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In the electricity sector in the United Kingdom, the National Grid is the high-voltage electric power transmission network serving

Great Britain Great Britain is an island in the North Atlantic Ocean off the northwest coast of continental Europe. With an area of , it is the largest of the British Isles, the largest European island and the ninth-largest island in the world. It i ...

, connecting

power station A power station, also referred to as a power plant and sometimes generating station or generating plant, is an industrial facility for the generation of electric power. Power stations are generally connected to an electrical grid. Many ...

s and major substations and ensuring that electricity generated anywhere on it can be used to satisfy demand elsewhere. The network covers the great majority of Great Britain and several of the surrounding islands. It does not cover

Northern Ireland Northern Ireland ( ga, Tuaisceart Éireann ; sco, label= Ulster-Scots, Norlin Airlann) is a part of the United Kingdom, situated in the north-east of the island of Ireland, that is variously described as a country, province or region. Nort ...

, which is part of a single electricity market with the

Republic of Ireland Ireland ( ga, Éire ), also known as the Republic of Ireland (), is a country in north-western Europe consisting of 26 of the 32 Counties of Ireland, counties of the island of Ireland. The capital and largest city is Dublin, on the eastern ...

. The GB grid is connected as a wide area synchronous grid nominally running at 50 hertz. There are also undersea interconnections to other grids in the

Isle of Man ) , anthem = "O Land of Our Birth" , image = Isle of Man by Sentinel-2.jpg , image_map = Europe-Isle_of_Man.svg , mapsize = , map_alt = Location of the Isle of Man in Europe , map_caption = Location of the Isle of Man (green) in Europe ...

, Northern Ireland, the Republic of Ireland, France, Belgium, the Netherlands and Norway. On the breakup of the

Central Electricity Generating Board The Central Electricity Generating Board (CEGB) was responsible for electricity generation, transmission and bulk sales in England and Wales from 1958 until privatisation of the electricity industry in the 1990s. It was established on 1 Janua ...

in 1990, the ownership and operation of the National Grid in

England and Wales England and Wales () is one of the three legal jurisdictions of the United Kingdom. It covers the constituent countries England and Wales and was formed by the Laws in Wales Acts 1535 and 1542. The substantive law of the jurisdiction is Eng ...

passed to National Grid Company plc, later to become National Grid Transco, and now National Grid plc. In Scotland the grid was already split into two separate entities, one for southern and central Scotland and the other for northern Scotland, with interconnectors. The first is owned and maintained by SP Energy Networks, a subsidiary of

Scottish Power Scottish Power is a vertically integrated energy company based in Glasgow, Scotland. It is a subsidiary of Spanish utility firm Iberdrola. ScottishPower is the distribution network operator for Central and Southern Scotland, Merseyside, Nor ...

, and the other by SSE. However, National Grid plc continues to be the

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for the whole GB grid.

History

Electricity Pylon - Chatsworth Crescent - geograph

At the end of the 19th century,

Nikola Tesla Nikola Tesla ( ; ,"Tesla"
''

three-phase high-voltage

electric power distribution Electric power distribution is the final stage in the delivery of electric power; it carries electricity from the transmission system to individual consumers. Distribution substations connect to the transmission system and lower the transmissi ...

while he was working for Westinghouse in the United States. The first to use this system in the United Kingdom was Charles Merz, of the

Merz & McLellan Merz and McLellan was a leading British electrical engineering consultancy based in Newcastle. History The firm was founded by Charles Merz and William McLellan in Newcastle upon Tyne in 1902 when McLellan joined Merz's existing firm establishe ...

consulting partnership, at his

Neptune Bank Power Station Neptune Bank Power Station was a coal-fired power station situated on the River Tyne at Wallsend near Newcastle upon Tyne. Commissioned in 1901 by the Newcastle upon Tyne Electric Supply Company, the station was the first in the world to provid ...

near

Newcastle upon Tyne Newcastle upon Tyne ( RP: , ), or simply Newcastle, is a city and metropolitan borough in Tyne and Wear, England. The city is located on the River Tyne's northern bank and forms the largest part of the Tyneside built-up area. Newcastle is ...

. This opened in 1901, and by 1912 had developed into the largest integrated power system in Europe. The rest of the country, however, continued to use a patchwork of small supply networks. In 1925, the British government asked Lord Weir, a Glaswegian industrialist, to solve the problem of Britain's inefficient and fragmented electricity supply industry. Weir consulted Merz, and the result was the Electricity (Supply) Act 1926, which recommended that a "national gridiron" supply system be created. The 1926 Act created the Central Electricity Board, which set up the UK's first synchronised, nationwide AC grid, running at 132 kV, 50 Hz. National Grid helicopter

The grid was created with of cables – mostly overhead cables – linking the 122 most efficient power stations. The first "grid tower" was erected near Edinburgh on 14 July 1928, and work was completed in September 1933, ahead of schedule and on budget. It began operating in 1933 as a series of regional grids with auxiliary interconnections for emergency use. Following the unauthorised but successful short term parallelling of all regional grids by the night-time engineers on 29 October 1937, by 1938 the grid was operating as a national system. The growth by then in the number of electricity users was the fastest in the world, rising from three quarters of a million in 1920 to nine million in 1938. It proved its worth during the

Blitz Blitz, German for "lightning", may refer to: Military uses *Blitzkrieg, blitz campaign, or blitz, a type of military campaign *The Blitz, the German aerial campaign against Britain in the Second World War *, an Imperial German Navy light cruiser b ...

, when South Wales provided power to replace lost output from Battersea and Fulham power stations. The grid was nationalised by the

Electricity Act 1947 The Electricity Act 1947 (10 & 11 Geo. 6. c. 54.) was an Act of the Parliament of the United Kingdom which nationalised, or bought into state control, the electricity supply industry in Great Britain. It established a central authority called t ...

, which also created the British Electricity Authority. In 1949, the British Electricity Authority decided to upgrade the grid by adding 275 kV links. At its inception in 1950, the 275 kV Transmission System was designed to form part of a national supply system with an anticipated total demand of 30,000 MW by 1970. The predicted demand was already exceeded by 1960. The rapid load growth led the

to carry out a study in 1960 of future transmission needs. The report was completed in September 1960, and its study is described in a paper presented to the

Institution of Electrical Engineers The Institution of Electrical Engineers (IEE) was a British professional organisation of electronics, electrical, manufacturing, and Information Technology professionals, especially electrical engineers. It began in 1871 as the Society of T ...

by E. S. Booth, D. Clark, J. L. Egginton and J. S. Forrest in 1962. Considered in the study, together with the increased demand, was the effect on the transmission system of the rapid advances in generator design resulting in projected power stations of 2,000–3,000 MW installed capacity. These new stations were mostly to be sited where advantage could be taken of a surplus of cheap low-grade fuel and adequate supplies of cooling water, but these situations did not coincide with the load centres. West Burton with 4 × 500 MW machines, sited at the

Nottinghamshire Nottinghamshire (; abbreviated Notts.) is a landlocked county in the East Midlands region of England, bordering South Yorkshire to the north-west, Lincolnshire to the east, Leicestershire to the south, and Derbyshire to the west. The trad ...

coalfield near the

River Trent The Trent is the third-longest river in the United Kingdom. Its source is in Staffordshire, on the southern edge of Biddulph Moor. It flows through and drains the North Midlands. The river is known for dramatic flooding after storms and ...

, is a typical example. These developments shifted the emphasis on the transmission system, from interconnection to the primary function of bulk power transfers from the generation areas to the load centres, such as the anticipated transfer in 1970 of some 6,000 MW from

the Midlands The Midlands (also referred to as Central England) are a part of England that broadly correspond to the Kingdom of Mercia of the Early Middle Ages, bordered by Wales, Northern England and Southern England. The Midlands were important in the In ...

to the

home counties The home counties are the counties of England that surround London. The counties are not precisely defined but Buckinghamshire and Surrey are usually included in definitions and Berkshire, Essex, Hertfordshire and Kent are also often included ...

. Continued reinforcement and extension of the existing 275 kV systems was examined as a possible solution. However, in addition to the technical problem of very high fault levels, many more lines would have been required to obtain the estimated transfers at 275 kV. As this was not consistent with the

's policy of preservation of amenities a better solution was sought. Consideration was given to 400 kV and 500 kV schemes: both gave a sufficient margin for future expansion. The decision in favour of a 400 kV system was made for two main reasons. Firstly the majority of the 275 kV lines could be uprated to 400 kV, and secondly it was envisaged that operation at 400 kV could begin in 1965 compared with 1968 for a 500 kV scheme. Design work was started and in order to meet the programme for 1965 it was necessary for the contract engineering for the first projects to run concurrently with the design. One of these projects was the West Burton 400 kV Indoor Substation, the first section of which was commissioned in June 1965. From 1965, the grid was partly upgraded to 400 kV, beginning with a line from Sundon to West Burton, to become the Supergrid. In the 2010 issue of the code that governs the National Grid, the ''

Grid Code A grid code is a technical specification which defines the parameters a facility connected to a public electric grid has to meet to ensure safe, secure and economic proper functioning of the electric system. The facility can be an electricity gener ...

'', the Supergrid is defined as those parts of the British electricity transmission system that are connected at voltages in excess of 200 kV. In 2013 the construction of the 2.2 GW undersea Western HVDC Link from Scotland to North Wales started, which was completed in 2018. This is the first major non-

alternating current Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude continuously with time in contrast to direct current (DC) which flows only in one direction. Alternating current is the form in whic ...

grid link within GB, though interconnectors to foreign grids already use

HVDC A high-voltage direct current (HVDC) electric power transmission system (also called a power superhighway or an electrical superhighway) uses direct current (DC) for electric power transmission, in contrast with the more common alternating curre ...

. In 2021 a new non-lattice design of electricity pylon, the T-pylon, was built near East Huntspill, Somerset for the new 35 mile

Hinkley Point C Hinkley Point C nuclear power station (HPC) is a two-unit, 3,200 MWe EPR nuclear power station under construction in Somerset, England. The site was one of eight announced by the British government in 2010, and in November 2012 a nuclear site ...

Avonmouth Avonmouth is a port and outer suburb of Bristol, England, facing two rivers: the reinforced north bank of the final stage of the Avon which rises at sources in Wiltshire, Gloucestershire and Somerset; and the eastern shore of the Severn Es ...

connection.

Characteristics of the grid

Electricity-production-by-sourceUK1985-2020

The contiguous synchronous grid covers England (including the

Isle of Wight The Isle of Wight ( ) is a Counties of England, county in the English Channel, off the coast of Hampshire, from which it is separated by the Solent. It is the List of islands of England#Largest islands, largest and List of islands of England#Mo ...

Scotland Scotland (, ) is a Countries of the United Kingdom, country that is part of the United Kingdom. Covering the northern third of the island of Great Britain, mainland Scotland has a Anglo-Scottish border, border with England to the southeast ...

(including some of the Scottish islands such as Orkney, Skye and the

Western Isles The Outer Hebrides () or Western Isles ( gd, Na h-Eileanan Siar or or ("islands of the strangers"); sco, Waster Isles), sometimes known as the Long Isle/Long Island ( gd, An t-Eilean Fada, links=no), is an island chain off the west coast ...

which have limited connectivity),

Wales Wales ( cy, Cymru ) is a country that is part of the United Kingdom. It is bordered by England to the east, the Irish Sea to the north and west, the Celtic Sea to the south west and the Bristol Channel to the south. It had a population in ...

, and the

Network size

The following figures are taken from the 2005 Seven Year Statement (SYS) * Maximum demand (2005/6): 63 GW (approx.) (81.39% of capacity) * Minimum demand (2020 May): 15.3 GW * Annual electrical energy used in the UK is around * Capacity (2005/6): 79.9 GW (or 80 GW per the 2008 Seven Year Statement) * Number of large

s connected to it: 181 * Length of 400 kV grid: 11,500 km (circuit) * Length of 275 kV grid: 9,800 km (circuit) * Length of 132 kV (or lower) grid; 5,250 km (circuit) Total generating capacity is supplied roughly equally by

renewable A renewable resource, also known as a flow resource, is a natural resource which will replenish to replace the portion depleted by usage and consumption, either through natural reproduction or other recurring processes in a finite amount of ti ...

gas Gas is one of the four fundamental states of matter (the others being solid, liquid, and plasma). A pure gas may be made up of individual atoms (e.g. a noble gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or ...

fired, nuclear,

coal Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is mostly carbon with variable amounts of other elements, chiefly hydrogen, sulfur, oxygen, and nitrogen. Coal is formed when ...

fired

power stations A power station, also referred to as a power plant and sometimes generating station or generating plant, is an industrial facility for the generation of electric power. Power stations are generally connected to an electrical grid. Many po ...

. Annual energy transmitted in the UK grid is around , with an average load factor of 72% (i.e. 3.6×10¹¹/(8,760 × 57×10⁶).

Decarbonisation

The national grid has a stretch target to be

carbon neutral Carbon neutrality is a state of net-zero carbon dioxide emissions. This can be achieved by balancing emissions of carbon dioxide with its removal (often through carbon offsetting) or by eliminating emissions from society (the transition to the "p ...

or negative by 2033, well ahead of the UK's national target to achieve this by 2050. It also aims to have the capability to be ''zero carbon'' as early as 2025; 'zero carbon' meaning that if energy suppliers are able to produce sufficient green power, the grid could theoretically run without any

greenhouse gas emissions Greenhouse gas emissions from human activities strengthen the greenhouse effect, contributing to climate change. Most is carbon dioxide from burning fossil fuels: coal, oil, and natural gas. The largest emitters include coal in China and ...

at all (i.e. no

carbon capture Carbon capture may refer to: * Carbon capture and utilization, where the captured carbon dioxide is used * Carbon sequestration, where the captured carbon dioxide is stored ** Carbon capture and storage, referring to carbon sequestration from point ...

or offsetting would be needed as is the case with 'net zero'). In 2020 about 40% of the grid's energy came from burning natural gas, and it is not expected that anywhere close to sufficient green power will be available to run the grid on zero carbon in 2025, except perhaps on the very windiest days. Analysts such as Hartree Solutions consider even getting to 'net zero' by 2050 will be challenging, even more so to reach 'net zero' by 2033. There has, however, been sustained progress towards carbon neutrality, with

carbon intensity An emission intensity (also carbon intensity or C.I.) is the emission rate of a given pollutant relative to the intensity of a specific activity, or an industrial production process; for example grams of carbon dioxide released per megajoule o ...

falling by 53% in the five years to 2020. The phase out of coal is progressing rapidly with only 1.6% of the UK's electricity coming from coal in 2020, compared with about 25% in 2015. 2020 saw the UK go more than two months without needing to burn any coal for electricity, the longest period since the

industrial revolution The Industrial Revolution was the transition to new manufacturing processes in Great Britain, continental Europe, and the United States, that occurred during the period from around 1760 to about 1820–1840. This transition included going f ...

Losses

Figures are again from the 2005 SYS. * Joule heating in cables: 857.8 MW * Fixed losses: 266 MW (consists of

corona Corona (from the Latin for 'crown') most commonly refers to: * Stellar corona, the outer atmosphere of the Sun or another star * Corona (beer), a Mexican beer * Corona, informal term for the coronavirus SARS-CoV-2, which causes the COVID-19 di ...

and iron loss; can be 100 MW higher in adverse weather) * Substation transformer heating losses: 142.4 MW * Generator transformer heating losses: 157.3 MW * Total losses: 1,423.5 MW (2.29% of peak demand) Although overall losses in the national grid are low, there are significant further losses in onward

electricity distribution Electric power distribution is the final stage in the delivery of electric power; it carries electricity from the transmission system to individual consumers. Distribution substations connect to the transmission system and lower the transmissi ...

to the consumer, causing a total distribution loss of about 7.7%. Losses differ significantly for customers connected at different voltages; connected at high voltage the total losses are about 2.6%, at medium voltage 6.4% and at low voltage 12.2%. Generated power entering the grid is metered at the high-voltage side of the generator transformer. Any power losses in the generator transformer are therefore accounted to the generating company, not to the grid system. The power loss in the generator transformer does not contribute to the grid losses.

Power flow

In 2009–10 there was an average power flow of about 11 GW from the north of the UK, particularly from Scotland and northern England, to the south of the UK across the grid. This flow was anticipated to grow to about 12 GW by 2014. Completion of the Western HVDC Link in 2018 added capacity for a flow of 2.2 GW between Western Scotland and North Wales. Because of the power loss associated with this north to south flow, the effectiveness and efficiency of new generation capacity is significantly affected by its location. For example, new generating capacity on the south coast has about 12% greater effectiveness due to reduced transmission system power losses compared to new generating capacity in north England, and about 20% greater effectiveness than in northern Scotland.

Interconnectors

There is a 40 MW AC cable to the Isle of Man, the

Isle of Man to England Interconnector The Isle of Man to England Interconnector is a submarine power cable connecting the transmission system of the Manx Electricity Authority to that of Great Britain. With an undersea section of approximately , it is the second longest AC undersea ...

. The UK grid is connected to adjacent European electrical grids by submarine power cables at an electricity interconnection level (transmission capacity relative to production capacity) which was 6% .COM/2015/082 final: "Achieving the 10% electricity interconnection target
TextPDF
page 2-5. ''

European Commission The European Commission (EC) is the executive of the European Union (EU). It operates as a cabinet government, with 27 members of the Commission (informally known as "Commissioners") headed by a President. It includes an administrative body ...

'', 25 February 2015
ArchiveMirror
/ref> As of 2022, the total capacity of these connectors is about 7.7 GW. They include direct-current cables to northern France (2 GW

HVDC Cross-Channel The HVDC Cross-Channel (french: Interconnexion France Angleterre IFA 2000) is the 73 km long high-voltage direct current (HVDC) interconnector that operates since 1986 under the English Channel between the continental European grid at Bonningues ...

, 1 GW HVDC

IFA-2 IFA-2 (Interconnexion France-Angleterre 2) is a subsea electrical interconnector, running beneath the English Channel between France and the United Kingdom. The high voltage DC (HVDC) cable operates at +/-320kV with the capacity to transmit 1,00 ...

, 1 GW ElecLink via the Channel Tunnel); Belgium (1 GW HVDC

Nemo Link Nemo Link is an HVDC submarine power cable between Richborough Energy Park in Kent, the United Kingdom and Zeebrugge, Belgium. The project is a joint venture between British National Grid and Belgian Elia. The electrical interconnector is the ...

); the Netherlands (1 GW HVDC BritNed); Norway (1.4 GW HDVC

North Sea Link The North Sea Link is a 1,400MW high-voltage direct current submarine power cable between Norway and the United Kingdom. At it is the longest subsea interconnector in the world. The cable became operational on 1 October 2021. Route The cable ...

); Northern Ireland (500 MW HVDC Moyle Interconnector); and the Republic of Ireland (500 MW HVDC East–West Interconnector). A link is under construction to Denmark (1.4 GW Viking Link) which is due to be completed in 2023. A further 500 MW link with the Republic of Ireland ( Greenlink) is scheduled for 2024. Further potential schemes include links with Germany ( NeuConnect, 1.4 GW); Iceland (

Icelink Icelink is a proposed electricity interconnector between Iceland and the United Kingdom via Great Britain. At , the 8001,200MW high-voltage direct current (HVDC) link would be the longest sub-sea power interconnector in the world. The project pa ...

, around 1 GW) and

Morocco Morocco (),, ) officially the Kingdom of Morocco, is the westernmost country in the Maghreb region of North Africa. It overlooks the Mediterranean Sea to the north and the Atlantic Ocean to the west, and has land borders with Algeria t ...

(3.6 GW from new battery-backed solar generation).

Grid storage

The UK grid has access to large pumped storage systems, notably Dinorwig Power Station which can provide 1.7 GW for 5–6 hours, and the smaller Cruachan and

Ffestiniog Ffestiniog () is a community in Gwynedd in Wales, containing several villages, in particular the settlements of Llan Ffestiniog and Blaenau Ffestiniog. It has a population of 4,875. History Ffestiniog was a parish in Cantref Ardudwy; in 1284 ...

. There are also some grid batteries. As of May 2021, 1.3 GW of battery storage was operating in the United Kingdom, with 16 GW of projects in the pipeline potentially deployable over the next few years. A 100 MW power Chinese-financed plant at

Minety Minety is a village in north Wiltshire, England, between Malmesbury – to the west – and Swindon. It takes its name from the water mint plant found growing in ditches around the village, and has previously been known as Myntey. It has a prima ...

, Wiltshire was reported to be the largest in Europe when it opened in July 2021; when a 50 MW extension is completed, the site's storage capacity will be 266 MWh.

Reserve services and frequency response

National Grid is responsible for contracting short term generating provision to cover demand prediction errors and sudden failures at power stations. This covers a few hours of operation giving time for market contracts to be established to cover longer term balancing. Frequency-response reserves act to keep the system's AC frequency within ±1% of 50 Hz, except in exceptional circumstances. These are used on a second by second basis to either lower the demand or to provide extra generation. Reserve services are a group of services each acting within different response times: * Fast Reserve: rapid delivery (within two minutes) of increased generation or reduced demand, sustainable for a minimum of 15 minutes. * Fast Start: generation units that start from a standstill and deliver power within five minutes automatically, or within seven minutes of a manual instruction, with generation maintained for a minimum of four hours. * Demand Management: reduction in demand of at least 25 MW from large power users, for at least an hour. * Short Term Operating Reserve (STOR): generation of at least 3 MW, from a single or aggregation of sites, within four hours of instruction and maintained for at least two hours. * BM Start-Up: mainstream major generation units maintained in either an ''energy readiness'' or ''hot standby'' state. These reserves are sized according to three factors: * The largest credible single generation failure event, which is currently either Sizewell B nuclear power station (1,260 MW) or one cable of the

interconnector (1,000 MW) * The general anticipated availability of all generation plants * Anticipated demand prediction errors

Control of the grid

The English and Welsh parts of the National Grid are controlled from the National Grid Control Centre which is located in St Catherine's Lodge, Sindlesham,

Wokingham Wokingham is a market town in Berkshire, England, west of London, southeast of Reading, north of Camberley and west of Bracknell. History Wokingham means 'Wocca's people's home'. Wocca was apparently a Saxon chieftain who may ...

in Berkshire. It is sometimes described as being a 'secret' location. the system is under consistent

cyber attack A cyberattack is any offensive maneuver that targets computer information systems, computer networks, infrastructures, or personal computer devices. An attacker is a person or process that attempts to access data, functions, or other restricted ...

. Although the transmission network in Scotland is owned by separate companies – SP Transmission plc (part of

) in the south, and Scottish Hydro Electric Transmission plc (part of

Scottish and Southern Electricity Networks Scottish and Southern Electricity Networks (the trading name of Scottish and Southern Energy Power Distribution Limited, Scottish Hydro Electric Transmission plc, Scottish Hydro Electric Power Distribution plc and Southern Electric Power Distr ...

) in the north – overall control rests with National Grid Electricity System Operator.

Transmission costs

The costs of operating the National Grid System are recouped by National Grid Electricity System Operator (NGESO) through levying of Transmission Network Use of System (TNUoS) charges on the users of the system. The costs are split between the generators and the users of electricity. Tariffs are set annually by NGESO, and the country is divided into zones, each with a different tariff for generation and consumption. In general, tariffs are higher for generators in the north and consumers in the south since there is generally a north–south flow of electricity.

Triad demand

'Triad demand' is a metric of demand which reports retrospectively three numbers about peak demand between November and February (inclusive) each winter. In order to encourage usage of the National Grid to be less 'peaky', the triad is used as the basis for extra charges paid by the users (the licensed electricity suppliers) to the National Grid: the users pay less if they can manage their usage so as to be less peaky. For each year's calculation, historic system demand metrics are analysed to determine three half-hour periods of high average demand; the three periods are known as triads. The periods are (a) the period of peak system demand, and (b) two other periods of highest demand which are separated from peak system demand and from each other by at least ten days. For power stations, the chargeable demand is only the net site demand (per CUSC rule 14.17.10), so when the site is net exporting (i.e. total metered generation at that site exceeds total separately-metered station demand), that separately-metered station demand shall not be liable for demand TNUoS charges in relation to the station demand at triad. Triad dates in recent years were: In April of each year, each licensed electricity supplier (such as Centrica, BGB, etc.) is charged a yearly fee for the load it imposed on the grid during those three half-hours of the previous winter. Exact charges vary depending on the distance from the centre of the network, but in the South West it is £21,000/MW. The average for the whole country is about £15,000/MW. This is a means for National Grid to recover some of its costs, and to impose an incentive on users to minimise consumption at peak, thereby easing the need for investment in the system. It is estimated that these charges reduced peak load by about 1 GW out of say 57 GW. This is the main source of income which National Grid uses to cover its costs for high-voltage long-distance transmission (lower voltage distribution is charged separately). The grid also charges an annual fee to cover the cost of generators, distribution networks and large industrial users connecting. Triad charges encourage users to cut load at peak periods; this is often achieved by using diesel generators. Such generators are also routinely used by National Grid.

Estimating costs per kW⋅h of transmission

If the total TNUoS or Triad receipts (say £15,000/MW·year × 50,000 MW = £750 million/year) is divided by the total number of units delivered by the UK generating system in a year (the total number of units sold – say .), then a crude estimate can be made of transmission costs, and one gets the figure of around 0.2p/kW⋅h. Other estimates also give a figure of 0.2p/kW⋅h. However, Bernard Quigg notes: "According to the 06/07 annual accounts for NGC UK transmission, NGC carried 350TW⋅h for an income of £2012m in 2007, i. e. NGC receives 0.66p per kW hour. With two years inflation to 2008/9, say 0.71p per kW⋅h.", but this also includes generators' connection fees.

Generation charges

In order to be allowed to supply electricity to the transmission system, generators must be licensed (by BEIS) and enter into a connection agreement with NGET which also grants Transmission Entry Capacity (TEC). Generators contribute to the costs of running the system by paying for TEC, at the generation TNUoS tariffs set by NGET. This is charged on a maximum-capacity basis. In other words, a generator with 100 MW of TEC who only generated at a maximum rate of 75 MW during the year would still be charged for the full 100 MW of TEC. In some cases, there are negative TNUoS tariffs. These generators are paid an amount based on their peak net supply over three ''proving runs'' over the course of the year. This represents the reduction in costs caused by having a generator close to the centre of demand of the country. National Grid uses a grid services market. "Dynamic Containment" started in October 2020, initially priced at £17 per MW per hour, and Dynamic Regulation (DR) started in April 2022.

Demand charges

Consumers of electricity are split into two categories: half-hourly metered (HH) and non-half-hourly metered (NHH). Customers whose peak demand is sufficiently high are obliged to have a HH meter, which, in effect, takes a meter reading every 30 minutes. The rates at which charges are levied on these customers' electricity suppliers therefore varies 17,520 times a (non-leap) year. The TNUoS charges for a HH metered customer are based on their demand during three half-hour periods of greatest demand between November and February, known as the Triad. Due to the nature of electricity demand in the UK, the three Triad periods always fall in the early evening, and must be separated by at least ten clear working days. The TNUoS charges for a HH customer are simply their average demand during the triad periods multiplied by the tariff for their zone. Therefore, () a customer in London with a 1 MW average demand during the three triad periods would pay £19,430 in TNUoS charges. TNUoS charges levied on NHH metered customers are much simpler. A supplier is charged for the sum of their total consumption between 16:00 and 19:00 every day over a year, multiplied by the relevant tariff.

Constraint payments

Constraint payments are payments to generators above a certain size, where the National Grid gives them dispatch instructions that they are unable to take the electricity that the generators would normally provide. This can be due to a lack of transmission capacity, a shortfall in demand, or unexpected excess generation. A constraint payment is recompense for the reduction in generation.

Major incidents

Power cuts because of problems with the infrastructure of the Supergrid or lack of generation to supply it with sufficient power are very rare. The nominal standard of security of supply is for power cuts due to lack of generation to occur in nine winters in a hundred. The overall performance measure for electricity transmission is published on NGET's website and includes a simple high-level figure on transmission availability and reliability of supply. For 2008–9 this was 99.99979%. Issues affecting the low voltage distribution systems – for which National Grid is not responsible – cause almost all the 60 minutes or so per year, on average, of domestic power cuts. Most of these low voltage distribution interruptions are in turn the fault of third parties such as workmen drilling through the street mains (or subterranean higher voltage) cables; this does not happen to major transmission lines, which are for the most part overhead on pylons. For comparison with Supergrid availability, Ofgem, the electricity regulator, has published figures on the performance of 14 electricity distributors. Since 1990, there have been three power cuts of high national prominence that were linked to National Grid, two due to generation issues.

August 2003

The first case was in August 2003, and related to the condition of National Grid's assets. In the early evening of 28 August, a power cut affected 476,000 customers in South London, as well as the

London Underground The London Underground (also known simply as the Underground or by its nickname the Tube) is a rapid transit system serving Greater London and some parts of the adjacent counties of Buckinghamshire, Essex and Hertfordshire in England. The ...

and some rail services. Some news reports accused National Grid of under-investment in new assets at the time. A transformer oil leak had been left untreated, except for top-ups, for many months, pending a proper fix. It also transpired that there was a significant error in a protection relay setting which became evident, resulting in a power cut, only when the first fault, the oil leak, had a real effect. National Grid took some time to admit to these aspects of the incident. A second event happened on 5 September 2003 which affected 220,000 customers in East Birmingham. This was attributed to an error made by National Grid when commissioning protection systems.

May 2008

The second case was in May 2008, and related to generation issues for which National Grid was not responsible. A power cut took place in which a protective shutdown of parts of the network was undertaken by the distribution network operators, under pre-arranged rules, due to a sudden loss of generating capacity causing a severe drop in system frequency. First, two of Britain's largest power stations, Longannet in Fife and Sizewell B in Suffolk, shut down unexpectedly ('tripped') within five minutes of one another. There was no relationship between the two trips: the first did not cause the second. Such a loss is most unusual; at that time, Grid secured only against the loss of 1320 MW – the "infrequent infeed loss limit" (which rose to 1800 MW from 2014). The two shutdowns caused a sudden 1,510 MW adverse change in the balance of generation and demand on the Supergrid, and the frequency dropped to 49.2 Hz. Whilst the frequency was dropping to 49.2 Hz, or just after it reached that point, 40 MW of wind farms and more than 92 MW of other embedded generation (meaning, connected to the distribution system, rather than directly connected to the supergrid), such as landfill plant, tripped on the basis of the rate of change of frequency ('ROCOF') being high, just as it is supposed to do under the G 59/2 connection rules. The frequency stabilised at 49.2 Hz for a short while. This would have been an acceptable frequency excursion, even though it was below the usual lower limit of 49.5 Hz, and recovery would not have been problematic. The fact that frequency stabilised at this level in spite of a beyond-design-basis event could be viewed as reassuring. Ireland, which is a smaller system and has a more temperamental (and therefore less stable) grid, sees about 10 frequency excursions below 49.5 Hz per year. (Its target frequency is 50 Hz, just as in Britain). Consumers would not have noticed the small drop in system frequency; other aspects of their supply, such as voltage, remained perfect. There would, therefore, have been no consumer detriment; all would have been well at this point, had nothing further untoward occurred. However, further issues affecting smaller generators arose because the frequency remained below 49.5 Hz for more than a few seconds, and because some generators' control settings were wrong. The connection standard G 59/2 for embedded generation states that they must not trip (cease generating) as a result of sustained low frequency, until frequency has fallen below 47 Hz. However, a number of embedded generators used out-of-date control software that is not compliant with G59/2, as it erroneously trips them (as per the previous standard, G/59, in force when they were designed and specified) if frequency falls below 49.5 Hz for a few seconds. For this reason, another 279 MW of embedded generation tripped as a result of the low frequency whilst it was at 49.2 Hz. This was a problem as the Grid had no remaining available fast-acting generation, or demand-response, reserve margins. The frequency fell as a result to 48.792 Hz. Grid rules state that as frequency falls below 48.8 Hz, distribution network operators must apply compulsory demand control. This should start, if time permits, with voltage reduction, rapidly followed by the compulsory disconnection of, in stages, up to a final total of 60 percent of all distribution-connected customers (a very small number of very large customers are connected directly to the supergrid; for them, other measures apply). There was no time to use voltage reduction (which keeps customers on supply, but subtly reduces their demand through reducing the voltage slightly); as a result, 546 MW of demand was automatically disconnected by distribution network operators. None of the directly supergrid-connected customers were cut off. National Grid had by now taken other measures to increase output at other generation sites (and demand had been reduced at those customer sites where the customer has volunteered for this to happen, in return for reimbursement, under demand-side response contracts with National Grid, or with their supplier). National Grid was then able to restore system frequency. The average duration of loss of supply to the 546 MW of mostly low-voltage-connected (e.g. domestic) demand affected was 20 minutes. National Grid had time to issue a warning to all users of the supergrid – "demand control imminent" – which is one step away from its most serious warning "demand disconnection warning". During these incidents, the system was at risk to further generation loss which could have resulted in parts of the network being automatically disconnected by the operation of low frequency protection to ensure frequency is maintained within mandatory limits.

August 2019

The third event occurred on 9 August 2019, when around a million customers across Great Britain found themselves without power. Lightning struck a transmission line at 4:52 pm, causing the loss of 500 MW embedded (mostly solar) generation. Almost immediately, Little Barford Power Station and

Hornsea Wind Farm Hornsea Wind Farm is a Round 3 wind farm which began construction in 2018. Sited in the North Sea off the east coast of England, the eventual wind farm group is planned to have a total capacity of up to 6 gigawatt (GW). The development ha ...

tripped within seconds of each other, removing 1.378 GW of generation, which was in excess of the 1 GW of backup power (the size of the largest single expected loss) that the operator was maintaining at the time.https://www.nationalgrideso.com/document/151081/download Interim Report into the Low Frequency Demand Disconnection (LFDD) following Generator Trips and Frequency Excursion on 9 Aug 2019 - 16 August 2019 The grid frequency fell to 48.8 Hz before automatic load-shedding disconnected 5% of the local distribution networks (1.1 million customers) for 15 to 20 minutes; this action stabilised the remaining 95% of the system and prevented a larger blackout. Although power was maintained at all times to the railway network (but not to the signalling system), the reduction in frequency caused 60 Thameslink Class 700 and 717 trains to fail. Half were restarted by the drivers but the others required a technician to come out to the train to restart it. This led to substantial travel disruption for several hours on the East Coast Main Line and Thameslink services. The supply to Newcastle Airport was also disrupted and a weakness was exposed in backup power arrangements at

Ipswich Hospital Ipswich Hospital is a large district general hospital in Heath Road, Ipswich, Suffolk, England. It is now managed by East Suffolk and North Essex NHS Foundation Trust which was formed on 1 July 2018 by the merging of Ipswich Hospital NHS Trust w ...

. An investigation by

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concluded in January 2020. It found that Little Barford and Hornsea One had failed to remain connected to the grid following the lightning strike, and their operators – RWE and Ørsted respectively – agreed to each pay £4.5 million to Ofgem's redress fund. Additionally, Ofgem fined distribution network operator UK Power Networks £1.5M for beginning to reconnect customers before being cleared to do so, although this breach of procedure did not affect the recovery of the system.

Minor incidents

November 2015

On 4 November 2015 National Grid issued an emergency notice asking for voluntary power cuts because of "multiple plant breakdowns". No power cuts occurred but wholesale electricity prices rose dramatically, with the grid paying up to £2,500 per megawatt-hour.

References

External links

The Transmission System
National Grid's Seven Year Statement (2008)
The Statement of the Use of System Charging Methodology
, National Grid
Useful Information
National Grid

ELEXON
UK National Grid Status

''UK Electricity Networks: The nature of UK electricity transmission and distribution networks in an intermittent renewable and embedded electricity generation future''
Scott Butler
''The electricity supply industry and the Central Electricity Generating Board''
UK Competition Commission Report 1987
Map of GB power stations and national grid
BBC website, but map revised by Deloitte & Touche, 2003
Archive

{{DEFAULTSORT:National Grid (Uk) Electric power transmission in the United Kingdom 1933 establishments in the United Kingdom Wide area synchronous grids