When we refer to the electricity network as a “grid”, it lends a sense of order to the complex, labyrinthine network of transmission and distribution lines, substations and infrastructure that transports power from where it’s generated to where it’s used in homes and businesses.
Interconnected, finely balanced and increasingly automated to meet fluctuating generation and demand, grids can cover whole countries and continents. So what are the elements that make up a typical electricity grid?
Power generation
In the past, generation typically occurred at gas, hydroelectric and nuclear power stations. Now, offshore and onshore wind farms, solar parks and domestic solar panels and turbines all contribute to the UK’s electricity production. Some facilities also burn biomass – such as wood, farm waste and other organic material – to generate electricity, though these plants will be phased out in the longer term in favour of cleaner sources. Geothermal heat from the Earth is also used to generate electricity as well as for heating and cooling. Remote parts of the UK still use diesel generators, which provide emergency backup.
Transmission grid
These are the “motorways” of power. The transmission system transports electricity across regions, countries and continents, losing as little energy as possible en route. It operates at very high voltages to make transmission more efficient over long distances. In the UK, the steel pylons and wires you see tend to be 400 kV and 275 kV AC lines.
Distribution systems
By contrast, distribution systems – the wires carried by shorter poles in rural parts or underground in urban areas – are the “B roads” that carry electricity to consumers. Distribution is generally 132 kV and below.
Substations
Substations connect transmission and distribution grids. They’re where generated electricity is transformed to different voltages. Substations also contain switchgear and control equipment including fuses, switches and circuit breakers. These allow the substation – or distribution lines – to be disconnected from the transmission grid for maintenance and repairs. Control equipment regulates the flow of energy through the system. More than 300 substations connect transmission and distribution grids in England and Wales (the two territories that comprise the National Grid). ScottishPower operates more than 150 grid substations in central and southern Scotland and, in the north of Scotland, SSEN operates 148 grid substations.
Transformers
Transformers change voltages. Electricity is produced in power stations at relatively low voltages of 25 kV or below, which are unsuitable for long distance transmission. Step-up transformers at large substations close to the source of generation raise voltages to 400 kV for efficient transport.
Once electricity reaches its destination it must be converted to lower voltages in substations, for safe distribution. A network of smaller substations and transformers gradually reduce voltages further for different needs – households use lower voltages than some industries for instance.
Pylons
Pylons or transmission towers are the familiar latticed steel structures supporting high voltage overhead lines across land and natural obstacles and have been around in the UK for nearly a century. Pylon comes from the ancient Greek “pyle”, meaning entrance or gateway. The iconic A-frame design still seen today was the winning entry to a 1927 design competition from US engineering firm Milliken Brothers. This year, the first new pylon design since then has been installed in the south-west – these are T-pylons, with a single pole and cross arms, which have a smaller footprint.
Today’s pylons carry longer insulators for higher voltage cables and might be shorter near airfields or taller to cross rivers – the highest couple of pylons in the UK reach 190 metres and support a high voltage line that crosses the Thames from Essex to Kent.
There are an estimated 47,000 pylons across England, Wales and Scotland, each weighing an average 30 tonnes.
Transmission lines
Transmission lines or cables are conductors that transport electricity most of the way from generator to the end users. These lines – the ones you see strung high between pylons – use high voltages to minimise power losses over longer distances. Although they are mostly overhead, occasionally they’re underground or under the water. Today ultra high voltage cables, which have reached 1,000 kV in Japan, can carry electricity vast distances. Because they can transport electricity over huge distances, high voltage lines make it easier to integrate remote renewable energy into the grid.
In England and Wales, National Grid operates more than 4,500 miles of overhead power lines, compared with 900 miles of underground cables. In Scotland, ScottishPower and SSEN maintain between them more than 6,600 miles of overhead, underground and subsea power lines.
Ground wires
Ground wires are cables strung above live wires to protect them from lightning. If struck, the current passes through ground wires to the tower, which is connected to a mesh buried beneath the ground. Thinner and higher than transmission lines, ground wires are often fitted with bird deterrents such as globes and reflectors.
Insulators
Insulators made from materials such as porcelain, glass or polymers that resist the flow of electricity are used to prevent high voltage parts of the network leaking to the supporting structures. For safety, the higher the voltage, the higher the lines must be.
Distribution lines
Distribution lines are conductors that carry lower voltage lines to deliver electricity from substations to consumers – these are typically supported by shorter poles or are mostly underground in urban areas.
UK mains electricity, the power that enters homes, is typically 230 V in line with Europe, though this can fluctuate slightly – voltages in the UK usually measure at 240V, due to legacy infrastructure.
Interconnectors
Interconnectors allow electricity systems from neighbouring countries to connect and share energy. These are high voltage cables often beneath the sea and can be hundreds of miles long. National Grid operates six interconnectors linking the UK with Belgium, France, Ireland, the Netherlands, Norway and most recently Denmark.
