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BLACK OUTS NETWORK / 32 / NOVEMBER 2019 I n the unlikely event of a total or partial shut- down of the electricity transmission networks, system operators use Black Start services to re-energise the grid and restore power to consumers as quickly as possible. "Black Start" has traditionally relied on the use of designated power stations with self-starting capa - bility to re-energise sections of the network, then incrementally connect loads and additional generators to create "power islands". These "islands" are connected together to restore supply to consumers. Traditionally, Black Start services have been provided by large coal and gas power stations. However, over the last decade, the electricity genera - tion mix in Great Britain has seen a decline in coal and gas power station operation, creat- ing a need to focus attention on the future of our Black Start and system restoration capabilities. The future generation mix is expected to be dominated by non-synchronous renewable generation, which is inherently less suited to contribute to Black Start. UK nuclear plants, the alternative synchronous power option, have not traditionally been able (technically or from a safety perspective) to support emergency restoration. A range of Black Start op - tions can be explored, based on self-starting technologies enabled by power electronic converters; such devices can translate the power produced Electricity interconnectors are increasingly adding flexibility to the high voltage network in Great Britain. But can their convertor capacity also offer a Black Start service to the grid? Oluwole Daniel Adeuyi and colleagues at the National HVDC Centre, operated by Scottish Hydro Electricity, discusses its recent scenario planning around an interconnector Black Start. asynchronously or via direct current (DC) into alternating current (AC) power capable of restoring the grid. A converter-based Black Start comes with the benefit that the converters can start from "cold" which can be problematic for traditional power stations. How - ever, unlike a large conventional power station, converters do not have the same levels of over- load capabilities to deal with short term high-power needs across Black Start, and for many renewable technologies the intermittency or predictability of the energy resource could also be a limiting factor. These opportunities and constraints are currently being explored by National Grid ESO in its three-year programme to identify a mix of suitable candi - dates and devise protocols for a Black Start reliant on Distrib- uted Energy Resources [DER], including converter-based low-carbon technologies (see interview, p 30-31). However, one converter technology approach is to rely on high voltage direct current interconnector (HVDC) schemes that connect the GB grid to those of neighbouring countries, where the size of their DC to AC converter capacity provides the flexibility to support Black Start while also having the advan - tage of dependable power from another independent system. International connections HVDC technology is not a source of generation in itself; rather it is an alternative to high voltage alternating current (HVAC) used for the implementation of circuit connections in electricity trans - mission systems. There are five point-to-point HVDC interconnectors con- necting the GB grid to adjacent grids, with more in construc- tion and planned. Currently, the GB grid has connections to Northern Ireland, Ireland, France and the Netherlands. Sophisticated control of semi- conductor switches within the convertors enable a sequence of "gates" across which power can flow to construct DC voltages An alternative route to Black Start and current. Two principle technologies are used in these converters: • line commutated convert - ers (LCC), which require an AC voltage and a strong enough AC system to be present in order to construct a DC voltage; and •voltage source converters (VSC) which can construct their own AC and DC voltage and can self-start, hence it is more suit - able for Black Start restoration compared to LCC schemes. VSC-HVDC is expected to be the dominant technology in future developments on the GB grid. According to the National Grid ESO, the benefits of VSC technology include a reduced land footprint compared to an equivalent LCC installation, and the capability to provide services such as Black Start functionality to network opera - tors. HVDC interconnectors also offer access to generators in an area which is not blacked out, allowing access to a greater diversity of fuel sources, improv - ing overall resilience. Currently there is 5GW capac- ity from HVDC interconnec- tions to the five other national external networks, of which some 1.5GW is VSC technology. In the next 6 years, this capacity is expected to rise to between 8-17GW, based on the National Grid ESO's Future Energy Sce- narios (see panel, right). Learning laboratory The National HVDC Centre (hvd- ccentre.com), based in Cum- bernauld, is a Scottish Hydro Figure 1: Existing and planned HVDC schemes in Scotland and north east England