ing a simultaneous 'common mode' loss of large of amounts of generation. According to Ørsted, the so•ware on the wind turbines at Hornsea has since been updated so at least that particular control system failure should not happen again. Generally, less well under - stood – and managed – than the behaviour of transmission connected plant is that of dis- tribution connected generation, i.e. DG. At least some of this is known to trip as a result of dis- turbances on the transmission network though exactly how much depends on the nature of the disturbance. This com- pounded the problem on August 9th and was exacerbated by further trips that no-one seems to have been expecting. Normally, the ESO schedules enough frequency containment reserve to cover for the loss of whatever the 'single largest infeed' happens to be at the time – reported by Ofgem to have been 969MW on August 9th. However, 1561MW was lost within less than half a second. Nonetheless, it's possible that the system frequency might not have dropped as low as the 48.8Hz threshold for the first stage of LFDD if all the sched - uled frequency containment reserve had delivered. Unfor- tunately, it did not all deliver, perhaps as much as 17% of it in certain categories. The disconnection of 'es - sential services' by the action of LFDD seems less than ideal: could at least the first tranche of LFDD (there are nine of them in total) have been set up on sec- tions of the distribution network that fed no essential services? Some essential services' own protection, such as trains, also seemed to respond to the drop in system frequency and discon- nected themselves. What have the investigations concluded? Ofgem reported that Ørsted and RWE have made payments of £4.5m each to Ofgem's "volun- tary redress fund" in recognition of the impact that failure of their plant had on consumers. One of the DNOs, UKPN, also made a payment, of £1.5m, in recogni- tion of the adverse impact that its actions might have had when restoring disconnected demand before being authorised to do so by the ESO. There is a programme of work within the industry to re - place 'loss of mains' DG protec- tion with settings that are less sensitive to transmission system disturbances. However, this pro- gramme is scheduled to be com- pleted only in 2022. Ofgem has recommended a review of this timetable. It also recommended a review of various codes and standards including the Security and Quality of Supply Standard (SQSS) that sets out minimum requirements for how the ESO should manage the risk associ - ated with disturbances to the system. E3C made a number of the same recommendations as Ofgem. In addition, it promised to clearly define what 'essential services' are and provide guid - ance to those services, and to develop a new incident response communications strategy. It also asked the DNOs and the Energy Networks Association to under - take a "fundamental review" of the LFDD scheme. The ORR recommended that train operating companies should check the settings of train protection systems and that Network Rail should check the nature of their connections to DNOs' networks. Of particular note, I think, is that Ofgem has identified a number of issues with the ESO's existing processes and proce - dures. These include how the need for frequency management services is identified and the Keith Bell, Professor of Smart Grids University of Strathclyde & co-director of the UK Energy Research Centre