Editing Electric power transmission (section)

=== 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'.