National Grid (UK)

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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 Great Britain can be used to satisfy demand elsewhere. There are also undersea interconnections to northern France (HVDC Cross-Channel), Northern Ireland (HVDC Moyle), and the Isle of Man (Isle of Man to England Interconnector).

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 is owned by Scottish Power and Scottish and Southern Energy Group. These groups also operated the systems until April 1, 2005, when National Grid plc took control of day-to-day operations, though the network is still owned by the Scottish companies.

Contents

[edit] History

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,[1] and by 1912 had developed into the largest integrated power system in Europe.[2] 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.[3] 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, but by 1938 the grid was operating as a national system. 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".

[edit] Standing Reserve and Frequency Response

Main article: Reserve service

[edit] Grid description

[edit] Network size

The following figures are taken from the 2005 seven-year statement (SYS)[4]

  • Maximum Demand (2005/6): 63 GW (approx.) (81.39% of Capacity)
  • Capacity (2005/6): 77.4 GW (or 78 GW per the EIA 2005 figures[5])
  • Number of large power stations: 181
  • Length of 400 kV grid: 11,500 circuit km
  • Length of 275 kV grid: 9,800 circuit km
  • Length of 132 kV (or lower) grid; 5,250 circuit km

[edit] Losses

Figures are again from the 2005 SYS.

  • Joule heating in cables: 857.8 MW
  • Fixed losses: 266 MW (consists of corona and iron losses; can be 100 MW higher in adverse weather)
  • Substation transformer heating losses: 142.4 MW
  • Generator transformer heating losses: 157.3 MW
  • Total losses: 1423.5 MW (2.29% of peak demand)

Although 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%.[6] However losses differ significantly for customers connected at different voltages; connected at high voltage the grid losses are about 2.6%, at medium voltage 6.4% and at low voltage 12.2%.[7]

[edit] Power flow

There is an average power flow of about 8 GW from the north of the UK, particularly northern England, to the south of the UK across the grid. This flow is anticipated to grow to about 9 GW by 2011.[8]

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 11% greater effectiveness due to reducing transmission system power losses compared to new generating capacity in north England.[9]

[edit] Typical conductor currents[10]

  • 400 kV, 700 MW circuit: 1 kA
  • 132 kV, 70 MW circuit: 300 A
  • 11 kV, 3 MW circuit: may 150 A
  • 400 V, 150 kW final distribution circuit: 200 A

[edit] Control of the Grid

Main article: Control of the National Grid (UK)

[edit] Transmission costs

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.

[edit] Triad demand

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 (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 per year.

If averaged over all power supplied in the UK in one year then this is currently around 0.2p/kWh. This is calculated by taking the total annual triad charges, which are say £15,000/MW/year x 50 GW = £750 million and dividing it by the total number of units sold – say 3.6 trillion kWh = 0.2p/kWh.

This is the sole source of money which National Grid uses to cover its costs and these charges are commonly also known as TNUoS - Transmission Network Use of System charges. (Note this is for high voltage long distance transmission and the lower voltage distribution is charged separately).

Cost per kWh of Transmission If the total number of units delivered by the UK generating system in a year, are divided into the total TNUoS or Triad receipts, then one gets the surprisingly low figure of around 0.2p/kWh. This is calculated by taking the total annual Triad charges, which are say £15,000/MW/year x 50,000 MW = £750 million/year and divide it by the total number of units sold – say 3.6 trillion kWh.

[edit] Costs of reinforcing the National Grid to cope with e.g. Renewable Energy

It is often argued that the costs of reinforcing the National Grid (or any other grid) are prohibitively expensive. This is unlikely to be true. For example if it is assumed that the entire grid were duplicated to accommodate variable wind power then this would evidently only add an extra 0.2p/kWh to the cost of power.

[edit] Generation charges

In order to be allowed to supply electricity to the Transmission system, generators must obtain permission to do so from NGET. This permission is supplied in the form of 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.

[edit] 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 charges levied on these customers' electricity suppliers therefore vary 17520 times a 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 demand during the triad periods multiplied by the tariff for their zone. Therefore (as of 2007) a customer in London with a 1 MW cumulative 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.

[edit] Major incidents

In May 2008 National Grid was forced to perform a protective shutdown of parts of the network due to a sudden loss of generating capacity. Two of Britain's largest power stations, Sizewell B in Suffolk and Longannet in Fife, shut down unexpectedly, resulting in a 1,510 megawatt shortfall in supply. National Grid issued its most serious warning to its distribution customers — "demand control imminent" (DCI) before shutting down parts of the network to ensure that the frequency was kept within the agreed range. Details of the incident were not made public to retain confidence in the network. [11][12][13]

[edit] See also

[edit] References

  1. ^ Mr Alan Shaw (29 September, 2005). Kelvin to Weir, and on to GB SYS 2005. Royal Society of Edinburgh.
  2. ^ Survey of Belford 1995. North Northumberland Online.
  3. ^ Lighting by electricity. The National Trust.
  4. ^ National Grid
  5. ^ Energy Information Administration - International Electricity Installed Capacity Data
  6. ^ Electricity Stats
  7. ^ Time to Take a Fresh Look at CHP..., Simon Minett, Director, DELTA Energy and Environment, October 2005
  8. ^ Overview of Main Power Flows at Peak
  9. ^ Table 7.5 - Effectiveness of Marginal Generation due to Transmission Losses
  10. ^ A basic introduction to electricity, EMFs and the terminology used
  11. ^ "Blackouts hit thousands as generators fail", The Times, 2008-05-28. 
  12. ^ "Q&A: Blackout Britain", The Guardian, 2008-05-28. 
  13. ^ iPM. BBC Radio 4. 2008-05-31.

[edit] External links

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