Classification and types in detail

1. Three Phase Transformer
1

2. Classification of 3-phase transformers
• Transformers which are directly coupled to Generator of the power station
are called Generator transformers. Their power range goes up to far above
1000MVA and voltage range extends to approx. 1500 kV.
• Connections between different high voltage system levels is made via
network transformers. Their Power range exceeds 1000 MVA and voltage
range exceeds 1500 kV.
• Distribution transformers are within the range from 50 to 2500 kVA and
max. 36 kV.
• All transformers above 2.5 MVA are classified as Power Transformers.
2

3. Possible methods of three phase
transformer operation
Use of bank of single phase
transformers
Use of a single unit three phase
transformer
• Economical-saving in core material
• Less floor space required
• Ease of transportation and
maintenance.
• Three phase operation possible with
only two single phase transformers
e.g., open delta & Scott connection
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4. Bank of single phase transformers
A three phase unit
4

5. Three phase system-an overview
5

6. Three phase fundamentals
• Possible connections are Star (Y) or Delta( )
– For Delta: Iline= Iphase ; Vline= Vphase
– For star(Y): Iline= Iph ; VLL= Vph
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7. 7

8. 8

9. Core design
Shell type: provides better shielding ;
used in the applications such as arc
furnaces; rarely used.
Core type:
3-limbs core type- most widely used
design. Cross sectional area of Top and
bottom yoke equals that of the wound
limb.
5-limbs core type- used where the
height of transformer is to be reduced.
The top & bottom yoke is reduced to
50%of the x-sectional area of the
wound limb. Therefore flux return
path is provided by two extra side
limbs.
9

10. Three phase connections-
the transformer vector
group
Possible three phase connections
are:
Star-Delta Yd
Delta-Delta Dd
Delta-Star Dy
Star-Star Yy
Capital letter represents HV
winding
Small Letter represents LV
winding.
N stands for Neutral Brought out
10

11. Phase shift in three phase
transformer-Clock
convention
360°/12hr=30°
Possible configurations:
Dyn1
Dyn11
YNd1
YNd11
Yy
HV is represented by big needle at
12 i.e., as reference
For position of 1’O clock, LV(small
needle) lags HV by 30 °
For 11 O’clock position, LV leads HV
by 30 °
11

12. Phase shift in three phase
transformers (Yd or Dy)
There are two ways by which delta
connection can be formed
Phase- windings have terminals A1A2,
B1B2 and C1C2, where the number
marked-2 are positive or dotted
terminals.
If delta is formed by connecting a1b2,
b1c2 and c1 a2, then LV Delta winding
will lag Y by 30 This is shown by clock
convention as 1 O’clock position.
Hence termed as Yd1
If delta is formed a2b1, b2c1 and c2a1,
then LV delta-winding will lead HV Y
winding by 30°. This is shown by clock
convention as 11 O’clock position.
Hence termed as yd11
B1
a2a1
A2
A1
B2
C1
C2
b2b1
c2c1
Vab
Vca
Vbc
VAB
VBC
VCA
B1
a2a1
A2
A1
B2
C1
C2
b2b1
c2c1
Vab
Vca
Vbc
VAB
VBC
VCA
12

13. DY1
13

14. YY Transformer
If system is balanced, Line to line
voltages form an equilateral triangle,
and neutral is at zero voltage.
In case of unbalanced system, if
transformer has delta winding, it will
provide a circulating path for
unbalanced induced current.
Problems with YY transformer:
1. If both windings have Y
connection, and load is
unbalanced, neutral will not be at
zero potential but is free to shift.
1. Due to non- linear magnetizing
characteristics, 3rd harmonic
voltage is present in line voltages.
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15. Triplen Harmonics
3rd harmonic voltage components of all
three phases are in-phase and therefore
3rd harmonic current can not return
through phases. As a result high 3rd
harmonic voltage will be present in the
line in addition to fundamental voltage.
This causes higher line voltages.
Therefore a path must be provided for
third harmonic current.
To provide a return path for 3rd harmonic
current, YY transformer must either
-have neutral terminal earthed, specially
on primary side
OR
- A delta connected additional winding
called Tertiary winding must be
provided. Delta winding provides a
circulating path for third harmonic
current.
0 1 2 3 4 5 6 7 8 9 10
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
Phase A and its third harmonic
0 1 2 3 4 5 6 7 8 9 10
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
Phase B and its third harmonic
0 1 2 3 4 5 6 7 8 9 10
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
Phase C and its third harmonic
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16. Parallel Operation of Transformers
• Parallel operation is required
– To cater for increased load by sharing it between parallel transformers
– For improving the reliability of system
• Conditions necessary for parallel operation
– Primary & secondary voltage ratings of all transformers should be suitable for
system voltage
– Both transformers should have same transformation ratio, otherwise
circulating current will flow between windings of parallel transformers
– Transformers must be connected properly according to their polarities so that
net loop voltage should be zero. This will make circulating current zero.
– For three phase transformers, attention must be given for vector groups of
parallel transformers. Such as a Dd can not be connected in parallel with a Dy
transformer because of the phase shift in Dy transformer.
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17. Importance of recognizing vector
group of transformers
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18. Exercise Problem- Three phase
transformer
18

19. Three phase transformation using Two transformers
• One of the advantage of using bank of single phase
transformers is the possible three phase
transformation with only two transformers.
• Power rating of the bank however reduces to 57.7%
( instead of 2/3rd or 66.67%) of the Power rating with
all three transformers in operation.
• Possible configuration are
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20. The open delta (V-V) Connection
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21. 21

22. TRANSFORMER COOLING PROBLEM
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23. • Prolong use of transformer at high
temperatures is particularly damaging to its
insulation.
• Small transformer up to few kVA can be
adequately cooled by natural convection of air
, sometimes assisted by fans.
• Larger physical size of power transformers
require careful design to transport heat from
the interior.
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24. Transformer tank
To avoid ingress of dust and to
assist transformer cooling,
transformer core and winding
are placed in a metal tank.
Transformers up to 30 kVA use
plain tank. However, with
higher ratings, tank surface is
corrugated, or have fins,
tubes and radiator tanks.
Tanks from 50 to
approximately 4000 kVA are
preferably of the corrugated
steel design.
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25. Transformer core & winding
inside tank Generator transformer
25

26. Methods of cooling
• Dry type:
– Cooling by surrounding air through natural
convection (AN- air natural) or supplemented by
cooling fans (AF-air forced)
• Liquid cooled; either oil cooled or Askeral cooled
– Oil Natural (ON), Oil natural-air natural(ONAN), Oil
natural-air forced (ONAF)
– At places of Fire hazards, transformers are cooled
by non-flammable synthetic liquid called askeral
26

27. Dry type transformer
Air natural –AN
surrounding air is used
for cooling. Small transformers
below 25 kVA can be readily
cooled by air natural cooling.
Cooling takes place by natural
convection
Air Forced- AF
when temperature of
winding increases above a
permissible level, fans at the side
of transformer tank are switched
on. Air is forced on the tank
surface to increase the rate of
heat dissipation.

28. Oil cooled transformer- tank with
tubes-up to 3000 kVA
• Transformer oil is usually a highly refined mineral oil that is stable at high
temperature and has excellent insulating properties.
• Oil must posses following characteristics:
- High dielectric strength
- Free from moisture
- Free from acid, alkalies, corrosive sulphur,
- free from sludging i.e., solid hydrocarbons produced due to decomposition of oil
as a result of heating

29. Oil natural cooling
Oil circulation inside tank is due to
natural convection. Oil circulates via
side tubes and dissipates its heat to
surrounding. It then settles down
through side tubes and re enter into
the tank.
Over the top of main tank is an
auxiliary tank, called conservator.
Which assists breathing action and
preserves oil level in the main tank.
To prevent moisture entering into oil
inside main tank, oil from conservator
is passed through a breather
containing blue color silica gel which
absorbs moisture from oil and turns
into pink.

30. Transformer with radiator tubes
Oil natural cooling ONAF cooling
30

31. ONAF cooling ON cooling

32. Coil with ducts- to facilitate heat
removal

33. Transformer core & winding

34. Transformer accessories
Breather Double float Buchloz relay
34

35. Bushings
Transformer
parts
Buchloz relay
35

36. Buchloz relay
Connection Internal mechanism
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37. AUTO TRANSFORMER &
INSTRUMENT TRANSFORMER
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38. The auto transformer
For small voltage changes, such as
from 13.2 kV to 13.8 kV, use of 2-
winding transformer is expensive.
For such applications, another
type of transformer, called auto
transformer, is used.
Auto transformer has a single
winding, part of which is
connected to the load and acts
like secondary of a 2-winding
transformer.
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39. Auto transformer winding has
two parts:
winding common to load and
source is termed as Common
winding &
rest of the winding , being in
series with common winding, is
called Series winding.
Like other types, the
autotransformer also has Step-up
or step down functions.
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40. 40

41. Volt-Ampere rating of
autotransformer-
comparison with 2
winding transformer
Input and output VA ratings are
VA rating of individual winding
As
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43. Instrument transformer
Potential transformer PT Current transformer
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44. CT operation
Primary winding is a single turn
winding
Secondary is multi-turn.
Secondary is rated either at 5A or
1A.
Common CT ratios are
50:5,100:5,150:5,200:5 up to
1000:5.
Secondary should never be left
open.
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45. Combined CT & PT
application
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