Grid balancing

Grid balancing is the grid's one hard rule: the electric power put into the grid must equal the power taken out of it every second. If the two do not match, the frequency of the supply drifts away from 50 hertz, equipment trips to protect itself, and the lights can go out over a whole region. A grid can store almost no electricity, so this balance has to be kept all the time.

Here is how it works in general. Control rooms, called load dispatch centres, forecast how much power people will use over the day. They plan which power plants will run, and they adjust the plants every few seconds as demand and supply change. If demand rises, they raise the output of a plant, call on stored power or ask large users to use less. If supply rises, for example when the sun is strong, they reduce another plant or store the extra power.

The frequency is the gauge that tells operators whether the balance is right. When power taken out is more than power put in, the spinning generators slow slightly and the frequency falls below 50 hertz. When supply is more than demand, the frequency rises. Operators keep it within a narrow band around 50 hertz. They also keep some plants running below full output, as a reserve, so that output can be raised within seconds. In India, each state has a state load dispatch centre, and the regional and national centres coordinate between states.

Take, for example, an evening in an Indian city when offices close and homes switch on lights and fans. Demand climbs within an hour, and hydro and gas plants are started to meet it. Consider a cloud passing over a large solar plant. Its output falls in minutes, and a grid battery supplies power for that time; see grid batteries. Say a state wants to shift demand. It offers cheaper power at noon, when solar output is high.

Solar and wind make this harder, because their output changes with the weather. Tools for this include grid control software, virtual power plants and smart meters. The scene the grid shows the whole system, and the grid edge explains why rooftop solar adds new complexity.

How is it different from energy trading? Balancing keeps the physical supply and demand equal each second. Trading is the buying and selling of power at agreed prices.