Electric power transmission
The Grand Coulee Dam in Washington State stands as a monument to bulk energy movement. Six circuits of five-hundred kilovolt lines carry the entire 6809 megawatt nameplate generation capacity from this hydroelectric plant. This facility demonstrates how electricity travels vast distances without losing its power. The transmission network connects generating sites like power plants to electrical substations. These interconnected lines form the backbone of the electrical grid. Local wiring between high-voltage substations and customers remains distinct from this bulk movement process. Efficient long-distance transmission requires high voltages to reduce losses produced by strong currents. Transformers change voltage levels for both transmission and distribution phases.
Commercial electric power initially traveled at the same voltage used by lighting and mechanical loads. This restriction limited distance between generating plants and loads. In 1882, direct current voltage could not easily increase for long-distance transmission. Different classes of loads required different voltages and separate generators. Generators were sited near their loads, creating what later became known as distributed generation using large numbers of small generators. Lucien Gaulard and John Dixon Gibbs built an early transformer called the secondary generator in 1881. The first long distance alternating current line stretched one mile for the 1884 International Exhibition of Electricity in Turin, Italy. It was powered by a two kilovolt, 130 hertz Siemens & Halske alternator. The system proved feasibility of alternating current electric power transmission over long distances. World War I spurred interconnection of local generation plants when governments built large electrical generating plants to power munitions factories. These historical developments transformed how societies moved electricity across vast landscapes.
High-voltage overhead conductors lack insulation and rely on air for clearance. Conductor material is usually an aluminum alloy formed of several strands possibly reinforced with steel strands. Copper was sometimes used but aluminum costs much less while reducing yields only marginally. Adverse weather conditions like high winds interrupt transmission. Wind speeds as low as twenty miles per hour can permit conductors to encroach operating clearances resulting in flashover and loss of supply. Oscillatory motion of the physical line is termed conductor gallop or flutter depending on frequency and amplitude. Underground cables take up no right-of-way and have lower visibility than overhead lines. Cable and excavation costs are much higher than overhead construction. Faults in buried transmission lines take longer to locate and repair. In some metropolitan areas, cables are enclosed by metal pipe insulated with dielectric fluid that is either static or circulated via pumps. If an electric fault damages the pipe and leaks dielectric, liquid nitrogen freezes portions of the pipe enabling draining and repair. This extends the repair period and increases costs significantly compared to overhead alternatives.
Most North American transmission lines use high-voltage three-phase alternating current though single phase AC appears in railway electrification systems. Direct current technology provides greater efficiency over longer distances typically hundreds of miles. High-voltage direct current technology serves submarine power cables typically longer than thirty miles. HVDC links stabilize power distribution networks where sudden new loads or blackouts might otherwise cause synchronization problems. The Pacific DC Intertie located in the Western United States exemplifies long distance direct current transmission. Converting alternating current in Seattle into high-voltage direct current allows cross-country transmission before converting back to locally synchronized alternating current in Boston. Submarine connections up to one hundred kilometers in length connect electricity grids between islands like Great Britain and continental Europe. These submarine HVDC systems interconnect grids without the limitations imposed by cable capacitance on alternating current systems. Phase angle limits do not exist for direct current links allowing full rated power transfer regardless of system loading conditions.
Electrical energy must be generated at the same rate it is consumed requiring sophisticated control systems. If demand exceeds supply, generation plants and transmission equipment automatically disconnect to prevent damage. In the worst case this leads to cascading series of shutdowns causing major regional blackouts. The US Northeast faced blackouts in 1965, 1977, 2003, and other regions experienced failures in 1996 and 2011. Transmission companies determine maximum reliable capacity of each line typically less than physical or thermal limits. Spare capacity remains available in event of failure elsewhere in network. Interconnected networks provide multiple redundant alternative routes for power flow should shutdowns occur. Rolling blackouts distribute insufficient power to various loads in turn during emergency situations. Brownouts occur when power supplied drops below demand while complete grid failure creates blackouts. Grid operators use microwaves, optical fibers, and power-line communication to monitor power flow so faulted conductors can quickly de-energize restoring balance to entire system.
Spain became first country to establish a regional transmission organization separating transmission operations from electricity markets. Red Eléctrica de España operates Spain's transmission system while Operador del Mercado Ibérico de Energía manages wholesale market. FERC Order 888 issued in 1996 spurred establishment of Regional Transmission Organizations in United States. Electric transmission companies operate independently of generation companies though Southern United States maintains vertical integration. Merchant transmission projects include Cross Sound Cable running from Shoreham New York to New Haven Connecticut. Neptune RTS Transmission Line connects Sayreville New Jersey to New Bridge New York. Path 15 serves California as another example of merchant infrastructure. Australia has Basslink as one unregulated market interconnector between Tasmania and Victoria. Major barrier to wider adoption involves difficulty identifying who benefits from facility so beneficiaries pay toll. FERC Order 1000 issued in 2010 attempted reducing barriers to third party investment where public policy need found. In UK transmission costs about 0.2 pence per kilowatt-hour compared to delivered domestic price around 10 pence per kilowatt-hour. Level capital expenditure in electric power T&D equipment market estimated at 128.9 billion dollars in 2011.
Mainstream scientific evidence suggests low-power electromagnetic radiation associated with household currents does not constitute short or long-term health hazard. A 1997 study reported no increased risk of cancer or illness from living near a transmission line. The New York State Public Service Commission measured electric field strength at edge of existing right-of-way on seven-hundred sixty-five kilovolt transmission line. Field strength reached 1.6 kilovolts per meter becoming interim maximum strength standard for new lines in state. Policy also limited voltage of new transmission lines built in New York to three hundred forty-five kilivolts. Magnetic field standard established at two hundred milligauss at edge of right-of-way using winter-normal conductor rating. One study reported limited evidence of carcinogenicity in humans and less than sufficient evidence for experimental animals particularly childhood leukemia associated with average exposure above 0.3 microtesla to 0.4 microtesla. Tree growth regulators and herbicides may be used in transmission line rights of ways potentially creating additional environmental effects. Earth natural geomagnetic field strength varies between 0.035 millitesla and 0.07 millitesla across planet surface while international standard for continuous exposure set at 40 millitesla for general public.
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Common questions
What is the nameplate generation capacity of the Grand Coulee Dam?
The Grand Coulee Dam has a 6809 megawatt nameplate generation capacity. Six circuits of five-hundred kilovolt lines carry this entire power output from the hydroelectric plant in Washington State.
When did Lucien Gaulard and John Dixon Gibbs build their early transformer?
Lucien Gaulard and John Dixon Gibbs built an early transformer called the secondary generator in 1881. This invention preceded the first long distance alternating current line which stretched one mile for the 1884 International Exhibition of Electricity in Turin, Italy.
How does high voltage reduce losses during electric power transmission?
Efficient long-distance transmission requires high voltages to reduce losses produced by strong currents. Transformers change voltage levels for both transmission and distribution phases to maintain efficiency across vast distances.
Which countries use submarine HVDC systems to connect electricity grids between islands?
Submarine connections up to one hundred kilometers in length connect electricity grids between islands like Great Britain and continental Europe. These submarine HVDC systems interconnect grids without the limitations imposed by cable capacitance on alternating current systems.
What were the blackout years experienced by the US Northeast region?
The US Northeast faced blackouts in 1965, 1977, 2003, and other regions experienced failures in 1996 and 2011. Transmission companies determine maximum reliable capacity of each line typically less than physical or thermal limits to prevent such events.