Electricity and the power grid

Most electricity is made using a principle discovered by Michael Faraday in 1831: moving a magnet near a coil of wire pushes electrons along the wire. A generator spins a magnet inside coils, or coils inside a magnetic field, and produces an electric current.

What differs between power stations is what does the spinning. Coal, gas and nuclear plants boil water into high-pressure steam that turns a turbine. Hydroelectric dams use falling water, and wind turbines use moving air. Solar panels are the main exception: photovoltaic cells turn sunlight directly into electric current with no moving parts.

In the 1880s Thomas Edison promoted direct current, which flows in one direction. George Westinghouse, using designs by Nikola Tesla, backed alternating current, which reverses direction many times a second. Their companies fought a bitter public battle.

Alternating current won for long-distance power because its voltage can easily be raised and lowered with transformers. Today most grids run at 50 cycles per second, as in Europe, or 60, as in North America. Direct current has made a comeback for very long high-voltage links and undersea cables, where it loses less energy.

Wires resist current and turn some energy into heat. The energy lost depends on the square of the current, so halving the current cuts losses to a quarter. Power equals voltage times current, so sending the same power at a much higher voltage allows a much smaller current.

That is why electricity leaves power stations through transformers that raise it to hundreds of thousands of volts for transmission lines on tall pylons. Near towns, substations step it down in stages, and a final transformer on a pole or in a street cabinet brings it to household levels, such as 230 volts in Europe or 120 volts in the United States.

The grid has very little storage built in, so the amount of electricity generated must match the amount used almost exactly, second by second. When demand rises faster than supply, the spinning generators slow slightly and the grid frequency drops. Operators watch frequency closely as a live signal of balance.

Grid operators forecast demand, which follows daily and seasonal patterns, and schedule power stations accordingly. Some plants run constantly, while others, such as gas turbines and hydro plants, can ramp up quickly to meet peaks. If frequency moves too far, protective systems disconnect equipment, and a severe imbalance can cascade into a blackout.

• Wind and solar output varies with weather and time of day, which makes balancing harder.
• Large battery systems can now respond in fractions of a second to stabilise frequency.
• Pumped hydro storage moves water uphill when power is cheap and releases it through turbines when needed.
• Interconnectors link national grids so surplus power in one region can supply another.
• Smart meters and flexible demand let some users shift consumption to times when power is plentiful.

Household wiring branches into circuits protected by fuses or circuit breakers, which cut power if the current becomes dangerously high. An earth wire gives fault current a safe path to the ground, and residual current devices switch off a circuit within milliseconds if they detect current leaking, for example through a person.

Many devices, such as phones and computers, actually run on low-voltage direct current. Their chargers and power supplies convert the alternating current from the wall into the steady direct current their electronics need.