Transmission lines

Electric power is transmitted at high voltage (110 kV or above) to reduce the energy lost in long-distance transmission. Power is usually transmitted through overhead power lines. Underground power transmission has a significantly higher cost and greater operational limitations but is sometimes used in urban areas or sensitive locations.

Transmission lines normally use high voltage three-phase alternating current. High voltage direct current (HVDC) technology is used for greater efficiency in very long distances (typically several hundreds of kilometers). HVDC links are also used to stabilize against control problems in large power distribution networks where sudden new loads or blackouts in one part of a network can otherwise result in synchronization problems and cascading failures.

Generally, several conductors are strung on a transmission tower for each electrical circuit. Conductors are constructed primarily of twisted metal conductors. High voltage overhead conductors are not covered by insulation. The conductor material is usually aluminum conductor steel reinforced (ACSR) which is a specific type of high-capacity, high-strength stranded conductor. The outer strands are made from hard drawn aluminum wire manufactured from not less than 99.5% pure electrolytic aluminum rods of EC grade and copper content not exceeding 0.04%.

High purity aluminum alloy is chosen for its excellent conductivity, low weight and low cost. The center strands are of steel for the strength required to support the weight without stretching the aluminum due to its ductility. This gives the conductor an overall high tensile strength. Copper was earlier used for overhead transmission but aluminum is lighter, yields only marginally reduced performance and costs much less.

Most transmission lines are high-voltage three-phase alternating current (AC), although single phase AC is sometimes used in railway electrification systems. High-voltage direct-current (HVDC) technology is used for greater efficiency at very long distances (typically hundreds of miles (kilometers)), or in submarine power cables (typically longer than 30 miles (50 km)). HVDC links are also used to stabilize and control problems in large power distribution networks where sudden new loads or blackouts in one part of a network can otherwise result in synchronization problems and cascading failures.

Electricity is transmitted at high voltages (120 kV or above) to reduce the energy losses in long-distance transmission. Power is usually transmitted through overhead power lines. Underground power transmission has a significantly higher cost and greater operational limitations but is sometimes used in urban areas or sensitive locations.

A key limitation of electric power is that, with minor exceptions, electrical energy cannot be stored, and therefore must be generated as needed. A sophisticated control system is required to ensure electric generation very closely matches the demand.

Much analysis is done by transmission companies to determine the maximum reliable capacity of each line (ordinarily less than its physical or thermal limit) to ensure spare capacity is available should there be any such failure in another part of the network.