How the Electric Grid Works: A Crash Course

by David Bacisin, published 22 January 2023

The modern electricity grid delivers power to billions of people around the world, enabling everything from indoor lighting and refrigeration to advanced supercomputing and aluminum smelting. How does electricity get from generators to outlets in a reliable and seemingly endless way, and what causes blackouts and brownouts?

Parts of the electric grid

The electric grid is made up of many connected pieces that work together to produce, transmit, distribute, store, and use electricity. From a bird’s-eye view, generators produce electricity for end users to consume.

Generators

Power plants, also called generating stations, are the source of electricity on the grid. They convert other forms of energy, such as the chemical energy stored in natural gas or the radiant energy carried by sunlight, into electricity. There are many types of generators, and we can classify them by their energy sources: for example, natural gas plants, solar panels, nuclear fission plants, wind turbines, or hydroelectric dams.

Some generator types have greater monetary, environmental, or social costs compared to other types. Coal plants damage the environment and nearby communities with unhealthy air pollution from burning coal. Technological advancements in solar and resource constraints on gas have now made solar cheaper than natural gas. All generators have at least some impact on the environment and society: the extraction of metals for building this equipment damages plant and animal habitats, and the construction and location of facilities disrupts even more habitats. As a society, while we must certainly favor safer and cleaner electric generation with technologies like solar and wind, we must also prioritize using less electricity overall to reduce the negative impacts of even these clean technologies.

The amount of electricity that generators add to the grid must always closely match the amount of electricity being consumed from the grid. Too little or too much supply can cause brownouts, blackouts, and even equipment failure. Therefore, a resilient grid system has:

Base load generators to provide a stable minimum amount of power
Load-following power plants to meet additional demand
Peaking power plants to provide even more supply during an hour or two of peak usage
Storage systems to capture excess energy and to return that energy back to the grid as needed
Demand-side management to encourage electricity consumers to shift their usage away from peak hours, reducing the need for expensive peaker plants

The energy sources vary for each of these duty cycles. Nuclear power, for example, is slow and expensive to start and stop because the nuclear reaction must be managed safely. As a result, nuclear is used for generating base load and never for peak load. On the other hand, hydroelectric and gas turbine systems are relatively easy to start and stop, making them ideal for meeting peak demand.

Transformers and Converters

After generation, the next step is to prepare the electricity to travel. Transformers and converters enable efficient transmission of electricity over long distances. A step-up transformer near a generator increases the voltage of the AC power so that it can travel through transmission lines with minimal energy loss. Transmission voltages usually range from 110 kV to 765 kV. A step-down transformer decreases the voltage to a safer, more usable level for distribution to consumers.

Converters are used to turn AC power into DC power, and vice versa, for transmission over high-voltage direct current (HVDC) systems. Converters are more expensive to build than transformers, but they enable more efficient transmission over very long distances.

Transmission lines

Electric transmission lines bring electricity from the generation station closer to where it will be used. These metal wires may be overhead or underground. Higher voltages require greater insulation to prevent dangerous arcs between the lines and other objects. Overhead high-voltage lines are much taller than lower-voltage distribution lines because they use the surrounding air as a form of insulation.

As electricity flows through power lines, some energy is lost due to resistance of the conducting material. Resistance is affected by ambient temperature, how the strands of wire are twisted together (called spiraling), which metal is used to form the wire, and the skin effect—the tendency for AC power to flow mainly along the outer surface of a wire rather than evenly through its cross-section. All of these factors contribute to line losses, which are estimated at about 5% of power in the United States.

Electrical substations

As transmission lines get closer to end users, a series of substations transform power into lower and lower voltages. Substations are the physical locations for transformers and converters, as well as for various other equipment used to stabilize and control the flow of electricity on the grid. Substations are located throughout the grid: next to power plants, next to residential areas, and in many places in between.

Distribution lines

From a distribution substation, a further network of lines delivers power to its final destination. Transformers along the way decrease voltage to sub-transmission levels (33 to 132 kV), to distribution levels (3.3 to 25 kV), and finally to residential (110 to 240 V) or industrial (380 to 480 V) levels.

What is a microgrid?

While the electric grid spans large geographic regions, the same equipment can be built and managed in a much smaller area: a microgrid may consist of small groups of diesel-driven turbines, wind turbines, solar panels, energy storage, or other generators, and it can operate both in isolation from and in connection with the larger grid. When the main grid experiences peak load, for example, the microgrid can disconnect and create a power island to serve its users while taking its burden off of the larger grid. This increases resilience: in the event of a brownout or blackout on the main grid, the microgrid can supply its community with 100% power.

Who use microgrids? Corporate or university campuses, some residential communities, and military bases might build and manage their own microgrids. Industrial facilities that require constant power for their manufacturing processes may also invest in a microgrid. Remote facilities that are prohibitively far from existing grid infrastructure may construct a microgrid without ever connecting to a main grid.

An institution or community that is part of a microgrid can have more control over its emissions. If the microgrid, for example, consists purely of wind, solar, geothermal, and storage, then the community can use that green energy even if the external grid relies on coal or natural gas. If any supplemental energy is needed during peak demand, then the microgrid could allow a limited flow of fossil-fuel-based energy into its system.

Grid Failures and Resilience

The massive, complex network of the electrical grid is susceptible to many failure modes. Falling tree branches can disconnect or short out wires. Metals can corrode, transformers can overheat, and demand can mismatch supply. Nefarious actors can conduct cyber or physical attacks. Failures can even cascade because of the tight interdependencies between all pieces.

Various mechanisms protect equipment during failure, thus making the grid more resilient. Redundancy is created by having multiple electric lines serve the same customers, so if any one line fails, then others will continue service. A circuit breaker may trip to protect downstream equipment from overheating, sparking, or arcing. While the failure may still have negative consequences, the circuit breaker protects against more severe consequences and enables maintenance workers to resolve the problem more quickly.

Brownouts occur when the supplied voltage drops below the standardized operating levels that are established by the country or region. Brownouts may be intentional to reduce load during an emergency, or unintentional if demand exceeds supply.

Blackouts are the total loss of power. Like brownouts, they may be intentional to manage load during periods of high demand—often in the form of rolling blackouts, where portions of the grid are shut off at preplanned times in an attempt to distribute the pain equitably.

The 2003 blackout in the northeastern United States and Canada is a dramatic example of cascading grid failure. Hot weather caused a power line to sag into vegetation, triggering a circuit breaker in the affected line. The remaining power lines were unable to safely carry the needed capacity, and software issues left grid operators with poor knowledge of the problem. They did not know that they needed to request more power or to shed load. As nearby power lines also exceeded their rated capacity, more circuit breakers tripped. The failures continued as more and more lines automatically disconnected from the grid, leaving fewer and fewer lines remaining to serve the same demand. Ultimately, the blackout affected 50 million people across 8 U.S. states and southeastern Canada.

One of the deadliest and most destructive wildfires in the United States started due to a faulty electric transmission line. Pacific Gas and Electric Company (PG&E) was responsible for maintaining power lines in Butte County, CA, that had been constructed in the early 1900s. In 2011, the California Public Utilities Commission (CPUC) had inspected some of these lines and found several thousand problems. On November 8, 2018, sparks from a worn hook on a transmission tower ignited what would become known as the Camp Fire. Over 240 square miles (621 km²) burned, destroying 18,000 buildings across multiple towns. PG&E pled guilty to 84 counts of involuntary manslaughter due to neglecting to maintain the power lines.

Grid Futures

The future of the grid is carbon-free. We must replace polluting generating stations with cleaner ones, add storage to meet demand when the sun sets and the wind calms, and upgrade aging infrastructure to increase reliability and prevent disaster. Increased electrification of personal automobiles, heavy industry, and other sectors requires effective management to ensure capacity meets demand.