Electric power transmission

Electric power transmission is the bulk movement of electrical energy. It moves energy from a generating site, like a power plant, to an electrical substation.

The long conductors used for this movement are called transmission lines. When these lines connect, they form a transmission network. This network is a vital part of the electrical grid. It is distinct from electric power distribution. Distribution is the local wiring between substations and customers.

Efficient long-distance transmission requires very high voltages. High voltage is necessary because it reduces currents. Lower currents reduce energy losses caused by resistance.

To achieve this, the voltage is often increased for transmission. It is then reduced for local distribution. This change in AC voltage level is often done with transformers.

High-voltage direct current (HVDC) technology is also used. HVDC is very efficient over long distances, typically hundreds of miles. It is also used in submarine power cables longer than 30 miles. HVDC links help stabilize networks during sudden new loads or blackouts.

Transmission lines can be overhead or underground. Most power is transmitted through overhead power lines. These conductors are usually an aluminum alloy. Aluminum is lighter than copper and costs much less.

Conductors can range from 12 mm2 to 1,092 mm2 in size. For large conductors, much current flows near the surface. This is known as the skin effect. To increase capacity, engineers use bundle conductors. These are multiple parallel cables used at high voltages. They help reduce energy loss caused by corona discharge.

Underground transmission is another option. It is common in urban or environmentally sensitive areas. Underground cables have lower visibility and are less affected by weather. However, they have higher installation costs. They also have greater operational limitations. For example, long underground AC cables have significant capacitance. This reduces their ability to provide useful power. DC cables are not limited in length by capacitance. If a fault occurs in a buried line, it can be hard to fix. In some cities, cables are in metal pipes with dielectric fluid. If a leak occurs, liquid nitrogen may be used to freeze the pipe for repairs.

Historically, electricity was difficult to move long distances. In 1882, DC voltage could not be easily increased. This meant generators had to be near the users. This was called distributed generation. In 1881, Lucien Gaulard and John Dixon Gibbs built an early transformer. This made AC transmission possible. The first long-distance AC line was built in 1884 in Turin, Italy.

In 1885, William Stanley, Jr. developed a practical series AC transformer. He demonstrated a system in Massachusetts with George Westinghouse. Later, Nikola Tesla and Galileo Ferraris independently invented induction motors. These ran on polyphase current.

Modern grids are massive and interconnected. North America has four major interconnections: Western, Eastern, Quebec, and Texas. One grid connects most of continental Europe.

These interconnections reduce the risk of failure. They provide redundant, alternative routes for power. If one part of the network shuts down, power can flow elsewhere. This helps prevent large-scale failures. Transmission companies also ensure they have spare capacity. They determine the maximum reliable capacity of each line to ensure safety.

Maintaining the grid requires a constant balance. Electrical energy must be generated at the same rate it is consumed. A sophisticated control system manages this balance. If demand exceeds supply, equipment may automatically disconnect. This prevents damage to the plants and lines. In the worst case, this leads to a cascading series of shutdowns. These are called major regional blackouts. The US Northeast faced blackouts in 1965, 1977, and 2003. Other major US blackouts occurred in 1996 and 2011.