Electric power transmission system

Electricity transmission is the bulk movement of electrical energy from a generating station (such as a power plant) to a substation.The interconnected lines that facilitate this movement form the transmission network.This is different from the local wiring between high voltage substations and customers, which is often called power distribution.The transmission and distribution network is the part of electricity delivery known as the grid.Efficient long-distance power transmission requires high voltages. This reduces losses from high currents.Transmission lines use alternating current (AC) or direct current (DC). The voltage level changes with the transformer.The voltage is boosted for transmission and then stepped down for local distribution.

Wide-area synchronous grids called "interconnections" in North America directly connect generators to provide alternating current to many consumers at the same relative frequency. There are four major interconnection points in North America: West, East,Quebec and Texas. A grid connects most of continental Europe.Historically, transmission and distribution lines were often owned by the same company, but starting in the 1990s, many countries deregulated electricity markets, resulting in separate companies handling transmission and distribution.

System 

Most North American transmission lines are high-voltage three-phase AC, although railroad electrification systems sometimes use single-phase AC.DC technology is used to increase efficiency over longer distances (typically hundreds of miles).High-voltage direct current (HVDC) technology is also used for subsea power cables (typically longer than 30 miles (50 kilometers)), and for the exchange of power between grids that are not synchronized with each other. HVDC links stabilize power distribution networks where sudden new loads or outages in one part of the network could otherwise cause synchronization problems and cascading failures.Electricity is transmitted at high voltage to reduce energy loss that occurs over long distances.Electricity is usually transmitted over overhead power lines.Underground power transmission has significantly higher installation costs and greater operational constraints,but reduces maintenance costs. Underground transmission is more common in urban areas or environmentally sensitive areas.

Electric energy must generally be produced at the same rate as it is consumed.A complex control system is required to ensure that generation is closely matched to demand.If supply exceeds demand, this imbalance can cause power plants and transmission equipment to automatically disconnect or shut down to prevent damage. In a worst-case scenario, this could lead to a cascade of shutdowns and widespread power outages.The Northeastern United States faced blackouts in 1965, 1977, 2003, and the rest of the country in 1996 and 2011.Electricity transmission networks are interconnected to regional, national or even continental networks by providing multiple redundant backup power paths in the event of such outages.The transmission company determines the maximum reliable capacity of each line (usually less than its physical or thermal limit) to ensure that spare capacity is available if another part of the network fails.