Download Demonstrate knowledge of single and three phase transformers

Demonstrate knowledge of single and
three phase transformers used in the
electricity industry
US 19323
Training and Assessment Resource
Level 3
Credits 4
Electricity Supply Industry Training Organisation
Page 1 of 37
www.esito.org.nz
Contents
Introduction to Training Assessment Resource
Glossary
Symbols
1.0 3
4
5
Introduction to Transformers........................................................................................................... 7
2. 0
Inductance ...................................................................................................................................... 8
2.1 Self inductance.................................................................................................................................. 8
2.2 Mutual inductance.............................................................................................................................. 9
3.0
Transformer Construction............................................................................................................... 10
4.0
Transformer Load ........................................................................................................................... 13
4.1 Transformer at no load...................................................................................................................... 13
4.2 Loading a transformer....................................................................................................................... 14
5.0
5.1
5.2
5.3
5.4
Transformer Losses and Leakage Flux.......................................................................................... 15
Transformer losses............................................................................................................................ 15
Efficiency ..................................................................................................................................... 16
Transformer rating............................................................................................................................. 16
Leakage Flux..................................................................................................................................... 17
6.0 Voltage regulation........................................................................................................................... 18
6.1Tap-changers.................................................................................................................................... 19
7.0 Auto-transformers.......................................................................................................................... 21
8.0 Three-phase transformers.............................................................................................................. 23
8.1 Three phase transformer connections............................................................................................... 26
8.2 Three phase transformer voltage ratings and turns ratios.................................................................. 30
Answers to activities..................................................................................................................................... 32
Getting started on the ESITO Training & Assessment Resources
Activity: A written or spoken exercise or assignment.
Keypoint: Important information to remember.
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TAR 19323 | Rev. 1, 2010
Introduction to Training
Assessment Resource
This Training Assessment Resource (TAR) contains the information that you require to complete the written assignment
in the assessment pack for this unit standard.
Purpose
People who obtain credit for this unit standard are able to:
Demonstrate knowledge of single phase transformers in the electricity supply industry.
Demonstrate knowledge of three phase transformer theory.
Demonstrate knowledge of transformer outputs and efficiencies.
TAR 19323 | Rev. 1, 2010
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Glossary
When I see this word
It means
Approximation
Not quite exact
Catastrophic failure
Total failure resulting in a lot of damage and risk of injury
Configuration
The way the parts of something are arranged and how they fit together
Counteract
To prevent something having an effect, or to lessen its effect
Dissipate
To cause something to disappear or reduce
Equivalent
Being the same, or effectively the same
Faraday’s law
This is a basic law of relating voltage and flux. It forms the operating principles of transformers, inductors, and many types of electrical motors and generators
Fundamental principle
An important underlying law or theory
Fundamental
Underlying or deep-seated
Fundamentally
Basically
Instantaneous
Occurring at a given moment in time
Load The power drawn or power required
Mitigate
To lessen the effect, to make something less harsh or severe
Proportional
There is a constant ratio between two variables.
Static
Not moving, fixed in position
Subscript Printed on a lower level than other characters in a line of type
Summation
Add together
Variable
Capable of change
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TAR 19323 | Rev. 1, 2010
Symbols
There are different symbols used in this training manual.
Cu means copper
Fe means iron
ph means phase (phase to neutral)
l means line (phase to phase)
The following subscripts are used to associate a symbol with a phase or winding.
1 means associated with winding 1
2 means associated with winding 2
P means associated with primary winding
S means associated with secondary winding
a means associated with phase A
b means associated with phase B
c means associated with phase C
Combining symbols and subscripts
Ia means current in A phase
Vp is voltage associated with primary winding
N2 is the number of turns associated with winding 2
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Table 1 shows the symbols used, their meaning and the units that are used to measure them.
Symbol
V
V1
=
N
V1
=
N
Meaning
Unit
Voltage
Volt (V)
I
Current
Ampere (A)
P
Real power
Watt (W)
S
Apparent power
Volt-Ampere (VA)
N
Number of turns
θ
Power angle (angle between
voltage and current)
Degrees
Magnetic flux
Webers (Wb)
Rate of change of flux
Webers per second (wb/s)
Resistance
Ohm ( Ω )
dØ
dt
dØ
dt
R
=
=
Output Power
Input Power
Table 1: Symbols
Pout
Pout
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+
Efficiency
PLosses
TAR 19323 | Rev. 1, 2010
1.0 Introduction to Transformers
Transformers are static devices that transfer energy from one circuit into another by electromagnetic induction. The
energy transferred from the primary circuit to the secondary circuit is equal, excluding losses. The frequency between
the circuits is unchanged. However, the voltage and the current of each circuit is either increased or decreased, unless
the transformer is being used as a one-to-one isolating transformer.
Transformers are used for three reasons:
1.
Step-up the voltage.
A step-up transformer is used to increase the voltage. An example of a step-up transformer is a generator
transformer. In this example the transformer changes the voltage from the generator voltage (eg. 11kV) to the voltage of the transmission system (eg. 220kV).
2.
Step-down the voltage.
A step-down transformer is used to decrease the voltage. An example of a step-down transformer is a
transformer used to supply a rural house. In this example the transformer changes the voltage of the distribution system (eg.11kV) to the mains voltage, 230V.
3.
One-to-one (Isolating Transformer).
A one-to-one transformer is commonly referred to as an isolating transformer. Isolating transformers are used where it is necessary to have electrical isolation between the primary and secondary circuits. An example of a one-to-one transformer is an isolating transformer used to isolate hand-held power tools on a job site.
The construction of all of these transformers is fundamentally the same, with the difference being the relative number of
turns on each winding.
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2.0Inductance
Current passing through a wire will produce a magnetic field around the wire. The creation of any line of magnetic field
around a conductor creates an electrical property called “inductance”.
2.1 Self inductance
If a magnetic field line is created by the current itself in the wire then this is called “self inductance”. When the wire
is wound to form a coil, the magnetic field lines from the individual conductors add together to form larger loops that
surround the coil. This summation of the individual self induced magnetic lines is shown in Figure 1.
P
x
Figure 1: Flux formed by a coil
Should the current in the coil change, then there would be a corresponding change in the magnetic field. A change
in the magnetic field will produce an electromotive force (emf) or voltage that will oppose the change in current. This
voltage is often referred to as a back emf. As the back emf is induced by the current in the coil itself, it is referred to as
a self induced voltage, and the inductance formed by the coil as self inductance.
The back emf can be calculated by Faraday’s Law.
V1
=
N
dØ
dt
Equation 1: Faraday’s Law
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2.2 Mutual inductance
If the self induced flux lines of a coil spread out and interact with a second coil then induction will occur in the second
coil. This type of induction is given the label “Mutual Inductance”. Any change in the magnetic flux produced by the first
coil will induce a voltage into the second coil to oppose the changing magnetic flux. Mutual inductance is a fundamental
principal of a transformer. The voltage produced by mutual inductance can also be calculated by Faraday’s Law,
Equation 1.
With the aid of a diagram explain the terms self inductance and mutual inductance.
TAR 19323 | Rev. 1, 2010
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3.0 Transformer Construction
The fundamental components of a transformer are two or more windings wound around a magnetic steel core.
An alternating voltage in one winding will induce an alternating magnetic field in the core. The core is used to transport
the magnetic flux to the second winding. The alternating magnetic field will induce an alternating voltage in the second
winding, by mutual inductance. A diagram of a transformer is shown in Figure 2.
Primary
winding
Secondary
winding
Np turns
Primary
current
Ns turns
IP
Magnetic
Flux
+
Secondary
current
Is
+
Primary
voltage
VP
Secondary
voltage
Vs
_
_
Transformer
core
Figure 2: Diagram of the ideal transformer
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TAR 19323 | Rev. 1, 2010
In an “ideal” transformer there would be no losses and perfect magnetic flux coupling of both the primary and
secondary windings. The relationship between the voltage and the number of turns in each winding in an ideal
transformer can be expressed by Equation 2.
Vp
Vs
=
Np
Ns
Equation 2: Voltage / Turns Radio
As the transformer is a static device (i.e no rotating parts), and if you ignore losses then “power in” must equal “power
out.” Power can be calculated by P=VIcosθ. Using this power equation together with Equation 2, a relationship can be
formed between the number of turns and the current in the winding.
Vp
Vs
=
Np
Ns
=
Is
Ip
Equation 3: Voltage / Turns / Current Ratio
From Equation 3 it can be seen that the ratio of primary to secondary voltage is equal to the ratio of the number of
turns on the primary winding to the number on the secondary winding. It is also inversely proportional to the ratio of
the currents in each winding. The ratio of the number of turns on the primary winding to the number of turns on the
secondary winding is referred to as the Transformer Turns Ratio.
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In a practical transformer design, the windings would not be wound on separate legs (or limbs) of the core as shown in
Figure 2. They would be wound one over the other as shown in Figure 3. Typically, the low voltage winding will be wound
next to the core with the high voltage winding over the top. The two windings will be separated by solid insulation and
insulating fluid. Core - Yoke
Core - Limb
Note: There is also a limb of
the core in the centre of the
winding set.
Note:
Limb covered with insulation,
core is laminated steel
Winding set - Both
low voltage and high
voltage winding
Figure 3: Photo of a single phase transformer
Draw a diagram that shows the fundamental components of a single phase transformer.
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TAR 19323 | Rev. 1, 2010
4.0 Transformer Load
4.1 Transformer at no load
The self induced voltage of the primary winding (V /1 in Figure 4) will be only slightly less than the supply voltage (V1)
(due to small internal winding losses) and will oppose any change in the magnetic field and therefore V1. Only a small
current will flow; this is required to drive the magnetic flux lines around the core. This small current is known as the
magnetising current.
The flux will flow through the secondary winding and induce a voltage (V /2) via mutual inductance. The value of this V /2
can be calculated by the formula in Equation 2.
Magnetic flux
Laminated
core
Primary
winding
V1
I
Secondary
winding
I0
/
V1
/
V2
V2
Figure 4: Transformer diagram
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4.2 Loading a transformer
When a load is connected to the secondary winding, current will flow within the secondary winding. There is a
transformer fundamental principle known as “Lenz’s Law” which states that in mutually coupled coils if current flows in
one coil then an opposing current will flow in the other coil. For transformers this means that if current flows out of the
loaded side then current will flow into the supply side.
Given these opposing electrical currents and opposing self induced magnetic flux lines the mutual flux linking the
primary and secondary coils remains constant independent of load current.
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5.0 Transformer Losses and
Leakage Flux
Up to now we have only discussed the ideal transformer which has no losses, and all the flux is coupled between the
two windings. In the real world the “ideal” transformer does not exist. All transformers will have losses and leakage flux
(i.e. magnetic field lines that do not link both the primary and secondary coils). Both losses and leakage flux will affect
the ideal transformer equations. However, these equations are a good approximation of most practical transformers.
5.1 Transformer losses
Transformer losses are split into two components; copper losses and iron losses or load and no-load losses respectively.
Copper Losses
The resistance of each winding is relatively low; however each winding will have resistance. The current flowing through
the resistance of the windings will result in heat and therefore losses. These losses are known as copper losses.
The copper losses will be equal to:
Pcu
=
Copper losses of primary winding
=
2
+
Copper losses of secondary winding
2
+
Ip R p
Is R s
Equation 4: Copper Losses
As copper losses are proportional to the load current they are often referred to as load losses.
Iron Losses
There are also losses associated with magnetising the transformer iron core. These losses are not affected by load
current. Iron losses are often referred to as no-load losses.
Total Losses
Total transformer losses are equal to the sum of copper and iron losses, as shown in equation 5.
Stray losses and auxiliary losses have been excluded.
P Losses
=
P cu
+
P fe
Equation 5: Transformer Losses
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5.2 Efficiency
As with all machines, the efficiency of a transformer can be expressed by Equation 6.
=
=
Output Power
Input Power
Pout
Pout
+
PLosses
Equation 6: Transformer Efficiency
Transformers have very high efficiency, typically 97-99%.
5.3 Transformer rating
The rating of a transformer is expressed in terms of its Volt-Ampere capability. As the power factor of the load is
unknown, transformer ratings are expressed in terms of Apparent Power that can be supplied to the load. In general
terms the voltage is set by its insulation capability and the current is set by the transformer’s ability to dissipate copper
losses.
Transformer rating can be expressed in Equation 7.
Transformer rating =
Ss
Vs
Output Power
rated
rated
Is
rated
Equation 7: Transformer Rating
Input Power
Pout
Pout
+
PLosses
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TAR 19323 | Rev. 1, 2010
5.4 Leakage flux
In the “ideal transformer” equations it is assumed that all of the magnetic flux lines link both windings. However, in a
real transformer not all the flux lines produced by one winding will link the other winding and vice-versa. Magnetic flux
lines that do not link both windings are known as leakage flux.
Leakage flux is shown in Figure 5. As not all of the flux links both windings the voltage ratio of the ideal equations will
only be an approximation.
Primary
winding
Secondary
winding
12
Main flux
1
2
Leakage
flux
21
Figure 5: Transformer diagram showing leakage flux
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6.0 Voltage Regulation
Voltage regulation is a term that is used to quantify the change in output voltage with load. Voltage regulation is
expressed as a percentage, and is shown in Equation 8.
Voltage Regulation
=
Output Voltage
No Load
-
Output Voltage
Output Voltage
Full Load
%
No Load
Equation 8: Voltage Regulation
Voltage regulation is caused by leakage flux lines. As the load increases, the amount of leakage flux produced by each
winding also increases. For that reason there will be a reduction in the mutual flux as the load increases. Therefore, by
Faraday’s Law (Equation 1) the output voltage will decrease as the load increases.
If a transformer has a no-load secondary voltage of 230V and a voltage regulation of 10%, what is the full
load voltage?
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TAR 19323 | Rev. 1, 2010
With the aid of a diagram explain how leakage flux affects voltage regulation.
6.1Tap-changers
Transformers are fitted with tap-changers to counteract the effects of voltage regulation, and to allow for system voltage
movement. In other words, tap-changers can be used to regulate the output voltage of the transformer, for example to
keep it at a required level.
Tap-changers are a means of changing the number of turns on either the primary or secondary winding. By changing
the number of turns on one of the windings the turns ratio will change. Therefore, by Equation 2, the ratio of primary
voltage to secondary voltage can also be changed.
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Figure 6 shows a diagrammatic example of a tap-changer winding.
Vp
Vs
Figure 6: Circuit diagram of a tap-changer
Tap-changers are referred to as off-load or on-load. An on-load tap-changer is designed to ensure that when the
tap-changer is operated the winding is never open-circuited, and therefore the tap-change can occur when the
transformer is energised and on load.
WARNING: Off-load tap-changers must not be operated when the transformer is energised. Should an
off-load tap-changer be operated when the transformer is energised then the winding with the
tap-changer would be open circuited, even if the transformer is unloaded. This would result in a large
voltage being produced across the tap-changer contacts, and the catastrophic failure of the transformer.
Explain the term tap-changer and how it can be used to mitigate the effects of voltage regulation.
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TAR 19323 | Rev. 1, 2010
7.0Auto-Transformers
The transformers discussed up to now are known as double wound transformers. In a double wound transformer the
voltage is changed by having two separate windings each with a differing number of turns. Each winding is electrically
isolated from the other.
Double wound transformers are often referred to as two winding transformers.
An auto-transformer is made from a single coil. Change in the voltage between the primary and secondary circuits is
achieved by having the primary and secondary windings tapped off at different points, as shown in Figure 7.
load
Figure 7: Step-down autotransformer diagram
In certain applications, autotransformers are used in preference to double wound transformers. The main disadvantage
of an autotransformer is the fact that there is no electrical isolation between the primary and secondary winding. In
a step-down autotransformer failure of the common winding would result in the primary voltage being applied to the
secondary circuit.
An autotransformer can be used as either a step-up or step-down transformer. An example of a step-up autotransformer
is a line booster. A line booster is used to boost the voltage at the end of a long distribution circuit, where the voltage
can start to fall away. Another common use for a step-up autotransformer is as a starter in a fluorescent lamp. An
example of a step-down autotransformer is an old starter on an induction motor. In this example the transformer can
be used to reduce the voltage to the motor, this is then stepped through different taps until the motor is running at full
speed and supplied with full voltage.
Another common use of an autotransformer is a variac. A variac is a variable output transformer. These transformers
can supply a load with a variable output voltage. The output range of a variac autotransformer is typically 0-110% of the
input voltage.
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Page 21 of 37
Draw a diagram for a variac with an output range of 0-110%, set at 50%.
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TAR 19323 | Rev. 1, 2010
8.0 Three Phase Transformers
The transformers discussed this far have been single phase transformers. The power grid in New Zealand is a three
phase system. A three phase system can be supplied by either:
Three single phase transformers, which make a three phase bank of transformers, or
One three phase transformer.
Three phase transformers have the following advantages over using three single phase transformers:
Lower no load losses
Lower manufacturing cost
Less overall weight
Smaller footprint (Less area required)
Lower maintenance cost
Less insulating oil
The disadvantages are:
Higher transport weight and size. (The total weight of all three single phase units is more, however, each single phase unit can be transported separately. The single phase transformer used to make a three phase bank of transformers will have a lower weight than a three phase transformer of equivalent rating.)
Higher cost of a spare unit. (With a single phase bank only one single phase unit is required to be held as a spare unit.)
The theory discussed for single phase transformers also applies to three phase transformers. The construction of a
three phase transformer is similar to a single phase unit. Both the primary and secondary windings of each phase are
wound onto a common limb. The limbs of the core are attached to each other by the yoke. This forms a magnetic circuit
between each phase. Since the flux in each limb is 120o out-of-phase with the other two phases, the instantaneous sum
of all three fluxes in the yoke is zero. Therefore, there is no need for a separate return path for each phase.
TAR 19323 | Rev. 1, 2010
Page 23 of 37
Figures 8a-c show the construction process of a three phase transformer.
Core limbs
Note: The core is made up
of the vertical limbs and the
horizontal yokes. The yokes
complete the magnetic circuit
between the limbs.
Core yoke
Note: The upper yoke has
not been fitted and the lower
yoke is not clearly visible. It is
made up of steel laminations,
like the limbs, and runs
horizontally between the three
limbs. It is located under the
wooden end rings and behind
the white end frame.
Figure 8a: Transformer core minus the upper yoke
The winding
sets are placed
over the core
limbs
Note: One winding set per
phase, and a winding set
contains both primary and
secondary windings.
Figure 8b: Winding sets placed over core limbs, upper yoke not fitted
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TAR 19323 | Rev. 1, 2010
The upper core yoke
has been fitted
Note: The upper yoke is
laminated core steel. It links
the three limbs, and is located
behind the white end frame
Tap-charger
Figure 8c: Upper yoke and tap changer fitted
A2
B2
C2
a2
b2
c2
N
n
Figure 9:
Graphic representation of a star/star three phase transformer
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Page 25 of 37
Explain why the instantaneous sum of flux in the yoke equals zero.
8.1 Three phase transformer connections
The three phase electrical system in use today around the world requires the phases of transformers to be connected
together. This applies equally to a three phase transformer bank made from three single phase units, or a single three
phase transformer. There are two basic connection configurations used; delta and star. Each connection has particular
features which affect the design and operation of transformers. The basic configuration of each is shown in Figure10.
Delta
Star
Figure 10: Delta and Star configurations
Delta winding connection
Delta connected windings are constructed by connecting each end of a winding to a different winding, resulting in
a triangular arrangement with three connection points or terminals. There are only three terminals brought through
the transformer tank with each connection point connecting to one line terminal of the power grid. This means each
winding sees the full phase-to-phase (line) voltage of the power grid.
1
Due to the nature of three phase system, the current in each winding is of the line current. 3
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TAR 19323 | Rev. 1, 2010
The relationships between the line (phase to phase) voltage and the voltage across a delta connected winding as well
as the current in the line and the current in a delta connected winding are expressed in the following equations and in
Figure 11.
Voltage across a delta winding =
Current in a delta winding =
Line Voltage
Line Current
3
Iline
Ia = Iline
3
Va = Vline
Vline
Vline
Figure 11: Delta winding configurations
The apparent power of a delta connected winding can be calculated as shown in Equation 9.
S
=
=
S a+ S b+ Sc
Va Ia + Vb Ib + Vc I c
=
3VI (I I / 3 )
=
3 VI II
Equation 9: Apparent power of a delta connected winding
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Star or Y connected windings
Star connected windings are constructed by connecting one end of each winding together. This connection point is
known as the star point or neutral point of the winding. The neutral connection, along with the three phase connections,
is usually brought out of the transformer. The neutral connection is usually earthed.
1
The voltage across a star connected winding is only the phase-to-neutral voltage which is of the line voltage.
3
The current through the windings is the same as the line current. The relationships between the line (phase to phase)
voltage and the voltage across a star connected winding as well as the current in the line and the current in a star
connected winding are expressed in the following equations and in Figure Line
12. Voltage
Voltage across a star winding =
Line Voltage
3
Voltage across a star winding =
3
Current in a star winding =
Line Current
Current in a star winding =
Iline
Line Current
Ia = Iline
Va = Vline
3
Vline
Vline
Figure 12: Star winding configurations
The apparent power of a star connected winding can be calculated as shown in Equation 10.
S
=
=
S a+ S b+ Sc
Va Ia + Vb Ib + Vc Ic
=
3Vph Iph
=
3( VI / 3 )II
=
3 VI II
Equation 10: Apparent power of a star connected winding
From equations 9 and 10 it can be seen that although the voltages and currents in a star or delta connected winding
differ, the formula used to calculate the apparent power is the same. This is also true for real and reactive power.
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P=
3 VL I L Cos θ
Q=
3 VL I L Sin θ
TAR 19323 | Rev. 1, 2010
Transformer connections
It is possible to connect both windings in either star or delta, therefore there are four three phase winding configurations
used. These are shown in Figure 13. These configurations are also used when connecting three single phase
transformers to form a three phase bank.
Primary
Secondary
(a) Star-star
Primary
Secondary
(b) Delta-delta
Primary
Secondary
(c) Delta-star
Primary
Secondary
(d) Star - delta
Figure 13: Three phase transformer connections
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Page 29 of 37
Draw a diagram of a delta/star three phase transformer. On the diagram label: Primary winding, secondary
winding, core yoke, and core limb.
8.2 Three phase transformer voltage ratings and turns ratios
Three phase systems are referred to in terms of line voltages. Likewise, three phase transformers are referred to in
terms of the line voltage on either side of the winding.
However, if the winding is star connected then the voltage applied to the winding will be
1
the line voltage.
3
Therefore, for a star/delta or a delta/star transformer the actual winding turns ratio will not equal the rated voltage ratio
of the transformer. This is demonstrated by the example in Table 2. This example is for a generator transformer that is
used to transform the generator voltage of 11kV to the system voltage of 110kV. This transformer has a delta connected
primary winding and a star connected secondary winding.
Primary
System voltage
Secondary
11,000 V
110,000 V
Winding configuration
Delta
Star
Winding voltage
= System voltage
= System Voltage / 3
11,000 V
63,500 V
173
999
Number of turns
Ratio
1:10
1:5.77
1:5.77
Table 2: Three phase transformer voltage ratio example
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TAR 19323 | Rev. 1, 2010
Complete the table below
Primary
Secondary
Rated voltage
33,000 V
11,000 V
Rated current
141
420
Winding Configuration
Delta
Star
Transformer Power Rating
Number of turns on Nominal Tap
5000
Full load voltage on Nominal Tap
33,000 V
9,900 V
Tapping range to acheive rated
voltage at no load to 5%
overvoltage at full load
Not tapping winding
% to %
Copper losses st full load
40kW
35kW
Iron Losses at rated voltage
15kW
0
Voltage Regulation on Nominal Tap
Number of turn required to
increase output voltage at full load
to 5% over rated.
Efficiency at full load
Step up or Step down
Example of where this tarnsformer
might be used
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Page 31 of 37
Answers to Activities
Activity (page 9)
With the aid of a diagram explain the terms self inductance and mutual inductance.
Current flowing in a coil will produce a magnetic field, should that magnetic field change a voltage will
be produced in the coil to oppose the change. This effect is called self inductance.
Should a second coil be placed in the magnetic field of a primary coil, should the magnetic field of the
primary coil change then a voltage will be produced in the secondary coil. This is called mutual inductance.
Activity (page 10)
Draw a diagram that shows the fundamental components of a single phase transformer.
Note: Diagram needs to include a core and two windings.
Primary
winding
Secondary
winding
Np turns
Primary
current
Ns turns
IP
Magnetic
Flux
+
Secondary
current
Is
+
Primary
voltage
VP
Secondary
voltage
Vs
_
_
Transformer
core
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TAR 19323 | Rev. 1, 2010
Activity (page 18)
If a transformer has a no-load secondary voltage of 230V and a voltage regulation of 10%, what is the full load voltage?
Voltage Regualtion =
Output Voltage
Output Voltage
No Load
Output Voltage
10% =
230 - Output Voltage
0.1x230 =
TAR 19323 | Rev. 1, 2010
%
No Load
Full Load
230
230 - Output Voltage
%
Full Load
Full Load
= 230 - 0.1x230
Output Voltage
Full Load
= 230 - 23
Output Voltage
Full Load
= 207V
Output Voltage
Full Load
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Activity (page 19)
With the aid of a diagram explain how leakage flux affects voltage regulation.
Primary
winding
Secondary
winding
12
Main flux
1
2
Leakage
flux
21
Leakage flux is flux that does not couple both windings. As the current in each winding increases the
leakage flux will also increase. As there is a larger percentage of the flux produced by each winding is
not coupled then the voltage produced in the secondary winding will also decrease.
Activity (page 20) Explain the term tap-changer and how it can be used to mitigate the effects of voltage regulation.
A tap-changer is a device that is used to change the number of turn on one of the coils. By changing the
turns ratio the voltage ratio will also change. Therefore a tap-changer can be used to increase the output
voltage when the load is increased.
Activity (page 22)
Draw a diagram for a variac with an output range of 0-110%, set at 50%.
Note: Winding extends past
the primary connect. This is to
enable 110% output voltage.
Secondary winding connected
half way down primary winding.
Therefore 50% output voltage.
load
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Activity (page 26)
Explain why the instantaneous sum of flux in the yoke equals zero.
Like the current in each coil the flux is 120 degree apart. Therefore the sum of all three phases equals 0.
Activity (page 30)
Draw a diagram of a delta/star three phase transformer. On the diagram label the: Primary winding, secondary winding, core yoke, and core limbs.
Note:
Primary winding as the end of one coil connected to the start of the second (Delta).
The secondary winding has all the ends connected together (Star)
Core Yoke is the horizontal part of the core that links the limbs together
Core Limb is the vertical part of the core that has the windings wound over it.
Core Yoke
A2
B2
C2
Primary Winding
Core Limb
n
Secondary Winding
a2
TAR 19323 | Rev. 1, 2010
b2
c2
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Activity (page 31)
Complete the table below.
Primary
Secondary
Rated voltage
33,000 V
11,000 V
Rated current
141
420
Winding Configuration
Delta
Star
Transformer Power Rating
= √3 x 11,000 x 420 = 8MVA
Number of turns on Nominal Tap
5000
5,000 x (11,000/√3)/33,000 = 962
Full Load Voltage on Nominal Tap
33,000 V
9,900 V
Voltage Regulation on Nominal Tap
(11,000 – 9,900)/11,000 = 10%
Number of turn required to
increase output voltage at full load
to 5% over rated.
Not tapping winding
=11,550 – 9,900 =
1650 volts increase
Additional turns =
5000 x (1650/√3)/33000= 144
Total number of turns required = 962
+ 144 = 1106
Tapping range to achieve
rated voltage at no load to 5%
overvoltage at full load
Not tapping winding
0 % to (144/962)=15%
Copper losses at full load
40kW
35kW
Iron Losses at rated voltage
15kW
0
Efficiency at full load
=8000/(8000 + 40 + 35 + 15) = 98.9%
Step up or Step down
Step down. Output voltage is lower than input
Example of where this transformer
might be used.
Distribution transformer.
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TAR 19323 | Rev. 1, 2010
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