I Can’t Sleep - Electric Power Distribution | Calm Bedtime Reading

Episode Date: June 26, 2023

Drift off with this calm bedtime reading as Benjamin explores electric power distribution, helping you relax and ease insomnia. You’ll learn how electricity travels from power plants to homes and businesses, the systems and infrastructure that make it possible, and the history of its development. Benjamin’s soothing cadence transforms technical details into peaceful storytelling that reduces stress and quiets the mind. This isn’t whispering or hypnosis—just gentle, fact-filled narration designed to guide you into rest. Press play, settle in, and let this exploration of electric power distribution carry you into sleep. Want More? Request a Topic: https://www.icantsleeppodcast.com/request-a-topic Ad-Free Episodes: https://icantsleep.supportingcast.fm/ Shop Sleep-Friendly Products: https://www.icantsleeppodcast.com/sponsors Join the Discussion on Discord: https://discord.gg/myhGhVUhn7 This content is derived from the Wikipedia article on Electric Power Distribution, available under the Creative Commons Attribution-ShareAlike (CC BY-SA) license. Read the full article: Wikipedia – Electric Power Distribution. Happy sleeping! Learn more about your ad choices. Visit megaphone.fm/adchoices

You're listening to a Glassbox media podcast. What if I told you that most of the modern day self-help advice you've been hearing could actually make you worse? The key to a better life isn't about feel-good gimmicks that sound catchy. The Mentally Stronger Podcast gives you access to a licensed therapist who shares science-backed tools that will actually change your life. Hi, I'm Amy Morin, psychotherapist, mental strength trainer, and international best-selling author. In each episode, we cover research-back strategies, like how to stop relying on willpower and start creating habits for lasting change. And the five mental strength-building exercises you can do from your couch.

I also speak to world-class experts like Dr. Nicole Kane, who shares how to permanently heal anxiety by addressing the root cause. With over 200 episodes in our catalog, this podcast is for you if you're ready to crush self-doubt, conquer challenges, and become stronger than ever with therapist-approved strategies that can change your life. Listen to Mentally Stronger with Therapist Amy Morin, wherever you get your podcasts. Welcome to the I Can't Sleep Podcast. I'm your host, Benjamin Boster. Before we begin today's episode, I'd like to share a few updates. We recently partnered with Glassbox media to reach more listeners who struggle with sleep.

You may have noticed a change a few months ago when we started incorporating more ad placements at the beginning and partway through each episode. I appreciate your feedback and your patience as we resolve the audio issues that many have kindly reported during this new phase of the podcast. With the help of ad revenues, we've been able to increase our monthly production of episodes. While I understand this format might not be ideal for a podcast of this nature, we're making every effort to ensure that the ads do not interrupt your sleep. In a moment, you will hear another set of ads.

I hope to narrate most, if not all, of these ads in the future and would like to suggest a couple options for an improved listening experience. Firstly, Spotify and Apple Podcasts have a sleep timer feature that halts playback after a predetermined period or at the end of the episode. This prevents the audio from automatically playing the next episode and its corresponding ads, so you won't be disturbed if you're already asleep. Secondly, I've included a link in the show notes where you can sign up for an ad-free experience for just $2.99 a month, irrespective of your listening platform.

Your questions and feedback are always welcome, and I appreciate your patience as we work through this next phase of the podcast. Here are the next set of ads, after which we'll delve right into today's episode. Welcome to the I Can't Sleep Podcast, where I read random articles from across the web to bore you to sleep with my soothing voice. I'm your host, Benjamin Boster. Today's episode is from a Wikipedia article titled Electric Power Distribution. Electric Power Distribution is the final stage in the delivery of electrical. electricity. Electricity is carried from the transmission system to individual consumers.

Distribution substations connect to the transmission system and lower the transmission voltage to medium voltage ranging between 2 kilovolts and 33 kilovolts with the use of transformers. Primary distribution lines carry this medium voltage power to distribution transformers, located near the customer's premises. Distribution transformers again lower the voltage to the utilization voltage used by lighting, industrial equipment, and household appliances. Often several customers are supplied from one transformer through secondary distribution lines.

Commercial and residential customers are connected to the secondary distribution lines through service drops. Customers demanding a much larger amount of power may be connected directly to the primary distribution level or the sub-transmission level. The transition from the transmission to distribution happens in a power substation, which has the following functions.

Circuit breakers and switches enable the substation to be disconnected from the transmission grid or for distribution lines to be disconnected. Transformers step down transmission voltages, 35 kilovults or more, down to primary distribution voltages. These are medium voltage circuits, usually 600 to 35,000 volts.

From the transformer, power goes to the bus bar that can split the distribution power off in multiple directions. The bus distributes power to distribution. lines which fan out to customers. Urban distribution is mainly underground, sometimes in common utility ducts. Rural distribution is mostly above ground with utility poles, and suburban distribution is a mix. Closer to the customer, a distribution transformer steps the primary distribution power down

to a low voltage secondary circuit, usually 120 to use. 40 volt in the U.S. for residential customers. The power comes to the customer via a service drop and an electricity meter. The final circuit in an urban system may be less than 15 meters, but may be over 91 meters for a rural customer. Electric power distribution didn't become necessary until the 1880s, when electricity started being generated at power stations. Before that, electricity was usually used to.

generated where it was used. The first power distribution systems installed in European and U.S. cities were used to supply lighting, arc lighting running on very high voltage, around 3,000 volts, alternating current, AC, or direct current, DC, and incandescent lighting running on low voltage, 100 volts direct current. Both were supplying gas lighting systems, with arc lighting taking over a large area and street lighting, and incandescent lighting replacing gas lights for business and residential users. Due to the high voltages used in arc lighting, a single generating station could supply a long string of lights up to seven miles long, and each doubling of voltage would allow a given cable to transmit the same amount of power four times a distance than at the lower voltage

with the same power loss. By contrast, direct current indoor incandescent lighting systems, for example Edison's first power station installed in 1882, had difficulty supplying customers more than a mile away. This was due to the low voltage 110 volts it used throughout from generation to end use. This low voltage translated to higher current, requiring thick copper cables for transmission. In practice, Edison's DC generating plants needed to be within about 1.5 miles of the furthest customer

to avoid even thicker, more expensive conductors. The problem of transmitting electricity over longer distances became a recognized engineering roadblock to electric power distribution, with many less-than-satisfactory solutions tested by lighting companies. But the mid-1880s saw a breakthrough with a development of functional transformers that allowed AC power to be stepped up to a much higher voltage for transmission, then dropped down to a lower voltage near the end user. Compared to direct current, AC had much cheaper transmission costs and greater economies of scale, with large AC generating plants capable of supplying whole cities and regions,

which led to the use of AC spreading rapidly. In the U.S., the competition between direct current and alternating current took a personal turn in the late 1800s in the form of a war of currents when Thomas Edison started attacking George Westinghouse and his development of the first U.S. AC transformer systems, highlighting the deaths caused by high-voltage AC systems over the years, and claiming any AC system was inherently dangerous. Edison's propaganda campaign was short-lived, with his company switching over to AC in 1892.

AC became the dominant form of transmission of power with the innovations in Europe and in the U.S., an electric motor design, and the development of engineered universal systems, allowing the large number of legacy systems to be connected to large AC grids. In the first half of the 20th century in many places, the electric power and energy, industry was vertically integrated, meaning that one company did generation, transmission, distribution, metering, and billing. Starting in the 1970s and 80s, nations began the process of deregulation and privatization, leading to electricity markets. A distribution system would remain regulated, but generation, retail, and sometimes transmission systems were transformed into competitive markets. Electric power begins at a generating station where the potential difference can be as high as 33,000 volts.

AC is usually used. Users of large amounts of DC power, such as some railway electrification systems, telephone exchanges, and industrial processes such as aluminum smelting, use rectifiers to derive DC from the public AC supply, or may have their own generation systems. High voltage DC can be advantageous for isolating alternating alternating current systems or controlling the quantity of electricity transmitted. For example, Hydro-Cubek has a direct current line

which goes from the James Bay region to Boston. From the generating station, it goes to the generating station switchyard, where a step-up transformer increases the voltage to a level suitable for transmission, from 44 kilovolts to 765 kilovolts. Once in the transmission system, electricity from each generating station is combined with electricity produced elsewhere. Electricity is consumed as soon as it is produced. It is transmitted at a very high speed, close to the speed of light.

light. Primary distribution voltages range from 4 kilovolts to 35 kilovolts phase to phase. Only large consumers are fed directly from distribution voltages. Most utility customers are connected to a transformer, which reduces the distribution voltage to the low voltage utilization voltage, supply voltage, or mains voltage, used by lighting and interior wiring systems. Distribution networks are divided into two types, radial or network. A radial system is arranged like a tree where each customer has one source of supply. A network system has multiple sources of supply operation in parallel. Spot networks are used for concentrated loads.

loads. Radial systems are commonly used in rural or suburban areas. Radial systems usually include emergency connections where the system can be reconfigured in case of problems, such as a fault or planned maintenance. This can be done by opening and closing switches to isolate a certain section from the grid. Long feeders experience voltage drop, power factor distortion. requiring capacitors or voltage regulators to be installed. Reconfiguration by exchanging the functional links between the elements of the system represents one of the most important measures which can improve the operational performance of a distribution system. The problem of optimization through the reconfiguration of a power distribution system

in terms of its definition is a historical single-objective problem with constraints. Since 1975, when Merlin and Black introduced the idea of distribution system reconfiguration for active power loss reduction, until nowadays, a lot of researchers have proposed diverse methods and algorithms to solve the reconfiguration problem as a single objective problem. Some authors have proposed Pareto-optimality-based approaches, including active power losses, and reliability indices as objectives. For this purpose, different artificial intelligence-based methods have been used, microgenetic, branch exchange, particle swarm optimization, and non-dominated sorting genetic algorithm.

Rural electrification systems tend to use higher distribution voltages because of the longer distances covered by distribution lines. 7.2, 12.47, 25, and 34.5 kilovolte distribution is common in the United States. 11 kilovolts and 33 kilovolts are common in the UK, Australia, and New Zealand. 11 kilovolts and 22 kilovolts are common in South Africa. 10, 20, and 35 kilovolts are common in China. Other voltages are occasionally used. Rural services normally try to minimize the number of poles and wires.

It uses higher voltages than urban distribution, which in turn permits use of galvanized steel wire. The strong steel wire allows for less expensive wide pole spacing. In rural areas, a pole mount transformer may serve only one customer. In New Zealand, Australia, Saskatchewan, Canada, and South Africa, South Africa, single-wire earth-return systems, SWER, are used to electrify remote rural areas. Three-phase service provides power for large agricultural facilities, petroleum pumping facilities, water plants, or other customers that have large loads three-phase equipment. In North America, overhead distribution systems may be three-phase, four-wire, with a neutral

conductor. Rural distribution system may have long runs of one-phase conductor and a neutral. In other countries, or in extreme rural areas, the neutral wire is connected to the ground to use that as a return, single-wire earth return. Electricity is delivered at a frequency of either 50 or 60 hertz, depending on the region. It is delivered to domestic customers as single-phase electric power. In some countries, as in Europe, a three-phase supply may be made available for larger properties. Seeing with an oscilloscope, the domestic power supply in North America would look like a sine wave, oscillating between negative 170 volts and 170 volts, giving an effective voltage of 120 volts RMS. Three-phase electric power is more efficient in terms of power delivered,

per cable used, and is more suited to running large electric motors. Some large European appliances may be powered by three-phase power, such as electric stoves and clothes triers. A ground connection is normally provided for the customer's system, as well as for the equipment owned by the utility. The purpose of connecting the customer system to ground is to limit the voltage that may develop if high voltage conductors fall down. down onto lower voltage conductors, which are usually mounted lower to the ground, or if a

failure occurs within a distribution transformer. Earthing systems can be T-T-T-N-S, T-N-C-S, or TNC. Most of the world uses 50-hertz, 220, or 230-volt single phase, or 400-volt-3-phase for residential and light industrial services. In this system, the primary distribution network supplies a few substations per area, and the 230-volt-400-volt power from each substation is directly distributed to end users over a region of normally less than one kilometer radius. Three live hot wires and the neutral are connected to the building for a three-phase service.

single-phase distribution with one live wire and the neutral is used domestically where total loads are light. In Europe, electricity is normally distributed for industry and domestic use by the three-phase four-wire system. This gives a phase-to-phase voltage of 400-volt-Y service and a single-phase voltage of 230-folds between any one phase and neutral. In the UK, a typical urban or sub-urban low-voltage substation would normally be rated between 150 kilovolta-ampiers and one megavolt ampere, and supply a whole neighborhood of a few hundred houses. Transformers are typically sized on an average load of 1 to 2 kilowatt per household, and the service fuses and cable is sized to allow any one property to draw a peak load of perhaps. 10 times this. For industrial customers, three-phase 690-400-volt is also available, or maybe generated locally. Large industrial customers have their own transformers with an input from 11-kilovolts to 220-kilovolds. Most of the Americas use 60-hertz AC, the 122-40-volt split-phase system domestically, and three-phase for larger

installations. North American transformers usually power homes at 240 volts, similar to Europe's 230 volts. It is the split phase that allows use of 120 volts in the home. In the electricity sector in Japan, the standard voltage is 100 volts, with both 50 and 60 hertz AC frequencies being used. Parts of the country use 50 hertz, while other parts use 60 hertz. This is a relic from the 1890s. Some local providers in Tokyo imported 50-hertz German equipment, while the local power providers in Osaka brought in 60-hertz generators from the United States. The grids grew until eventually the entire country was wired.

Today the frequency is 50 hertz in eastern Japan, including Tokyo, Yokohama, Tohoku, and Hokkaido, and 60 hertz in western Japan, including Nagoya, Osaka, Kyoto, Hiroshima, Shikoku, and Kyushu. Most household appliances are made to work on either frequency. The problem of incompatibility came into the public eye when the 2011 Tohoku earthquake and tsunami knocked out about a third of the east's capacity, and power in the West could not be fully shared with the East, since the country does not have a common frequency. There are four high-voltage direct current HVDC converter stations that move power across Japan's AC frequency border.

Shin Shin-Sinno is a back-to-back HVDC facility in Japan, which forms one of four frequency changer stations that link Japan's western and eastern power grids. The other three are at Higashi Shimutsi, Minami Fukumitsu, and Sakuma Dam. Together they can move up to 1.2 gigawatts of power east or west. Most modern North American homes are wired to receive 240 volts from the transformer, and through the use of split-phase electrical power can have both 120-volt receptacles and 240-volt receptacles. The 120 volts is typically used for lighting and most wall outlets.

The 240-volt circuits are typically used for appliances requiring high watt heat output, such as ovens and heaters. They may also be used to supply an electric car charger. Traditionally, the distribution systems would only operate as simple distribution lines, where the electricity from the transmission networks would be shared among the customers. Today's distribution systems are heavily integrated with renewable energy generations at the distribution level of the power systems by the means of distributed generation resources such as solar energy and wind energy. As a result, distribution systems are becoming more independent from the transmission networks day by day. Balancing the supply-demand relationship at these modern distribution networks sometimes referred to as,

microgrids is extremely challenging, and it requires the use of various technological and operational means to operate. Such tools include battery storage power station, data analytics, optimization tools, etc. Electric power transmission is the bulk movement of electrical energy from a generating side such as a power plant to an electrical substation. The interconnected lines that facilitate this movement form a transmission network. This is distinct from the local wiring between high voltage substations and customers, which is typically referred to as electric power distribution. The combined transmission and distribution network is part of electricity delivery, known as the electrical grid. Efficient long-distance transmission of electric power

requires high voltages. This reduces the losses produced by strong currents. Transmission lines use either alternating current, AC, or direct current, DC. The voltage level is changed with transformers. The voltage is stepped up for transmission, and reduced for local distribution. A wide area synchronous grid known as an interconnection in North America, directly connect to the generators delivering AC power with the same relative frequency to many customers. North America has four major interconnections. Western, Eastern, Quebec, and Texas. One 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 liberalized the regulation of the electricity market and ways that led to separate companies handling transmission and distribution. Most North American transmission lines are high-voltage three-phase AC, although single-phase AC is sometimes used in railway electrification systems. DC technology is used for greater efficiency over longer distances, typically hundreds of miles. High-voltage direct current HVDC technology is also used in some of the same. submarine power cables, typically longer than 30 miles, and in the interchange of power between

grids that are not mutually synchronized. HVDC-Link stabilized power distribution networks where sudden new loads or blackouts in one part of a network might otherwise result in synchronization problems and cascading failures. Electricity is transmitted at high voltages to reduce the energy loss due to resistance that occurs over long distances. Power is usually transmitted through overhead power lines. Underground power transmission has a significantly higher installation cost and greater operational limitations, but lower maintenance costs. Underground transmission is more common in urban areas or environmentally sensitive locations.

electrical energy must typically be generated at the same rate at which it is consumed. A sophisticated control system is required to ensure the power generation closely matches demand. If demand exceeds supply, the imbalance can cause generation plants and transmission equipment to automatically disconnect or shut down to prevent damage. In the worst case, this may lead to a cascading series of shutdowns and a major region. blackout. The U.S. Northeast faced blackouts in 1965, 1977, 2003, and major blackouts in other U.S. regions in 1996 and 2011. Electric transmission networks are interconnected into regional, national, and even continent-wide networks to reduce the risk of such a failure by providing

multiple redundant alternative routes for power to flow should such shutdowns occur. transmission companies determine the maximum reliable capacity of each line, ordinarily less than its physical or thermal limit, to ensure that spare capacity is available in the event of a failure in another part of the network. High voltage overhead conductors are not covered by insulation. The conductor material is nearly always an aluminum alloy, formed of several strands and possibly reinforced with steel strands. Copper was sometimes used for overhead transmission, but aluminum is lighter, reduces yields only marginally, and costs much less. Overhead conductors are supplied by several companies. Conductor material and shapes are regularly improved to increase capacity. Conductor sizes range from 12mm squared to 750mm squared, with varying resistance and current carrying capacity.

For large conductors, more than a few centimeters in diameter, much of the current flow is concentrated near the surface due to the skin effect. The center of the conductor carries little current, but contributes weight and cost. Thus, multiple parallel cables, called bundle conductors, are used for higher capacity. bundle conductors are used at high voltages to reduce energy lost caused by corona discharge. Today, transmission level voltages are usually 110 kilovolts and above. Lower voltages, such as 66 kilovolts and 33 kilovolts, are usually considered sub-transmission voltages, but are occasionally used on long lines with light loads.

Voltages less than 33 kilovolts are usually used for distribution. Voltages above 765 kilovolts are considered extra high voltage and require different designs. Overhead transmission wires depend on air for insulation, requiring that lines maintain minimum clearances. Adverse weather conditions such as high winds and low temperature, interrupt transmission. Wind speeds as low as 23 knots can permit conductors to encroach

operating clearances, resulting in a flashover and loss of supply. Oscillatory motion of the physical line is termed conductor gallop or flutter, depending on the frequency and amplitude of oscillation. Electric power can be transmitted by underground power cables. Underground cables take up no right of way, have lower visibility, and are less affected by weather. However, cables must be insulated.

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 and insulated with dielectric fluid, usually an oil, that is either static or circulated via pumps. If an electric fault damages the pipe and leaks to electric, liquid nitrogen is used to freeze portions of the pipe to enable draining and repair. This extends the repair period and increases costs. The temperature of the pipe and surroundings are monitored throughout the repair period. Underground lines are limited by their thermal capacity, which present.

as less overload or re-rating lines. Long underground AC cables have significant capacitance, which reduces their ability to provide useful power beyond 50 miles. DC cables are not limited in length by their capacitance. Commercial electric power was initially transmitted at the same voltage used by lighting and mechanical loads. This restricted the distance between generating plant and loads. In 1882, DC voltage could not easily be increased for long-distance transmission. Different classes of loads, for example lighting, fixed motors, and traction railway systems

required different voltages, and so used different generators and circuits. Thus, generators were cited near their loads, a practice that later became known as distributed generation using large numbers of small generators. Transmission of alternating current, AC, became possible after Lucien Gullard and John Dixon Gibbs built what they called the secondary generator, an early transformer provided with one-to-one turn ratio, an open magnetic circuit in 1881. The first long distance AC line was 34 kilometers long, built for the 1884 International Exhibition of Electricity in Turin, Italy.

It was powered by a 2-kilovolt, 130-hertz Siemens, and Halski alternator, and featured several Guller transformers with primary windings connected in series, which fed incandescent lamps. The system proved the feasibility of AC electric power transmission over long distances. The first commercial AC distribution system entered service in 1885, and Via de Cherokee, Rome, Italy. for public lighting. It was powered by two Siemens and Halski alternators, rated 30 horsepower, 2 kilovolt at 120 hertz, and used 19 kilometers of cables and 200 parallel connected 2 kilovolt

to 20 volt step-down transformers, provided with a closed magnetic circuit, one for each lamp. A few months later, it was followed by the first British AC system, serving Grazvener Gallery, It also featured Siemens' alternators and 2.4 kilovolts to 100 volts step-down transformers, one per user with shunt-connected primaries. Working to improve what he considered an impractical Galard Gibbs design, electrical engineer William Stanley Jr. developed the first practical series AC Transformer in 1885. Working with the support of George Westinghouse, in 1886, he demonstrated a transformer-based AC lighting system in Great Barrington, Massachusetts.

It was powered by a steam engine-driven 500-volt seamen generator. Voltage was stepped down to 100 volts using the Stanley transformer to power incandescent lamps at 23 businesses over 4,000 feet. This practical demonstration of a transformer and alternating current lighting system led Westinghouse to begin installing AC systems later that year. In 1888, the first designs for an AC motor appeared. These were induction motors running on polyphase current, independently invented by Galileo Ferraris and Nikola Tesla.

Westinghouse licensed Tesla's design. Practical three-phase motors were designed by Mikhail Delivo Dobrovolski and Charles Eugene Lancelot Brown. Widespread use of such motors were delayed many years by development problems and the scarcity of polyphase power systems needed to power them. In the late 1880s and early 1890s, smaller electric companies merged into larger corporations, such as Gons and A.E.G. in Europe

and General Electric and Westinghouse Electric in the U.S. These companies developed AC systems, but the technical difference between direct and alternating current systems required a much longer technical merger. Alternating currents, economies of scale with large generating plants, and long-distance transmission, slowly added the ability to link all the loads. These included single-phase AC systems, polyphase AC systems, low voltage, incandescenters, low-voltage, incandes. lighting, high-voltage arc lighting, and existing DC motors in factories and streetcars. In what became a universal system, these technological differences were temporarily bridged via the rotary converters and motor generators that allowed the legacy systems to connect to the

AC grid. These stopgaps were slowly replaced as older systems were retired or upgraded. The first transmission of single-phase alternating current using high voltage came in Oregon in 1890 when power was delivered from a hydroelectric plant at Willamette Falls to the city of Portland, 14 miles downriver. The first three-phase alternating current using high voltage took place in 1891 during the International Electricity Exhibition in Frankfurt. A 15-kilovolt transmission line approximately 175 kilometers long, connected Laofan on the Necker and Frankfurt.

Transmission voltages increased throughout the 20th century. By 1914, 55 transmission systems operating at more than 70 kilovolts were in service. The highest voltage then used was 150 kilovolts, interconnecting multiple generating plants over a wide area reduced costs. The most efficient plans could be used to supply varying loads during the day. Reliability was improved and capital costs were reduced. Because standby generating capacity could be shared over many more customers and a wider area. Remote and low-cost sources of energy such as hydroelectric power or mine-moutryly.

coal could be exploited to further lower costs. The 20th century's rapid industrialization made electrical transmission lines and grids critical infrastructure. Interconnection of local generation plants and small distribution networks was spurred by World War I, when large electrical generating plants were built by governments to power munitions factories. These networks use components such as power lines, cables, circuit-bran makers, switches, and transformers. The transmission network is usually administered on a regional basis by an entity such as a regional transmission organization or transmission system operator.

Transmission efficiency is improved at higher voltage and lower current. The reduced current reduces heating losses. Jules' first law states that energy losses are proportional to the square of the current. thus reducing the current by a factor of two lowers the energy lost to conductor resistance by a factor of four for any given size of conductor. The optimum size of a conductor for a given voltage and current can be estimated by Kelvin's law for a conductor size, which states that size is optimal when the annual cost of energy wasted in resistance is equal to the annual capital charges of, providing the conductor. At times of lower interest rates and low commodity costs, Kelvin's Law indicates that thicker wires are optimal, otherwise thinner conductors are

indicated. Since power lines are designed for long-time use, Kelvin's Law is used in conjunction with long-term estimates of the price of copper and aluminum, as well as interest rates. Higher voltage is achieved in AC circuits by using a step-up transformer. High-voltage direct current HVDC systems require relatively costly conversion equipment that may be economically justified for particular projects, such as submarine cables and longer distance, high-capacity point-to-point transmission. HVDC is necessary for sending energy between unsynchronized grids. A transmission grid is a network of power stations, transmission lines, and substations.

Energy is usually transmitted with a grid with three-phase AC. Single-phase AC is used only for distribution to end users, since it is not usable for large polyphase induction motors. In the 19th century, two-phase transmission was used but required either four wires or three-wire. wires with unequal currents. Higher order phase systems require more than three wires, but deliver little or no benefit. While the price of generating capacity is high, energy demand is variable, making it often cheaper to import needed power than to generate it locally. Because loads often rise and fall together across large areas, power often comes from

distant sources. Because of the economic benefits of load sharing, wide area transmission grids may span countries and even continents. Interconnections between producers and consumers enables power to flow even if some lengths are inoperative. The slowly varying portion of demand is known as the base load and is generally served by large facilities with constant operating costs term firm power. Some facilities are nuclear, coal, or hydroelectric, which other energy sources such as concentrated solar thermal and geothermal power have the potential to provide firm power. Renewable energy sources such as solar photovoltaics, wind, wave, and tidal are due to their interminancy, not considered to be firm. The remaining or peak power demand is supplied by

peaking power plants, which are typically smaller, faster responding, and higher cost sources, such as combined cycle or combustion turbine plants typically fueled by natural gas. Long distance transmission hundreds of kilometers is cheap and efficient, with costs of $.005 to 0.02 per kilowatt hour, compared to annual average large producer costs of $1 to $025 per kilowatt hour. Retail rates upwards of 10 cents per kilowatt hour, and multiples of retail for instantaneous high-demand moments. New York often buys over 1,000 megawatt, of low-cost hydropower from Canada. Local sources, even if more expensive and infrequently used,

can protect the power supply from weather and other disasters that can disconnect distant suppliers. Hydro and wind sources cannot be moved closer to big cities, and solar costs are lowest in remote areas where local power needs are nominal. Connection costs can determine whether any particular renewable alternative is economically realistic. Costs can be prohibitive for transmission lines, but high-capacity long-distance supergrid transmission network costs

could be recovered with modest usage fees.