Generating Electricity

From Fuel to Electricity

When you boil your kettle, the electricity that heats the element began its journey kilometres away — perhaps in a wind farm on a hillside, or inside a large power station. Understanding how it got there involves two key ideas: the generator and the National Grid.

In most power stations, fuel (such as natural gas, coal, nuclear fuel, or the heat from geothermal sources) is used to produce steam. The steam pushes against the blades of a turbine, making it spin at high speed. The spinning turbine is connected to a generator, which converts the rotational energy into electrical energy. In a wind turbine, the wind itself turns the blades directly — no steam is needed.

A generator works by electromagnetic induction: when a coil of wire rotates inside a magnetic field (or when a magnet rotates inside a coil), a changing magnetic field induces a potential difference and drives a current. The output is alternating current (AC) because as the coil spins, the direction of the induced current reverses every half turn. The UK mains supply is AC at a frequency of 50 Hz and a voltage of about 230 V.

Transformers and Voltage

Once electricity leaves the generator, its voltage must be changed before it travels long distances. A transformer uses electromagnetic induction in an iron core to increase or decrease voltage. There are two types in the National Grid system:

• A step-up transformer at the power station increases the voltage to very high levels (typically hundreds of thousands of volts) before electricity enters the transmission cables. At higher voltage, the current is lower.
• A step-down transformer at a local substation reduces the voltage back to 230 V for safe use in homes and small businesses.

Transformers only work with AC — they rely on a changing magnetic field in the iron core to induce a voltage in the secondary coil. DC produces a constant (non-changing) field and would not induce any current. This is one of the main reasons the National Grid uses AC rather than DC.

Why Transmit at High Voltage?

The cables in the National Grid have resistance. Any current flowing through resistance causes energy to be wasted as heat: \(P_{\text{loss}} = I^2 \times R\). Because current is squared in this equation, even a small reduction in current makes a dramatic difference to energy losses.

By stepping voltage up before transmission, current is reduced proportionally (since power = voltage × current, and power must stay the same). Lower current means far less heat is produced in the cables. This is why transmission losses in the National Grid are small — the grid is a highly efficient system even though the cables themselves stretch for thousands of kilometres across the country.

The National Grid in Practice

The National Grid is the system that connects electricity producers to consumers across the country. Electricity enters the grid at high voltage from a range of sources — large thermal power stations, offshore wind farms, solar farms, and others. It travels along overhead power lines (on tall pylons) or underground cables.

At substations, step-down transformers reduce the voltage in stages. Industrial users may draw electricity at an intermediate voltage (e.g. 11,000 V), while homes and shops receive it at 230 V. The Grid is a national system, so electricity generated in one part of the country can be directed to areas of high demand elsewhere. This makes the supply more reliable and responsive to peaks in demand (such as during half-time of a major televised sporting event, when millions of people simultaneously use kettles).