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