National Grid (Great Britain)

The National Grid is the high-voltage electric power transmission network in the United Kingdom, 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.

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, 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 grid, in 1937 (approx.) by the night time engineers, 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.

Grid description

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)
• Annual electrical energy used in the UK is around 360 TWh (1.3 EJ)
• Capacity (2005/6): 79.9 GW (or 80 GW per the 2008 Seven Year Statement)
• Number of large power stations: 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)

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: 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 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%.

Power flow

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.

Standing Reserve and Frequency Response

Main article: Reserve service

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.

These reserves are sized according to three factors:

• The largest credible single generation failure, which is currently either Sizewell B nuclear power station (1,260 MW) or one cable of the HVDC Cross-Channel interconnector (1,000 MW)
• The general anticipated availability of all generation plants
• Anticipated demand prediction errors

Control of the Grid

Main article: Control of the National Grid

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.

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 (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 it 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 a fee to generators to connect.

These Triad charges incentive users to cut load at peak periods, and this is often done using diesel generators. Such generators are also routinely used by National Grid

Estimating costs 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 a crude estimate can be made of transmission costs, and one gets the figure of around 0.2p/kWh. This is calculated by taking the total annual Triad charges, which are say £15,000/MW·year × 50,000 MW = £750 million/year and dividing it by the total number of units sold–say 360 terawatt-hours (1.3 EJ). Other estimates give a much lower 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.

Upgrading National Grid to deal with renewables

On the basis of the above figures if the cost of the UK Grid were to be hypothetically doubled, this would add about 0.71p/kWh which is about 7% of the domestic charge for power in the UK.

Generation charges

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.

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 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.

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, 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.

See also

• Brittle Power
• Demand response
• Relative cost of electricity generated by different sources
• Economics of new nuclear power plants (for cost comparisons)
• Energy security and renewable technology
• High-voltage direct current
• Intermittent energy source
• Low-cost solar cell
• Northeast Blackout of 2003
• List of power outages
• Potential energy
• Calculating the cost of the UK Transmission network: cost per kWh of transmission
• Calculating the cost of back up: See spark spread
• Load management
• National Grid Reserve Service
• Energy use and conservation in the United Kingdom
• Control of the National Grid
• Relative cost of electricity generated by different sources
• Diesel-electric transmission
• Motor-generator
• Three-phase electric power
• Load bank

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
• Final Tariffs 2006, National Grid
• 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
• Carbon-intensity variations in the GB grid