A distribution transformer is a transformer

Transformer

A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...

 that provides the final voltage

Voltage

Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...

 transformation in the electric power distribution

Electric power distribution

File:Electricity grid simple- North America.svg|thumb|380px|right|Simplified diagram of AC electricity distribution from generation stations to consumers...

 system, stepping down the voltage used in the distribution lines to the level used by the customer. If mounted on a utility pole

Utility pole

A utility pole is a pole used to support overhead power lines and various other public utilities, such as cable, fibre optic cable, and related equipment such as transformers and street lights. It can be referred to as a telephone pole, power pole, hydro pole, telegraph pole, or telegraph post,...

, they are called pole-mount transformers (or colloquially a pole pig). If the distribution lines are located underground, distribution transformers are mounted on concrete pads and locked in steel cases, thus known as pad-mount transformers. Because of weight restrictions transformers for pole mounting are only built for primary voltages under 30 kV.

Classification

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

• Type of insulation - liquid-immersed distribution transformers or dry-type distribution transformers
• Number of Phases - single-phase distribution transformers or three-phase distribution transformers
• voltage class (for dry-type) – Low voltage distribution transformers or medium voltage distribution transformers
• Basic impulse insulation level (BIL), for medium-voltage, dry-type.

Use

Distribution transformers are normally located at a service drop

Service Drop

In an electric power distribution grid, a service drop is an overhead electrical line running from a utility pole, to a customer's building or other premises. It is the point where electric utilities provide power to their customers. The customer connection to an underground distribution system...

, 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

Pumping station

Pumping stations are facilities including pumps and equipment for pumping fluids from one place to another. They are used for a variety of infrastructure systems, such as the supply of water to canals, the drainage of low-lying land, and the removal of sewage to processing sites.A pumping station...

s at voltage

Voltage

Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...

s below 30kV. Another application is the power supply of the overhead wire of railways electrified with AC. In this case single phase distribution transformers are used.

In North American utility practice, single-phase transformers are widely used to power individual homes, while in Europe three-phase transformers are more common, which can supply several buildings. Pad-mount transformers are used in urban areas and neighborhoods where the primary distribution lines run underground. Many large buildings have electric service provided at primary distribution voltage. These buildings have customer-owned transformers in the basement for step-down purposes.

High voltage hobbyists often use these transformers in reverse (step-up) by feeding 120 or 240 volts into the secondary and drawing the resulting high voltage off the primary bushings, using it to power devices like Jacob's Ladders and Tesla coil

Tesla coil

A Tesla coil is a type of resonant transformer circuit invented by Nikola Tesla around 1891. It is used to produce high voltage, low current, high frequency alternating current electricity. Tesla coils produce higher current than the other source of high voltage discharges, electrostatic machines...

s, and many other high voltage experiments.

Connections

Both pole-mount and pad-mount transformers convert the high 'primary' voltage of the overhead or underground distribution lines to the lower 'secondary' voltage of the distribution wires inside the building. The primaries use the three-phase

Three-phase electric power

Three-phase electric power is a common method of alternating-current electric power generation, transmission, and distribution. It is a type of polyphase system and is the most common method used by grids worldwide to transfer power. It is also used to power large motors and other heavy loads...

 system. Main distribution lines always have three wires, while smaller "laterals" (close to the customer) may include one or two phases, used to serve all customers with single-phase power. If three-phase service is desired, one must have a three-phase supply. Primaries provide power at one of a wide range of standard voltages from 4 to 33 kilovolts, but the most widely used are 7,200 or 14,400 volts.

Primary

The high voltage primary windings are brought out to bushings

Bushing (electrical)

A bushing is a hollow insulating liner through which a conductor may pass. Bushings appear on switchgear, transformers, circuit breakers and other high voltage equipment.- Description :...

 on the top of the case.

• Single phase transformers, generally used in the USA system, are attached to the overhead wires with two different types of connections:
  • If a primary neutral wire is available, a 'wye' or 'phase to neutral' transformer can be used. This usually has only one bushing on top, connected to one of the primary phases. The other end of the primary winding is 'grounded' to the transformer's case, which is connected to the neutral wire of the 3 phase system, and also earth ground. This type of distribution system, called 'grounded wye', is preferred because the transformers present unbalanced loads on the line, causing currents in the neutral wire. With the 'delta' connection, this can cause variations in the voltages on the 3 phase wires.
  • If no neutral wire is available, a 'delta' or 'phase to phase' transformer must be used. This has two bushings on top which are connected to two of the three primary wires, so the voltage across the primary winding is the phase-to-phase voltage. This type is used on long distribution lines where it is uneconomical to run a fourth neutral wire.
• Transformers providing three-phase secondary power, which are used for residential service in the European system, have three secondary windings and are 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 together and grounded.

The transformer is always connected to the primary distribution lines through protective fuses

Fuse (electrical)

In electronics and electrical engineering, a fuse is a type of low resistance resistor that acts as a sacrificial device to provide overcurrent protection, of either the load or source circuit...

 and disconnect switch

Switch

In electronics, a switch is an electrical component that can break an electrical circuit, interrupting the current or diverting it from one conductor to another....

es. For pole-mounted transformers this usually takes the form of a 'fused cutout'. An electrical fault causes the fuse to melt, and the device drops open to give a visual indication of trouble. It can also be manually opened while the line is energized by lineworker

Lineworker (occupation)

A lineman or linesman , also occasionally called a lineworker or a powerline technician , is a tradesman who constructs and maintains electric power transmission and distribution facilities...

s using insulated hot stick

Hot stick

In the electric power distribution industry, a hot stick is an insulated pole, usually made of fiberglass, used by electric utility workers when engaged on live-line working on energized high-voltage electric power lines, to protect them from electric shock...

s.

Secondary

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

• In the USA and countries using its system, the secondary is most often the split-phase 240/120 volt system. The 240 V secondary winding is center-tapped and the center neutral wire is grounded, making the two end conductors "hot" with respect to the center tap. These three wires run down the service drop to the electric meter and service panel inside the building. Connecting a load between either hot wire and the neutral gives 120 volts. Connecting between both hot wires gives 240 volts.
• In Europe and countries using its system, the secondary is often the three phase 416Y/240 system. There are three 240 V secondary windings, each receiving power from a primary winding attached to one of the primary phases. One end of the 3 secondary windings are connected together to a 'neutral' wire, which is grounded. The other end of the 3 secondary windings, along with the neutral, are brought down the service drop to the service panel. 240 V loads are connected between any of the three phase wires and the neutral.

Higher secondary voltages, such as 480 volts, are sometimes required for commercial and industrial uses. Some industrial customers require three-phase power at secondary voltages. To provide this, three-phase transformers can be used. In the US, which uses mostly single phase transformers, three identical single phase transformers are often wired in a transformer bank in either a wye or delta connection, to create a three phase transformer.

Construction

The transformer

Transformer

A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...

s for these are made much the same way smaller transformers are made. Most use a "C" or "E" shaped core made from laminations of sheet steel

Steel

Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...

 stacked and either glued together with resin or banded together with steel straps. The low current, high voltage primaries are wound from enamel coated copper

Copper

Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

 wire and the high current, low voltage secondaries are wound using a thick ribbon of aluminum or copper insulated with resin-impregnated paper. The entire assembly is baked to cure the resin then submerged in a large (usually gray) powder coated

Powder coating

thumb|right|Aluminium extrusions being powder coatedPowder coating is a type of coating that is applied as a free-flowing, dry powder. The main difference between a conventional liquid paint and a powder coating is that the powder coating does not require a solvent to keep the binder and filler...

 steel tank which is then filled with high purity mineral oil

Mineral oil

A mineral oil is any of various colorless, odorless, light mixtures of alkanes in the C15 to C40 range from a non-vegetable source, particularly a distillate of petroleum....

, which is inert and non-conductive. The mineral oil helps dissipate heat and protects the transformer from moisture, which will float on the surface of the oil. The tank is temporarily depressurized to remove any remaining moisture that would cause arcing and is sealed against the weather with a gasket at the top.