Distribution Transformer - Wikipedia, The Free Encyclopedia_1

7/30/2019 Distribution Transformer - Wikipedia, The Free Encyclopedia_1

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/11/12 Distribut ion transformer - Wikipedia, the free encyclopedia

.wikipedia.org/wiki/Dist ribution_transf ormer

Single-phase distribution transformer

in Canada

Distribution transformerFrom Wikipedia, the free encyclopedia

A distribution transformeris a transformer that provides the final

voltage transformation in the electric power distribution system, stepping

down the voltage used in the distribution lines to the level used by the

customer. If mounted on a utility pole, they are called pole-mount

transformers (or colloquially a pole pig). If the distribution lines arelocated at ground level or 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.

Contents

1 Classification2 Use

3 Connections

3.1 Primary

3.2 Secondary

4 Construction

5 References

Classification

Distribution transformers[1] 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, where wires run from a utility pole or undergroun

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 stations at voltages 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
http://en.wikipedia.org/wiki/File:Polemount-singlephase-closeup.jpghttp://en.wikipedia.org/wiki/File:Polemount-singlephase-closeup.jpghttp://en.wikipedia.org/wiki/Railwayhttp://en.wikipedia.org/wiki/Pumping_stationhttp://en.wikipedia.org/wiki/Voltagehttp://en.wikipedia.org/wiki/Service_drophttp://en.wikipedia.org/wiki/Service_drophttp://en.wikipedia.org/wiki/Distribution_transformer#Referenceshttp://en.wikipedia.org/wiki/Distribution_transformer#Constructionhttp://en.wikipedia.org/wiki/Distribution_transformer#Primaryhttp://en.wikipedia.org/wiki/Distribution_transformer#Connectionshttp://en.wikipedia.org/wiki/Distribution_transformer#Usehttp://en.wikipedia.org/wiki/Padmount_transformerhttp://en.wikipedia.org/wiki/Padmount_transformerhttp://en.wikipedia.org/wiki/File:Polemount-singlephase-closeup.jpghttp://en.wikipedia.org/wiki/File:Polemount-singlephase-closeup.jpghttp://en.wikipedia.org/wiki/File:Polemount-singlephase-closeup.jpghttp://en.wikipedia.org/wiki/File:Polemount-singlephase-closeup.jpghttp://en.wikipedia.org/wiki/Voltagehttp://en.wikipedia.org/wiki/Transformerhttp://en.wikipedia.org/wiki/Railwayhttp://en.wikipedia.org/wiki/Voltagehttp://en.wikipedia.org/wiki/Pumping_stationhttp://en.wikipedia.org/wiki/Service_drophttp://en.wikipedia.org/wiki/Distribution_transformer#cite_note-0http://en.wikipedia.org/wiki/Distribution_transformer#Referenceshttp://en.wikipedia.org/wiki/Distribution_transformer#Constructionhttp://en.wikipedia.org/wiki/Distribution_transformer#Secondaryhttp://en.wikipedia.org/wiki/Distribution_transformer#Primaryhttp://en.wikipedia.org/wiki/Distribution_transformer#Connectionshttp://en.wikipedia.org/wiki/Distribution_transformer#Usehttp://en.wikipedia.org/wiki/Distribution_transformer#Classificationhttp://en.wikipedia.org/wiki/Padmount_transformerhttp://en.wikipedia.org/wiki/Utility_polehttp://en.wikipedia.org/wiki/Electric_power_distributionhttp://en.wikipedia.org/wiki/Voltagehttp://en.wikipedia.org/wiki/Transformerhttp://en.wikipedia.org/wiki/File:Polemount-singlephase-closeup.jpg


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Phase-to-phase

transformer in Britain

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 coils, 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 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 voltage

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 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 tothe 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 Europea

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 and

grounded.

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

switches. 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
http://en.wikipedia.org/wiki/Fuse_cutouthttp://en.wikipedia.org/wiki/Switchhttp://en.wikipedia.org/wiki/Fuse_(electrical)http://en.wikipedia.org/wiki/Ground_(electrical)http://en.wikipedia.org/wiki/Bushing_(electrical)http://en.wikipedia.org/wiki/Three-phase_electric_powerhttp://en.wikipedia.org/wiki/Tesla_coilhttp://en.wikipedia.org/wiki/Spark_gap#Visual_entertainmenthttp://en.wikipedia.org/wiki/File:240v_transformer_-_geograph.org.uk_-_1062014.jpg


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Three phase distribution

transformer in Syria

while the line is energized by lineworkers using insulated hot sticks.

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 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 the 3 secondary windings are connected to a

'neutral' wire, which is grounded. The other end of the 3 secondary windings, along with the neutral, arebrought down the service drop to the service panel. 230 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 bankin either a wye or delta connection, to create a three phase transformer.

Construction

The transformer (http://powertransformers.in/high_voltage_transformers.html) 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

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 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 t

cure the resin then submerged in a large (usually gray) powder coated steel tank which is then filled with high purity

mineral oil, which is inert and non-conductive. The mineral oil helps dissipate heat and protects the transformer fro

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.

References

1. ^ Distribution transformers (http://www.pacificcresttrans.com/liquid-filled-distribution-transformers.html)

List of Distribution Tranformers (http://www.electricitysaver.com/distributionandpowertransformers.html)

Retrieved from "http://en.wikipedia.org/w/index.php?title=Distribution_transformer&oldid=513861094"

Categories: Transformers (electrical) Electric power distribution
http://en.wikipedia.org/wiki/Special:Categorieshttp://en.wikipedia.org/w/index.php?title=Distribution_transformer&oldid=513861094http://www.electricitysaver.com/distributionandpowertransformers.htmlhttp://www.pacificcresttrans.com/liquid-filled-distribution-transformers.htmlhttp://en.wikipedia.org/wiki/Distribution_transformer#cite_ref-0http://en.wikipedia.org/wiki/Mineral_oilhttp://en.wikipedia.org/wiki/Powder_coatinghttp://en.wikipedia.org/wiki/Aluminumhttp://en.wikipedia.org/wiki/Copperhttp://en.wikipedia.org/wiki/Steelhttp://powertransformers.in/high_voltage_transformers.htmlhttp://en.wikipedia.org/wiki/Three-phase_powerhttp://en.wikipedia.org/wiki/Electrical_service_panelhttp://en.wikipedia.org/wiki/Split-phase_electric_powerhttp://en.wikipedia.org/wiki/Hot_stickhttp://en.wikipedia.org/wiki/Lineworker_(occupation)http://en.wikipedia.org/wiki/Category:Electric_power_distributionhttp://en.wikipedia.org/wiki/Category:Transformers_(electrical)http://en.wikipedia.org/wiki/File:Pylon_transformer_in_Syria.jpg


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Oil-cooled three-phase distribution

transformer, similar to one in abovephoto, with housing off, showing

construction.

This page was last modified on 21 September 2012 at 12:08.

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Distribution Transformer - Wikipedia, The Free Encyclopedia_1

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