Editing Electric power transmission (section)

== Bulk transmission ==
[[File:Transmissionsubstation.jpg|thumb|A [[Electrical substation|transmission substation]] decreases the voltage of incoming electricity, allowing it to connect from long-distance high-voltage transmission, to local lower voltage distribution. It also reroutes power to other transmission lines that serve local markets. This is the [[PacifiCorp]] Hale Substation, [[Orem, Utah]], US.]]

These networks use components such as power lines, cables, [[circuit breaker]]s, switches and [[transformer]]s. The transmission network is usually administered on a regional basis by an entity such as a [[regional transmission organization]] or [[transmission system operator]].<ref>{{cite web|title=Distribution Substations - Michigan Technological University|url=https://pages.mtu.edu/~avsergue/EET3390/Lectures/CHAPTER6.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://pages.mtu.edu/~avsergue/EET3390/Lectures/CHAPTER6.pdf |archive-date=2022-10-09 |url-status=live|access-date=20 April 2019}}</ref>

Transmission efficiency is improved at higher voltage and lower current. The reduced current reduces heating losses. [[Joule's 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 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-term 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 station]]s, transmission lines, and [[Electrical substation|substations]]. Energy is usually transmitted within a grid with [[Three-phase electric power|three-phase]] [[alternating current|AC]]. Single-phase AC is used only for distribution to end users since it is not usable for large polyphase [[induction motor]]s. In the 19th&nbsp;century, two-phase transmission was used but required either four wires or three wires with unequal currents. Higher order phase systems require more than three wires, but deliver little or no benefit.

[[File:European electricity grid.svg|thumb|The [[wide area synchronous grid|synchronous grids]] of Europe]]

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 synchronous grid|wide area transmission grids]] may span countries and even continents. Interconnections between producers and consumers enables power to flow even if some links 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, termed [[firm power]]. Such facilities are nuclear, coal or hydroelectric, while 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 intermittency, not considered to be firm. The remaining or ''peak'' power demand, is supplied by [[peaking power plant]]s, 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 US$0.005–0.02 per kWh, compared to annual averaged large producer costs of US$0.01–0.025 per kWh, retail rates upwards of US$0.10 per kWh, and multiples of retail for instantaneous suppliers at unpredicted high demand moments.<ref name="limits-of-very-long-distance"/> New York often buys over 1000&nbsp;MW of low-cost hydropower from Canada.<ref>{{cite web|title=NYISO Zone Maps|url=http://www.nyiso.com/public/markets_operations/market_data/maps/index.jsp|publisher=New York Independent System Operator|access-date=10 January 2014|archive-date=December 2, 2018|archive-url=https://web.archive.org/web/20181202015308/http://www.nyiso.com/public/markets_operations/market_data/maps/index.jsp|url-status=dead}}</ref> Local sources (even if more expensive and infrequently used) can protect the power supply from weather and other disasters that can disconnect distant suppliers.

[[File:Electicaltransmissionlines3800ppx.JPG|thumb|A high-power electrical transmission tower, 230&nbsp;kV, double-circuit, also double-bundled]]

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 [[super grid]] transmission network costs could be recovered with modest usage fees.

=== Grid input ===
{{Unreferenced section|date=November 2022}}
At [[power station]]s, power is produced at a relatively low voltage between about 2.3&nbsp;kV and 30&nbsp;kV, depending on the size of the unit. The voltage is then stepped up by the power station [[transformer]] to a higher voltage (115&nbsp;kV to 765&nbsp;kV AC) for transmission.

In the United States, power transmission is, variously, 230&nbsp;kV to 500&nbsp;kV, with less than 230&nbsp;kV or more than 500&nbsp;kV as exceptions.

The [[Western Interconnection]] has two primary interchange voltages: 500&nbsp;kV AC at 60&nbsp;Hz, and ±500&nbsp;kV (1,000&nbsp;kV net) DC from North to South ([[Columbia River]] to [[Southern California]]) and Northeast to Southwest (Utah to Southern California). The 287.5&nbsp;kV ([[Hoover Dam]] to [[Los Angeles]] line, via [[Victorville]]) and 345&nbsp;kV ([[Arizona Public Service]] (APS) line) are local standards, both of which were implemented before 500&nbsp;kV became practical.

=== Losses ===
Transmitting electricity at high voltage reduces the fraction of energy lost to [[Joule heating]], which varies by conductor type, the current, and the transmission distance. For example, a {{convert|100|mile|abbr=on}} span at 765&nbsp;kV carrying 1000&nbsp;MW of power can have losses of 0.5% to 1.1%. A 345&nbsp;kV line carrying the same load across the same distance has losses of 4.2%.<ref>{{cite web|website=American Electric Power|title=Transmission Facts, p.&nbsp;4| url=https://www.aep.com/about/transmission/docs/transmission-facts.pdf |archiveurl=https://web.archive.org/web/20110604181007/https://www.aep.com/about/transmission/docs/transmission-facts.pdf|archivedate=2011-06-04}}</ref> For a given amount of power, a higher voltage reduces the current and thus the [[resistive loss]]es. For example, raising the voltage by a factor of 10 reduces the current by a corresponding factor of 10 and therefore the <math>I^2 R</math> losses by a factor of 100, provided the same sized conductors are used in both cases. Even if the conductor size (cross-sectional area) is decreased ten-fold to match the lower current, the <math>I^2 R</math> losses are still reduced ten-fold using the higher voltage.

While power loss can also be reduced by increasing the wire's [[Electrical Conductance|conductance]] (by increasing its cross-sectional area), larger conductors are heavier and more expensive. And since conductance is proportional to cross-sectional area, resistive power loss is only reduced proportionally with increasing cross-sectional area, providing a much smaller benefit than the squared reduction provided by multiplying the voltage.

Long-distance transmission is typically done with overhead lines at voltages of 115 to 1,200&nbsp;kV. At higher voltages, where more than 2,000&nbsp;kV exists between conductor and ground, [[corona discharge]] losses are so large that they can offset the lower resistive losses in the line conductors. Measures to reduce corona losses include larger conductor diameter, hollow cores<ref>[http://www.cpuc.ca.gov/environment/info/aspen/deltasub/pea/16_corona_and_induced_currents.pdf California Public Utilities Commission] Corona and induced currents</ref> or conductor bundles.

Factors that affect resistance and thus loss include temperature, spiraling, and the [[skin effect]]. Resistance increases with temperature. Spiraling, which refers to the way stranded conductors spiral about the center, also contributes to increases in conductor resistance. The skin effect causes the effective resistance to increase at higher AC frequencies. Corona and resistive losses can be estimated using a mathematical model.<ref>{{cite web |title=AC Transmission Line Losses |author=Curt Harting |date=October 24, 2010 |publisher=[[Stanford University]] |url=http://large.stanford.edu/courses/2010/ph240/harting1/ |access-date=June 10, 2019}}</ref>

US transmission and distribution losses were estimated at 6.6% in 1997,<ref name="tonto.eia.doe.gov">{{cite web |url=http://tonto.eia.doe.gov/tools/faqs/faq.cfm?id=105&t=3 |archive-url=https://archive.today/20121212061118/http://tonto.eia.doe.gov/tools/faqs/faq.cfm?id=105&t=3 |url-status=dead |archive-date=12 December 2012 |title=Where can I find data on electricity transmission and distribution losses? |date=19 November 2009 |work=Frequently Asked Questions – Electricity |publisher=[[U.S. Energy Information Administration]] |access-date=29 March 2011 }}</ref> 6.5% in 2007<ref name="tonto.eia.doe.gov"/> and 5% from 2013 to 2019.<ref name="eia.gov">{{cite web |url=https://www.eia.gov/tools/faqs/faq.php?id=105&t=3|title=How much electricity is lost in electricity transmission and distribution in the United States? |date=9 January 2019 |work=Frequently Asked Questions – Electricity |publisher=[[U.S. Energy Information Administration]] |access-date=27 February 2019}}</ref> In general, losses are estimated from the discrepancy between power produced (as reported by power plants) and power sold; the difference constitutes transmission and distribution losses, assuming no utility theft occurs.

As of 1980, the longest cost-effective distance for DC transmission was {{convert|7000|km|mi|abbr=off}}. For AC it was {{convert|4000|km|mi|abbr=off}}, though US transmission lines are substantially shorter.<ref name="limits-of-very-long-distance">{{cite web |url=http://www.geni.org/globalenergy/library/technical-articles/transmission/cigre/present-limits-of-very-long-distance-transmission-systems/index.shtml |title=Present Limits of Very Long Distance Transmission Systems | first1 = L. | last1 = Paris | first2 = G. | last2 = Zini | first3 = M. | last3 = Valtorta | first4 = G. | last4 = Manzoni | first5 = A. | last5 = Invernizzi | first6 = N. | last6 = De Franco | first7 = A. | last7 = Vian |year=1984 |work=[[CIGRE]] International Conference on Large High Voltage Electric Systems, 1984 Session, 29&nbsp;August &ndash; 6&nbsp;September |publisher=Global Energy Network Institute |access-date=29 March 2011 |format=PDF}} 4.98&nbsp;MB</ref>

In any AC line, conductor [[inductance]] and [[capacitance]] can be significant. Currents that flow solely in reaction to these properties, (which together with the [[Electrical resistance|resistance]] define the [[electrical impedance|impedance]]) constitute [[reactive power]] flow, which transmits no power to the load. These reactive currents, however, cause extra heating losses. The ratio of real power transmitted to the load to apparent power (the product of a circuit's voltage and current, without reference to phase angle) is the [[power factor]]. As reactive current increases, reactive power increases and power factor decreases.

For transmission systems with low power factor, losses are higher than for systems with high power factor. Utilities add capacitor banks, reactors and other components (such as [[phase-shifter]]s; [[static VAR compensator]]s; and [[flexible AC transmission system]]s, FACTS) throughout the system help to compensate for the reactive power flow, reduce the losses in power transmission and stabilize system voltages. These measures are collectively called 'reactive support'.

=== Transposition ===
{{Unreferenced section|date=November 2022}}
Current flowing through transmission lines induces a [[magnetic field]] that surrounds the lines of each phase and affects the [[inductance]] of the surrounding conductors of other phases. The conductors' mutual inductance is partially dependent on the physical orientation of the lines with respect to each other. Three-phase lines are conventionally strung with phases separated vertically. The mutual inductance seen by a conductor of the phase in the middle of the other two phases is different from the inductance seen on the top/bottom.

Unbalanced inductance among the three conductors is problematic because it may force the middle line to carry a disproportionate amount of the total power transmitted. Similarly, an unbalanced load may occur if one line is consistently closest to the ground and operates at a lower impedance. Because of this phenomenon, conductors must be periodically transposed along the line so that each phase sees equal time in each relative position to balance out the mutual inductance seen by all three phases. To accomplish this, line position is swapped at specially designed [[transposition tower]]s at regular intervals along the line using various [[Transposition (telecommunications)|transposition schemes]].

=== Subtransmission ===
[[File:Cavite, Batangas jf0557 11.jpg|thumb|175px|A 115&nbsp;kV subtransmission line in the [[Philippines]], along with 20&nbsp;kV [[Electric power distribution|distribution]] lines and a [[street light]], all mounted on a wood [[Utility pole|subtransmission pole]]]]
[[File:Wood Pole Structure.JPG|thumb|173px|115 kV H-frame transmission tower]]{{Unreferenced section|date=November 2022}}
Subtransmission runs at relatively lower voltages. It is uneconomical to connect all [[electrical substation|distribution substation]]s to the high main transmission voltage, because that equipment is larger and more expensive. Typically, only larger substations connect with this high voltage. Voltage is stepped down before the current is sent to smaller substations. Subtransmission circuits are usually arranged in loops so that a single line failure does not stop service to many customers for more than a short time.

Loops can be ''normally closed'', where loss of one circuit should result in no interruption, or ''normally open'' where substations can switch to a backup supply. While subtransmission circuits are usually carried on [[Overhead power line|overhead lines]], in urban areas buried cable may be used. The lower-voltage subtransmission lines use less right-of-way and simpler structures; undergrounding is less difficult.

No fixed cutoff separates subtransmission and transmission, or subtransmission and [[Electric power distribution|distribution]]. Their voltage ranges overlap. Voltages of 69&nbsp;kV, 115&nbsp;kV, and 138&nbsp;kV are often used for subtransmission in North America. As power systems evolved, voltages formerly used for transmission were used for subtransmission, and subtransmission voltages became distribution voltages. Like transmission, subtransmission moves relatively large amounts of power, and like distribution, subtransmission covers an area instead of just point-to-point.<ref>Donald G. Fink and H. Wayne Beaty. (2007), ''Standard Handbook for Electrical Engineers'' (15th&nbsp;ed.). McGraw-Hill. {{ISBN|978-0-07-144146-9}} section&nbsp;18.5</ref>

=== Transmission grid exit ===
[[electrical substation|Substation]] transformers reduce the voltage to a lower level for [[Electric power distribution|distribution]] to customers. This distribution is accomplished with a combination of sub-transmission (33 to 138&nbsp;kV) and distribution (3.3 to 25&nbsp;kV). Finally, at the point of use, the energy is transformed to end-user voltage (100 to 4160 volts).