Editing Distribution transformer (section)

==Construction==
[[Image:Ölgekühlter Transformator ohne Gehäuse.jpg|thumb|upright|Oil-cooled three-phase distribution transformer, similar to one in above photo, with housing off, showing construction.]]Distribution transformers consist of a [[magnetic core]] made from [[lamination]]s of sheet [[silicon steel]] ([[transformer steel]]) stacked and either glued together with resin or banded together with steel straps, with the primary and secondary wire windings wrapped around them. This core construction is designed to reduce [[core loss]]es and dissipation of magnetic energy as heat in the core, an economically important cause of power loss in utility grids. Two effects cause core losses: [[hysteresis loss]] in the steel and [[eddy current]]s. Silicon steel has low [[hysteresis loss]], and the laminated construction prevents [[eddy current]]s from flowing in the core, which dissipates power in the resistance of the steel. The efficiency of typical distribution transformers is between about 98 and 99 percent.<ref name="De Keulenaer2001">{{harvnb|De Keulenaer|Chapman|Fassbinder|McDermott|2001|}}</ref><ref>{{Cite book| last1 = Kubo| first1 = T.|last2 = Sachs| first2 = H.| last3 = Nadel| first3 = S.| title = Opportunities for New Appliance and Equipment Efficiency Standards| publisher = [[American Council for an Energy-Efficient Economy]] | at = p. 39, fig. 1| year = 2001| url = http://www.aceee.org/research-report/a016| access-date = June 21, 2009}}</ref> Where large numbers of transformers are made to standard designs, a wound C-shaped core is economical to manufacture. A steel strip is wrapped around a former, pressed into shape, and then cut into two C-shaped halves re-assembled on the copper windings.{{sfn|Harlow|2012|p=3-3}}

The primary coils are wound from enamel-coated [[copper]] or aluminum wire, and the high-current, low-voltage secondaries are wound using a thick ribbon of [[aluminum]] or copper. The windings are insulated with resin-impregnated paper. The entire assembly is baked to cure the resin and then submerged in a [[Powder coating|powder-coated]] steel tank, which is then filled with [[transformer oil]] (or other insulating liquid), which is inert and non-conductive. The transformer oil cools and insulates the windings and protects them from moisture. The tank is temporarily evacuated during manufacture to remove any remaining moisture that would cause arcing and is sealed against the weather with a gasket at the top.{{citation needed|date=July 2023}}

Formerly, distribution transformers for indoor use would be filled with a [[polychlorinated biphenyl]] (PCB) liquid. Because these chemicals persist in the environment and adversely affect on animals, they have been banned. Other fire-resistant liquids such as [[silicone]]s are used where a liquid-filled transformer must be used indoors. Certain vegetable oils have been applied as transformer oil; these have the advantage of a high fire point and are completely biodegradable in the environment.{{sfn|Harlow|2012|p=3-5}}

Pole-mounted transformers often include accessories such as surge arresters or protective fuse links. A self-protected transformer consists of an internal fuse and surge arrester; other transformers have these components mounted separately outside the tank.{{sfn|Pansini|2005|p=63}} Pole-mounted transformers may have lugs allowing direct mounting to a pole or may be mounted on cross-arms bolted to the pole. Aerial transformers, larger than around 75 kVA, may be mounted on a platform supported by one or more poles.{{sfn|Pansini|2005|p=61}} A three-phase service may use three identical transformers, one per phase.

Transformers designed for below-grade installation can be designed for periodic submersion in water.{{sfn|Harlow|2012|p=3-9}}

Distribution transformers may include an off-load tap changer, which slightly adjusts the ratio between primary and secondary voltage to bring the customer's voltage within the desired range on long or heavily loaded lines.{{Citation needed|date=January 2023}}

Pad-mounted transformers have secure locked, bolted' and grounded metal enclosures to discourage unauthorized access to live internal parts. The enclosure may also include fuses, isolating switches, load-break bushings, and other accessories as described in technical standards. Pad-mounted transformers for distribution systems typically range from around 100 to 2000 kVA, although some larger units are also used.{{Citation needed|date=January 2023}}