Editing Distribution transformer (section)

==Connections==
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[[Image:240v transformer - geograph.org.uk - 1062014.jpg|thumb|upright|Phase-to-phase transformer in Britain]]
[[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 grounded. 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, it will remain live as the transformers try to return current. It could be a hazard to line workers.
*Transformers providing three-phase secondary power, 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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[[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 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" concerning 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]].
*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.