Single - Phase TRANSMISSION & DISTRIBUTION Transformer ... 2 Book 1/11. SINGLE-PHASE... · Explain the purpose of transformation. 2. Explain the basic operating principle of a transformer

T R A I N I N G

M A N U A L0 2 / 0 7 / 0 5

Single - PhaseTransformer

OperatingPrinciples

Training Development Centre

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INSULATINGTUBES

SS I N G L E - P H A S E

T R A N S F O R M E R

O P E R A T I N G

P R I N C I P L E S

OBJECTIVE:From memory, you will be able todescribe the parts, purpose andoperating principle of single-phasetransformers.

WHY?Transformers are an integral part of theelectrical system. Delivering quality voltageto customers requires the installation andmaintenance of transformers. To work safelyon this apparatus requires a knowledge oftheir components and operatingcharacteristics.

LEARNING OBJECTIVES:1. Explain the purpose of transformation.

2. Explain the basic operating principle of atransformer.

3. Describe the five main components of atransformer.

4. Describe the three main classifications oftransformers.

5. List five methods used for transformercooling.

6. Describe transformer losses.

Preface

Start Here

Intro to Hotsticks

Alternating Current

Street Lights

Single Phase Transformation

Three Phase Systems

Class 1 Rubber Gloves

DCRACF

RMSRINRIR

PACCXC

RXCSRX

PRX

SLIISL

MSLSLP

TOP

TRRPTR

TRLTWC

Standard Protection

Code

TPSYCO

DCOCTP

Copyright © 1998 by the Training and Development Centre, SaskPower. All rights reserved. 0 2 / 0 7 / 0 5

IN THIS MODULE:1. Purpose of Transformation

2. Components of a Transformer

3. Transformer Classifications

4. Transformer Cooling

5. Transformer Losses

RESOURCES:

ESTIMATED TIME:45 minutes

PREREQUISITES:• “Introduction to AC Fundamentals”

module

•

LEARNING STEPS:1. Cover the module.

2. Complete the Review Questions.

3. Clarify any questions you may have.

Copyright © 1998 by the Training and Development Centre, SaskPower. All rights reserved.0 2 / 0 7 / 0 5

Purpose of TransformationTransformation allows us to either raise or lower voltages bytransferring electrical energy from one AC circuit to another.

Operating PrincipleA transformer is a very unique piece of equipment simply becauseit has no moving parts. We now know that the generation ofelectricity requires relative motion and a magnetic field. In powerplants, this motion is supplied by generators. In a transformer, thismechanical energy or motion, is accomplished by an expandingand collapsing magnetic field. When a transformer is connected toan AC source, the magnetic field oscillates back and forth. Theexpanding and collapsing field cuts the opposite coil and induces avoltage across the ends.

For this reason, DC current cannot be used to operate a transformer.There is no relative motion in a DC circuit, therefore the magneticfield remains constant.

A transformer operates on the principle of “Mutual Induction”.That is, the ability of one coil to induce a voltage into another coil.The induced voltage will oppose the motion that produced it -Lenz’s Law.

Basic transformer

4 T R A I N I N G M A N U A L .

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Voltages are stepped up or down depending on the voltage required,whether it be for efficient transportation of electricity or to supplyvoltages to residences.

Typical distributionsystem

Transfer of energy

S I N G L E - P H A S E T R A N S F O R M E R O P E R A T I N G P R I N C I P L E S 5 .

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Components of a TransformerA transformer is composed of five main parts:

• Case

• Coils

• Core

• Insulation

• Name plate

CaseThe case is the shell or container of the transformer which sealsout the elements. It is always grounded.

CoilsThe coils (windings) of a transformer are made of square orrectangular copper wire which provides better conductivity for theheat to flow out of the windings. Some smaller transformers stillutilize round wire. The coils are covered with an insulatingcompound to insulate each turn from the adjoining one.

The primary coil is the input coil and is connected to the energysource. The secondary coil is the output coil and is connected tothe load.

CoreThe core is a device designed to channel or concentrate thealternating lines of flux. In a poorly designed core, such as a solidiron bar, some lines of flux are able to leak away. There was alsothe presence of eddy currents and hysteresis. We will look closerat this later on in the module. These problems were remedied bythe use of a laminated silicon steel core.



Examples of Cores

A shell typetransformer

A core typetransformer

A transformer withan H-type core



InsulationInsulation provides an isolation to ground for the live line whichallows the transformer to function without shorting to ground.

Bushings

Bushings are usually made of porcelain to prevent the primaryand secondary leads from contacting the grounded tank.

Primary�Bushings

Secondary�Bushings

Four bushingtransformer



Insulation Tubes

Kraft PaperKraft paper provides an insulating barrier between the core andcoil windings, as well as the primary and secondary windingsthemselves.

Enamel or VarnishThe enamel covers each individual wrap of wire in the coil toprevent it from contacting adjacent coils.

Oil or LiquidOil is important in providing protection between the coil and coreassembly as well as the case of the transformer. High di-electricstrength oil allows the manufacturing of smaller, more compacttransformers.

The insulating tubes protect the leads which extend from thecoils to the tap changers and bushings.

INSULATINGTUBES

Inside a transformer



Name Plate

The name plate of a transformer is located on the outside of thecase. It provides pertinent information which is vital to theinstallation, operation and maintenance of the transformer.

Most transformer name plates contain the following fourteen itemsof information, if available:

• Manufacturer

• Serial number

• Capacity (kVA)

• Polarity

• High and low voltage

• Low voltage connections

• High voltage tap connections

• Weight

• Gallons of oil

• Oil or liquid filled

• Frequency

• Insulated for “Y” connections

• Impedance at 75 degrees

• Temperature rise at 55 degrees

Typical transformername plate



Transformer ClassificationsTransformers are classified according to their primary andsecondary voltages, as well as their capacity (usually in kVA).

Example:

14,400V/120/240V - 25kVA

14,400V = primary voltage

120/240V = secondary voltage

25kVA = capacity

There are three basic classifications of transformer –– power,distribution and instrument.

Power TransformersPower transformers are larger transformers over 250kVA. Theymay supply a three-phase line or a single-phase line. Substationtransformers fit this classification.

Distribution TransformersDistribution transformers are used to supply loads of less than250kVA. These transformers supply residential loads and smallcommercial services.

Instrument TransformersInstrument transformers can be either potential or current. Theyare used in metering applications and are not designed to carryanything but minute secondary loads.

Current transformer(bar type)



Instrument transformers are used to step down voltage and current.Current transformers are sometimes used to isolate a meter fromhigh voltage rather than lower current values.

Current transformer(window type)



Transformer CoolingAs a transformer nears full load operating conditions, aconsiderable amount of heat is produced by the internal losses. Forthis reason, transformers must be kept cool to allow them tomaintain a high operating efficiency.

Dry transformers are cooled by the natural circulation of the airaround the windings or coils.

Distribution transformers are liquid or oil filled. Oil within atransformer allows the heat to be carried away from the coils as itcirculates, while acting as a good insulator.

Let’s now look at the five methods of transformer cooling.

FansSince radiator size is limited, fans are added to assist in circulatingair through them. Fans are generally thermostatically controlledand start up when the oil reaches a certain temperature. Fans arefound on substation transformers.

FinsFins are attached to the tank to increase the surface area and allowheat from the tank to dissipate more rapidly into the surroundingair.

TubesTubes are attached to the exterior of the tank and allow oil to passthrough them. Heated oil enters the tubes near the top of the tankand as it cools, the oil falls down the tubes to re-enter the tanknear the bottom. This circulation process continues as thetransformer functions.

RadiatorsRadiators are used to provide a maximum amount of radiatingsurface and allows a more efficient cooling system.



Natural ConvectionAs the heated oil rises to the top of the container away from thecoils, it cools off and naturally circulates back to the bottom of thecontainer. This process continues and the oil circulates naturally.



Transformer LossesAlthough transformers tend to be 90 to 99 percent efficient, smalllosses do occur during their regular operation.

Copper LossThese losses are in the form of heat given off when the currentencounters resistance as it flows through the copper coils.

Core LossesCore losses occur in the form of eddy currents and hysteresis.

Eddy CurrentsEddy currents are low voltage, short circuit currents in the corethat cause heat. This problem is usually rectified by using alaminated core.

HysteresisHysteresis is heat which is produced by the rapidly reversingmolecules in the core and caused by the alternating flux. It isminimized by using silicon steel in the core to reduce friction.

Excitation CurrentExcitation current or charging current, as it is often called, is theamount of current required to magnetize the core of a transformer.From no load to full load, the charging current remains constant.As a rule, excitation current is so small, it is often ignored in kVAload calculations.



SummaryTo summarize this module, you have learned:

• The purpose and operating principle of transformation.

• Five methods used for transformer cooling.

• Main types of transformer losses.

Now...

• Complete the Review Questions.

• Clarify any concerns or questions you may have.



REVIEW QUESTIONS

Single-Phase Transformer OperatingPrinciples

Explain the purpose of transformation.

1. The purpose of transformation is to _______________ or_______________ voltages by transferring electrical energy fromone AC circuit to another.

2. Transformers are vital in ( stepping up / stepping down ) voltagesfor efficient transportation of electricity by ( increasing / decreasing ) the current and thereby preventing anoverload or damage to conductors and equipment.

3. ( AC / DC ) cannot be used to operate a transformer because themagnetic field remains constant.

Explain the basic operating principle of atransformer.

4. A transformer operates on the principle of_________________________.

5. An induced voltage is in the ( opposite / same ) direction as theapplied voltage and the current is in the ( opposite / same )direction as the applied current.

This section is a review for your Power Line Apprenticeshipwritten exams. Complete the questions, and verify your answerswith the Review Question Solutions located at the end of thismodule.



6. What is one advantage of a “step up” transformer as it pertains tothe transmission of power?

Describe the five main components of atransformer.

7. Match the components of the transformer with a description onthe right.

8. The part of the transformer which is always grounded is the:

a) coil.

b) case.

c) none of the above.

d) all of the above.

___ Laminated strips of silicon steelused to channel the magneticlines of flux.

___ Includes such things as oil,bushings, enamel and kraft paper.

___ The container or “shell” of thetransformer.

___ Usually copper wire covered withvarnish and separated by kraftpaper.

___ States high and low voltages,capacity of the transformer,polarity, manufacturer, etc.

a) Case

b) Coils

c) Core

d) Insulation

e) Name plate



9. The name plate contains information on:

a) polarity.

b) the serial number.

c) the capacity in kVA.


10. The primary winding in a transformer is:

a) the winding with the most turns of wire.

b) the winding supplied with energy.

c) the winding from which energy is taken.

d) the winding with the least turns of wire.

11. The secondary winding in a transformer is:

a) the winding with the most turns of wire.

b) the winding supplied with energy.

c) the winding from which energy is taken.

d) the winding with the least turns of wire.

12. Transformer cores are laminated:

a) because they cost less to manufacture.

b) to prevent core heat from eddy currents.

c) to prevent core breakdowns from hysteresis.

d) because they are more light weight than a solid steel core.

13. List the five types of insulation used in a transformer.



Describe the three main classifications oftransformers.

14. Transformers are classed according to:

a) capacity (kVA).

b) high-voltage.

c) low voltage.


15. Power transformers have a kVA rating of:

a) under 100kVA.

b) 100kVA and higher.

c) under 250kVA.

d) 250kVA and higher.

16. Instrument transformers are utilized to:

a) decrease the voltage to a level which can safely be metered.

b) decrease the current to a level which can safely be metered.

c) act as a voltage protection between their source and load.


17. Distribution transformers have a kVA rating of:

a) under 250kVA.

b) 250kVA and higher.

c) under 100kVA.

d) 100kVA and higher.



List five methods used for transformercooling.

18. Match the following terms on the left with the information on theright.

19. This characteristic of oil allows minimized clearance between theoil and windings to the tank.

a) High resistance to water emulsion

b) Low viscosity

c) High di-electric strength

d) Free of impurities

20. A transformer should contain oil with a ( high / low ) viscosity topermit rapid circulation and a ( high / low ) freezing point to keepthe oil in a liquid state during cold weather.

___ Increases metal surface area.

___ Used to increase the coolingeffect of a radiator by forcingmore air through it.

___ A series of tubes and fins usedso the maximum amount ofradiating surface is available.

___ Heat rising from oil causingthe oil to circulate through thecore.

___ Oil circulates freely throughthem and cools in the process.

a) Naturalconvection

b) Fins

c) Tubes

d) Radiator

e) Fans



Describe transformer losses.

21. List the two types of transformer losses.

22. The use of a laminated core helps minimize ( eddy currents /hysteresis ) in a transformer.

23. A copper loss in a transformer is caused by:

a) eddy currents.

b) exciting current.

c) hysteresis.

d) none of the above.

24. Stray currents which bounce around in the core and cause heat arecalled:

a) eddy currents.

b) exciting currents.

c) hysteresis.

25. _______________ is a heat build up from the change in directionof current in an AC circuit transformer core.

26. Hysteresis can be minimized through the use of:

a) lightning arrestors.

b) a laminated core.

c) silicon steel in the core.




S I N G L E P H A S E T R A N S F O R M E R O P E R A T I N G P R I N C I P L E S 23 .


1. RaiseLower

2. Stepping upDecreasing

3. DC

4. Mutual induction

5. OppositeOpposite

6. For efficient transportationof electrical energy

7. c, d, a, b, e

8. b

9. d

10. b

11. c

12. b

13. BushingsInsulation tubesKraft paperEnamel or varnishOil or liquid

14. d

15. d

16. d

17. a

18. b, e, d, a, c

19. c

20. LowLow

21. Copper lossesCore losses

22. Eddy currents

23. d

24. a

25. Hysteresis

26. c

REVIEW QUESTION SOLUTIONS

Single Phase Transformer OperatingPrinciples


Copyright © 1998 by the Training and Development Centre, SaskPower. All rights reserved. 0 2 / 0 7 / 0 5

Documents

Single - Phase TRANSMISSION & DISTRIBUTION Transformer ... 2 Book 1/11. SINGLE-PHASE... · Explain the purpose of transformation. 2. Explain the basic operating principle of a transformer