The Distribution Grid

For power to be useful in a home or business, it comes off the transmission grid and is stepped-down to the distribution grid. This may happen in several phases. The place where the conversion from "transmission" to "distribution" occurs is in a power substation. A power substation typically does two or three things:

• It has transformers that step transmission voltages (in the tens or hundreds of thousands of volts range) down to distribution voltages (typically less than 10,000 volts).
• It has a "bus" that can split the distribution power off in multiple directions.
• It often has circuit breakers and switches so that the substation can be disconnected from the transmission grid or separate distribution lines can be disconnected from the substation when necessary.

A typical small substation

The box in the foreground is a large transformer. To its left (and out of the frame but shown in the next shot) are the incoming power from the transmission grid and a set of switches for the incoming power. Toward the right is a distribution bus plus three voltage regulators.

The transmission lines entering the substation and passing through the switch tower

The switch tower and the main transformer

Now the distribution bus comes into the picture.