Unit 2 - Transformers.pdf

Chapter Two
Transformer
 Introduction
 Types and Construction of Transformers
 The Ideal Transformer
 Theory of Operation of Real Single-Phase Transformers
 The Equivalent Circuit of a Transformer
 The Per-Unit System of Measurements
 Transformer Voltage Regulation and Efficiency
 Three-Phase Transformer Connections

Introduction
 A transformer is a static device that changes ac electric power at one
voltage level (primary) to ac electric power at another voltage level
(secondary) through the action of a magnetic field without the change
in frequency.
 Transformer consists of
a core assemble of laminated ferromagnetic core
 two or more coils of wire wrapped around a core(windings).
 The transformer has not rotating part; hence it is often called a static
device. Although a transformer is not classified as an electrical
machine, the principles of its operation are fundamental for the
induction motor and synchronous machines. 2

Cont.…
 It consists of one or more coils of wire wrapped around a common
ferromagnetic core.
 The transformer winding connected to the power source is called the
primary winding or input winding.
 The transformer winding connected to the loads is called the
secondary winding or output winding.
 The primary and secondary windings are not connected electrically, but
coupled magnetically.
 The transformer with three windings is called triple or three –winding
transformer. The third winding on the transformer is called the tertiary
winding. 3

Cont.…
Schematic diagram of a two winding transformer
4

Types and Construction of Transformers
 All transformers have the following essential parts.
 Electrical windings or coils
 Laminated magnetic Core
 The two basic type of transformer construction are the shell type
and core type transformer construction.
 The two types of construction differ in their relative arrangement of
copper conductor and the iron cores.
 The magnetic core of a transformer is made up of stacks of thin
laminations (0.35mm) of cold-rolled grain oriented silicon steel sheets
lightly insulated with varnish. Silicon steel has the desirable properties
of low cost, low core loss and high permeability at high flux.
5

Core type transformer
 Consists of a simple rectangular or L shaped laminated piece of steel
 To reduce the leakage flux problem -half of the low voltage winding
over one leg and other half over the second limb.
 The copper conductor surrounds the core.
 It has a longer mean length of core and a shorter mean length of coil
turns.
 Therefore, it requires more iron core and fewer conductors for coils.
 Has more rooms for ventilation –is better adopt for low voltage
transformers
 concentric coils types are used for core type transformers 6

Cont.…
Figure 2.2 core type transformer
7

Shell type transformer:
 Consists of a three-legged laminated core with the windings wrapped
around the center leg.
 Core surrounds a major part of the windings.
 It has better provision for mechanically supporting and bracing the coil.
 The core is either E or F shaped and stacked to give a rectangular
figure eight.
 To reduce the amount of the high voltage insulation required, the
low voltage coils place adjacent to the iron core.
 Interleaved (or sand witched) coils types are used for shell type
transformers.
8

Cont.…
 Power transformers used in power systems are sometimes referred as
follows:
 A power transformer connected to the output of a generator and used to
step its voltage up to the transmission level (110 kV and higher) is called a
unit transformer.
 A transformer used at a substation to step the voltage from the
transmission level down to the distribution level (2.3 … 34.5 kV) is called a
substation transformer.
 A transformer converting the distribution voltage down to the final level
(110 V, 220 V, etc.) is called a distribution transformer.
 In addition to power transformers, other types of transformers are used. 11

Working Principle of a Transformer
 The basic principle on which the
transformer works is Faraday’s Law
of Electromagnetic Induction or
mutual induction between the two
coils.
 The transformer consists of two
separate windings placed over the
laminated silicon steel core.
 It works on the alternating current only
because an alternating flux is required for
mutual induction between the two
windings.
12

Ideal Transformer
 An ideal transformer is a lossless device with an input winding and
an output winding. The idealizing assumptions made are:
i) The primary and secondary windings have zero resistance (no
ohmic power loss and no resistive voltage drop).
ii) There is no leakage flux so that all the flux is confide to the core and
links both windings.
iii) The core has infinite permeability so that zero magnetizing current is
needed to establish the requisite amount of flux in the core.
iv) The core loss (hysteresis and eddy current loss) is considered zero
13

Cont.…
 We consider a lossless transformer
with an input (primary) winding
having 𝑁𝑝 turns and a secondary
winding of 𝑁𝑠 turns.
 The relationship between the
voltage applied to the primary
winding 𝑉
𝑝(𝑡) and the voltage
produced on the secondary winding
𝑉
𝑠 𝑡 is
𝑉
𝑝(𝑡)
𝑉
𝑠 𝑡
=
𝑁𝑝
𝑁𝑠
= 𝑎
14
Where a is the turns ration of the transformer.

Cont.…
 The relationship between the
primary 𝑖𝑝(𝑡) and secondary 𝑖𝑠(𝑡)
currents is
𝑁𝑝𝑖𝑝 𝑡 = 𝑁𝑠𝑖𝑠(𝑡)
𝑖𝑝(𝑡)
𝑖𝑠 𝑡
=
1
𝑎
 In the phasor notation
𝑉
𝑝
𝑉
𝑠
= 𝑎 𝑎𝑛𝑑
𝐼𝑃
𝐼𝑆
=
1
𝑎
 The phase angles of primary and
secondary voltages are the same.
 The phase angles of primary and
secondary currents are the same
also.
 The turns ratio of the ideal
transformer affects the magnitudes
of the voltages and currents, but not
their angles.
15

Cont.…
 One winding’s terminal is usually
marked by a dot used to determine
the polarity of voltages and
currents.
1) If the voltage is positive at the
dotted end of the primary winding at
some moment of time, the voltage at
the dotted end of the secondary
winding will also be positive at the
same time instance.
2) If the primary current flows into
the dotted end of the primary
winding, the secondary current will
flow out of the dotted end of the
secondary winding.
16

The current ratio on a transformer
 The last approximation is valid for well-
designed unsaturated cores. Therefore:
𝑁𝑝𝑖𝑝 = 𝑁𝑠𝑖𝑠
𝑖𝑝
𝑖𝑠
≈
𝑁𝑠
𝑁𝑝
=
1
𝑎
 An ideal transformer (unlike the real one) can
be characterized as follows:
1.The core has no hysteresis or eddy currents.
2.The magnetization curve is
3.The leakage flux in the core is zero.
4.The resistance of the windings is zero.
17
Magnetization
curve of an ideal
transformer

Power in an ideal transformer
 Assuming that 𝜃𝑝 and 𝜃𝑠 are the angles between voltages and currents on
the primary and secondary windings respectively, the power supplied to the
transformer by the primary circuit is:
𝑃𝑖𝑛 = 𝑉
𝑝𝐼𝑝 cos 𝜃𝑝
 The power supplied to the output circuits is
𝑃𝑜𝑢𝑡 = 𝑉
𝑠𝐼𝑠 cos 𝜃𝑠
 Since ideal transformers do not affect angles between voltages and currents:
𝜃𝑝 = 𝜃𝑠 = 𝜃
 The primary and secondary windings of an ideal transformer have the same
power factor.
18

Cont.…
 Since for an ideal transformer the following holds:
𝑉
𝑠 =
𝑉𝑝
𝑎
𝐼𝑠 = 𝑎𝐼𝑝
 Therefore:
𝑠𝐼𝑠 cos 𝜃𝑠 =
𝑉
𝑝
𝑎
𝑎𝐼𝑝 cos 𝜃 = 𝑉𝑃𝐼𝑝 cos 𝜃 = 𝑃𝑖𝑛
 The output power of an ideal transformer equals to its input power – to be
expected since assumed no loss. Similarly, for reactive and apparent powers:
𝑄𝑜𝑢𝑡 = 𝑉
𝑠𝐼𝑠 sin 𝜃 = 𝑉
𝑝𝐼𝑝 sin 𝜃 = 𝑄𝑖𝑛
𝑆𝑜𝑢𝑡 = 𝑉
𝑠𝐼𝑠 = 𝑉
𝑝𝐼𝑝 = 𝑆𝑖𝑛
19

Impedance Transformation
 The impedance of a device or an
element is defined as the ratio of
the phasor voltage across it to the
phasor current flowing through it:
𝑍𝐿 = ൗ
𝑉𝐿
𝐼𝐿
 A transformer changes voltages
and currents and, therefore, an
apparent impedance of the load
that is given by
𝑍𝐿 = ൗ
𝑉
𝑠
𝐼𝑠
 The apparent impedance of the
primary circuit is: 𝑍𝐿
′
= ൗ
𝑉𝑝
𝐼𝑝
 Which is
𝑍𝐿
′
=
𝑉𝑃
𝐼𝑃
=
𝑎𝑉𝑆
Τ
𝐼𝑠 𝑎
= 𝑎2 𝑉𝑠
𝐼𝑠
= 𝑎2𝑍𝐿
 With a transformer, it is possible
to match the magnitude of a load
impedance to a source impedance
simply by picking the proper turns
ratio. 20

Cont.…
21
b)Impedance scaling through a transformer
a) Definition of impedance

Analysis of Circuits Containing Ideal Transformers
 A simple method to analyze a circuit containing an ideal transformer is by
replacing the portion of the circuit on one side of the transformer by an
equivalent circuit with the same terminal characteristics.
 Next, we exclude the transformer from the circuit and solve it for voltages
and currents.
 The solutions obtained for the portion of the circuit that was not replaced
will be the correct values of voltages and currents of the original circuit.
 Then the turns ratio of the transformer can be used to determine the
voltages and currents on the other side of the transformer.
 The process of replacing one side of a transformer by its equivalent at the other side's
voltage level is known as referring the first side of the transformer to the second side. 22

The voltage ratio across a real transformer
 If the source voltage 𝑉
𝑝(𝑡) is applied to
the primary winding, the average flux in
the primary winding will be:
∅𝑝 =
1
𝑁
න 𝑉𝑃 𝑡 𝑑𝑡
 A portion of the flux produced in the
primary coil passes through the secondary
coil (mutual flux); the rest is lost (leakage
flux): ∅𝑝 = ∅𝑚 + ∅𝐿𝑝
 Similarly, for the secondary coil:
∅𝑠 = ∅𝑚 + ∅𝐿𝑠
 The portion of the flux that
goes through one of the
transformer coils but not the
other one is called leakage
flux.
average primary flux mutual flux
average secondary flux
mutual flux
23

Cont.…
 The flux in the primary coil of the
transformer can thus be divided into
two components:
 a mutual flux, which remains in the
core and links both windings, and a
small leakage flux, which passes
through the primary winding but
returns through the air, bypassing
the secondary winding:
24

Cont.…
 From the Faraday’s law, the primary coil’s voltage is:
𝑉
𝑝 𝑡 = 𝑁𝑝
𝑑∅𝑝
𝑑𝑡
= 𝑁𝑝
𝑑∅𝑚
𝑑𝑡
+ 𝑁𝑝
𝑑∅𝐿𝑝
𝑑𝑡
= 𝑒𝑝 𝑡 + 𝑒𝐿𝑝(𝑡)
 The secondary coil’s voltage is:
𝑉
𝑠 𝑡 = 𝑁𝑠
𝑑∅𝑠
𝑑𝑡
= 𝑁𝑠
𝑑∅𝑚
𝑑𝑡
+ 𝑁𝑝
𝑑∅𝐿𝑠
𝑑𝑡
= 𝑒𝑠 𝑡 + 𝑒𝐿𝑠(𝑡)
 The primary and secondary voltages due to the mutual flux are:
𝑒𝑝 𝑡 = 𝑁𝑝
𝑑∅𝑚
𝑑𝑡
𝑒𝑠 𝑡 = 𝑁𝑠
𝑑∅𝑚
𝑑𝑡
 Combining the last two equations:
𝑒𝑝(𝑡)
𝑁𝑝
=
𝑑∅𝑚
𝑑𝑡
=
𝑒𝑠(𝑡)
𝑁𝑠 25

Cont.…
 Therefore:
𝑒𝑝(𝑡)
𝑒𝑠(𝑡)
=
𝑁𝑝
𝑁𝑠
= 𝑎
That is, the ratio(turn ratio)
 For well-designed transformers:
∅𝑚 ≫ ∅𝐿𝑝 𝑎𝑛𝑑 ∅𝑚 ≫ ∅𝐿𝑠
 Therefore, the following approximation normally holds:
𝑉
𝑝(𝑡)
𝑉
𝑠(𝑡)
≈
𝑁𝑝
𝑁𝑠
≈ 𝑎
26

The magnetization current in a real transformer
 Even when no load is connected to the secondary coil of the transformer, a
current will flow in the primary coil. This current consists of:
 The magnetization current 𝑖𝑚 needed to produce the flux in the core;
 The core-loss current 𝑖ℎ+𝑒 hysteresis and eddy current losses.
 Ignoring flux leakage and assuming time-harmonic primary voltage, the
average flux is:
ഥ
∅ =
1
𝑁𝑝
න 𝑉
𝑝 𝑡 𝑑𝑡 =
1
𝑁𝑝
න 𝑉
𝑚 cos 𝜔𝑡𝑑𝑡 =
𝑉
𝑚
𝜔𝑁𝑝
sin 𝜔𝑡 [𝑊𝑏]
 If the values of current are comparable to the flux they produce in the
core, it is possible to sketch a magnetization current. We observe:
27

Cont.…
1. The magnetization current in the transformer is not sinusoidal. The higher
frequency components in the magnetization current are due to magnetic
saturation in the transformer core.
2. Once the peak flux reaches the saturation point in the core, a small increase
in peak flux requires a very large increase in the peak magnetization current.
3. The fundamental component of the magnetization current lags the voltage
applied to the core by 90°.
4. The higher-frequency components in the magnetization current can be
quite large compared to the fundamental component. In general, the further a
transformer core is driven into saturation, the larger the harmonic
components will become. 28

Cont.…
29
Typical magnetization curve
Flux causing the
magnetization current
Assuming a sinusoidal flux in the core, the eddy currents will be largest when flux passes zero.

Cont.…
 The other component of the no-
load current in the transformer is
the current required to supply
power to make up the hysteresis
and eddy current losses in the core.
this is the core-loss current.
 Core-loss current is:
1. The core- loss current is nonlinear
because of the nonlinear effects of
hysteresis.
2. The fundamental component of
the core- loss current is in phase with
the voltage applied to the core. Non
linear due to nonlinear effects of
hysteresis;
30
Core-loss current

Cont.…
 The total no-load current in the
core is called the excitation current
of the transformer: It is just the
sum of the magnetization current
and the core- loss current in the
core:
𝑖𝑒𝑥 = 𝑖𝑚 + 𝑖ℎ+𝑒
31
Total excitation current in a transformer

The Current Ratio on a Transformer
 If a load is connected to the
secondary coil, there will be a
current flowing through it.
 A current flowing into the dotted
end of a winding produces a
positive magnetomotive force F:
𝐹𝑝 = 𝑁𝑝𝑖𝑝 𝐹𝑠 = 𝑁𝑠𝑖𝑠
 The net magnetomotive force in
the core
𝐹𝑛𝑒𝑡 = 𝑁𝑝𝑖𝑝 − 𝑁𝑠𝑖𝑠 = ∅𝑅
 Where 𝑅 is the reluctance of the
transformer core. For well-designed
transformer cores, the reluctance is
very small if the core is not
saturated. Therefore:
𝐹𝑛𝑒𝑡 = 𝑁𝑝𝑖𝑝 − 𝑁𝑠𝑖𝑠 ≈ 0
32

The Equivalent Circuit of a Transformer
 To model a real transformer accurately, we need to account for the
following losses:
 Copper losses(𝐼2𝑅) – resistive heating losses in the windings:
 Eddy current losses – resistive heating losses in the core: proportional to
the square of voltage applied to the transformer.
 Hysteresis losses – energy needed to rearrange magnetic domains in the
core: nonlinear function of the voltage applied to the transformer.
 Leakage flux – the fluxes ∅𝐿𝑝 and ∅𝐿𝑆that escapes from the core and flux
that passes through one winding only. These escaped fluxes produce a self-
inductance in the primary and secondary coils.
33

The Exact Equivalent Circuit of a Real Transformer
 Copper losses are modeled by the resistors 𝑅𝑝 and 𝑅𝑠.
 Leakage flux in a primary winding produces the voltage:
 Since much of the leakage flux pass through air, and air has a constant
reluctance that is much higher than the core reluctance, the primary coil’s
leakage flux is:
𝑒𝐿𝑝 𝑡 = 𝑁𝑝
𝑑∅𝐿𝑝
𝑑𝑡
∅𝐿𝑝 = 𝑝𝑁𝑝𝑖𝑝 where 𝑝 is permeance of flux path
Therefore: 𝑒𝐿𝑝 𝑡 = 𝑁𝑝
𝑑
𝑑𝑡
𝑝𝑁𝑝𝑖𝑝 = 𝑁𝑝
2
𝑝
𝑑𝑖𝑝
𝑑𝑡
34

Cont.…
 Recognizing that the self-inductance of the primary coil is
𝐿𝑝 = 𝑁𝑝
2
𝑝
 The induced voltages in the primary coil and secondary coil are:
𝑒𝐿𝑝 𝑡 = 𝐿𝑝
𝑑𝑖𝑝
𝑑𝑡
𝑒𝐿𝑠 𝑡 = 𝐿𝑠
𝑑𝑖𝑠
𝑑𝑡
 The leakage flux can be modeled by primary and secondary inductors.
 The magnetization current can be modeled by a reactance 𝑋𝑀 connected
across the primary voltage source.
 The core-loss current can be modeled by a resistance 𝑅𝐶 connected across
the primary voltage source.
 Both currents are nonlinear; therefore, 𝑋𝑀 and 𝑅𝐶 are just approximations.35

Cont.…
 However, the exact circuit
is not very practical.
 Therefore, the equivalent
circuit is usually referred
to the primary side or the
secondary side of the
transformer.
36
The exact equivalent circuit of real transformer

Cont.…
37
Equivalent circuit referred to the primary side
Equivalent circuit referred to the secondary side

Approximate equivalent circuit of a transformer
 For many practical applications,
approximate models of transformers
are used.
a) Referred to the primary side.
b) Referred to the secondary side.
c)Without an excitation branch
referred to the primary side.
d) Without an excitation branch
referred to the secondary side.
38
 The values of components of the
transformer model can be
determined experimentally by an
open-circuit test or by a short-
circuit test.

Determining the values of components
The open-circuit test
 Full line voltage is applied to the
primary side of the transformer. The
input voltage, current, and power are
measured.
 From this information, the power factor
of the input current and the magnitude
and the angle of the excitation
impedance can be determined.
 To evaluate 𝑅𝐶 and 𝑋𝑀, we determine
the conductance of the core-loss
resistor is: 𝐺𝐶 =
1
𝑅𝐶
 The susceptance of the
magnetizing inductor is:
BM =
1
XM 40

Cont.…
 Since both elements are in parallel, their admittances add. Therefore, the
total excitation admittance is:
𝑌𝐸 = 𝐺𝐶 − 𝑗𝐵𝑀 =
1
𝑅𝐶
− 𝑗
1
𝑋𝑀
 The magnitude of the excitation admittance in the open-circuit test is:
𝑌𝐸 =
𝐼𝑜𝑐
𝑉
𝑜𝑐
 The angle of the admittance in the open-circuit test can be found from the
circuit power factor (PF):
cos 𝜃 = 𝑃𝐹 =
𝑃𝑜𝑐
𝑉
𝑜𝑐𝐼𝑜𝑐
41

Cont.…
 In real transformers, the power factor is always lagging, so the angle of the
current always lags the angle of the voltage by 𝜃 degrees. The admittance is:
𝑌𝐸 =
𝐼𝑜𝑐
𝑉
𝑜𝑐
< −𝜃 =
𝐼𝑜𝑐
𝑉
𝑜𝑐
< − cos−1 𝑃𝐹
 The power-factor angle (𝜃) is given by
𝜃 = cos−1
𝑃𝑜𝑐
𝑉
𝑜𝑐𝐼𝑜𝑐
 Therefore, it is possible to determine values of 𝑅𝐶 and 𝑋𝑀 in the open-
circuit test.
42

The short-circuit test.
 Fairly low input voltage is applied to the primary
side of the transformer. This voltage is adjusted
until the current in the secondary winding equals
to its rated value.
 The input voltage, current, and power are again
measured.
 Since the input voltage is low, the current
flowing through the excitation branch is
negligible; therefore, all the voltage drop in the
transformer is due to the series elements in the
circuit. The magnitude of the series impedance
referred to the primary side of the transformer
is: 𝑍𝑆𝐶 =
𝑉𝑆𝑐
𝐼𝑆𝑐
 The power factor of the
current is given by:
PF = cos θ =
Psc
VscIsc
(Lagging)
43

Cont.…
 Therefore: 𝑍𝑆𝐸 =
𝑉𝑠𝑐<0°
𝐼𝑠𝑐<−𝜃°
=
𝑉𝑠𝑐
𝐼𝑠𝑐
< 𝜃°
 Since the series impedance 𝑍𝑆𝐸 is equal to
𝑍𝑆𝐸 = 𝑅𝑒𝑞 + 𝑗𝑋𝑀
𝑍𝑆𝐸 = 𝑅𝑝 + 𝑎2𝑅𝑠 + 𝑗(𝑋𝑝 + 𝑎2𝑋𝑝)
 It is possible to determine the total series impedance referred to the
primary side of the transformer. However, there is no easy way to split the
series impedance into primary and secondary components.
 The same tests can be performed on the secondary side of the transformer.
The results will yield the equivalent circuit impedances referred to the
secondary side of the transformer. 44

The per-unit system
 Another approach to solve circuits containing transformers is the per-unit system.
Impedance and voltage-level conversions are avoided. Also, machine and transformer
impedances fall within fairly narrow ranges for each type and construction of device
while the per-unit system is employed.
 The voltages, currents, powers, impedances, and other electrical quantities are measured
as fractions of some base level instead of conventional units.
𝑞𝑢𝑎𝑛𝑡𝑖𝑡𝑦 𝑝𝑒𝑟 𝑢𝑛𝑖𝑡 =
𝑎𝑐𝑡𝑢𝑎𝑙 𝑣𝑎𝑙𝑢𝑒
𝑏𝑎𝑠𝑒 𝑣𝑎𝑙𝑢𝑒 𝑜𝑓 𝑞𝑢𝑎𝑛𝑡𝑖𝑡𝑦
 Usually, two base quantities are selected to define a given per-unit system. Often, such
quantities are voltage and power (or apparent power). In a 1-phase system:
𝑃𝑏𝑎𝑠𝑒,𝑄𝑏𝑎𝑠𝑒 𝑜𝑟 𝑆𝑏𝑎𝑠𝑒 = 𝑉𝑏𝑎𝑠𝑒𝐼𝑏𝑎𝑠𝑒
𝑍𝑏𝑎𝑠𝑒 =
𝑉𝑏𝑎𝑠𝑒
𝐼𝑏𝑎𝑠𝑒
=
(𝑉𝑏𝑎𝑠𝑒)2
𝑆𝑏𝑎𝑠𝑒 45

Cont.…
𝑌𝑏𝑎𝑠𝑒 =
𝐼𝑏𝑎𝑠𝑒
𝑉𝑏𝑎𝑠𝑒
 Ones the base values of P (or S) and V are selected, all other base values can be
computed from the above equations.
 In a power system, a base apparent power and voltage are selected at the specific point
in the system. Note that a transformer has no effect on the apparent power of the
system, since the apparent power into a transformer equals the apparent power out of a
transformer. As a result, the base apparent power remains constant everywhere in the
power system.
 On the other hand, voltage (and, therefore, a base voltage) changes when it goes
through a transformer according to its turn ratio. Therefore, the process of referring
quantities to a common voltage level is done automatically in the per-unit system.
46

Cont.…
 When only one device (transformer or motor) is analyzed, its own ratings
are used as the basis for per-unit system. When considering a transformer
in a per-unit system, transformer’s characteristics will not vary much over a
wide range of voltages and powers. For example, the series resistance is
usually from 0.02 to 0.1 pu; the magnetizing reactance is usually from 10 to
40 pu; the core-loss resistance is usually from 50 to 200 pu. Also, the per-
unit impedances of synchronous and induction machines fall within
relatively narrow ranges over quite large size ranges.
 If more than one transformer is present in a system, the system base
voltage and power can be chosen arbitrary. However, the entire system must
have the same base power, and the base voltages at various points in the
system must be related by the voltage ratios of the transformers.
47

Cont.…
 System base quantities are commonly chosen to the base of the largest
component in the system.
 Per-unit values given to another base can be converted to the new base
either through an intermediate step (converting them to the actual values)
or directly as follows:
(𝑃, 𝑄, 𝑆)𝑝𝑢,𝑏𝑎𝑠𝑒 2= (𝑃, 𝑄, 𝑆)𝑝𝑢,𝑏𝑎𝑠𝑒 1∗
𝑉𝑝𝑢,𝑏𝑎𝑠𝑒 2 = 𝑉𝑝𝑢,𝑏𝑎𝑠𝑒1
𝑉𝑏𝑎𝑠𝑒 1
(𝑅, 𝑋, 𝑍)𝑝𝑢,𝑏𝑎𝑠𝑒 2= (𝑅, 𝑋, 𝑍)𝑝𝑢,𝑏𝑎𝑠𝑒 1∗
(𝑉𝑏𝑎𝑠𝑒 1)2(𝑆𝑏𝑎𝑠𝑒 2)
2(𝑆𝑏𝑎𝑠𝑒 1)
48

Voltage Regulation and Efficiency of Transformers
 Since a real transformer contains series impedances, the transformer’s
output voltage varies with the load even if the input voltage is constant. To
compare transformers in this respect, the quantity called a full-load voltage
regulation (VR) is defined as follows:
𝑉𝑅 =
𝑉𝑠,𝑛𝑙 − 𝑉𝑠,𝑓𝑙
𝑉𝑠,𝑓𝑙
× 100% =
Τ
𝑉
𝑝 𝑎 − 𝑉𝑠,𝑓𝑙
𝑉𝑠,𝑓𝑙
× 100%
 In a per-unit system:
𝑉𝑅 =
𝑉
𝑝,𝑝𝑢 − 𝑉𝑠,𝑓𝑙,𝑝𝑢
𝑉𝑠,𝑓𝑙, 𝑝𝑢
× 100%
Where 𝑉𝑠,𝑛𝑙 and 𝑉𝑠,𝑓𝑙 are the secondary no load and full load voltages.
 Note, the VR of an ideal transformer is zero. 49

The transformer phasor diagram
 To determine the VR of a transformer, it is necessary to understand the
voltage drops within it. Usually, the effects of the excitation branch on
transformer VR can be ignored and, therefore, only the series impedances
need to be considered.
 The VR depends on the magnitude of the impedances and on the current
phase angle.
 A phasor diagram is often used in the VR determinations.
 The phasor voltage 𝑉
𝑠 is assumed to be at 0° and all other voltages and
currents are compared to it.
50

Cont.…
 Considering the diagram referred
to the secondary side and by
applying the Kirchhoff’s voltage
law, the primary voltage is:
𝑉
𝑝
𝑎
= 𝑉
𝑠 + 𝑅𝑒𝑞𝐼𝑠 + 𝑗𝑋𝑒𝑞𝐼𝑠
 A transformer phasor diagram is a
graphical representation of this
equation.
51

Cont.…
 A transformer operating at a
lagging power factor. It is seen
that Τ
𝑉
𝑝 𝑎 > 𝑉
𝑠, 𝑉𝑅 > 0
 A transformer operating at a unity
power factor. It is seen that 𝑉𝑅 > 0
 A transformer operating at a
leading power factor. If the
secondary current is leading, the
secondary voltage can be higher
than the referred primary voltage;
𝑉𝑅 < 0. 52

Derivation of the approximate equation for Τ
𝑽𝑷 𝒂
53
 For lagging loads the vertical component of 𝑅𝑒𝑞 and 𝑋𝑒𝑞 will partially
cancel each other. Due to that the angle of Τ
𝑉
𝑝 𝑎 will be very small, hence
we can assume that Τ
𝑉
𝑝 𝑎 is horizontal. Therefore the approximation will be

The Transformer Efficiency
 The efficiency of a transformer is defined as:
𝜂 =
𝑃𝑜𝑢𝑡
𝑃𝑖𝑛
× 100% =
𝑃𝑜𝑢𝑡
𝑃𝑜𝑢𝑡 + 𝑃𝑙𝑜𝑠𝑠
× 100%
 Note: the same equation describes the efficiency of motors and generators.
 Considering the transformer equivalent circuit, we notice three types of
losses:
 Copper (𝐼2𝑅) losses – are accounted for by the series resistance
 Hysteresis losses – are accounted for by the resistor 𝑅𝐶.
 Eddy current losses – are accounted for by the resistor 𝑅𝐶.
54

Cont.…
 Since the output power is
𝑠𝐼𝑠 cos 𝜃𝑠
 The efficiency of the transformer can be expressed by:
𝜂 =
𝑉𝑠𝐼𝑠 cos 𝜃
𝑃𝑐𝑢+𝑃𝑐𝑜𝑟𝑒+𝑉𝑠𝐼𝑠 cos 𝜃
× 100%
55
Example?

Transformer taps and voltage regulation
 Taps allow adjustment of the transformer in the field to accommodate for
local voltage variations.
 Sometimes, transformers are used on a power line, whose voltage varies
widely with the load (due to high line impedance, for instance). Normal
loads need fairly constant input voltage though…
 One possible solution to this problem is to use a special transformer called
a tap changing under load (TCUL) transformer or voltage regulator. TCUL
is a transformer with the ability to change taps while power is connected to
it. A voltage regulator is a TCUL with build-in voltage sensing circuitry that
automatically changes taps to keep the system voltage constant.
 These “self-adjusting” transformers are very common in modern power systems. 56

The Autotransformer
 Sometimes, it is desirable to change the voltage by a small amount (for
instance, when the consumer is far away from the generator and it is needed
to raise the voltage to compensate for voltage drops).
 In such situations, it would be expensive to wind a transformer with two
windings of approximately equal number of turns. An autotransformer (a
transformer with only one winding) is used instead.
 Diagrams of step-up and step-down autotransformers are shown below:
 Output (up) or input (down) voltage is a sum of voltages across common
and series windings.
57

Cont.…
Series winding
Common winding
Series
winding
Common
winding
Output (up) or input (down) voltage is a sum of voltages across
common and series windings.
58
A step-down autotransformer connection
A step-up autotransformer connection

Cont.…
 Since the autotransformer’s coils are physically connected, a different terminology is
used for autotransformers:
 The voltage across the common winding is called a common voltage VC, and the current
through this coil is called a common current 𝐼𝐶. The voltage across the series winding is
called a series voltage 𝑉𝑆𝐸, and the current through that coil is called a series current 𝐼𝑆𝐸.
 The voltage and current on the low-voltage side are called 𝑉𝐿 and 𝐼𝐿; the voltage and
current on the high-voltage side are called 𝑉𝐻 and 𝐼𝐻.
 For the autotransformers:
𝑉𝐶
𝑉𝑆𝐸
=
𝑁𝐶
𝑁𝑆𝐸
𝑁𝐶𝐼𝐶 = 𝑁𝑆𝐸𝐼𝑆𝐸
𝑉𝐿 = 𝑉𝐶 𝐼𝐿 = 𝐼𝐶 + 𝐼𝑆𝐸
𝑉𝐻 = 𝑉𝐶 + 𝑉𝑆𝐸 𝐼𝐻 = 𝐼𝑆𝐸
59

Voltage and Current relationships in an Autotransformer
60

The apparent power advantage of Autotransformers
 Not all the power traveling from the primary to the secondary winding of the
autotransformer goes through the windings. As a result, an autotransformer can handle
much power than the conventional transformer (with the same windings).
 Considering a step-up autotransformer, the apparent input and output powers are:
𝑆𝑖𝑛 = 𝑉𝐿𝐼𝐿 𝑆𝑜𝑢𝑡 = 𝑉𝐻𝐼𝐻
 It is easy show that 𝑆𝑖𝑛 = 𝑆𝑜𝑢𝑡 = 𝑆𝐼𝑂
 Where 𝑆𝐼𝑂 is the input and output apparent powers of the autotransformer.
 However, the apparent power in the autotransformer’s winding is
𝑆𝑊 = 𝑉
𝑐𝐼𝐶 = 𝑉𝑆𝐸𝐼𝑆𝐸
SW = VL IL − IH = VLIL − VLIH = VLIL − VLIL
NC
NSE+NC
= VLIL
(NSE + NC) − NC
NSE + NC
= SIO
NSE
NSE + NC
61

Cont.…
 Therefore, the ratio of the apparent power in the primary and secondary of
the autotransformer to the apparent power actually traveling through its
windings is
SIO
SW
=
NSE + NC
NSE
 The last equation described the apparent power rating advantage of an
autotransformer over a conventional transformer.
 𝑆𝑊 is the apparent power actually passing through the windings. The rest
passes from primary to secondary parts without being coupled through the
windings.
 Note that the smaller the series winding, the greater the advantage! 62

Cont.…
 For example, a 5 MVA autotransformer that connects a 110 kV system to a
138 kV system would have a turns ratio (common to series) 110:28. Such an
autotransformer would actually have windings rated at:
𝑆𝑊 = 𝑆𝐼𝑂
𝑁𝑆𝐸
𝑁𝑆𝐸+𝑁𝐶
= 5.
28
28+110
= 1.015 MVA
 Therefore, the autotransformer would have windings rated at slightly over 1
MVA instead of 5 MVA, which makes is 5 times smaller and, therefore,
considerably less expensive.
 However, the construction of autotransformers is usually slightly different.
In particular, the insulation on the smaller coil (the series winding) of the
autotransformer is made as strong as the insulation on the larger coil
to withstand the full output voltage.
63

Variable-voltage Autotransformers
 The effective per-unit impedance of an autotransformer is smaller than of
a conventional transformer by a reciprocal to its power advantage. This is
an additional disadvantage of autotransformers.
 It is a common practice to make variable voltage autotransformers.
64

Three Phase transformers
 The majority of the power generation/distribution systems in the world are
3-phase systems. Transformers for 3-phase circuits can be constructed in
two ways:
 Connect 3 single phase transformers
 Three sets of windings wrapped around a common core.
 A single three-phase transformer is lighter, smaller, cheaper, and slightly
more efficient, but using three separate single-phase transformers has the
advantage that each unit in the bank could be replaced individually in the
event of trouble.
65

Cont.…
66
A three-phase transformer bank composed
of independent transformers.
A three phase transformer wound on a single
three-legged core.

Three Phase Transformer Connections
 We assume that any single transformer in a 3-phase transformer (bank)
behaves exactly as a single-phase transformer. The impedance, voltage
regulation, efficiency, and other calculations for 3-phase transformers are
done on a per-phase basis, using the techniques studied previously for
single-phase transformers.
 Four possible connections for a 3-phase transformer bank are:
1. Wye-Wye(Y-Y)
2. Wye-Delta(Y-∆)
3. Delta-Delta(∆ - ∆ )
4. Delta-Wye(∆ -Y)
67

1. Y-Y Connection
 The primary voltage on each phase
of the transformer is
𝑉∅𝑃 =
𝑉𝐿𝑃
√3
 The secondary phase voltage is
𝑉𝐿𝑆 = √3𝑉∅𝑆
 The overall voltage ratio is
𝑉𝐿𝑃
𝑉𝐿𝑆
=
√3𝑉∅𝑃
√3𝑉∅𝑆
= a
68

Cont.…
The Y-Y connection has two very serious problems:
1). If loads on one of the transformer circuits are unbalanced, the voltages
on the phases of the transformer can become severely unbalanced.
2). The third harmonic issue.
The voltages in any phase of an Y-Y transformer are 120° apart from the
voltages in any other phase. However, the third-harmonic components of
each phase will be in phase with each other. Nonlinearities in the
transformer core always lead to generation of third harmonic! These
components will add up resulting in large (can be even larger than the
fundamental component) third harmonic component.
69

Cont.…
Both problems can be solved by one of two techniques:
1). Solidly ground the neutral of the transformers (especially, the primary
side). The third harmonic will flow in the neutral and a return path will be
established for the unbalanced loads.
2). Add a third ∆ -connected winding. A circulating current at the third
harmonic will flow through it suppressing the third harmonic in other
windings.
70

2. Y- ∆ Connection
𝑉∅𝑃 =
𝑉𝐿𝑃
√3
𝑉𝐿𝑆 = 𝑉∅𝑆
𝑉𝐿𝑃
𝑉𝐿𝑆
=
√3𝑉∅𝑃
𝑉∅𝑆
= √3𝑎
71

Cont.…
 The Y- ∆ connection has no problem with third harmonic components due
to circulating currents in ∆. It is also more stable to unbalanced loads since
the ∆ partially redistributes any imbalance that occurs.
 One problem associated with this connection is that the secondary voltage
is shifted by 30° with respect to the primary voltage. This can cause
problems when paralleling 3-phase transformers since transformers
secondary voltages must be in-phase to be paralleled. Therefore, we must
pay attention to these shifts.
 In the U.S., it is common to make the secondary voltage to lag the primary
voltage. The connection shown in the previous slide will do it.
72

3.  -Y Connection
𝑉∅𝑃 = 𝑉𝐿𝑃
𝑉𝐿𝑆 = √3𝑉∅𝑆
𝑉𝐿𝑃
𝑉𝐿𝑆
=
𝑉∅𝑃
√3𝑉∅𝑆
=
𝑎
√3
73

4.  -  Connection
𝑉∅𝑃 = 𝑉𝐿𝑃
𝑉𝐿𝑆 = 𝑉∅𝑆
𝑉𝐿𝑃
𝑉𝐿𝑆
=
𝑉∅𝑃
𝑉∅𝑆
= 𝑎
74

3-phase transformer: per-unit system
 The per-unit system applies to the 3-phase transformers as well as to single-
phase transformers. If the total base VA value of the transformer bank is
𝑆𝑏𝑎𝑠𝑒, the base VA value of one of the transformers will be
𝑆𝑙∅,𝑏𝑎𝑠𝑒 =
𝑆𝑏𝑎𝑠𝑒
3
 Therefore, the base phase current and impedance of the transformer are
𝐼∅,𝑏𝑎𝑠𝑒 =
𝑆𝑙∅,𝑏𝑎𝑠𝑒
𝑉∅,𝑏𝑎𝑠𝑒
=
3𝑉∅𝑏𝑎𝑠𝑒
𝑍𝑏𝑎𝑠𝑒 =
(𝑉∅,𝑏𝑎𝑠𝑒)2
𝑆1∅,𝑏𝑎𝑠𝑒
=
3(𝑉∅,𝑏𝑎𝑠𝑒)2
75

Cont.…
 The line quantities on 3-phase transformer banks can also be represented in
per-unit system. If the windings are in  :
𝑉𝐿,𝑏𝑎𝑠𝑒 = 𝑉∅,𝑏𝑎𝑠𝑒
 If the windings are in Y:
𝑉𝐿,𝑏𝑎𝑠𝑒 = √3𝑉∅,𝑏𝑎𝑠𝑒
 And the base line current in a 3-phase transformer bank is
𝐼𝐿,𝑏𝑎𝑠𝑒 =
√3𝑉𝐿,𝑏𝑎𝑠𝑒
 The application of the per-unit system to 3-phase transformer problems is
similar to its application in single-phase situations. The voltage regulation
of the transformer bank is the same.
76

Transformer ratings
 Transformers have the following major ratings:
Apparent power;
Voltage;
Current;
Frequency.
77

Transformer ratings: Voltage and Frequency
 Transformer ratings: Voltage and
Frequency
 If a steady-state voltage
is applied to the transformer’s
primary winding, the transformer’s
flux will be
78
 An increase in voltage will lead to a
proportional increase in flux. However,
after some point (in a saturation region),
such increase in flux would require an
unacceptable increase in magnetization
current!

Cont.…
79
Magnetization current
Fig. The effect of the peak flux in a
transformer core upon the required
magnetization current

Transformer ratings: Voltage and Frequency
 Therefore, the maximum applied voltage (and thus the rated voltage) is set
by the maximum acceptable magnetization current in the core.
 We notice that the maximum flux is also related to the frequency:
∅𝑚𝑎𝑥 =
𝑉
𝑚𝑎𝑥
𝜔𝑁𝑃
 Therefore, to maintain the same maximum flux, a change in frequency (say,
50 Hz instead of 60 Hz) must be accompanied by the corresponding
correction in the maximum allowed voltage. This reduction in applied
voltage with frequency is called derating. As a result, a 50 Hz transformer
may be operated at a 20% higher voltage on 60 Hz if this would not cause
insulation damage. 80

Transformer ratings: Apparent Power
 The apparent power rating sets (together with the voltage rating) the
current through the windings. The current determines the 𝑖2
𝑅 losses and,
therefore, the heating of the coils. Remember, overheating shortens the life
of transformer’s insulation!
 In addition to apparent power rating for the transformer itself, additional
(higher) rating(s) may be specified if a forced cooling is used. Under any
circumstances, the temperature of the windings must be limited.
 Note, that if the transformer’s voltage is reduced (for instance, the
transformer is working at a lower frequency), the apparent power rating
must be reduced by an equal amount to maintain the constant current.
81

Transformer ratings: Current inrush
 Assuming that the following voltage is applied to the transformer at the
moment it is connected to the line:
𝑣 𝑡 = 𝑉𝑀 sin(𝜔𝑡 + 𝜃)
 The maximum flux reached on the first half-cycle depends on the phase of
the voltage at the instant the voltage is applied. If the initial voltage is
𝑣 𝑡 = 𝑉𝑀 sin(𝜔𝑡 + 𝜃) = 𝑉𝑀 cos 𝜔𝑡
 and the initial flux in the core is zero, the maximum flux during the first
half-cycle is equals to the maximum steady-state flux (which is ok):
∅𝑚𝑎𝑥 =
𝑉𝑀
𝜔𝑁𝑃
 However, if the voltage’s initial phase is zero, i.e.
𝑣 𝑡 = 𝑉𝑀 sin(𝜔𝑡)
82

Cont.…
 The maximum flux during the first
half-cycle will be
∅𝑚𝑎𝑥 =
1
𝑁𝑃
න
0
Τ
𝜋
𝜔
𝑉𝑀 sin 𝜔𝑡 𝑑𝑡
=
𝑉𝑀
𝜔𝑁𝑃
cos 𝜔𝑡
=
2𝑉𝑀
𝜔𝑁𝑃
 Which is twice higher than a normal
steady-state flux!
 Doubling the maximum flux in the core
can bring the core in a saturation and,
therefore, may result in a huge
magnetization current!
 Normally, the voltage phase angle
cannot be controlled. As a result, a large
inrush current is possible during the first
several cycles after the transformer is
turned ON.
 The transformer and the power system
must be able to handle these currents.
83

Cont.…
84
The current inrush due to a transformer's magnetization current on starting.

Transformer ratings: Information Plate
 Rated voltage, currents, and (or)
power is typically shown on the
transformer’s information plate.
 Additional information, such as per-unit
series impedance, type of cooling, etc.
can also be specified on the plate.
85

Instrument Transformers
 Two special-purpose transformers are used to take measurements: potential
and current transformers.
 A potential transformer has a high-voltage primary, low-voltage secondary,
and very low power rating. It is used to provide an accurate voltage samples
to instruments monitoring the power system.
 A current transformer samples the current in a line and reduces it to a safe
and measurable level. Such transformer consists of a secondary winding
wrapped around a ferromagnetic ring with a single primary line (that may
carry a large current )running through its center. The ring holds a small
sample of the flux from the primary line. That flux induces a secondary
voltage. 86

Cont.…
 Windings in current transformers are loosely coupled: the mutual flux is
much smaller than the leakage flux. The voltage and current ratios do not
apply although the secondary current is directly proportional to the
primary.
 Current transformers must be short-circuited at all times since very high
voltages can appear across their terminals.
87

Unit 2 - Transformers.pdf

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