Distribution transformer: Difference between revisions

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

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

[[image:Polemount-singlephase-closeup.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'''. SupposeWhen theplaced distribution lines are locatedeither at ground level or underground. In that case, 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 typically have ratings less than 200 [[Volt-ampere|kVA]],{{sfn|Bakshi|2009|p=1-24}} although some national standards allow units up to 5000 kVA to be described as distribution transformers. Since distribution transformers are energized 24 hours a day (even when they don't carry any load), reducing [[Magnetic core#Core loss|iron loss]]es is vital in their design. They usually don't operate at full load, so they are designed to have maximum efficiency at lower loads. To have 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}}

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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 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 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 standard. For example, in the [[United States]], the most common voltage is 12.47 kV, with 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> It 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===

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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 – A ''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. However, with a delta distribution system, the unbalanced loads can cause variations in the voltages on the 3-phase wires.

**Delta – A ''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; this 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 significant safety advantage in a dry area like California, where soil conductivity is low. The main disadvantage is higher cost, e.g., needing at least two insulated '"hot'" phase wires even on a branch circuit. Another minor disadvantage is that if only one of the primary phases is disconnected upstream, itthe downstream side will remain live as the transformers try to return current. It could be a hazard to line workers.

*Transformers providing threeThree-phase secondary powertransformers, 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 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. [[lineworker (occupation)|Lineworker]]s can also manually open it while the line is energized 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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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 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" concerningcompared to the center tap. These three wires run down the service drop to the building's electric meter and [[Electrical service panel|service panel]]. Connecting a load between the hot wire and the neutral gives 120 volts, which is used for lighting circuits. Connecting both hot wires gives 240 volts 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 three secondary windings and 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 from each other, the voltage between any two phases is sqrt(3) * 230V = 400V, 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==

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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 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, 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}}

==Placement==

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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}}