Editing Transformer (section)

===Cooling===
[[Image:Drehstromtransformater im Schnitt Hochspannung.jpg|thumb|upright|right|Cutaway view of liquid-immersed transformer. The conservator (reservoir) at top provides liquid-to-atmosphere isolation as coolant level and temperature changes. The walls and fins provide required heat dissipation.]]

It is a rule of thumb that the life expectancy of electrical insulation is halved for about every 7&nbsp;°C to 10&nbsp;°C increase in [[operating temperature]] (an instance of the application of the [[Arrhenius equation]]).<ref name="Harlow2004-3">{{harvnb|Harlow|2004|loc=§3.4.8 in Section 3.4 Load and Thermal Performance by Robert F. Tillman in Chapter 3 Ancillary Topics}}</ref>

Small dry-type and liquid-immersed transformers are often self-cooled by natural convection and [[radiation]] heat dissipation. As power ratings increase, transformers are often cooled by forced-air cooling, forced-oil cooling, water-cooling, or combinations of these.<ref name="Pansini1999-32">{{harvnb|Pansini|1999|p=32}}</ref> Large transformers are filled with [[transformer oil]] that both cools and insulates the windings.<ref name="willis2004">H. Lee  Willis, ''Power Distribution Planning Reference Book'', 2004 CRC Press.  {{ISBN|978-0-8247-4875-3}}, pg. 403</ref> Transformer oil is often a highly refined [[mineral oil]] that cools the windings and insulation by circulating within the transformer tank. The mineral oil and [[electrical insulation paper|paper]] insulation system has been extensively studied and used for more than 100 years. It is estimated that 50% of power transformers will survive 50 years of use, that the average age of failure of power transformers is about 10 to 15 years, and that about 30% of power transformer failures are due to insulation and overloading failures.<ref name="Hartley (2003)">{{cite conference|last=Hartley|first=William H. (2003)|title=Analysis of Transformer Failures|url=http://www.bplglobal.net/eng/knowledge-center/download.aspx?id=191|conference=36th Annual Conference of the International Association of Engineering Insurers|access-date=30 January 2013|page=7 (fig. 6)|url-status=dead|archive-url=https://web.archive.org/web/20131020185815/http://www.bplglobal.net/eng/knowledge-center/download.aspx?id=191|archive-date=20 October 2013}}</ref><ref name="Hartley (~2011)">{{cite web|last=Hartley|first=William H. (~2011)|title=An Analysis of Transformer Failures, Part 1 – 1988 through 1997|url=http://www.hsb.com/TheLocomotive/AnAnalysisOfTransformerFailuresPart1.aspx|publisher=The Locomotive|access-date=30 January 2013|archive-date=18 June 2018|archive-url=https://web.archive.org/web/20180618175745/http://www.hsb.com/TheLocomotive/AnAnalysisOfTransformerFailuresPart1.aspx|url-status=dead}}</ref> Prolonged operation at elevated temperature degrades insulating properties of winding insulation and dielectric coolant, which not only shortens transformer life but can ultimately lead to catastrophic transformer failure.<ref name="Harlow2004-3"/> With a great body of empirical study as a guide, [[transformer oil testing]] including [[dissolved gas analysis]] provides valuable maintenance information.

Building regulations in many jurisdictions require indoor liquid-filled transformers to either use dielectric fluids that are less flammable than oil, or be installed in fire-resistant rooms.<ref name="De Keulenaer2001"/> Air-cooled dry transformers can be more economical where they eliminate the cost of a fire-resistant transformer room.

The tank of liquid-filled transformers often has radiators through which the liquid coolant circulates by natural convection or fins. Some large transformers employ electric fans for forced-air cooling, pumps for forced-liquid cooling, or have [[heat exchangers]] for water-cooling.<ref name="willis2004"/> An oil-immersed transformer may be equipped with a [[Buchholz relay]], which, depending on severity of gas accumulation due to internal arcing, is used to either trigger an alarm or de-energize the transformer.<ref name="Harlow2004-2"/> Oil-immersed transformer installations usually include fire protection measures such as walls, oil containment, and fire-suppression sprinkler systems.

[[Polychlorinated biphenyl]]s (PCBs) have properties that once favored their use as a [[coolant|dielectric coolant]], though concerns over their [[Persistent organic pollutant|environmental persistence]] led to a widespread ban on their use.<ref>{{Cite web| title = ASTDR ToxFAQs for Polychlorinated Biphenyls| year = 2001|url=https://wwwn.cdc.gov/TSP/ToxFAQs/ToxFAQsLanding.aspx?id=140&tid=26
| access-date = June 10, 2007 }}</ref>
Today, non-toxic, stable [[silicone]]-based oils, or [[fluorocarbon|fluorinated hydrocarbons]] may be used where the expense of a fire-resistant liquid offsets additional building cost for a transformer vault.<ref name="De Keulenaer2001"/><ref name="Kulkarni2004-2">{{harvnb|Kulkarni|Khaparde|2004|pp=2–3}}</ref> However, the long life span of transformers can mean that the potential for exposure can be high long after banning.<ref name=":0">{{Cite web|title=What silicone wristbands say about chemical exposure in Uruguayan children|url=http://www.buffalo.edu/news/releases/2020/07/015.html|access-date=2022-01-28|website=www.buffalo.edu|language=en}}</ref>

Some transformers are gas-insulated.  Their windings are enclosed in sealed, pressurized tanks and often cooled by [[nitrogen]] or [[sulfur hexafluoride]] gas.<ref name="Kulkarni2004-2"/>

Experimental power transformers in the 500–1,000&nbsp;kVA range have been built with [[liquid nitrogen]] or [[liquid helium|helium]] cooled [[superconductivity|superconducting]] windings, which eliminates winding losses without affecting core losses.<ref name="Mehta (1997)">{{cite journal|last=Mehta|first=S.P.|author2=Aversa, N.|author3=Walker, M.S.|title=Transforming Transformers [Superconducting windings]|journal=IEEE Spectrum|date=Jul 1997|volume=34|issue=7|pages=43–49|doi=10.1109/6.609815|url=http://www.superpower-inc.com/files/T141+IEEE+Spectrum+XFR.pdf|access-date=14 November 2012}}</ref><ref name="Pansini1999-66">{{harvnb|Pansini|1999|pp=66–67}}</ref>