EE028-Electrical Machines 2-Th-Inst.pdf

7/25/2019 EE028-Electrical Machines 2-Th-Inst.pdf

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SRI LANKA INSTITUTE of ADVANCED TECHNOLOGICAL EDUCATION

Training Unit

ELECTRICAL MACHINES 2Transformer

Theory

No: EE 028

INDUSTRIETECHNIKINDUSTRIETECHNIK

ELECTRICAL and ELECTRONIC

ENGINEERING

Instructor Manual


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Training Unit

Electr ical Machines 2 - Transformer

Theoretical Part

No.: EE 028

Edition: 2008Al l Rights Reserved

Editor: MCE Industrietechnik Linz GmbH & CoEducation and Training Systems, DM-1Lunzerst rasse 64 P.O.Box 36, A 4031 Linz / Aus triaTel. (+ 43 / 732) 6987 3475Fax (+ 43 / 732) 6980 4271Website: www.mcelinz.com


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ELECTRICAL MACHINES 2 - TRANSFORMER

CONTENTS Page

LEARNING OBJECTIVES...................................................................................................5

1 1. GENERAL ................................................................................................................6

1.1 A definition of terms .............................................................................................6

1.2

The principle of the transformer ...........................................................................6

2

THE CONSTRUCTION OF TRANSFORMERS ...........................................................7

2.1 The iron core........................................................................................................7

2.2 The coil former .....................................................................................................8

2.3

The windings........................................................................................................8

2.3.1

The cylindrical winding .....................................................................................9

2.3.2 The disc winding ............................................................................................10

3

THE METHOD OF OPERATION................................................................................11

3.1 Static induction (transformer principle) ..............................................................11

3.2

The basic transformer equation .........................................................................12

4 4. THE OPERATING CHARACTERISTICS ...............................................................13

4.1

No-load ..............................................................................................................13

4.1.1 No-Ioad phasor diagram ................................................................................15

4.2 The transformer on load.....................................................................................16

4.2.1

Phasor diagram - on load - resistive and inductive load ................................17

5 IMPEDANCE VOLTAGE ............................................................................................19

5.1

Impedance voltage - general .............................................................................19

5.2 Impedance voltage phasor diagram...................................................................21

6

SHORT-CIRCUIT CURRENT.....................................................................................22

7 STARTING CURRENT...............................................................................................23

8 THE EFFICIENCY OF THE TRANSFORMER...........................................................24

8.1 Power-flow diagram ...........................................................................................24

9

TYPES OF LOAD....................................................................................................... 25

10

SMALL TRANSFORMERS.....................................................................................26

10.1 Construction and operating characteristics........................................................26

10.1.1

The iron core ..............................................................................................26

10.1.2 The winding................................................................................................27

10.1.3 Short-circuit characteristics ........................................................................27

10.1.4 Voltage specification ..................................................................................27


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10.2

Safety transformers............................................................................................28

10.2.1 Transformers for toys .................................................................................28

10.2.2

Bell transformers ........................................................................................28

10.2.3 Transformers for defrosting........................................................................29

10.2.4 Isolating transformers (with separated windings) ......................................29

10.2.5 Mains transformers ....................................................................................30

10.3 Measurement transformers................................................................................31

10.4 Autotransformers ...............................................................................................31

10.5

Welding transformers.........................................................................................33

11

LARGE TRANSFORMERS.....................................................................................34

11.1 Power transmission............................................................................................34

11.2 High-voltage transmission system .....................................................................35

11.3

Types of large transformers ...............................................................................37

11.4

Protective devices for large transformers...........................................................38

11.4.1 Monitoring the oil temperature ...................................................................38

11.4.2

The transformer cooling system.................................................................39

11.4.3 Buchholz relays (gas pressure relays) .......................................................39

11.4.4

The circulating current protective system...................................................42

11.4.5 Excess current protection...........................................................................42

11.4.6

Excess voltage diverters ............................................................................43

11.4.7 The Peterson coil .......................................................................................43

11.4.8 Fire protection ............................................................................................43

12

SINGLE-PHASE TRANSFORMERS ......................................................................44

12.1 Shell-type transformers......................................................................................44

12.2

Limb-type transformers ......................................................................................45

13 THREE-PHASE TRANSFORMERS .......................................................................46

13.1

Winding connections (circuits) ........................................................................... 46

13.2 Switching groups (switching combinations) .......................................................47

13.2.1 The most common switching groups..........................................................48

13.3 Parallel operation of three-phase transformers..................................................49

13.4

Three-phase transformers as rectifier transformers...........................................49

13.5

The variable transformer....................................................................................50

14 AIR-COOLED TRANSFORMERS .......................................................................... 52

15

OIL-FILLED TRANSFORMERS..............................................................................53

15.1 Transformer oil...................................................................................................55

15.1.1 Di-electric strength .....................................................................................55

15.1.2 Maintaining the oil quality...........................................................................56


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15.2

The expansion vessel (conservator) ..................................................................56

15.3 Air dryer .............................................................................................................57

16

CLOPHENE-FILLED TRANSFORMERS................................................................58


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TRANSFORMERS

LEARNING OBJECTIVES

The trainee should . . .

. . . state the purpose of transformers.

. . . describe the construction of single-phase transformers.

. . . describe the no-Ioad operation of a single-phase transformer.

. . . classify transformers according to their switching, core

construction winding system and type of cooling.

. . . state the fundamental equation for a transformer.

. . . sketch a power flow diagram for a transformer.

. . . state the requirements for parallel operation of single-phasetransformers.

. . . describe the construction of a three-phase transformer.

. . . name the different circuit groups for a three-phase transformer.


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T R A N S F O R M E R S

1 1. GENERAL

1.1 A definition of terms

Transformers are used to change alternating voltages of a certain voltage and frequency

to higher or lower voltages of the same frequency.

1.2 The principle of the transformer

Transformers are classified according to their application.

Example:

- power transformers

- voltage transformers

- current transformers

- audio-frequency transformers etc.


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2 THE CONSTRUCTION OF TRANSFORMERS

2.1 The iron core

This consists of laminations, which are insulated from each other by layers of paper, or

oxide coating of the laminations.

This construction is necessary in order to reduce eddy currents, which occur due to the

continual magnetic reversals in the iron core.

Layered core laminations

single-phase

transformer

3-phase transformer stepped core

cross section

The material of the iron core should be a relatively poor conductor of electricity, in order to

reduce the eddy currents. Its magnetic properties must be good, however. This is

achieved by alloying the iron with silicon (depending an the properties required - about 2

to 5 % silicon).

Cold rolled sheets are used to improve the magnetic properties.

One must take care that the direction of the magnetic flux in each part of the iron core is

the same as the direction of rolling, as the cold rolled sheet can be easily magnetized inthe direction of rolling.


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2.2 The coil former

This consists of press pan, synthetic resin bonded paper and plastics etc.

2.3 The windings

These consist of copper wires insulated with varnish or silk (For large transformers the

windings may consist of shaped copper bars).

The primary and secondary windings are named according to the direction of energy flow.

Windings are classified as high voltage or low voltage windings according to the voltage in

the winding.


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Either the primary or the secondary winding can be the high-voltage or Iow-voltage

winding.

Windings are classified according to their shape as cylindrical windings or disc windings.

2.3.1 The cylindrical winding

Application:

The cylindrical winding is used mainly for mains transformers up to a voltage of 400 V.


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2.3.2 The disc winding

Application:

The disc winding is used mainly for high-voltage windings. The individual coils are

connected in series.


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3 THE METHOD OF OPERATION

If coil N1 is connected to an alternating voltage U1 , then an alternating magnetic field is

produced. This is Iinked to the windings of coil N2 , and a voltage is thus induced in this

coil.

3.1 Static induction (transformer principle)

The magnetic field constantly changes in size and direction - it is an alternating field.

Both coils are wound on the same iron core. The magnetic flux is contained almost

entirely inside the iron circuit.

The level of the induced voltage depends on:

- the number of turns N1, N2

- the level of the magnetic flux

- the frequency f


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3.2 The basic transformer equation

Uo = no-load rms voltage in V

Bmax = maximum flux density in Tesla

A = iron cross section in m

f = frequency in Hertz (Hz)

N = number of turns

4.44 = constant

= magnetic flux in Webers

This equation applies to the primary and the secondary winding. The effective cross

section of the iron is less than the measured cross section of the core due to the insulating

Iayer between the laminations. This space factor is usually 0.9.

Uo= 4.44 xBmaxxA x f xN


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4 4. THE OPERATING CHARACTERISTICS

4.1 No-load

If the circuit on the secondary winding of a transformer is open and the primary winding is

connected to an alternating voltage U1, then the current flowing is the no-Ioad current Io.

The current flowing is small, as the transformer behaves like an inductance on no-Ioad.

The no-load current Ioconsists of the phasor sum of the magnetizing current Imand the

iron loss component Icadded geometrically.

The magnetizing current Improduces the magnetic flux .

lt is a purely reactive current and is displaced by 90 in phase relative to the voltage.


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The fron loss current Icis in phase with voltage U1and is caused by the hysteresis losses

Ph, the eddy current loss Pe and the negligible 1R loss in the primary winding. lt is a

wattful current.

The magnetic flux maxproduced by the magnetizing current induces a voltage in both the

primary and the secondary windings. The voltage induced in the primary winding has a

180 phase displacement from the input voltage.

U0 = opposing rms voltage in V

f = frequency in Hz

max= maximum magnetic flux in Webers

N = number of turns

= 4.44 = constant


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4.1.1 No-Ioad phasor diagram

The opposing voltage U10is almost equal to the applied input voltage U1.

Voltage U02 is produced in the secondary winding, which is also likewise Iinked by the

magnetic flux .

U1= U01= 4.44 xmaxxf xN1

U02= 4.44 xmaxxf xN2


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The amount of this voltage depends on the number of turns in the secondary winding.

If one divides the two equations one obtains the turns ratio n.

The voltages are in the same ratio as the number of turns.

4.2 The transformer on load

If a consumer is connected to the secondary winding if the transformer, i.e., the

transformer is loaded, then a current I2flows.

If one multiplies this current by the number of turns on the secondary winding, then the

ampere-turns F2 are obtained. This produces the magnetic flux 2 , which opposes the

primary magnetic flux 1 (Lenz's Law). The sum of the fluxes 1and 2 gives the total

magnetic flux in the core .

Since the direction of energy flow is from the primary to the secondary, the primary flux 1

must be greater than the secondary flux2, by the amount of the no-load magnetic flux.


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4.2.1 Phasor diagram - on load - resistive and inductive load

UXL = Volt drop in leakage reactance

UR = Volt drop in winding resistance

UK = Total volt drop between input voltage and induced voltage in primary on load


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In addition to the no-Ioad current I0 , the primary winding of the transformer must take a

current, the magnetizing effect of which cancels the secondary flux 2.

If one disregards the no-Ioad current, the flux 1must be equal to the flux 2.

i.e. the primary and secondary currents are inversely proportional to the number of turns

in their windings.


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5 IMPEDANCE VOLTAGE

The impedance voltage of a transformer is the primary voltage required to circulate full

load current in the secondary circuit on short circuit.

5.1 Impedance voltage - general

The winding resistances of a transformer are negligible and will have little effect on the

impedance voltage. The leakage flux, and hence the leakage will have a greater effect

and it can be used to determine the behaviour of a transformer on short circuit.

If a transformer is to have a low impedance voltage, the windings are arranged so thateven the flux leaving the iron core passes through both windings (cylindrical windings). In

transformers with high impedance voltages, the windings are arranged so that leakage

flux only passes through one winding. To obtain very high impedance voltages, an

additional yoke may be fitted between the coils. Transformers are built for low or high

impedance voltages according to their use. The output voltage of a transformer with a low

impedance voltage drop on load is lower than that for a transformer with a high

impedance voltage.


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Low impedance voltage:

Small distribution transformers and voltage transformers.

High impedance voltage:

Bell transformers and welding transformers.

If a short circuit occurs on the secondary side of a transformer, then the transformer

supplies short-circuit current.

The short-circuit current on transformers with low impedance voltage is high because of

the low voltage drop, whereas for transformers with high impedance voltage it is low.

A short-circuit on a transformer with a low impedance voltage is, therefore, more

dangerous than on one with high impedance voltage.


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5.2 Impedance voltage phasor diagram

IK = short-circuit current

UXL = leakage reactance

voltage drop

UR = resistive voltage drop

UK = impedance voltage drop

The leakage reactance voltage drop can be affected by:

- suitable arrangement of the coils

- arrangement of iron core

For convenience in calculations, the impedance voltage UK is normally given as a

percentage of the no-Ioad voltage of the transformer.


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6 SHORT-CIRCUIT CURRENT

If there is a short-circuit an the secondary side of a transformer, then the transformer

supplies short-circuit current.

The current which Iast for several cycles is called the continuous short-circuit current ISC.

Short-circuit currents can lead to the destruction of electrical equipment (switches and

busbars).

Example:


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7 STARTING CURRENT

When the transformer is switched on, a very high current will flow, even when the

transformer is not on load.

The starting surge current can be 10 times the rated current. The magnitude of the surge

will depend on the instantaneous values of the supply voltage when switching on.

Transformers must, therefore, be fused for double the rated current on the primary side.


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8 THE EFFICIENCY OF THE TRANSFORMER

Electrical energy is transformed when a transformer is used and losses occur as with all

transformations of energy.

The losses in transformers have various causes.

The iron core is heated due to the continual magnetic reversals (iron losses).

When a transformer is on-load the windings are heated by the currents flowing through

them (copper losses).

8.1 Power-flow diagram


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9 TYPES OF LOAD

The output voltage depends on the type of load. If the load is resistive, the voltage drop is

relatively small.

The voltage drop with an inductive load is greater than that with a resistive load due to the

inductive volt drop. If the load is capacitive, the output voltage rises slightly. Large

capacitors, therefore, should not be connected to the mains supply on their own.

The dependence of output voltage on the type of load


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10 SMALL TRANSFORMERS

These are transformers with ratings up to 16 kVA for use on mains voltages up to 1000 V

and frequencies up to 500 Hz.

Application:

Small transformers are used to provide safe low voltages in toys, bells, as isolating and as

mains transformers.

10.1 Construction and operating characteristics

10.1.1 The iron core

This usually consists of layers of laminations or of a core made of cut steel tape.

M section E and I sections U and I sections

laminations

core made of cut steel tape


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10.1.2 The winding

This is usually wound from varnished copper wire. The permitted current density is

normally 2.5 A/mm.

Varnished paper is usually used for insulation between the Layers of the winding.

10.1.3 Short-circuit characteristics

In the manufacturers specifications, these will be data on how resistant a transformer is to

short circuits.

Short-circuit proof transformers have a high impedance voltage.

There are:

- fully short-circuit proof transformers

- semi short-circuit proof transformers

- transformers which are not short-circuit proof

10.1.4 Voltage specification

The no-load voltage is up to 10 % higher than the voltage on-load.


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10.2 Safety transformers

These are used if "safety extra low voltage" (SELV) is specified as a protective measure

(42 V maximum). As safety transformers they are subject to strict regulations concerning

their construction.

10.2.1 Transformers for toys

The rated output voltage may not exceed 24 \/. The no-load voltage must be less than33 V. The maximum output is 200 VA. Transformers for toys must be double wound and

double insulated.

10.2.2 Bell transformers

The maximum no-Ioad voltage is 33 V. Bell transformers must be fully short-circuit proof.


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10.2.3 Transformers for defrosting

The nominal voltages are 250/24 V. These transformers must be absolutely short-circuit

proof, double wound and double insulated.

10.2.4 Isolating transformers (with separated windings)

These transformers must correspond to the "double insulated" standard. They have

electrically separated windings. The turn ratio is usually 1:1.


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10.2.5 Mains transformers

These are used mainly in radios, television sets, recorders and battery charges etc.

They produce a low voltage, for use in electronic equipment.

Other small transformers

Head lamp transformers, ignition transformers, control transformers and transformers for

medical equipment etc.

Autotransformers and chokes are also included among small transformers.


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10.3 Measurement transformers

These are transformers which are used in meter circuits and for relay circuits.

Measurement (instrument) transformers reduce high currents and high voltages to values

suitable for measuring equipment.

They also provide separation from high voltage circuits.

10.4 Autotransformers

The high and low-voltage windings are electrically connected.


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Step-down autotransformer 250/200 V

Low-voltage winding = parallel winding = 200 V

High-voltage winding = series and parallel winding = 250 V.

Step-up autotransformer 250/300 V

Low-voltage winding = parallel winding = 250 V

High-voltage winding = parallel and series winding = 300 V.


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Small autotransformers are constructed as regulating transformers with a ring core.

Autotransformers can be compared with voltage dividers, but resistive voltage dividers

can only reduce or divide the applied voltage. Autotransformers, in a suitable circuit, can

be used as step-up transformers.

10.5 Welding transformers

The no-load voltage should not exceed 70 V.

The welding current can be set by adjusting the stray yoke (changing the leakage flux).


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11 LARGE TRANSFORMERS

These are used primarily for the electrical power supply in a 3-phase AC system.

Transformer banks are used for outputs above 1,000 MVA. These consists of 3 single-

phase transformers which are switched together. The windings of large transformers must

withstand very high voltages (high-voltage windings) and very high currents (Iow-voltage

windings). At the power station, electrical power is stepped up to high voltage by

transformers, transmitted over transmission lines and then stepped down to the load

voltage in the area supplied with power.

11.1 Power transmission

The economic importance of transformers lies in the fact that if electrical power is stepped

up to a very high voltage, it can be transmitted over long distances.


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lt is very uneconomical to transmit electrical power over large distances at low voltage.

Large conductor cross sections would be necessary for this. Therefore, power has to be

transmitted at high voltages over large distances (e.g. 110, 220 or 380 kV).

By using a transformer any level of alternating voltage can be achieved. The frequency,

however, remains unchanged.

Voltages of 220, 380 and 500 V are used primarily in houses and in industry.

Higher voltages would be unsuitable due to the heavy conductor insulation and equipment

insulation which would be required.

11.2 High-voltage transmission system

A high-voltage transmission system for supplying power all over the country would bepractically impossible without transformers.


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11.3 Types of large transformers

Three-phase power transformers are always used for large transformers. Three separate

single-phase transformers may be connected together to make a three-phase transformer.

Transformer banks

These are frequently used for large transformers and are easier to transport than three-

phase transformers.

Autotransformers are used for high voltages


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11.4 Protective devices for large transformers

Large oil-filled transformers are such expensive pieces of equipment, that constant

monitoring is necessary, in order to detect any damage at an early stage.

Monitoring or protective devices include:

- Oil temperature monitoring devices

- Circulating current protective systems

- Excess current protective relays

- Transformer cooling system

- Buchholz relays (gas pressure relays)

- Excess voltage protective relays

- Fire protection

11.4.1 Monitoring the oil temperature

Contact, bimetal or resistance thermometers are used, with remote indication in the power

station control room.


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Bridge circuit for remote temperature indication

11.4.2 The transformer cooling system

The heat generated in a transformer varies according to the load. Transformer oil

temperature is controlled by temperature-sensitive elements which start the pumps and

fans of forced-circulation cooling equipment.

The life of a transformer is assumed to be about 30 years. Constantly exceeding thepermitted working temperature will shorten this life. A working temperature of 80C is

reached with 90 % load (ambient temperature 25C) 105C must not be exceeded.

11.4.3 Buchholz relays (gas pressure relays)

Buchholz relays enable the processes occurring inside a transformer to be monitored.


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The Buchholz relay is installed between the transformer and the expansion vessel

(conservator). The relay operates if gas is formed in the oil. If there is a voltage

breakdown (e.g. if there is a short circuit in the windings), the oil is decomposed and gas

is formed. This gas rises and collects in the upper part of the gas collector. The resulting

pressure lowers the oil level.

A mercury contact is fixed to the float connected to visual and audible alarms. These will

operate if the oil level is too low.


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If a heavy short circuit occurs inside the transformer, then there will be a gas pressure

wave near the fault. This wave moves an oil surge towards the expansion vessel

(conservator). The impact flap is actuated and closes the mercury contact, which switches

off the transformer.

The Buchholz relay operates in the following conditions:

- Gas formation due to local overheating.

- Short circuit between two windings.

- Overheating of laminations.

- Voltage breakdowns.

- Open-circuited windings.

- Air entering the tank.

- Oil losses.

A small amount of gas is formed in transformer oil during normal operation. Therefore, the

gas bubble in the gas collector must be released at regular intervals. An analysis of the

composition of this gas gives some indication of general conditions and the possible

causes of faults.


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11.4.4 The circulating current protective system

1 = transformer

HV = high-voltage winding

LV = low-voltage winding

2 = limiting system

3 = tripping system

4 = measuring transformers (CT's)

This is an important addition to the Buchholz relay. By comparing the currents on the

primary side with the currents on the secondary side via current transformers, one can

discover whether there is an undue difference between them.

11.4.5 Excess current protection

Excess current protection relays protect the transformer against overload by operating the

circuit breakers.


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11.4.6 Excess voltage diverters

These are arranged near the input terminals of the transformer.

11.4.7 The Peterson coil

This is connected between neutral and earth is designed to neutralise voltage surges

occurring when the circuit breakers operate after an earth fault in the transformer or the

supply system.

11.4.8 Fire protection

This is effected by erecting fire walls and oil drainage pits.

Fire protection shut off valves, which are released by fire protection cords, must be

mounted indoors.


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12 SINGLE-PHASE TRANSFORMERS

These are operated with single-phase AC.

There are two types of construction:

- Shell-type transformers

- Limb-type transformers

12.1 Shell-type transformers

The iron core surrounds the coil like a shell.

If the two windings are situated one above the other, there is a low impedance voltage UK.There is little stray flux.

The arrangement of the windings as indicated above is used for mains and low-voltage

transformers etc.

The arrangement of the windings shown above has a high impedance voltage. The stray

flux is high.

This arrangement is used for toy and bell transformers.


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12.2 Limb-type transformers

In this type of transformer the coils surround the Iimbs of the iron core.

The separate arrangement of the windings gives a high leakage flux and a high

impedance voltage.


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13 THREE-PHASE TRANSFORMERS

One phase of the primary and secondary windings is accommodated on each limb of the

iron core.

13.1 Winding connections (circuits)

Three-phase transformers can be connected in star or in delta. The high-voltage and low-

voltage sides need not be connected in the same way. The high-voltage windings are

labelled 1U , 1V and 1Wand the low-voltage and neutral windings 2U ,2V , 2Wand 2N

respectively.


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Zigzag connections

Advantage:

Like the delta connection the circuit will take an unbalanced load and like the star

connection it produces two voltages on the secondary side.

Disadvantage:

lt requires 15 % more material.

13.2 Switching groups (switching combinations)

The switching group code letters indicate the connections to the primary and the

secondary windings. In addition, a code number gives the phase displacement betweenthe primary and the secondary windings e.g. Dy5.

High-voltage side = delta

Low-voltage side = star

5 x 30 = 150 phase displacement between high voltage and low voltage.


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13.2.1 The most common switching groups


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13.3 Parallel operation of three-phase transformers

The conditions for parallel operation:

- The primary and the secondary must have the same rated voltages.

- The identical terminals must be connected to each other.

- The transformers must belong to the same switching group.

- The transformers must have approximately the same impedance voltage.

- The ratios of rated power should be within 3 : 1.

Check the same terminals (phases) of transformer T-2 with a voltmeter.

13.4 Three-phase transformers as rectifier transformers

In rectification circuits, the more phases there are in the AC supply, the lower will be the

ripple in the DC output.

Transformers are built to serve this purpose. The secondary windings consist of several

switching groups (phase transformers).

E.g. Dy5 and Dy11 give a double star connection.


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Example:

Six-phase current for three-phase rectification by a matched transformer circuit.

13.5 The variable transformer

The construction is similar to that of a three-phase slip-ring motor, the rotor of which is

stalled and rotated by means of a worm gear.


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The rotor winding produces a rotating field, which induces an additional voltage Uz in the

stator winding.

The phase of the additional voltage relative to the basic voltage U1or the output voltage

U2, changes according to the position of the rotor. The resultant output voltage magnitude

can vary within limits.


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14 AIR-COOLED TRANSFORMERS

Air-cooled transformers are self cooled as a result of convection currents of air.

Air-cooled transformers can be moulded in resin for loads up to 5,000 kVA.

Application:

Air-cooled transformers are used indoors in large buildings, mines (underground) and

ships.

No particular fire protection or oil drainage pits are required.

The current density of the windings should not exceed 2.5 A/mm. The heat generated in

operation is released into the ambient air by convection and radiation. Such transformers

may only be erected in dry areas. Additional cooling (external cooling) is possible using a

blower.


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15 OIL-FILLED TRANSFORMERS

Large transformers are always oil-filled.

The transformer core, consisting of laminations, is situated with the windings in an oil-filled

tank.

The oil tank for medium sized transformers is designed with cooling fins (larger surface).

For larger Ioads, in order to increase the natural oil circulation, the oil tank is built either as

a cylindrical or rectangular tank, fitted with radiating tubes in which the oil circulates by

convection.


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In transformers with very high Ioads, the cooling oil is circulated by a circulating pump

through a separate oil cooler, which can be air or water cooled.


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15.1 Transformer oil

This is the best refined oil. lt must be free of foreign matter, acids, salts and alkalis etc.

lt has to be boiled at 120C before use, in order to exclude any water particles. The oil is

used for both insulation and cooling purposes. The winding is designed with an open

structure, so that the oil can flow through cooling channels of sufficient size.

Advantage:

- Heat is removed more effectively than by air.

- lt has very good insulating properties.

- lt prevents moisture from entering.

15.1.1 Di-electric strength

For transformer oil this is 20 to 30 kV/mm for a period of 1 minute.

Ed = di-electric strength kV/mm

Ud = breakdown voltage in kV

s = thickness of the material sample in mm.

The di-electric strength depends on the duration of the stress.


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15.1.2 Maintaining the oil quality

Oil samples must be taken from the transformer at regular intervals. If the oil no longer

complies with the requirements it must be replaced or purified

The oil is freed of suspended matter by centrifuging and filtering. Water is removed by

boiling under reduced pressure.

15.2 The expansion vessel (conservator)

lt is important to fit the expansion vessel (conservator) above the transformer and to

connect it to the tank by only 1 pipe. This arrangement prevents oil circulation through the

expansion vessel, consequently. The oil in the expansion vessel remains cool.

Transformer oil has a coefficient of thermal expansion of 0.000725 per 0C, which is

equivalent to a 7.25 per cent volume change over an oil temperature range of 0-100C.

The expansion may be accommodated in a free breathing or sealed space at the top of

the tank or in a conservator tank mounted on the tank cover. lt is desirable to avoid

oxidation of the oil which causes slugging and acidity to develop.


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With the sealed tank of the conservator, a nitrogen cushion can be maintained above the

surface of the coil to prevent oxidation. A free-breathing conservator is preferable to a

sealed tank as the temperature of the oil in contact with air is lower and this in itself

reduces the rate of oxidation.

15.3 Air dryer

If an oil-filled transformer is on load, then it will become warm. The oil expands through

heating and displaces air from the conservator through the ventilation opening. When the

oil cools, the oil level drops and air is sucked in. The transformer "breathes".

A constant change of air in the conservator would result in humidity. An air dryer filled with

cobalt salt can prevent this.

The blue salt absorbs humidity from the air, and in doing so turns pale pink.

lt must then be replaced.


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16 CLOPHENE-FILLED TRANSFORMERS

Clophene is a liquid transformer coolant, which does not burn and has good insulating

properties. lt complies with strict fire protection regulations.

Particular care must be taken when working on transformers filled with clophene as it is

very injurious to health.

Clophene breathed in vapour is absorbed by the human body and remains in the body.


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EE 028Electr ical Machines 2 -

Transformers

Theoretical Test


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EE 028


T E S T 1

1. State the function of a transformer.

2. Why should the material of an iron core be a relatively poor conductor?

3. State the direction of energy flow in a transformer.

4. Name the main use of a disc winding.

5. State what is meant by the term "space factor" in an iron core.

6. Which component of no-load current is caused by the iron losses PFe?

7. State the relationship between the magnitudes of the magnetic flux due to the primary

current and that due to the secondary current.

8. State the relationship between output and input current in a transformer.

9. State the factors which limit the short circuit current in a transformer.

10. Why must transformers be fused for double the rated current an the primary side?


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EE 028


T E S T 2

1. What type of load causes the output voltage in a transformer to rise?

2. Which type of transformer has electrically separated windings with a turn ratio of 1:1?

3. What is the purpose of using measurement transformers in electrical circuits?

4. In which type of transformer are the high and low voltage windings electrically

connected?

5. State a typical no load voltage for the output of a welding transformer?

6. For what reason is electrical power stepped up to a very high voltage?

7. Are autotransformers used in high voltage circuits?

8. Which type of transformers are used for high power transformers?

9. Name five protective devices for transformers.

10. Which devices are used to monitor the temperature of transformer oil?


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EE 028


T E S T 3

1. State the different fault conditions which would cause a Buchholz relay to operate.

2. Describe the principle of operation of a circulating current protective system.

3. Do shell type transformers have a low impedance voltage or a high impedance

voltage?

4. Which type of transformer has a very high stray flux and a high impedance voltage?

5. State the meaning of the Symbol Yy6.

6. Name five conditions for parallel transformer operation.

7. What characteristics should transformer oil have?

8. Give three advantages of oil filled transformers.

9. How much must the di-electric strength for transformer oil be?

10. Where, and for what purpose is an air-drier used?


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EE 028


T E S T 1

( S o l u t i o n )

1. Transformers transform the alternating voltage of a certain voltage and frequency to

higher or lower voltages of the same frequency.

2. In order to reduce eddy currents.

3. The direction of energy flow is always from the primary winding to the secondary

winding.

4. The disc winding is used mainly for high-voltage windings.

5. The effective cross section of the iron is less that the measured cross section of the

core, due to the insulation of the laminations. This space factor is usually 0.9.

6. The active component.

7. The primary flux, must be greater by the amount of the no-Ioad magnetic flux.

8.

9. Mainly the leakage inductance and the winding resistances.

10. Due to the starting current.


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EE 028


T E S T 2

( S o l u t i o n )

1. Capacitive load.

2. Isolating transformer.

3. To reduce high voltages and high currents to a value suitable for measuring, and also

to provide separation from high voltage.

4. Autotransformers.

5. 70 volts.

6. lt is uneconomical to transmit low voltage electrical power over a long distance.

7. Yes.

8. Three separate single-phase transformers are connected together to form a three-

phase transformer.

9. Circulating current protective system.

Excess current protective system.

Gas pressure relays.

Excess voltage protective relays.

Fire protection.

10. Bimetal or resistance thermometers.


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EE 028


T E S T 3

( S o l u t i o n )

1. The Buchholz relay operates in the event of:

- Gas formed due to local overheating.

- Short-circuit between two windings.

- Overheating of laminations.

- Voltage breakdowns.

- Open circuit windings.

- Air entering the tank.

- Oil losses.

2. By comparing the currents on the primary side with the currents on the secondary side

via current transformers, one can discover whether there is an undue difference within

them.

3. Low impedance voltage.

4. Limb-type transformers.

5. High voltage side is connected in star.

Low voltage side is connected in star.

Phase displacement between high voltage and low voltage is 6 x 30 = 180.

6. The primary and the secondary must have the same rated voltages. The identical

terminals must be connected to each other. The transformers must belong to the same

switching group. The transformers must have approximately the same short-circuit

voltage.

The ratios of rated power should be within 3:1.


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7. lt must be free of foreign matter, acids, salts, and alkalis.

8. Heat is removed more effectively than by air. lt has very good insulating properties. lt

prevents humidity from entering.

9. lt is 20 to 30 kV/mm for a period of one minute.

10. In an oil filled transformer with expansion vessel.

In order to absorb the humidity in an expansion vessel.


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KEY TO EVALUATION

PER CENT MARK

88 100 1

75 87 2

62 74 3

50 61 4

0 49 5

EE028-Electrical Machines 2-Th-Inst.pdf

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