Distribution transformer: Difference between revisions

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{{Short description|Final stage in power distribution to users}}

{{Use American English|date=January 2025}}

[[Fileimage:SinglePolemount-phasesinglephase-pole-transformer-d335closeup.jpg|thumb|Single-phase distribution transformer in Canada]]

A '''distribution transformer''' or '''service''' '''transformer''' is a [[transformer]] that provides a final [[voltage]] transformationreduction in the [[electric power distribution]] system, stepping down the voltage used in the distribution lines to the level used by the customer.{{sfn|Harlow|2012|p=3-4}} The invention of a practical, efficient transformer made [[AC power]] distribution feasible; a system using distribution transformers was demonstrated as early as 1882.<ref>{{Cite web |title=Lightcast |url=https://lightcast.io/open-skills/skills/KS1232366XC0QSF5RGMJ/distribution-transformer |access-date=2024-09-11 |website=Lightcast |language=en}}</ref>

The invention of a practical efficient transformer made AC power distribution feasible; a system using distribution transformers was demonstrated as early as 1882.

If mounted on a [[utility pole]], they are called '''pole-mount transformers'''. IfWhen theplaced distribution lines are locatedeither at ground level or underground, distribution transformers are mounted on concrete pads and locked in steel cases, thus known as distribution tap [[Padmount transformer|pad-mountmounted transformerstransformer]]s.

Distribution transformers normallytypically have ratings less than 200 [[Volt-ampere|kVA]],{{sfn|Bakshi|2009|p=1-24}} although some national standards can allow for units up to 5000 kVA to be described as distribution transformers. Since distribution transformers are energized for 24 hours a day (even when they don't carry any load), reducing [[Magnetic core#Core loss|iron loss]]es hasis an important rolevital in their design. As theyThey usually don't operate at full load, so they are designed to have maximum efficiency at lower loads. To have a better efficiency, [[voltage regulation]] in these transformers should beis kept to a minimum. Hence, they are designed to have small [[leakage reactance]].{{sfn|Bakshi|2009|p=1-25}}

==Types==

Distribution transformers are classified into different categories based on factors such as:<ref>https://wzy.ece.iastate.edu/Courses/EE555/05%20Chapter%202%20Distribution%20System%20Transformers.pdf {{Bare URL PDF|date=July 2025}}</ref>

{{Unreferenced section|date=January 2023}}

Distribution transformers are classified into different categories based on factors such as:

* Mounting ___location – pole, pad, underground vault

* Type of insulation – liquid-immersed or dry-type

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==Use==

[[File:CP-SLOPE-wb-gantry-power-supply-transformer-wiki.JPG|thumb|[[Rail transport|Railroad]] signal power supply transformer (CP-SLOPE [[interlocking]]) in west of [[Altoona, Pennsylvania|Altoona]] with a warning label indicating that it contains [[Polychlorinated biphenyl|PCBs]].]]

Distribution transformers are normally located at a [[service drop]], where wires run from a utility pole or underground power lines to a customer's premises. They are often used for the power supply of facilities outside settlements, such as isolated houses, farmyards or [[pumping station]]s at [[voltage]]s below 30 kV. Another application is the power supply of the overhead wire of [[railway]]s electrified with AC. In this case single phase distribution transformers are used.{{sfn|Harlow|2012|p=3-17}}

Distribution transformers are normally located at a [[service drop]], where wires run from a utility pole or underground power lines to a customer's premises. They are often used for the power supply of facilities outside settlements, such as isolated houses, farmyards, or [[pumping station]]s at [[voltage]]s below 30 kV. Another application is the power supply of the overhead wire of [[railway]]s electrified with AC. In this case, single -phase distribution transformers are used.{{sfn|Harlow|2012|p=3-17}}

The number of customers fed by a single distribution transformer varies depending on the number of customers in an area. Several homes may be fed from a single transformer in urban areas. Rural distribution may require one transformer per customer, depending on mains voltage. A large commercial or industrial complex will have multiple distribution transformers. In urban areas and neighborhoods where the primary distribution lines run underground, [[padmount transformer]]s, transformers in locked metal enclosures mounted on a concrete pad, are used. Many large buildings have electric service provided at primary distribution voltage. These buildings have customer-owned transformers in the basement for step-down purposes.{{sfn|Harlow|2012|p=3-17}}

The number of customers fed by a single distribution transformer varies depending on the number of customers in an area. Several homes may be fed from a single transformer in urban areas.; depending on the mains voltage, Ruralrural distribution may require one transformer per customer, depending on mains voltage. A large commercial or industrial complex will have multiple distribution transformers. In urban areas and neighborhoods where the primary distribution lines run underground, [[padmount transformer]]s, transformers inand locked metal enclosures are mounted on a concrete pad, are used. Many large buildings have electric service provided at primary distribution voltage. These buildings have customer-owned transformers in the basement for step-down purposes.{{sfn|Harlow|2012|p=3-17}}

Distribution transformers are also found in the power collection networks of [[wind farm]]s, where they step up power from each wind turbine to connect to a substation that may be several miles (kilometres) distant.{{sfn|Harlow|2012|p=3-10}}

Distribution transformers are also found in the power collection networks of [[wind farm]]s power collection networks, where they step up power from each wind turbine to connect to a substation that may be several miles (kilometreskilometers) distant.{{sfn|Harlow|2012|p=3-10}}

==Connections==

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[[File:Pole mounted single-phase transformer cutaway.jpg|thumb|upright|Primary line on the right toward the front and secondary lines in the back of this single-phase transformer]]

Both pole-mounted and pad-mounted transformers convert the high 'primary' voltage of the overhead or underground distribution lines' high ''primary'' voltage to the lower ''secondary'' or 'utilization' voltage inside the building. The primary distribution wires use the [[Three-phase electric power|three-phase]] system. Main distribution lines always have three '"hot'" wires plus an optional neutral. In the North American system, where single-phase transformers connect to only one phase wire, smaller 'lateral' lines branching off on side roads may include only one or two 'hot' phase wires. (When there is only one phase wire exists, a neutral will always be provided as a return path.) Primaries provide power at the standard distribution voltages used in the area; these range from as low as 2.3 kV to about 35 kV depending on local distribution practice and standards;, often 11 kV (50&nbsp;Hz systems) and 13.8 kV (60&nbsp;Hz systems) are used, but many other voltages are commonstandard. For example, in the [[United States]], the most common voltage is 12.47 kV, which haswith a line-to-ground voltage of 7.2 kV.<ref>{{cite web |title=Introduction to Distribution Systems |url=https://home.engineering.iastate.edu/~jdm/ee455/notes1_intro.doc |website=Iowa State University |access-date=29 December 2023}}</ref> ThisIt has a 7.2 kV phase-to-neutral voltage, exactly 30 times the 240 V on the [[Split-phase electric power|split-phase]] secondary side.

===Primary===

{{unreferenced section|date=January 2023}}

The high -voltage primary windings are brought out to [[Bushing (electrical)|bushings]] on the top of the case.

*Single -phase transformers, generally used in the North American system, are attached to the overhead distribution wires with two different types of connections:

**Wye &ndash;– On a wye distribution circuit, aA ''wye'' or 'phase -to -neutral' transformer is used on a wye distribution circuit. A single -phase wye transformer usually has only one bushing on top, connected to one of the three primary phases. The other end of the primary winding is connected to the transformer case, which is connected to the neutral wire of the wye system, and is also [[ground (electrical)|grounded]]. A wye distribution system is not preferred because the transformers present unbalanced loads on the line that cause currents in the neutral wire and are then groundedthe ground. ButHowever, with a delta distribution system, the unbalanced loads can cause variations in the voltages on the 3 -phase wires.

**Delta &ndash;– On a delta distribution circuit, aA ''delta'' or 'phase -to -phase' transformer is used on a delta distribution circuit. A single -phase delta transformer has two bushings connected to two of the three primary wires, so the primary winding sees the phase-to-phase voltage.; Thisthis avoids returning primary current through a neutral that must be solidly grounded to keep its voltage near earth's potential. Since the neutral is also provided to customers, this is a bigsignificant safety advantage in a dry area like California, where soil conductivity is low. The main disadvantage is higher cost, e.g., from needing at least two insulated '"hot'" phase wires even on a branch circuit. Another smallerminor disadvantage is that if only one of the primary phases is disconnected upstream, the downstream itside will remain live as the transformers try to return current through it. ThisIt could be a hazard to line workers.

*Transformers providing threeThree-phase secondary powertransformers, which are used for residential service in the European system, have three primary windings attached to all three primary phase wires. The windings are almost always connected in a ''wye'' configuration, with the ends of the three windings connected and grounded.

The transformer is always connected to the primary distribution lines through protective [[Fuse (electrical)|fuses]] and disconnect [[switch]]es. For pole-mounted transformers, this is usually a '[[Fuse cutout|fused cutout]]'. An electrical fault melts the fuse, and the device drops open to give a visual indication of trouble. It[[lineworker (occupation)|Lineworker]]s can also be manually openedopen it while the line is energized by [[lineworker (occupation)|lineworker]]s using insulated [[hot stick]]s. In some cases, completely self -protected transformers are used, which have a [[circuit breaker]] built in, so a fused cutout isn't needed.

===Secondary===

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[[Image:Utility pole transformers.jpg|thumb|upright|A "transformer bank", widely used in North America: three single-phase transformers connected to make a 3-phase transformer.]]

The low -voltage secondary windings are attached to three or four terminals on the transformer's side.

*In North American residences and small businesses, the secondary is most often the [[split-phase electric power|split-phase]] 120/240 -volt system. The 240 V secondary winding is center-tapped, and the center neutral wire is grounded, making the two end conductors "hot" with respectcompared to the center tap. These three wires run down the service drop to the building's electric meter and [[Electrical service panel|service panel]] inside the building. Connecting a load between eitherthe hot wire and the neutral gives 120 volts, which is used for lighting circuits. Connecting between both hot wires gives 240 volts, which is used for heavy loads such as air conditioners, ovens, dryers, and [[Charging station|electric vehicle charging stations]]. Two, single-phase transformers are sometimes attached to a single utility pole with their secondaries wired in parallel.{{sfn|Shoemaker|Mack|2012|p=379}} This allows greater (single phase) capacity than could be obtained from either transformer alone.

*In Europe and other countries using its system, the secondary is often the three -phase 400Y/230 system. There are three 230 V secondary windings, each receiving power from a primary winding attached to one of the primary phases. One end of each secondary winding is connected to a 'neutral' wire, which is grounded. The other end of the 3three secondary windings, along withand the neutral, are brought down the service drop to the service panel. 230 V loads are connected between any of the three -phase wires and the neutral. Because the phases are 120 degrees with respect tofrom each other, the voltage between any two phases is sqrt(3) * 230V = 400V, as compared to the 2 * 120V = 240V in the North American split phase system. While three-phase power is almost unheard of in individual North American residences, it is common in Europe for heavy loads such as kitchen stoves, air conditioners, and electric vehicle chargers.

==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, which are an economically important cause of power loss in utility grids. CoreTwo losseseffects arecause causedcore by two effects;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 dissipatedissipates power in the resistance of the steel. EfficiencyThe 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 economiceconomical to manufacture. A steel strip is wrapped around a former, pressed into shape, and then cut into two C-shaped halves, which are 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 haveadversely adverse effectsaffect 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 includesconsists 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 crossarmscross-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 designedengineered for below-grade installationapplications canmay be designed for periodic submersion, particularly in areas with high water tables or flood risks.{{sfn|Harlow|2012|ppp=3-93–9}}

Distribution transformers may include an off-load tap changer, towhich allowslightly slight adjustment ofadjusts 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}}

==Placement==

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[[File:Jelenia_Góra_-_fotopolska.eu_(218205).jpg|thumb|Distribution substations inside a small tower-like building are common in Europe. Near [[Jelenia Góra]], Poland]]In the United States, distribution transformers are often installed outdoors on wooden poles.

In Europe, it is most common to place them in buildings. These look like towers, ifIf the feeding lines are overhead, these look like towers. If all lines running to the transformer are underground lines, small buildings are used. In rural areas, sometimes distribution transformers are mounted on poles, and the pole is usually made of concrete or iron due to the weight of the transformer.

==See also==

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* {{cite book|last=Pansini|first=Anthony J.|title=Guide to Electrical Power Distribution Systems |

publisher=The Fairmont Press, Inc.|year=2005|isbn=088173506X}}

* {{cite book|last1=Shoemaker|first1=Thomas M. |last2=Mack |first2=James E.|title=The Lineman's and Cableman's Handbook |edition=12th|publisher=McGraw Hill|year=2012|isbn=9780071742580}}

{{Electric transformers}}