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Glo ssary of Power Transformer Terms (photo by radar.guy via Flickr)

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Glossary of Power Transformer Terms

There are many terms associated with power and dist ributive transf ormers used in electrical networks.

These are just some of the most important.

Feel f ree to add any of missing terms in the comment below

Glossary

ABCDEFGHIKLMNOPRSTUVW

Top

A

AA

An Ansi (American Nat ional Standard Inst itute) cooling class designation indicating open, natural-draf t

ventilated transf ormer const ruction, usually f or dry-t ype t ransf ormers.

Ambient Temperature

The temperature of the surrounding atmosphere into which the heat of the transf ormer is dissipated.
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Ampere

unit o f current f low.

ANSI (American National Standards Institute)

An organization that provides writ ten standards on transf ormer [6OOv and below (ANSI C89.1), 601~ and

above (ANSI C57.12)].

Autotransformer

A transf ormer in which part of the winding is common to both the primary and the secondary circuit s.

Back to Index |A

B

BIL

Basic Impulse Level, the crest (peak) value that the insulation is required to withstand without f ailure.

Bushing

An electrical insulator (porcelain, epoxy, etc.) that is used to control the high voltage stresses that occur

when an energized cable must pass through a grounded barrier.

Buck transformer

Step down the Voltage f rom Primary Winding to Secondary Winding i.e. 460V to 230V.

Boost transformer

Step up t he Voltage f rom Primary Winding to Secondary Winding i.e. 230V to 460V.

Back to Index | B

C

Cast-coil Transformer

A transf ormer with high-voltage coils cas t in an epoxy resin. Usually used with 5 to 15 kV transformers.

Continuous Rating

Gaines the constant load that a t ransf ormer can carry at rated primary voltage and frequency without

exceeding the specif ied temperature rise.

Copper Losses

See Load Losses.

Core-Form Construction
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A type of core construct ion where the winding materials co mpletely enclose the core.

Current Transformer

A transf ormer generally used in instrumentation circuits that measure or contro l current.

Back to Index | C

D

Delta

A standard t hree- phase connect ion with the ends of each phase winding connected in series to f orm a

closed loop with each phase 120 degrees f rom the o ther. Sometimes ref erred to as 3-wire.

Delta Wye

A term or symbol indicat ing the primary connected in deltaand the secondary in wyewhen pertaining to

a three-phase t ransf ormer or transf ormer bank.

Distribution Transformers

Those rated 5 to 120 kVon the high-voltage side and normally used in secondary dist ribution systems. An

aplicable standard isANSI C-57.12.

Dripproof

Constructed or protected so that successf ul operation is not interf ered with by f alling moisture or dirt. Atransf ormer in which the transf ormer core and coils are not immersed in liquid.

Back to Index | D

E

Excit ing Current (No-load Current)

Current that f lows in any winding used to excite the t ransf ormer when all other windings are open circuited.It is usually expressed inpercent of the rated current of a windingin which it is measured.

Back to Index | E

F

FA

AnANSI cooling class designation indicating a forced air ventilated transf ormer, usually f or dry type

transf ormers and typically to increae the transf ormers and typically to increase the transf ormers KVA

rating above the natural ventilation or AA rating.

Fan Cooled
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Cooled mechanicallyto stay within rated temperature rise by addition of f ans internally and/or externally.

Normally used on large transformers only.

FOA

An ANSI cooling class designation indicat ing forced oil coolingusing pumps to circulate the o il fo r

increased cooling capacity.

FOW

An ANSI cooling class designation indicat ing f orced o il water cooling using a separate water loop in the oil

to take the heat to a remote heat exchanger. Typically used where air cooling is difficult such as

underground.

Frequency

On AC circuits, designate number of times that polarity alternates f rom pos itive to negative and back again,

such as 60 hertz(cycles per second).

Back to Index | F

G

Grounds or Grounding

Connecting one side of a circuit t o the earth through low-resistance or low-impedance paths. This help

prevent t ransmitt ing electrical shock to personnel. Also aids in the diss ipation or mitigation of Noise (High

f requency or other).

Ground Strap

A Flat Strap o f varying dens ity, width and length to aid in the dissipation of High f requency no ise, commonly

generated by Switching Power Supplies, Lighting Ballasts, Inverters or Variable Frequency Drives.

Back to Index | G

H

High-voltage and Low-voltage Windings

Terms used to distinguish the wind that has the greater voltage rating f rom that having the lesser in two-

winding transf ormers. The terminations on t he high-vo ltage windings are identif ied by H1, H2, etc., and on

the low-voltage byX1, X2, etc.

Back to Index | H

I

Impedance
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Retarding forces o f current f low in AC circuits.

Indoor transformer

A transf ormer that , because of its construct ion, is not suitable f or outdoor service.

Insulat ing Materials

Those materials used to electrically insulate the transformer windings from each other and to ground.

Usually classif ied by degree of st rength or voltage rating (0, A, B, C, and H).

Isolation transformer

For the purpose of isolating the Source Supply f rom the Consumer(s), aids in prevention o f noise

transmission, adds impedance, can also provide an isolated Ground on t he secondary.

Back to Index | I

K

kVA or Volt-ampere Output Rating

The kVA or volt-ampere output rating designates the output that a transf ormer can deliver f or a specif ied

time at rated secondary voltage and rated f requency without exceeding the specif ied temperature rise (1

kVA = 1000 VA).

Back to Index | K

L

Liquid-immersed Transformer

A transf ormer with the core and co ils immersed in liquid (as opposed to a dry-type transformer).

Load

The amount of electricity, in kVA or volt- amperes, supplied by the transf ormer. Loads are expressed as af unction of the current f lowing in the transf ormer, and not according to the watts consumed by the

equipment the transf ormer f eeds.

Load Losses

Those losses in a transf ormer that are incident to load carrying. Load losses include the I2R loss in the

winding,core clamps, etc., and the circulating currents (if any) in parallel windings.

Back to Index | L

M

Mid-tap
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A reduced- capacity tap midday in a winding usually the secondary.

Moisture-resistant

Const ructed or treated so as t o reduce harm by exposure to a moist atmosphere.

Natural-draft or Natural-draft Ventilated

An open transf ormer cooled by the draf t created by the chimney ef f ect of the heated air in it s enclosure.

Back to Index | M

N

No-load Losses (Excitat ion Losses)

Loss in a t ransf ormer that ls excited at its rated voltage and f requency, but which is not supplying load. No-load losses include core loss, dielectric loss , and copper loss in the winding due to exciting current.

Back to Index | N

O

OA

An ANSI cooling class designation indicat ing an oil filled transformer.

Back to Index | O

P

Parallel Operation

Single and three-phase transf ormers having appropriate terminals may be operated in parallel by

connecting similarly-marked terminals, provided their rat ios, voltages, resistances, reactances, and ground

connections are designed to permit paralleled operation and provided their angular displacements are the

same in the case of three-phase transf ormers.

Polarity Test

A standard t est perf ormed on transf ormers to determine instantaneous direct ion of the voltages in the

primary compared to the secondary (see Transformer Tests).

Poly-phase

More t han one phase.

Potential (Voltage) Transformer
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A transf ormer used in instrumentation circuits t hat measure or contro l voltage.

Power Factor

The ratio o f watts to volt- amps in a circuit.

Primary Taps

Taps added in the primary winding (see Tap).

Primary Voltage Rating

Designates the input circuit voltage f or which the primary t iding is designed.

Primary Winding

The primary winding on the energy input (supply) side.

Back to Index | P

R

Rating

The output or input and any other characteristic, such asprimary and secondary voltage, current,

frequency,power factorand temperature riseassigned to the transf ormer by the manufacturer.

Ratio Test

A standard t est of transf ormers used to determine the rat io of the primary to the secondary vo ltage.

Reactance

The eff ect of inductive and capacitive components o f the circuit producing other than unity power factor.

Reactor

A device f or int roducing inductive reactanceinto a circuit f or: motor starting, operating transformers in

parallel, and controlling current.

Back to Index | R

S

Scott Connect ion

Connection f or polyphase t ransf ormers. Usually used to change f rom two-phase to three-phase to three-

phase to two-phase.

Sealed Transformer
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A transf ormer completely sealed f rom outside atmosphere and usually contains an inert gas that is slight ly

pressurized.

Secondary Taps

Taps located in the secondary winding (see Tap).

Secondary Voltage Rating

Designates the load-circuit voltage f or which the secondary winding (winding on the output side) is

designed.

Series/Multiple

A winding of two similar co ils t hat can be connected f or series operat ion or multiple (parellel) operation.

Shell-type Construction

A type of transf ormer construct ion where the core completely surro unds the coil.

Star Connection

Same as wye connections.

Step-down Transformer

A transf ormer in which the energy t ransf er is f rom the high- voltage winding to the low-voltage winding or

windings.

Step-up transformer

A transf ormer in which the energy t ransf er is f rom the low-voltage winding to a high-voltage winding or

windings.

Back to Index | S

T

T-Connection

Use of Scot t Connection f or three-phase operation. A connection brought out o f a winding at some point

between its extremities, usually to permit changing the voltage or current rat io.

Temperature Rise

The increase o ver ambient temperature o f the winding due to energizing and loading the transformer.

Total Losses

The losses represented by the sum of the no-load and the load losses.

Transformer
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An electrical device, without continuously moving parts, which, by electro-magnetic induct ion, transf orms

energy f rom one or more circuits t o other circuits at the same frequency, usually with changed values of

voltage and current.

Turns Ratio (of a transformer)

The ratio o f turns in the primary winding to the number of turns in the secondary winding.

Back to Index | T

V

Volt-amperes

Circuit volts multiplied by circuit amperes.

Voltage Ratio (of a transformer)

The ratio of the RMS primary terminal voltage to the RMS secondary terminal voltage under specif ied

conditions o f load.

Voltage Regulation (of a t ransformer)

The change in secondary voltage that occurs when the load is reduced fro m rated value to zero , with the

values of all other quant ities remaining unchanged. The regulation may be expressed in percent (or per unit)

on the basis o f the rated secondary voltage at f ull load.

Back to Index | V

W

Winding Losses

See Load Losses.

Winding Voltage Rating

Designates the voltage f or which the winding is designed

Wye Connect ion (Y)

A standard t hree- phase connect ion with similar ends of the single-phase coils co nnected to a common

point. This common point f orms the electrical neutral point and may be grounded.

Back to Index | W

Reference:Power transformer maintenance and accepting testing Department of the Army TM 5686
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