From Power Plant to Outlet

Flip a light switch and the electricity that lights the bulb left a generator a fraction of a second earlier, possibly hundreds of miles away. Nothing was stored along the way. The whole trip is a chain of machines and wires, and each link has its own owner, its own rules and its own way of failing.

The chain, start to finish

1. Generation

Nearly every big generator works the same way: something spins a magnet inside coils of copper wire. Steam from burning coal or gas, steam from a nuclear reactor, falling water, the wind, or the hot exhaust of a gas turbine. The spinning makes alternating current. Solar panels and batteries are the exceptions. They make direct current, and an inverter converts it to grid-ready AC.

2. Step-up transformer

A generator's output is typically in the tens of thousands of volts. Right outside the plant, a step-up transformer raises it to transmission voltage. The reason is physics you can state in one line: the power lost as heat in a wire grows with the square of the current. Double the current, four times the loss.

At 100,000 volts (100 kV): 100,000,000 W / 100,000 V = 1,000 amps
At 500,000 volts (500 kV): 100,000,000 W / 500,000 V =   200 amps

3. Transmission

The tall steel towers you see crossing open country carry transmission, usually 115 kV to 500 kV in the U.S., with a few lines higher. EIA describes the system as thousands of miles of high-voltage lines feeding millions of miles of low-voltage ones. Transmission works as a network, so power from many plants mixes and flows along the paths of least resistance. You cannot trace your electrons to one plant. Contracts say who paid for what. Physics decides where it goes.

4. Substations

Transmission substations connect lines together and switch them on and off. Distribution substations step the voltage down to feeder levels, commonly somewhere around 4 kV to 35 kV. The fenced yard at the edge of town with the humming gray boxes is usually a distribution substation. Large industrial customers and big data centers often get their own substation and take power straight off transmission.

5. Distribution

Feeders run along streets on poles or underground. A final transformer, the gray can on a pole or green box on a pad, drops the voltage to 120/240 volts for a home. That last transformer might serve 5 to 15 houses. Most outages customers notice happen here, from wind, trees, cars hitting poles, squirrels and equipment failure, not on the big transmission lines.

Along the whole path, some energy turns into heat. EIA estimates transmission and distribution losses averaged about 5 percent of the electricity moved in the U.S. in 2018 through 2022.

AC and frequency

In alternating current, the flow reverses direction many times a second. In North America it completes 60 full cycles a second: 60 hertz (Hz). Every big spinning generator in an interconnection turns in lockstep with every other one, synchronized to that frequency.

Frequency is the grid's speedometer. When demand rises faster than generation, the spinning machines slow down slightly and frequency dips below 60 Hz. When generation exceeds demand, frequency creeps up. Operators watch it every second. A drop of a few tenths of a hertz is serious. Automatic protection starts shedding load before frequency falls far enough to damage equipment. Lesson 3 covers how operators keep it steady.

Why AC at all? Because transformers only work on AC, and cheap transformers are what make the voltage stepping above possible. High-voltage DC lines do exist for very long runs and for linking grids that are not synchronized, including the Pacific DC Intertie that carries Northwest hydropower toward Southern California.

Three grids, not one

The lower 48 is split into three interconnections: the Eastern, the Western and ERCOT, which covers most of Texas. Each runs at 60 Hz, but not in sync with the others, so they trade only small amounts of power through a few DC ties. In EIA's 2016 tally, the Western Interconnection had 37 balancing authorities and the Eastern 36; ERCOT is one. Everything from the Rocky Mountains to the Pacific coast, including Idaho, sits in the Western Interconnection.