What is a substation? Transformer? Common power-related terminology, explained

When power outages occur — whether they’re due to natural disasters or other causes — you may hear unfamiliar words and terminology as officials explain the situation or provide updates.

To provide some clarity, we’ve compiled definitions for some key, common power-related terms, mostly using information from the U.S. Department of Labor’s Occupational Safety and Health Administration (OSHA) and Leonard White, an electrical engineering Professor of the Practice at NC State University.

The terms are listed in alphabetical order.

What is an electric or power grid?

The terms power grid and electric grid describe the “big view” of the electric system, White said — it’s the complex overall system through which electricity moves and connects electricity producers and customers.

The grid includes substations, transformers, power lines and more.

What is an insulator?

An insulator is a component of the electric system that does not conduct electricity. Insulators are used to route and direct energized conductors — which carry electric currents — through metal enclosures, such as transformers, White said.

“Insulators are also used to support conductors from transmission towers or power poles,” White said.

White said insulators are typically made from ceramics and, though they are strong, they “are usually quite brittle.”

What is a substation?

A substation is “a high-voltage electric system facility,” OSHA says, where power — specifically, voltages — becomes suitable for distribution and supply to customers.

Substations can also be places where transmission lines are combined or split as needed to obtain the desired power distribution, White said.

Some power substations are indoors or underground, particularly in crowded cities. But the vast majority are outdoors, protected by fences, cameras and other security measures. The stations can withstand all types of weather, and utilities find it easier to keep them cool and to make repairs and replace equipment if they’re outdoors.

OSHA defines four main types of substations:

▪ Step-up transmission substations use a large power transformer to increase the voltage of electric power — which is generally received from a nearby generating facility — for transmission to distant locations.

▪ Step-down transmission substations are located at “switching points” in the power grid, connecting various parts of the grid and serving as a source of power for subtransmission lines — the smaller power lines, often seen along roadways, that carry voltages that have been reduced from a major transmission line system.

▪ Distribution substations are located near to the end-users, or customers, of the power. The transformers at these stations lower the voltage of power to make it suitable for use by customers.

▪ Underground distribution substations, like their above-ground counterparts, are located near to the end-users, or customers, of power and lower the voltage to make it suitable for use by customers.

What is a transformer?

A transformer converts electricity from one voltage to another.

Transformers can be found at various points in the electric grid. In some spots, they step up voltage for transmission over long distances. At others, they step down voltage to make it suitable for use by customers.

At a typical distribution substation, White said, there will be a step-down transformer to change the voltage from the high level used in transmission lines to the lower level used for distribution.

What is a transmission line?

Transmission lines carry electric energy from one point to another in the power system, or grid.

Generally seen overhead, high above the ground, transmission lines can vary in size and the corresponding voltage of energy they carry.

Though transmission lines can also be referred to as power lines, transmission lines “are at much higher voltage than those used for general distribution,” White said.

“In general, transmission line voltages are over 69 kV (thousand volts) while feeders are lower than 69 kV,” White said. “Typical feeder voltages would be 12.5 kV or 22.8 kV.”

Transmission lines generally “run on protected right-of-ways,” with “no trees within falling-distance of the lines,” White said — making tree damage to transmission lines less likely in storms, like hurricanes. The poles and towers that support larger transmission lines are also “much more robust than the structures that are used to support distribution feeders,” White said.

In storm or natural disaster scenarios, the smaller distribution lines — such as those you might see along a roadway or in your neighborhood — may be more likely to suffer damage from trees or other falling debris.

What is voltage?

“Voltage is one of the basic ways to classify an electrical system,” White said.

You could think of it as being similar to “pressure” in a water system, he said.

“High voltages are used for transmission lines because the power losses are less than they would be at lower voltages. The transmission line voltage is stepped down at substations, then again at the point-of-use transformer at a residence or business,” White said.

“The high voltages used for transmission lines — and to a lesser extent the lower voltages used for neighborhood distribution — are quite dangerous. The ultimate goal of the distribution system is to deliver voltage levels at the residence or business that are relatively save but still useful to drive lighting and equipment.”

Reporter Richard Stradling contributed to this story.