TRANSFORMER
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HOW DOES TRANSFORMER WORKS
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Why and where to use X’mer
Principle
Types of transformer
Construction
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Mutual Inductance
i2
i1
v1
N1
N2
v2
Faraday's Law
In linear range, flux is proportional to current
di1
di2
v1 (t )  L11
 L12
dt
dt
self-inductance
mutual inductance
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TRANSFORMER PRINCIPLE
• So the transformer is static device that can
transfer power from one level to another
without change in frequency.
CORE
COIL1
COIL2
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Working principle
• Physical basis is mutual induction
between two coils linked by common
magnetic circuit with low reluctance
path and high mutual inductance.
• AC source on primary cause
alternating flux in core and will
induce mutual emf in secondary coil.
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Construction
• There are basically two types of
transformer Core type and Shell type.
Winding surrounds
considerable part
of core.
Core
Winding
Core surrounds
considerable part of
winding
Core
Winding
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Construction
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Construction
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Core-Construction
• Coils wound on core are of cylindrical type. It
may be circular, rectangular or oval.
• For small size rectangular core used with
cylindrical coils.
• For large size transformer round coil fitted on
cruciform core.
• Cylindrical coil have higher strength and
wound in helical layer with different layer
insulated by paper, micarta, cloth.
• Because of lamination net or effective area
reduced by 10%
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Core-Construction
Helical coil
wound on core
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Core-Construction
• For large size transformer round coil
fitted on cruciform core reason is to
give high space factor as shown in
figure.
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Core-Construction
• Core stepping give high space factor
for coil but also reduces the length of
mean turn of winding so the I2R losses
also reduced.
• But stepped core increases labour
charges
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Shell-Construction
• Coils are form wound but are
multilayer disc type and insulated
from each other by paper.
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Core-Construction
• Basically CRGO(Cold Rolled Grain
Oriented silicon Steel) material is used
for the construction of core.
• It have high operating flux density
with lower loss per kg. so reduces
weight per kVA, and rigid in
construction, lower iron loss, lower
cost of manufacturing.
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For example : Mobile charger
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For example : Mobile charger
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Construction
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1. Transformer Tank
2. High Voltage Bushing
3. Low Voltage Bushing
4. Cooling Fins/Radiator
5. Cooling Fans –
6. Conservator Tank
7. System Ground Terminal
8. Drain Valve 9. Dehydrating Breather
10. Oil Temperature/Pressure gauges
11. Bushing Current Transformers
12. Control Panel
13. Surge Arresters
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Tank & Conservator
• Transofrmer assembly is placed inside
metal tank and immersed in oil which
acts as insulator as well as coolant.
• Oil is acting as cooling media, as well
winding should be protected from
moisture and dust, dirt so oil serve for
this purpose.
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Construction
• Bushings are provided for the purpose of
supporting conductor that are connected
to winding of transformer. That are made
of ceramic materials for better isolation
from tank to conductor and are provided
on both HV & LV windings.
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Construction
• Core is made up of sheet steel
laminations for continues magnetic path
with minimum airgap.
• Steel is of high silicon content and high
permeability with low hysteresis loss.
• Eddy current reduced by laminations.
• Thickness of lamination 0.35mm for
50Hz
• 0.5mm for 25Hz.
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Parts
• Good transformer
should free for
alkalies, sulphur,
and moisture. The
presence of moisture
lower down the
dielectric strength of
oil. This is avoided by
breather to permit oil
in tank to expand
and contract.
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Cooling of transformer
Oil Filled Self-Cooled Type
Oil filled self cooled type uses small and medium-sized distribution transformers. The
assembled windings and core of such transformers are mounted in a welded, oil-tight steel
tanks provided with a steel cover. The tank is filled with purified, high quality insulating oil as
soon as the core is put back at its proper place. The oil helps in transferring the heat from the
core and the windings to the case from where it is radiated out to the surroundings. For smaller
sized transformers the tanks are usually smooth surfaced, but for large size transformers a
greater heat radiation area is needed, and that too without disturbing the cubical capacity of the
tank. This is achieved by frequently corrugating the cases. Still larger sizes are provided with
radiation or pipes.
2. Oil Filled Water Cooled Type
This type is used for much more economic construction of large transformers, as the above told
self cooled method is very expensive. The same method is used here as well- the windings and
the core are immersed in the oil. The only difference is that a cooling coil is mounted near the
surface of the oil, through which cold water keeps circulating. This water carries the heat from
the device. This design is usually implemented on transformers that are used in high voltage
transmission lines. The biggest advantage of such a design is that such transformers do not
require housing other than their own. This reduces the costs by a huge amount. Another
advantage is that the maintenance and inspection of this type is only needed once or twice in a
year.
3. Air Blast Type
This type is used for transformers that use voltages below 25,000 volts. The transformer is
housed in a thin sheet metal box open at both ends through which air is blown from the bottom
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to the top
Radiator cooling
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Force Air Cooling
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Air cooling
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EMF equation
Let N1 =No. of turns in primary
N2 =No. of turns in secondary
φm =Maximum flux in core in Webbers = Bm × A
f =frequency of a.c. input in HZ
flux increases from its zero value to maximum
value φm in one quarter of the cycle i.e. in 1/4f second.
• Average rate of change of flux = φm / (1 / 4f)
= 4fφm Wb/s or volt
• Now, rate of change of flux per turn means induced
e.m.f. in volts.
• Average e.m.f. / turn = 4fφm volt
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Cntd.
• φm =Maximum flux in core in Webbers = Bm × A
• f =frequency of a.c. input in HZ
• flux increases from its zero value to maximum
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EMF equation
• If flux Ø varies sinusoidally, then r.m.s value of induced e.m.f is
obtained by multiplying the average value with form factor.
Form Factor = r.m.s. value/average value = 1.11
Therefore, r.m.s value of e.m.f/turn = 1.11 X 4f Ømax = 4.44f Ømax
• Now, r.m.s value of induced e.m.f in the whole of primary
winding= (induced e.m.f./turn) X Number of primary turns
Therefore,
E1 = 4.44f N1Ømax = 4.44fN1BmA
Similarly, r.m.s value of induced e.m.f in secondary is
• E2= 4.44f N2 Ømax = 4.44fN2BmA
• In an ideal transformer on no load,
• V1 = E1 and V2 = E2 , where V2 is the terminal voltage
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Transformation Ratio:
Ideal Transformer - Voltage
v1
d
v1 (t )  N1
dt
i2
i1
N1
N2
This changing flux through coil 2
induces a voltage, v2 across coil 2
d
v1 N1 dt
N1


v2 N 2 d N 2
dt
v2
The input AC voltage, v1,
produces a flux

1

v1 (t )dt

N1
v2 (t )  N 2
d
dt
N2
v2 
v1
N1
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Transformer - Current
i2
i1
v1
N1
N2
v2

Input power= output power
V1I1= V2I2
So,
where
N2
v2 
v1
N1
N1i1  N 2i2
i2 
N1
i1
N2
and
N2
v2 
v1
N1
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Ideal Transformer
• An Ideal Transformer is an imaginary transformer which does not have any loss in
it, means no core losses, copper losses and any other losses in transformer.
Efficiency of this transformer is considered as 100%.
• Ideal Transformer Model is developed by considering a transformer which does not
have any loss. That means the windings of the transformer are purely inductive and
core of transformer is loss free. There is zero Leakage Reactance of Transformer.
• As we said, whenever we place a low reluctance core inside the windings,
maximum amount of flux passes through this core; but still there is some flux
which does not pass through the core but passes through the insulation used in the
transformer.
• This flux does not take part in the transformation action of the transformer. This
flux is called leakage flux of transformer. In an Ideal Transformer, this leakage flux
is considered also nil.
• That means 100% flux passes through the core and linked with both primary and
secondary windings of transformer. Although every winding is desired to be purely
inductive but it has some resistance in it which causes voltage drop and I2R loss in
it. In such ideal transformer model, the winding are also considered, ideal that
means resistance of the winding is zero.
Ideal Transformer
Now if an alternating source voltage V1 is applied in the primary winding of that Ideal
Transformer, there will be a counter self emf E1 induced in the primary winding which is purely
180o in phase opposition with supply voltage V1.
For developing counter emf E1 across the the primary winding it draws electric current from the
source to produces required magnetizing flux. As the primary winding is purely inductive, that
current is in 90o lags from the supply voltage. This current is called magnetizing current of
transformer Iµ
Ideal Transformer
This alternating current, Iµ produces a alternating magnetizing flux Φ which is proportional to
that electric current and hence in phase with it. As this flux is also linked with secondary
winding through the core of transformer, there will be another emf E2 induced in the secondary
winding, this is mutually induced emf. As the secondary is placed on the same core where the
primary winding is placed, the emf induced in the secondary winding of transformer, E2 is in
the phase with primary emf E1 and in phase opposition with source voltage V1.
Practical Transformer
Practical Transformer
Losses In Transformer
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Core Loss or Iron losses
• Core loss or iron loss.
• Due to alternating flux magnetic core of
transformer subjected to magnetization
and demagnetization. Due to this there is
energy loss in transformer.
• Pcore=Ph + Pe
• Ph=Kh*Bm1.67fV
• Pe=Ke*Bm2*f2*t2
• Both the losses remains constant at any
load that’ s why its called constant losses.
• Both the losses minimized by high grade
steel and lamination.
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Copper loss
• This is due to power wasted in form of I2R
in winding.
• Pcu proportional to I2 or (kVA)2
• Total losses= Iron losses+Copper losses
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Total loss
• Total losses= Iron losses+Copper losses
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Equivalent Circuit of transformer
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Equivalent Circuit of transformer
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Equivalent Circuit of transformer
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Equivalent Circuit of transformer
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Equivalent Circuit of transformer
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Test
•The efficiency and voltage regulation of transformer at any load condition and
power
factor can be predermined by indirect loading method.
These test are
OPEN CIRCUIT TEST(gives exciting branch)
SHORT CIRCUIT TEST(give eq. parameter of transformer)
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Open circuit Test
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Short circuit Test
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Short circuit Test
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Regulation of transformer
• Change in terminal voltage from full
load to no load on secondary side
given as regulation of transformer
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Transformer Losses and Efficiency
Condition for maximum efficiency
All day efficiency
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Effect of pf on Efficicency
• As the power factor improves the
efficiency get higher and higher.
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Auto Transformer
• ab
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Auto Transformer
Auto Transformer
Auto Transformer-application
Parallel operation of transformers
Wrong connections give circulating between the windings that
can destroy transformers.
Transformer
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Parallel Operation of transformer

Parallel connection of transformer is necessary where load demand exceeds the
rated capacity of single transformer by paralleling we can cater the load demand as
well as it provides added advantage of more reliability compare to single one.
 But before parallel operation of two or more transformer certain condition must be
fulfilled
• For single phase transformers:
– Same polarity of transformers
– Same voltage ratio
• For 3 phase transformers:
– Same polarity
– Zero relative phase displacement
– Same phase sequence
– Same voltage ratio
Parallel Operation of transformer
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Polarity: The polarity of the transformers connected in parallel should be same
otherwise it may lead to dead short circuit.
Voltage Ratio: The voltage ratio of the 2 transformers should be kept equal in
order to avoid losses occurring in transformers due to load circulating currents.
If unequal voltage ratio is used it will give rise to circulating current in the
closed circuit formed by the secondary of the transformer even at no load
condition. The maximum permissible no-load circulating current should be
10% of its rated value.
Zero Relative Phase Displacement: This is the necessary condition for the 3
phase transformers. As the name suggests, the relative phase displacement
between the two transformers must be zero.
Phase Sequence: This is also an important condition for 3 phase transformers
which needs the phase sequence of the 2 transformers to be same otherwise it
may lead to short circuit of the each phase.
Parallel Operation of transformer
THREE PHASE
TRANSFORMER
Three phase transformer
3- Transformer Construction (3)
Transformer
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3- Transformer Construction(4)
Left: A 1300 MVA, 24.5/345 kV, 60Hz transformer with forced oil
and air (fan) cooling.
Right: A 60 MVA, 225/26.4 kV, 60 Hz showing the conservator.
Transformer
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3-phase transformers
The majority of the power generation/distribution systems in the world are 3-phase
systems. The transformers for such circuits can be constructed either as a 3-phase
bank of independent identical transformers (can be replaced independently) or as a
single transformer wound on a single 3-legged core (lighter, cheaper, more efficient).
3-phase transformers Type
• Core and Shell type
3-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.
2.
3.
4.
5.
6.
Y-Y
Y-
- 
-Y
Open Delta or V
Scott or T –T (3 phase to 3 phase conversion)
General things before Connections
• In Star Vph=VL/1.73 number of turns per phase
and insulation quantity is less(Suitable for large
HV transformer)
• In star Iph=IL winding carry high current cross
section area of winding high so winding
mechanical strong and can bear heavy loading
and short circuits.
• In delta Iph=IL/1.73 cross section of winding less
so connection economical for LV transformer
• In delta Vph=VL economical for large LV
transformer it increases number of turns per
phase.
General things before Connections
• In star line and phase voltage are 30o phase
shifted. So in star delta 30o phase shift between
primary and secondary voltage.
• In order to get secondary volt as sinusoidal the
magnetizing current of transformer must
contains 3rd harmonic component. The delta
connection provides close path for circulate 3rd
harmonic current.
• In star unbalance loading is problem.
• In star 3phase 4wire is avialable.
3-phase transformer Y-Y connections
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
3V P
VLP

a
VLS
3V S
3-phase transformer connections advantage
1. Due to star connection, phase voltages is (1/√3) times the line voltage. Hence
less
number of turns are required. Also the stress on insulation is less. This makes
the
connection economical for small high voltage purposes.
2. Due to star connection, phase current is same as line current. Hence windings
have to
carry high currents. This makes cross section of the windings high. Thus the
windings are
mechanically strong and windings can bear heavy loads and short circuit.
3. There is no phase shift between the primary and secondary voltages.
4. As neutral is available, it is suitable for three phase, four wire system.
3-phase transformer connections
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
1200 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.
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.
3 Transformer Interconnections
-Y
Y-
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3-phase transformer Y- connections
2. Y- connection:
The primary voltage on each phase of
the transformer is
V P 
VLP
3
(4.79.1)
The secondary phase voltage is
VLS  V S
(4.79.2)
The overall voltage ratio is
3V P
VLP

 3a
VLS
V S
(4.79.3)
3-phase transformer Y- connections
Advantages
The primary side is star connected. Hence fewer number of turns are required. This
makes the connection economical for large high voltage step down power
transformers.
The neutral available on the primary can be earthed to avoid distortion.
Large unbalanced loads can be handled satisfactory.
Disadvantages
In this type of connection, the secondary voltage is not in phase with the primary.
Hence it is not possible to operate this connection in parallel with star-star or deltadelta connected transformer
3-phase transformer Y- connections
= Line voltage on primary side.
VL2 = Line voltage on secondary side.
Vph1 = Phase voltage on primary side.
Vph2 = Phase voltage on secondary side.
K = Transformer ratio.
Vph1 = VL1/√3
Now
Vph2 /Vph1 = K
. ..
Vph2 = K Vph1 = K (VL1/√3)
Since secondary is connected in delta.
Vph2 = VL2
VL2 = K (VL1/√3) = ( K/√3) VL1
L1
3-phase transformer Delta-Star
3.  -Y connection:
The primary voltage on each phase of
the transformer is
V P  VLP
(4.81.1)
The secondary phase voltage is
VLS  3V S
(4.81.2)
The overall voltage ratio is
V P
VLP
a


VLS
3V S
3
(4.81.3)
The same advantages and the same
phase shift as the Y- connection.
Delta-Star
Let
VL1 = Line voltage on primary side.
VL2 = Line voltage on secondary side.
Vph1 = Phase voltage on primary side.
Vph2 = Phase voltage on secondary side.
K = Transformation ratio.
As primary in delta connected,
VL1 = Vph1
Now,
Vph2/Vph1 = K
. ..
Vph2 = K Vph1
Here secondary is connected in star
VL2 = √3 Vph2
VL2 = (√3 K) Vph1
VL2 = (√3 K) VL1
Delta-Star
Advantages
On primary side due to delta connection winding cross-section required is less.
On secondary side, neutral is available, due to which it can be used for 3-phase, 4
wire supply system.
There is no distortion due to third harmonic componenets.
The windings connected on star makes it economical due to saving in cost of
insulation.
Large unbalanced loads can be handled without any difficulty.
Disadvantages
Due to phase shift between primary and secondary voltages, the limitation of StarDelta connection continues for this type of connection as well.
3-phase transformer  -  connections
4.  -  connection:
The primary voltage on each phase of
the transformer is
V P  VLP
(4.82.1)
The secondary phase voltage is
VLS  V S
(4.82.2)
The overall voltage ratio is
VLP V P

a
VLS V S
No phase shift, no problems with
unbalanced loads or harmonics.
(4.82.3)
Delta-Delta
Advantages
On primary side due to delta connection winding cross-section required is less.
On secondary side, neutral is available, due to which it can be used for 3-phase, 4
wire supply system.
There is no distortion due to third harmonic componenets.
The windings connected on star makes it economical due to saving in cost of
insulation.
Large unbalanced loads can be handled without any difficulty.
Disadvantages
Due to phase shift between primary and secondary voltages, the limitation of StarDelta connection continues for this type of connection as well.
Delta-Delta
It can be seen that there is no phase shift between primary and secondary
voltages.
VL1 = Line voltage on primary side.
VL2 = Line voltage on secondary side.
Vph1 = phase voltage on primary side.
Vph2 = Phase voltage on secondary side.
K = Transformer ratio.
For delta connection, VL1 = Vph1
Now since
Vph2 /Vph1 = K
. ..
Vph2 = K Vph1
But again since secondary is connected in delta
VL2 = Vph2 = K VL1
Delta-Delta
Advantages
1.in order to get secondary voltage as sinusoidal, the magnetizing current of
transformer must contain a third harmonic component. The delta connection
provides a closed path for circulation of third harmonic component of current. The
flux remains sinusoidal which results in sinusoidal voltages.
2.Even if the load is unbalanced the three phase voltages remains constant. Thus it
allows unbalanced loading also.
3.The important advantage with this type of connection is that if there is bank of
single phase transformers connected in delta-delta fashion and if one of the
transformers is disabled then the supply can be continued with remaining tow
transformers of course with reduced efficiency.
4. There is no distortion in the secondary voltages.
5. Due to delta connection, phase voltage is same as line voltage hence winding
have more number of turns. But phase current is (1/√3) times the line current.
Hence the cross-section of the windings is very less. This makes the connection
economical for low voltages transformers.
Disadvantages
Due to the absence of neutral point it is not suitable for three phase four wire
system.
Open Delta connection
• The voltages are shown on phasor diagram. The
connection is used when the three phase load is very
very small to warrant the installation of full three
phase transformer.
• If one of the transformers fails in ∆ - ∆ bank and if it is
required to continue the supply eventhough at reduced
capacity until the transformer which is removed from
the bank is repaired or a new one is installed then this
type of connection is most suitable.
• When it is anticipated that in future the load increase,
then it requires closing of open delta. In such cases
open delta connection is preferred.
Open Delta connection
It can be seen from the Fig. 2(a)
∆ - ∆ capacity = √3 VL IL = √3 VL (√3 Iph )
∆ - ∆ capacity = 3 VL Iph
V- V capacity = √3 VL IL = √3 VL Iph
Thus the three phase load that can be carried without exceeding
the ratings of the transformers is 57.5 percent of the original load.
Hence it is not 66.7 % which was expected otherwise.
•
•
•
•
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Open
Delta
connection
Limitations
The limitation with V -V connection are given below :
The average p.f. at which V- V bank is operating is less than that with the
load . This power p.f is 86.6 % of the balanced load p.f.
The tow transformers in V -V bank operate at different power factor except
for balanced unity p.f .load.
The terminals voltages available on the secondary side become unbalanced.
This may happen eventhough load is perfectly balanced.
Thus in summary we can say that if tow transformers are connected in
V - V fashion and are loaded to rated capacity and one transformer is added
to increase the total capacity by √3 or 173.2 %. Thus the increase in
capacity is 73.2 % when converting from a V - V system to a ∆-∆ system.
With a bank of tow single phase transformers connected in V-V fashion
supplying a balanced 3 phase load with cosΦ asp.f., one of the transformer
operate at a p.f. of cos (30-Φ) and other at cos (30+Φ). The powers of tow
transformers are given by,
P1 = KVA cos (30-Φ)
P2 = KVA cos (30+Φ)
Scott or T-T connection or
3 phase to 2phase conversion
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•
•
•
•
There are two main reasons for the need to transform from three phases to
two phases,
To give a supply to an existing two phase system from a three phase supply.
To supply two phase furnace transformers from a three phase source.
Two-phase systems can have 3-wire, 4-wire, or 5-wire circuits. It is needed
to be considering that a two-phase system is not 2/3 of a three-phase
system. Balanced three-wire, two-phase circuits have two phase wires, both
carrying approximately the same amount of current, with a neutral wire
carrying 1.414 times the currents in the phase wires. The phase-to-neutral
voltages are 90° out of phase with each other.
Two phase 4-wire circuits are essentially just two ungrounded single-phase
circuits that are electrically 90° out of phase with each other. Two phase 5wire circuits have four phase wires plus a neutral; the four phase wires are
90° out of phase with each other.
Scott or T-T connection
•
•
•
The easiest way to transform three-phase voltages into two-phase
voltages is with two conventional single-phase transformers. The first
transformer is connected phase-to-neutral on the primary (three-phase)
side and the second transformer is connected between the other two
phases on the primary side.
The secondary windings of the two transformers are then connected to
the two-phase circuit. The phase-to-neutral primary voltage is 90° out of
phase with the phase-to-phase primary voltage, producing a two-phase
voltage across the secondary windings. This simple connection, called the
T connection, is shown in Figure
The main advantage of the T connection is that it uses transformers with
standard primary and secondary voltages. The disadvantage of the T
connection is that a balanced two-phase load still produces unbalanced
three-phase currents; i.e., the phase currents in the three-phase system
do not have equal magnitudes, their phase angles are not 120° apart, and
there is a considerable amount of neutral current that must be returned to
the source.
Scott or T-T connection
•
•
•
•
A Scott-T transformer (also called a Scott connection) is a type of circuit
used to derive two-phase power from a three-phase source or vice-versa.
The Scott connection evenly distributes a balanced load between the
phases of the source.
Scott T Transformers require a three phase power input and provide two
equal single phase outputs called Main and Teaser. The MAIN and Teaser
outputs are 90 degrees out of phase. The MAIN and the Teaser outputs
must not be connected in parallel or in series as it creates a vector current
imbalance on the primary side.
MAIN and Teaser outputs are on separate cores. An external jumper is also
required to connect the primary side of the MAIN and Teaser sections.
The schematic of a typical Scott T Transformer is shown below:
Scott or T-T connection
•
•
Scott T Transformer is built with two single phase transformers of equal power
rating. The MAIN and Teaser sections can be enclosed in a floor mount enclosure
with MAIN on the bottom and Teaser on top with a connecting jumper cable. They
can also be placed side by side in separate enclosures.
Assuming the desired voltage is the same on the two and three phase sides, the
Scott-T transformer connection consists of a center-tapped 1:1 ratio main
transformer, T1, and an 86.6% (0.5√3) ra o teaser transformer, T2. The centertapped side of T1 is connected between two of the phases on the three-phase
side. Its center tap then connects to one end of the lower turn count side of T2,
the other end connects to the remaining phase. The other side of the transformers
then connects directly to the two pairs of a two-phase four-wire system.
Scott or T-T connection
•
The Scott-T transformer connection may be also used in a back to back T to T
arrangement for a three-phase to 3 phase connection. This is a cost saving in the
smaller kVA transformers due to the 2 coil T connected to a secondary 2 coil T inlieu of the traditional three-coil primary to three-coil secondary transformer. In this
arrangement the Neutral tap is part way up on the secondary teaser transformer .
The voltage stability of this T to T arrangement as compared to the traditional 3
coil primary to three-coil secondary transformer is questioned
Instrument Transformer
• For measurement of high voltage and
current method of extension of range of
low range meter by providing suitable
shunts cannot be done. In such cases
accurate ratio transformer called
Instrument transformer are used. Also
provides advantage of isolation from high
voltage or current to meter, system and
person working with it.
• There are basically two types
• Current transformer(C.T )
• Potential Transformer(P.T)
Current Transformer
• A CT is a step up transformer with only one turn in
primary. There will be as many cores based on the
purposes like metering, protection etc. The secondary
of a CT should never be kept open circuited bcoz very
high flux will be developed in the secondary and hence
it may be damaged.
•
•
•
Bar-Type
» A fixed insulated straight conductor that is a single primary turn passing
through a core assembly with a permanently fixed secondary winding.
Wound Type
» A primary and secondary winding insulated from each other consisting
of one or more turns encircling the core. Constructed as multi-ratio CTs by
the use of taps on the secondary winding
Current Transformer
•
•
•
•
The CT basically wound with single full turn or more turn of thick wire on primary
and secondary with number of turns of fine wire.
So it acts as step up voltage transformer and from current point of view it is step
down current transformer. That low value of current can be use with low range
ammeter, energy meter, or wattmeter.
In this case primary is 1200 A and secondary is 5 Amps.
CT secondary never kept open that will cause I2=0 so secondary ampere turnN2I2
become zero so it will not oppose the primary (counter emf is zero) very high flux
will set in primary will cause more loss in core and more heat also there will be
very high voltage and primary and secondary. To avoid this CT secondary is always
short circuited.
Current Transformer
• ab
Current Transformer
• ab
Potential transformer
• A Voltage Transformer theory or Potential Transformer theory is
just like theory of general purpose step down transformer. Primary
of this transformer is connected across the phases or and ground
depending upon the requirement. Just like the transformer, used for
stepping down purpose, potential transformer i.e. PT has lowers
turns winding at its secondary. The system voltage is applied across
the terminals of primary winding of that transformer, and then
proportionate secondary voltage appears across the secondary
terminals of the PT. The secondary voltage of the PT is generally
110V. In an ideal Potential Transformer or Voltage
Transformer when rated burden connected across the secondary
the ratio of primary and secondary voltages of transformer is equal
to the turns ratio and furthermore the two terminal voltages are in
precise phase opposite to each other. But in actual transformer
there must be an error in the voltage ratio as well as in the phase
angle between primary and secondary voltages.
Potential transformer
Application of CT and PT
• ab
Book References
•
•
Electrical Technology by B.L Thareja vol-II
Electrical Machines by U.A Bakshi and M.V Bakshi. Techinical
publication pune.
Web References
•
•
•
•
http://www.electrical-forensics.com/Transformers/Transformers.html
http://www.powersystemsloss.com/2011/06/parts-of-power-transformer.html
http://www.agmcontainer.com/transformer_desiccant_breathers/index.htm
http://mplgmg.hubpages.com/hub/Power-Transformer
•
http://yourelectrichome.blogspot.in/2011/05/star-delta-connection-of-3hase.html
Thank You
Download available on
www.sumitrathor.co.in
Email: rathorsumit2006@gmail.com
Book References
•
•
Electrical Technology by B.L Thareja vol-II
Electrical Machines by U.A. Bakshi and M.V. Bakshi
105
Web References
•
•
•
•
•
http://www.electrical4u.com/ideal-transformer/
http://www.electrical-forensics.com/Transformers/Transformers.html
http://www.powersystemsloss.com/2011/06/parts-of-power-transformer.html
http://www.agmcontainer.com/transformer_desiccant_breathers/index.htm
http://mplgmg.hubpages.com/hub/Power-Transformer
106
Thank You
Download available on
www.sumitrathor.co.in
Email: rathorsumit2006@gmail.com
107