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The National Grid is the high-voltage electric power transmission network in Great Britain, connecting power stations and major substations and ensuring that electricity generated anywhere in England, Scotland and Wales can be used to satisfy demand elsewhere. There are also undersea interconnections to northern France (HVDC Cross-Channel), Northern Ireland (HVDC Moyle), the Isle of Man (Isle of Man to England Interconnector), the Netherlands (BritNed) and the Republic of Ireland (EirGrid).
On the breakup of the Central Electricity Generating Board in 1990, the ownership and operation of the National Grid in England and Wales passed to National Grid Company plc, later to become National Grid Transco, and now National Grid plc. In Scotland the grid split into two separate entities, one for southern and central Scotland and the other for northern Scotland, connected by interconnectors to each other. The first is owned and maintained by SP Energy Networks, a subsidiary of Scottish Power, and the other by SSE. However, National Grid plc remains the System Operator for the whole UK Grid.
At the end of the 19th century, Nikola Tesla established the principles of three-phase high-voltage electrical power distribution 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 consulting partnership, at his Neptune Bank Power Station near Newcastle upon Tyne. 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. It began operating in 1933 as a series of regional grids with auxiliary interconnections for emergency use. Following the unauthorised but successful short term paralleling of all regional grids by the night-time engineers in 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 when South Wales provided power to replace lost output from Battersea and Fulham power stations. The grid was nationalised by the Electricity Act 1947, which also created the British Electricity Authority. In 1949, the British Electricity Authority decided to upgrade the grid by adding 275 kV links. From 1965, the grid was partly upgraded to 400 kV, beginning with a 150-mile (241 km) line from Sundon to West Burton, to become the Supergrid. In the most recent issue of the code that governs the British Grid, the Grid Code, the Supergrid is defined as referring to those parts of the British electricity transmission system that are connected at voltages in excess of 200 kV (200,000 volts). British power system planners and operational staff therefore invariably speak of the Supergrid in this context; in practice the definition used captures all of the infrastructure owned by the National Grid company in England and Wales, and (in England and Wales) no other equipment.
The following figures are taken from the 2005 Seven Year Statement (SYS)
Figures are again from the 2005 SYS.
Although overall losses in the national grid are low, there are significant further losses in onward electricity distribution to the consumer, causing a total distribution loss of about 7.7%. However 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%.
There is 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 is anticipated to grow to about 12 GW by 2014.
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 northern Scotland.
The UK grid is connected to adjacent European and Irish electrical grids by submarine power cables, including for links to northern France (HVDC Cross-Channel), Northern Ireland (HVDC Moyle), Republic of Ireland (East–West Interconnector), the Isle of Man (Isle of Man to England Interconnector), and the Netherlands (BritNed). There are also plans to lay cables to link the UK with Iceland and Norway (Scotland–Norway interconnector) in the future.
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 reserve acts to keep the system AC frequency within 1% of 50 Hz, except in exceptional circumstances. This acts on a second by second basis to adjust demand and provide some extra generation.
Reserve services are a range of services on differing response times:
These reserves are sized according to three factors:
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 in Berkshire in South East England. 
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The costs of operating the National Grid System are recouped by National Grid Electricity Transmission plc (NGET) 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 NGET, and are zonal in nature—that is, the country is divided up into different 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. This is representative of the fact that there is currently a north-south flow of electricity, and the additional stresses on the system increasing demand in areas of currently high demand causes.
Triad demand is measured as the average demand on the system over three half hours between November and February (inclusive) in a financial year. These three half hours comprise the half hour of system demand peak and the two other half hours of highest system demand which are separated from system demand peak and each other by at least ten days.
These half hours of peak demand are usually referred to as Triads.
In April of each year, each licensed electricity supplier (such as Centrica, BGB, etc.) is charged a fee for the peak 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 for one year, or £7,000/MW for each of the three half hours, for convenience assuming they were identical (which is unlikely; however, they will be close). The average for the whole country is about £15,000/MW·year. This is a means for National Grid to recover its charges, 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 and these charges are commonly also known as Transmission Network Use of System charges (TNUoS). (Note this is for high voltage long distance transmission and the 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.
These Triad charges encourage users to cut load at peak periods; this is often done using diesel generators. Such generators are also routinely used by National Grid.
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 360 terawatt-hours (1.3 EJ).), then a crude estimate can be made of transmission costs, and one gets the figure of around 0.2p/kWh. Other estimates also give a figure of 0.2p/kWh.
However, Bernard Quigg notes: "According to the 06/07 annual accounts for NGC UK transmission, NGC carried 350TWh 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 kWh.", but this also includes generators' connection fees.
In order to be allowed to supply electricity to the transmission system, generators must be licensed (by DECC) 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 a sum 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 so close to the centre of demand of the country.
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 (as of 2007[update]) 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.
Power cuts due to either problems on the actual infrastructure of the supergrid (defined in the Grid Code, as the transmission system operated by National Grid, which in England and Wales, comprises lines energized at 275,000 volts and 400,000 volts), or due to lack of generation to supply it with sufficient energy at each point in time, are exceedingly 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: - this site includes a simple high level figure on transmission availability and reliability of supply – for 2008-9 this was 99.99979%. It is issues affecting the low voltage distribution systems, for which National Grid is not responsible, that cause almost all the 60 minutes or so per year, on average, of actual 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 for transmission lines, which are mostly 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 only been two power cuts of high national prominence, that were linked to National Grid (although one was actually due to generation issues) :
The first case was in 2003, and related to the condition of National Grid's assets. National Grid was implicated in a power cut affecting 10 per cent of London in August - see 2003 London blackout. In essence, some news stories accused Grid of under-investment in new assets at the time; this was not necessarily true, but it did transpire that 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. It is arguable either way whether, with more money spent on system planning and maintenance, the human error in the relay setting could have been prevented.
The second case was in May 2008, and related to some 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. What happened first, was that 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 definitely no relationship between the two trips - the first did not cause the second. Such a loss is most unusual; Grid currently secures only against the loss of 1320 MW (the "infrequent infeed loss limit", due to rise to 1800 MW from 2014). There was, as a result, a sudden 1,510 megawatt adverse change in the balance of generation and demand on the supergrid. Because National Grid only secures against the near-instantaneous loss of 1320 MW, 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 (meaning, connected to the distribution system, rather than directly connected to the supergrid) generation, 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. In fact, the fact that frequency stabilised at this level in spite of a beyond-design-basis event, could be viewed as reassuring. Ireland, which being a smaller system has a 'friskier' grid, sees about 10 frequency excursions below 49.5 Hz per year - her target frequency being 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, and the incident was a credit to National Grid - although the incident was sure to be closely studied to see if lessons could be learnt.
Further issues, however, affecting smaller generators, arose. The problem was because the frequency had remained below 49.5 Hz for more than a few seconds, and because some generators' control settings were wrong. The current 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 are still using 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. Because of the out of date software, 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. The grid rules state that as frequency falls below 48.8 Hz, distribution network operators must enact 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 per cent 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. As to communications during the incident, National Grid did just have time to issue its most serious warning to all users of the supergrid, (demand control imminent, or DCI) before parts of the network were automatically disconnected by the operation of low frequency protection to ensure frequency was maintained within mandatory limits.
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