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Understanding the Electric Power Grid

The U.S. electric grid is the network that connects electricity producers and consumers through a system of generation, transmission and distribution lines and facilities. It begins at a power plant and ends at your home or business.

According to the U.S. Energy Information Administration, the grid includes more than 300,000 miles of transmission and distribution lines and more than 7,200 power plants and generating facilities, each with at least 1 megawatt (MW) of generating capacity.

Santee Cooper’s grid is comprised of more than 7,800 miles of lines and 150 substations that must handle more than 5,000 MW of generation capacity flowing from power plants as large as the 2,370-MW Cross Generating Station and as small as the 311-kilowatt Grand Strand Solar Station.

The grid is a marvel of engineering that enables our way of life, from powering industry and business, to charging your smartphone, tablet, or electric vehicle.

The grid has come a long way since Thomas Edison introduced the first commercial power grid in 1882. Back then, electric utilities were isolated from one another, and many used low-voltage, direct current (DC) connections to transport their electricity to customers.

While Edison favored DC, Nikola Tesla championed alternating current (AC), which could be converted to different voltages using a transformer and therefore travel greater distances. AC would become the industry standard by the start of the 20th century and has been crucial for South Carolina electricity companies including, Santee Cooper’s, ability to serve its retail customers.

“Santee Cooper’s retail service area is concentrated in parts of Horry, Georgetown and  Berkeley counties,” says Vicky Budreau, manager of transmission operations. “We have 174,023 retail customers, and most of them are in the Grand Strand area. But our largest South Carolina power plants are in rural parts of Berkeley, Fairfield, Georgetown and even Anderson counties. Without alternating current, we couldn’t push the power from those generating stations over the many miles of lines to reach our retail customers.”

Transmission lines owned by an individual utility like Santee Cooper are no longer used exclusively by that utility. Santee Cooper operates its own South Carolina electricity grid, but it’s also connected to the grids of neighboring utilities: South Carolina Electric & Gas Co., Duke Energy Carolinas, Duke Energy Progress, Southern Company and the Southeastern Power Administration. This expanded and interconnected grid requires close coordination and cooperation.

“We’re responsible for maintaining our own grid just like SCE&G and Duke are responsible for their grids,” Budreau says. “It’s in our best interest and the interest of our interconnected neighbors to ensure the health, resiliency and integrity of our grid.”

Santee Cooper and other South Carolina power companies must comply with mandatory reliability standards, which are enforced by the North American Electric Reliability Corp. and regulated by the Federal Energy Regulatory Commission. In 2015, Santee Cooper maintained a transmission reliability factor of 99.9959 percent and a distribution reliability factor of 99.9956.

“What this means is that Santee Cooper’s transmission delivery points averaged just 21 minutes without service in 2015, and distribution customers averaged 23 minutes without service,” Budreau says.

Generating Electricity
Quick physics lesson: Electricity is the movement of electrons through a material. Power plants typically produce electricity using magnetic conduction, which occurs when a large number of conductive wires are spun around inside a magnetic field.

These days, power plants are distinguished by the type of fuel they use to generate electricity. Those fuels are mostly coal, natural gas and low-grade uranium, but the use of renewable resources like wind and solar is expanding. In 2015, Santee Cooper, a South Carolina power company, generated approximately 48 percent of its electricity from coal, 23 percent from natural gas, 9 percent from nuclear and   2 percent from renewables and hydro.

With fossil-fueled or nuclear-powered generating stations, electricity is produced using heat to convert water into steam. That highly pressurized steam then travels to the blades of a turbine. As that steam hits the blades, the turbine begins to turn. Giant wire coils inside the generator also begin to turn, which creates an electromagnetic field that forces electrons to move and start the flow of electricity. Some natural gas plants do not use steam. Instead, the gas is ignited and burned, and the heat creates the pressure that turns the turbine.

“Electricity is generated and used in real time, so that supply and demand must always remain in balance,” says Tom Abrams, vice president of planning and power supply. “Santee Cooper must be able to generate as much power as is needed at any given time to respond to our customers’ needs. The grid simply accommodates the flow.”

Moving Electricity
Santee Cooper’s transmission system is a high-voltage network of infrastructure that enables the movement of large amounts of electricity over long distances; think of it as the grid’s interstate highway system. Our transmission system spans more than 5,000 miles of lines and includes voltages of 230 kilovolts (kV), 115 kV, 69 kV and 34 kV.

“As a general rule, higher voltages are used for moving power longer distances or for moving larger quantities of power,” Abrams says.

Before that high-voltage electricity can be used at your home or business, it passes through a substation. A substation is placed at any location on the grid where one component connects with another (i.e. between transmission and distribution lines, from one transmission line to another, or between transmission lines and groups of customers). The Santee Cooper South Carolina power grid includes 105 transmission substations and 54 distribution substations.

“Substations are these points of control where we can adjust the voltage levels up or down, or disconnect or reconnect portions of the grid for improved reliability,” Budreau says. “They also contain equipment that meters the flow of electricity, protects the grid and provides information about traffic on the grid.” 

Substations can also connect Santee Cooper’s grid to larger industrial and municipal customers or to the electric cooperatives that distribute Santee Cooper’s power to its member owners.

If transmission lines are the interstate highways of the grid, distribution lines are the state roads. They represent the final stop from the power plant to the customer.

“Santee Cooper’s grid has more than 2,800 miles of distribution lines, and more than 50 percent are underground,” says Diane Bell, manager of distribution planning and technical operations. “Distribution voltages are 34 kV and 12 kV, which are reduced at substations to allow for safe local delivery of power. These voltages are further reduced by transformers before the power reaches a customer’s home or business. These are the can-like cylinders you see at the top of a power pole for overhead lines or those green boxes for underground lines.”

The typical U.S. household uses 120 volts, although electric clothes dryers, HVAC systems and water heaters require 240 volts.

“The U.S. electric grid might be over 100 years old, but it has evolved along the way,” Abrams says. “Processes that were once performed manually are now automatically executed in seconds. The grid will continue to change as new technologies emerge and more renewable-energy resources are integrated.”

Each evolution, like smart-grid technology and distributed generation, presents its own set of opportunities and challenges for utilities like Santee Cooper.

“Smart grids eventually can help utilities detect a power outage instantly, speeding up restoration. It can also help consumers learn more about their personal energy use and how they can conserve energy,” Bell says.

As more customers add renewable energy systems to their homes and businesses, Bell says the grid will also need to adapt to allow the two-way flow of electricity on a distribution system that wasn’t designed for such a purpose.

“Utilities will also need to adjust for the intermittent nature of renewable energy while still serving those customers whose renewable systems aren’t meeting all of their electric needs,” she says.

As technology continues to enhance the grid’s overall performance, it’s also made its safety and security paramount.

“Protecting the grid from physical and cybersecurity attacks has become a central priority for Santee Cooper and other utilities,” Abrams says.

The grid has seen several evolutions over the years, but it’s fundamentally the same system that enables Santee Cooper to fulfill its mission of improving the quality of life for the people of South Carolina.