Falling water turns a wheel, the wheel spins a magnet inside copper coils, and a flyball governor holds the speed. Transformers raise the voltage for the long line and lower it again for the town, where every window is lit by the same balance of supply and demand.
Each step turns energy from one form into the next. The numbers in each box are read live from the bench.
The grid's vital signs, then the voltage, current and power at each station along the way.
| Station | Voltage | Current | Power | What happens here |
|---|
On the left, the last minute of the grid: generation must follow demand, and frequency is the sign of how well it does. On the right, the same demand sent at each line voltage: losses fall with the square of the voltage, which is why lines run at hundreds of kilovolts.
The arithmetic. The swing equation sets frequency from the gap between mechanical and electrical power; a 4 % droop governor and slower automatic control answer it; the line's losses follow 3I²R with I from the power and the voltage, solved exactly, including the limit beyond which a line cannot carry more.
The sizes: one 150 MW machine with an inertia of 6 s, a line of about 110 km at 9 Ω per phase, and a town of 10 to 140 MW. The machine turns 100 times slower on the bench than in life, and the alternation is drawn at the same slowed rate.
Reactive power and power factor, line inductance, the three phases' separate waveforms, and the many other machines a real grid shares its load with. A real grid is far stiffer than one machine on its own.