Electric power transmission

• 1. Introduction

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  Electric power transmission is the system that makes modern civilization run, yet most people never think about it. Right now, somewhere in the northeastern United States, grid operators are watching the numbers tick. Supply and demand must match, almost perfectly, at every moment. A shortfall of even a few percent can trigger automatic disconnections that cascade outward, plunging whole regions into darkness. The US Northeast has experienced exactly that kind of catastrophic failure in 1965, 1977, and 2003, and major blackouts struck other US regions in 1996 and 2011. So how does electricity actually travel from a power plant to your home? What keeps the whole system from collapsing? And why do engineers transmit electricity at hundreds of thousands of volts just to immediately step it back down? Those are the questions this documentary will answer.

• 2. The Physics of Moving Power

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  Joule's first law sits at the heart of transmission engineering. It states that energy losses are proportional to the square of the current flowing through a conductor. That single relationship explains almost every design decision in the grid. Double the current and you lose four times the energy to heat. Cut the current by a factor of ten and losses drop by a factor of a hundred. The only way to cut current without cutting the power delivered is to raise the voltage. A 100-mile span of transmission line operating at 765 kV and carrying 1,000 MW loses somewhere between 0.5% and 1.1% of the power it carries. Run that same load over 345 kV lines and the loss jumps to 4.2%. Power stations generate electricity at relatively low voltages, typically 2.3 kV to 30 kV depending on the unit's size. Step-up transformers at the generating site lift that voltage to between 115 kV and 765 kV for the long journey across the grid. In the United States, most transmission runs at 230 kV to 500 kV, with voltages below 230 kV or above 500 kV as exceptions. Transmission-level voltages are generally considered to start at 110 kV. Voltages above 765 kV are classed as extra high voltage and require different design approaches entirely. At very high voltages, above about 2,000 kV between conductor and ground, a phenomenon called corona discharge becomes a serious problem. Charged air around the conductors bleeds off energy, and at those extreme voltages the corona losses can actually offset the resistive savings. Larger conductor diameters, hollow cores, and grouped conductor bundles are all measures engineers use to keep corona in check.

• 3. Wire, Steel, and the Overhead Grid

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  Aluminium is the dominant material in overhead transmission conductors today. It is lighter than copper, costs considerably less, and the reduction in electrical performance is only marginal. Traditional high-voltage lines use aluminium strands wrapped around a steel core, known as aluminium-conductor steel-reinforced cable. Conductor cross-sections span a wide range, from 12 square millimetres at the smallest to 1,092 square millimetres at the largest. For thicker conductors, the skin effect concentrates most of the current near the surface, meaning the conductor's core contributes little electrically but adds weight and cost. Engineers solve this by using bundles of parallel cables instead of a single massive conductor; bundle conductors also reduce corona discharge losses at high voltages. Overhead transmission wires rely on air alone for insulation, which means they must maintain strict minimum clearances from the ground and from each other. Wind speeds as low as 23 knots can cause conductors to swing close enough to trigger a flashover, a sudden arc discharge that knocks out power. The oscillatory motion of lines in wind has its own vocabulary: slow, large-amplitude swings are called conductor gallop, while faster, smaller oscillations are called flutter. Weather poses a constant challenge, and the consequences of a conductor sagging too close to dry vegetation have become starkly visible. Reconductoring, the practice of replacing existing wires with higher-capacity advanced conductors, addresses this problem directly. Replacing the steel core with a lighter, stronger composite material such as carbon fiber allows lines to run at higher temperatures with less sag, doubling transmission capacity. A reconductoring project in southeastern Texas upgraded 240 miles of transmission lines at a cost of $900,000 per mile. A greenfield 3,600-mile project over similar terrain averaged $1.9 million per mile, illustrating why reconductoring is attractive. A 2024 report found the United States behind countries like Belgium and the Netherlands in adoption of this technique.

• 4. Underground and Undersea

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  Underground cables eliminate visual impact and right-of-way conflicts, and they are far less vulnerable to weather. The trade-off is steep. Excavation and cable costs are far higher than overhead construction, and faults buried underground take significantly longer to find and repair. In some metropolitan areas, cables travel inside metal pipes insulated with a dielectric fluid, usually oil, that is either held static or circulated by pumps. When an electrical fault damages the pipe and the dielectric fluid leaks, engineers inject liquid nitrogen to freeze sections of the pipe, which allows draining and repair while extending the outage and increasing costs. Long underground AC cables face a physics problem overhead lines do not: significant capacitance, which reduces their ability to carry useful power. This capacitance effect limits practical AC underground cable runs to roughly 50 miles. DC cables carry no such restriction. High-voltage direct current cables are used wherever AC cannot go. Submarine HVDC links connect electricity grids across water: between Great Britain and continental Europe, between Great Britain and Ireland, between Tasmania and the Australian mainland, between the North and South Islands of New Zealand, and between New Jersey and New York City, among others. Submarine connections up to 600 km in length have been deployed. As of the 29th of December 2023, the longest operational land-and-subsea HVDC interconnector is Viking Link, running 765 km between the United Kingdom and Denmark, surpassing the North Sea Link at 720 km. The world record for overland power lines belongs to Inga-Shaba in the Democratic Republic of Congo, stretching 1,700 km.

• 5. From Gaulard to the War of Currents

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  In 1882, commercial electric power could only travel short distances because DC voltage could not be increased efficiently for long-distance transmission. Different types of loads, lighting, fixed motors, and traction systems each required different voltages, so generators were built close to whatever they powered. The change came from a device built by Lucien Gaulard and John Dixon Gibbs in 1881, which they called the secondary generator. It was an early transformer, though it used a 1:1 turn ratio and an open magnetic circuit, limiting its practical usefulness. The first long-distance AC line, 34 km long, appeared at the 1884 International Exhibition of Electricity in Turin, Italy. It was powered by a 2 kV, 130 Hz Siemens and Halske alternator connected to several Gaulard transformers wired in series, feeding incandescent lamps at the far end. The first commercial AC distribution system entered service in 1885 at via dei Cerchi in Rome, using two Siemens and Halske alternators and 19 km of cables with 200 parallel-connected step-down transformers, one for each lamp. A few months later, the first British AC system began serving Grosvenor Gallery using Siemens alternators and 2.4 kV to 100 V step-down transformers. Electrical engineer William Stanley Jr. found the Gaulard-Gibbs design impractical and set about improving it with the support of George Westinghouse. In 1886 in Great Barrington, Massachusetts, Stanley demonstrated a transformer-based AC lighting system powered by a steam-engine-driven 500 V Siemens generator. Voltage was stepped down to 100 volts to power incandescent lamps at 23 businesses across 4,000 feet. Westinghouse immediately began installing AC systems. In 1888 the induction motor appeared, independently invented by Galileo Ferraris and Nikola Tesla. Westinghouse licensed Tesla's design. Three-phase motors were later developed by Mikhail Dolivo-Dobrovolsky and Charles Eugene Lancelot Brown. By the late 1880s and early 1890s, smaller electric companies were merging into large corporations: Ganz and AEG in Europe, General Electric and Westinghouse Electric in the United States. By 1914, fifty-five transmission systems operating above 70 kV were in service, with the highest reaching 150 kV. World War I then accelerated the interconnection of local grids, as governments built large generating plants to power munitions factories.

• 6. High-Voltage Direct Current

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  HVDC systems trade a fundamental limitation of AC transmission for a significant advantage over very long distances. In an AC line, the power that can flow is tied to the phase angle between voltage at the sending end and voltage at the receiving end. Push that angle too close to 90 degrees and the connected systems risk falling out of synchronization, causing a cascade of failures. A DC link carries no such phase angle constraint. Power flow on an HVDC connection is controlled independently of the AC networks it bridges, so the link can always deliver its full rated power regardless of what is happening on either side. The Pacific DC Intertie in the Western United States is one operational example. The Western Interconnection's primary voltages include 500 kV AC at 60 Hz and a DC system rated at plus or minus 500 kV, equivalent to 1,000 kV net, running from the Columbia River to Southern California and from Utah to Southern California. HVDC is also necessary for linking grids that are not synchronized with each other. When sending power between such grids via AC is impossible, a DC link provides the bridge. The conversion equipment at each end is costly, but for long distances or submarine routes the economics can favor DC strongly. The record holder for transmission capacity is the Zhundong-Wannan HVDC system in China, rated at 12 GW at plus or minus 1,100 kV. The tallest transmission towers are the Yangtze River Crossing structures in China, standing 345 metres high. The longest single span of any power line anywhere is 5,376 metres at the Ameralik Span in Greenland.

• 7. Grid Stability and Market Structure

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  Electrical energy must be generated at the same rate it is consumed, at virtually every instant. A control system failure that allows demand to outrun supply can trigger automatic disconnections of generating plants and transmission equipment to prevent hardware damage, and in the worst case a cascading series of shutdowns follows. Brownouts occur when supply drops slightly below demand; full blackouts occur when the grid collapses. To prevent this, transmission networks are interconnected into regional, national, and continent-wide grids with redundant paths so power can reroute around failures. One grid connects most of continental Europe. North America has four major interconnections: Western, Eastern, Quebec, and Texas. Historically, transmission and distribution were owned by the same companies, but starting in the 1990s many countries restructured electricity markets to separate the two functions. Spain was the first country to establish a regional transmission organization. Its transmission system operator is Red Electrica de Espana, and its wholesale market operator is a separate entity. Spain's grid interconnects with those of France, Portugal, and Morocco. In the United States, the Federal Energy Regulatory Commission's Order 888, issued in 1996, drove the creation of independent regional transmission organizations. New York often buys more than 1,000 MW of low-cost hydropower from Canada as a direct result of these interconnection arrangements. US transmission and distribution losses were estimated at 6.6% in 1997, 6.5% in 2007, and fell to around 5% from 2013 to 2019. As of 2022, more than 10,000 power plant and energy storage projects were waiting for permission to connect to the US grid, and 95% of them were zero-carbon resources. New power lines can take ten years to plan, permit, and build, making grid expansion one of the binding constraints on the energy transition.