power transformers

1. TRANSFORMER
 Transformer is a static device used for transferring of
power from one circuit to another without change in
frequency.
 Operates on the principle of mutual induction between
two circuits linked by a common magnetic field.
 EMF induced in a winding is proportional to the flux
density in the core, cross section of the core, frequency
and no. of turns in the winding.

2. WORKING PRINCIPLE OF A POWER
TRANSFORMER
 A Transformer consists of two mutually
inductive coils that are electrically separated but
magnetically linked through a low reluctance
path .
 When one coil is connected to a source of
alternating voltage ,an alternating flux is set up
in the laminatrd core ,most of which links with
the second coil wound on the same core ,in which
it induces EMF according to Faraday ‘s laws of
electro magnetic induction
 If the second coil is connected to a load , a
current flows through it and thus the electric
energy is transfered completely magnetically
from 1st
coil to 2nd
coil at voltage depending upon
the no.of turns in these coils.

3. TRANSFORMER BASICALLY
CONSISTS OF:
 Magnetic Circuit comprising Limbs, yokes, clamping
structures
 Electrical circuit comprising primary, secondary
windings
 Insulation comprising of transformer oil and solid
insulation viz. paper, pressboard, wood etc. and bracing
devices
 Main tank housing all the equipment
 Radiators, Conservator tank
 On or Off load tap changer
 Vent pipe, Buchholz relay, Thermometers
 Fans, Cooling pumps connected piping
 Terminals i.e. connecting leads from windings to bushing
with supporting arrangements

5. FEATURES OF POWER
TRANSFORMERS
 Single Phase
 Three phase
 Star or Delta connected Primary
 Star or Delta connected Secondary
 With or without Tertiary winding
 Provided with Off-circuit tap switch or
On-load Tap Changer for voltage regulation

6. H.V Winding
L.V Winding
Core
Fundamental equation of transformerFundamental equation of transformer

7. EMF EQUATION OF A POWER
TRANSFORMER
 Induced EMF Primary (rms) =
E1 =4.44 f N1 Bm A
Induced EMF Secondary (rms) =
E2 = 4.44 f N2 Bm A
E1/ N1 =E2 /N2 = 4.44 f Bm A
E2 / E1 = N2 /N1 =K ,Transformation ratio.
if K is more than 1 , it is stepup transformer
if K is less than 1 ,it is step down transformer.
for an ideal tr. Input VA = output VA
V1 I1 = V2 I2.

8. Codes and Standards
• Codes or Regulations are mandatory requirements stipulated to
ensure the safety of the product during testing and service.
• Standards are the basis of agreement and can be used for
limited scope or even restricted. Standards also promote
interchangeability. Standards exist for material, product,
process, testing, calibration etc.
• Specifications are based on mandatory requirements of the
purchaser and agreed requirements of the standard.

9. DESIGN PARAMETERS – FROM USER
POINT
Voltage Ratio No. of phases
Flux density Rated capacity
Current density Insulation& cooling
medium
Insulation levels Tap changer
Vector group Cooling arrangement
Percentage Impedance Oil preservation system
Short circuit withstanding Operating conditions
capacity

10. NORMALLY FLUX DENSITY IS CHOSEN NEAR KNEE POINT OF
MAGNETIZATION CURVE LEAVING SUFFICIENT MARGIN TO TAKE
CARE OF VOLTAGE AND FREQUENCY VARIATIONS. CRGO STEEL
WITH SILICON CONTENT OF APPROX. 3% IS USED FOR
MAGNETIC CIRCUIT.
CHARACTERISTICS OF GOOD CORE ARE
I. MAX. MAGNETIC INDUCTION TO OBTAIN A HIGH INDUCTION
AMPLITUDE IN AN ALTERNATING FIELD.
II. MINIMUM SPECIFIC CORE LOSS AND LOW EXCITATION
CURRENT.
III. LOW MAGNETOSTRICTION FOR LOW NOISE LEVEL.
IV. GOOD MECHANICAL PROCESSING PROPERTIES.
MAGNETOSTRICTION IS CHANGE IN CONFIGURATION OF A
MAGNETIZABLE BODY IN A MAGNETIC FIELD WHICH LEADS TO
PERIODICAL CHANGES IN THE LENGTH OF THE BODY IN AN
ALTERNATING MAGNETIC FIELD. DUE TO MAGNETOSTRICTION
OF LAMINATIONS IN AN ALTERNATING FIELD CORE VIBRATES
GENERATING NOISE IN THE CORE.

11. CURRENT DENSITY IS AN IMPORTANT PARAMETER TO
DESIGN THE SECTION OF
THE CONDUCTOR FOR A SPECIFIED TEMPERATURE
RISE, RATED CAPACITY AND SHORT CIRCUIT
WITHSTAND CAPACITY OF THE TRANSFORMER.
DIFFERENT TYPES OF WINDINGS :
DISTRIBUTED CROSSOVER WINDING
SPIRAL WINDING
HELICAL WINDING
CONTINUOUS DISC WINDING
INTERLEAVED DISC WINDING
SHIELDED LAYER WINDING

12. Transformer oil serves as an electrical insulation and also as a
coolant to dissipate heat developed in the transformer.
CHARACTERISTICS OF TRANSFORMER OIL:
PHYSICAL
Appearance
The oil shall be clear, transparent and free from suspended
matter.
If color of oil is
a) Light - indicates degree of refining
b) Cloudy or foggy - Presence of moisture
c) Greenish tinge - Presence of copper salts
d) Acid smell - Presence of volatile acid. Can cause
corrosion

13. Density
At 27deg. c is 0.89gm/cu.cm. This ensures that water in the
form of ice present in oil remains at the bottom and does not
float up to a temp. of about – 10 deg. c.
Viscosity
Is a measure of oil resistance to continuous flow without the
effect of external forces. Oil must be mobile in transformers to
take away heat. Viscosity shall be as low as possible at low
temperatures.
Flash point
is the temperature at which oil gives so much vapor, which
when mixed with air forms an ignitable mixture and gives a
momentary flash on application of a flame. Minimum flash point
of a good oil shall be 140 deg. C.

14. Pour point
is the temperature at which oil will just flow under prescribed
conditions. If oil becomes too viscous or solidifies it will
hinder the formation of convection currents, thus cooling of
equipment will be affected.
Maximum pour point shall be -9 deg. C
Interfacial Tension
Is the measure of resultant molecular attractive force between
unlike molecules like water and oil at the interface. Presence of
soluble impurities decrease molecular attractive force between
oil and water. This gives an indication of degree of sludging of
oil.
Minimum value 40 dynes/M or 0.04 N/M
.

15. CHEMICAL
Neutralization Number
Is a measure of organic and inorganic acids present
in the oil. Expressed as mg. of KOH required to
neutralize the total acids in one gm. Of oil.
Limits for fresh oil - 0.03 mg KOH/gm - maximum
Limits for used oil - 0.05 mg KOH/gm - maximum
It leads to formation of sludge, metal surface
corrosion and lowering of di-electric strength.
Corrosive Sulphur
It indicates the presence of sulphur, sulphur
compounds, which are corrosive in nature and
corrode the copper surface.

16. Oxidation Stability
This is measured by ageing the oil by simulating actual service
condition of a transformer. Covers the evaluation of acid and
sludge forming tendency of new mineral oils. For used oil,
should be minimum to minimize electrical conduction and
corrosion
Water Content
By moisture entry into oil.
a) By accidental leakage
b) Breathing action
c) During oil filling or topping up
d) By chemical reaction
In unused oil - Maximum 30 ppm
Oil in transformer 145 KV & above - Maximum 15 ppm
Oil in transformer below 145 KV - Maximum 25 ppm
It reduces electrical strength and promotes degradation of oil as
well as paper.

17. ELECTRICAL
Electric Strength
Is the voltage at which arc discharge occurs between two electrodes
when oil is subjected to an electric field under prescribed conditions.
New oil unfiltered - 30 KV minimum (rms)
New oil filtered - 60 KV minimum (rms)
Resistivity
It is numerically equal to the resistance between opposite faces of a
centimeter cube of oil. Insulation resistance of the windings of
transformer is dependant on the resistivity of oil. A low value
indicates the presence of moisture and conducting contaminants.
Values for a new transformer are
(12)
At 27 deg. c 500x 10 ohm.cm
(12)
At 90 deg. c 30x 10 ohm.cm

18. Dielectric Dissipation Factor (Tan Delta & Loss Tangent)
Is measure of dielectric losses in oil & hence the amount of heat
energy dissipated.
It gives an indication as to the quality of insulation. A high
value indicates presence of contaminants or deterioration
products such as water, oxidation products, soluble
varnishes, and resins.
1) Tan delta at 90° for unused oil - maximum 0.2
2) Tan delta at 90° for oil before charging transformer -
maximum 0.005 (1/2%)
Low value of tan delta indicates low losses

19. TWO WINDINGS IS SAME. THIS IS CALLED
SUBTRACTIVE POLARITY. WHEN THE INDUCED EMFS
ARE IN OPPOSITE DIRECTION , THE POLARITY IS
CALLED ADDITIVE.
PRI. AND SEC. WINDINGS ON ANY ONE LIMB HAVE
INDUCED EMFS THAT ARE IN TIME PHASE. DIFFERENT
COMBINATIONS OF INTERNAL CONNECTIONS AND
CONNECTIONS TO TERMINALS PRODUCE DIFFERENT
PHASE DIVERGENCE OF SEC. VOLTAGE.
VECTOR GROUP OR CONNECTION SYMBOL OF A
TRANSFORMER DENOTES THE METHOD OF
CONNECTION OF PRI. AND SEC. WINDINGS AND THE
PHASE ANGLE DIVERGENCE OF SEC. WITH RESPECT TO
PRIMARY.

20. VECTOR GROUPS
1
3
4
U
VW
u
w
v
u
w
v
1 & 3 – YNd1
2 & 4 – YNd11
U
VW
2

21. U
VW
u
v
w
u
v
w
1
2
3
4
Vector Groups
U
V
W
1 & 3 – Dyn11
2 & 4 – Dyn5

22. NECESSARY TO REDUCE THERMAL DEGRADATION OF
INSULATION TO ENSURE LONGER LIFE. HEAT
GENERATED IN THE TR. IS TRANSMITTED TO
ATMOSPHERE THROUGH OIL.
DIFFERENT TYPES OF COOLING:
ONAN TYPE – OIL NATURAL AND AIR NATURAL. HOT
OIL IS CIRCULATED BY NATURAL MEANS DISSIPATING
HEAT TO ATMOSPHERE BY NATURAL MEANS.
ONAF TYPE – OIL NATURAL, AIR FORCED. HERE AIR IS
BLOWN ON TO THE COOLING SURFACES. FORCED AIR
TAKES AWAY HEAT AT A FASTER RATE.
OFAF TYPE – OIL FORCED, AIR FORCED. IF THE OIL IS
FORCE CIRCULATED WITHIN THE TR.AND RADIATOR
BY MEANS OF AN OIL PUMP, IN ADDITION TO FORCED
AIR, STILL BETTER RATE OF HEAT DISSIPATION IS
ACHIEVED OVER ONAF

23. OFWF TYPE – OIL FORCED, WATER FORCED. HERE
WATER IS EMPLOYED FOR COOLING OIL INSTEAD OF
AIR. AMBIENT TEMP. OF WATER IS LESS THAN
ATMOSPHERIC AIR. HENCE BETTER RATE OF COOLING
IS OBTAINED. IN THIS TYPE OIL TO WATER HEAT
EXCHANGERS ARE EMPLOYED. DIFFERENTIAL
PRESSURE BETWEEN OIL AND WATER IS MAINTAINED.
OIL IS CIRCULATED AT A HIGHER PRESSURE.
ODAF/ODWF TYPE – OIL DIRECTED, AIR/WATER
FORCED. IF THE OIL IS DIRECTED TO FLOW PAST THE
WINDINGS, LARGE QUANTITIES OF HEAT CAN BE
TAKEN AWAY BY OIL. COOL OIL IS DIRECTED TO FLOW
THROUGH THE WINDINGS IN PREDETERMINED PATHS.
OIL IS CIRCULATED BY A FORCED OIL SYSTEM LIKE OIL
PUMPS. THIS ENSURES FASTER RATE OF HEAT
TRANSFER.

24. ABSORBS MOISTURE. PRESENCE OF MOISTURE
REDUCES DIELECTRIC STRENGTH OF OIL. DIFFERENT
METHODS ARE AVAILABLE TO REDUCE
CONTAMINATION OF OIL WITH MOISTURE.
1. SILICAGEL BREATHER: IT IS CONNECTED TO THE
CONSERVATOR TANK. IT CONSISTS OF A CARTRIDGE
PACKED WITH SILICAGEL DESSICANT AND A SMALL
CUP CONTAINING OIL. AIR IS DRAWN INTO THE
CONSERVATOR THRO. OIL CUP AND BREATHER WHERE
MOST OF THE MOISTURE IS ABSORBED.
2. BELLOWS AND DIAPHRAGM SEALED
CONSERVATORS: A BELLOW TYPE BARRIER OR A
DIAPHRAGM TYPE BARRIER IS FITTED IN THE
CONSERVATOR. AIR ENTERING THE CONSERVATOR
TANK PUSHES THE DIAPHRAGM DOWNWARDS. AS OIL
EXPANDS THE DIAPHRAGM IS PUSHED UPWARDS.
POSITION OF DIAPHRAGM IS INDICATED BY OIL LEVEL
INDICATOR. DIAPHRAGM ACTS AS A BARRIER.

25. 3. GAS SEALED CONSERVATORS: IN THIS METHOD A CUSHION
OF AN INERT GAS LIKE NITROGEN IS PROVIDED OVER OIL
SURFACE IN THE CONSERVATOR. GAS PRESSURE IS ALWAYS
MAINTAINED HIGHER THAN ATMOSPHERIC PRESSURE.
NITROGEN GAS PRESSURE INSIDE THE CONSERVATOR IS
REGULATED BY NITROGEN CYLINDER AND PRESSURE REDUCING
VALVE WHICH ADMIT NITROGEN TO THE CONSERVATOR WHEN
THE PRESSURE FALLS. EXCESSIVE PRESSURE DEVELOPED
INSIDE THE CONSERVATOR IS RELIEVED THROUGH A RELIEF
VALVE.
4. REFRIGERATION BREATHERS: AN AIR DRYER IS FITTED TO
THE CONSERVATOR. AIR BREATHED THRO. THE UNIT IS DRIED IN
PASSING DOWN A DUCT COOLED BY A SERIES OF
THERMOELECTRIC MODULES BASED ON PELTIER EFFECT. TOP
AND BOTTOM ENDS OF THE DUCT ARE TERMINATED IN THE
EXPANSION SPACE ABOVE OIL LEVEL IN THE CONSERVATOR AND
AIR IS CONTINUOUSLY CIRCULATED THRO. THE DUCT BY
THERMOSYPHON FORCES.

26. SHORT CIRCUIT WITHSTAND CAPACITY:
EFFECTS OF SHORT CIRCUIT: ENERGY IN THE SYSTEM
GETS RELEASED IN THE FORM OF HEAVY FLOW OF
CURRENT WHEN FAULT OCCURS. EVERY FAULT FED
BY THE TRANSFORMER STRESSES THE WINDINGS.
THE STRESS DEVELOPED IN THE WINDING IS RELATED
TO THE INTENSITY OF FAULT. EACH FAULT CAUSES
SHARP RISE IN TEMPERATURE AND PRODUCES
MECHANICAL FORCES IN THE WINDING.
THESE FORCES ACT IN THE AXIAL AND RADIAL
DIRECTIONS OF THE WINDING, AND CAUSE
COMPRESSIVE OR TENSILE STRESSES ON THE
WINDING AND TEND TO DEFORM IT.

27. Radial forces: are due to flux in the space between coils. Tend to
burst coils and crush on the core.
Strengthening of winding
Axial forces: are due to radial component of flux which crosses
the winding at the ends and gives rise to axial compressive force
tending to squeeze the winding in middle.
Proper drying, compression and clamping

28. Thermal effect: rapid rise of temperature causes
I) mechanical weakening of insulation due to thermal ageing
– long term effect.
Ii) decomposition of insulation to produce gases – short term
effect.
Iii) conductor annealing – becomes brittle & cracks will be
formed.
Limit of max. Average temperature after short circuit is
2500
c for oil immersed transformer using copper winding.

29. TAP CHANGERS
ARE DEVICES FOR REGULATING THE VOLTAGE OF
TRANSFORMER.
OFF CIRCUIT TAP CHANGER : TAP CHANGING IS EFFECTED
WHEN TR. IS OFF. THESE ARE CHEAPER. THEY ARE USED
WHERE FREQUENCY OF TAP CHANGING IS VERY LESS.
ON LOAD TAP CHANGER : HERE TAP CHANGING IS EFFECTED
WITHOUT INTERRUPTING LOAD. ON LOAD TAP CHANGER
NORMALLY CONSISTS OF TRANSITION RESISTORS WHICH
BRIDGE THE CIRCUIT DURING TAP CHANGING OPERATION.
TWO TYPES OF OLTCS :
SINGLE COMPARTMENT TYPE – IN THIS TYPE SELECTION OF
TAPS AND SWITCHING ARE CARRIED OUT ON THE SAME
CONTACTS.
DOUBLE COMPARTMENT TYPE – IN THIS TAP SELECTION IS
DONE SEPARATELY AND SWITCHING IS DONE IN A SEPARATE
DIVERTER SWITCH.

30. TYPES OF TAP CHANGERS
 Based on applicationBased on application
Off-Circuit tap changerOff-Circuit tap changer
On Load Tap Changer (OLTC)On Load Tap Changer (OLTC)
 Based on mounting (for OLTC)
 Internal
 External

31. Internally Mounted OLTC

33. EXTERNALLY MOUNTED OLTC

34. EXTERNALLY MOUNTED OLTC

35. MAXIMUM THROUGH CURRENT
INSULATION LEVEL TO GROUND AND BETWEEN
VARIOUS CONTACTS NO OF STEPS AND BASIC
CONNECTIONS
TEMPORARY OVERLOADS AND SHORT CIRCUIT
STRENGTH
AUTOMATIC VOLTAGE REGULATING RELAYS ARE USED
FOR AUTOMATIC CONTROL OF BUS BAR VOLTAGE.
OUTPUT OF VOLTAGE TRANSFORMER CONNECTED TO
CONTROLLED VOLTAGE SIDE OF THE TR. IS USED TO
ENERGIZE AVR RELAY. WHEN VOLTAGE DEVIATION
EXCEEDS A PRESET LIMIT, A CONTROL SIGNAL TO
RAISE OR LOWER TAP OPERATION IS GIVEN. A TIME
DELAY UNIT IS CONNECTED IN THE CIRCUIT TO
PREVENT UNNECESSARY OPERATION OR HUNTING OF
TAP CHANGER DURING TRANSIENT VOLTAGE CHANGE.

36. BASIC CONDITIONS OF OPERATION
 Load current must not be interrupted during tap change
operation.
 Tap change must occur without short-circuiting the tap
winding directly.
 Positive change of tap position.
It means ‘make-before-break’ mechanism to be used.
This calls for a transition impedance.
Also the mechanism should be fast acting type –
spring loaded.

37. GENERAL DESIGN CONSIDERATIONS
 Capable to normal load/overloads on transformer.
 Maximum system voltage
 Step voltage & no. of steps
 Test voltage to earth and across tapping range
 Maximum surge voltage to earth and across range.
 Maximum test voltages between phases (where
applicable)
 Current rating – normal and overload

38. PARTS OF TAP CHANGER
 Selector switch
Tap selection takes place in this switch
 Diverter Switch
Make –before-break mechanism with transition
impedance. Arcing takes place and hence housed in a
separate compartment.
 Surge relay
 Conservator with oil level gauge.

39. TRANSITION IMPEDANCE
 Reactor type
 Resistor type

40. REQUIREMENTS OF TRANSITION
IMPEDANCE
 No voltage fluctuations during switching cycle
 Circulating currents should not be excessive
 Duration of arc should be minimum to minimize contact
erosion and reduce contamination of oil.

41. TAP CHANGER CONTROLS
 Manual / Electrical
 Local / Remote
 Manual / Automatic
 Independent Operation
 Parallel Operation
Group Control
 Master
 Follower

42. R2 R1
7
5
3
1
8
6
4
2
N
PRINCIPLE OF TAP CHANGER OPERATION
M2 T2 T1 M1

43. M2 T2 T1 M1 M2 T2 T1 M1 M2 T2 T1 M1
M2 T2 T1 M1 M2 T2 T1 M1 M2 T2 T1 M1
1 2 3
4 5 6

44. FEATURES OF TAP CHANGER
 Motor drive mechanism
Should rotate in both the directions
Step-by-step operation
Tap change in progress indication
Tap change complete indication
Sequence contact
Remote Tap position control & indication

45. TAP CHANGER OIL QUALITY
Use of tap changer Water content Dielectric strength
At neutral point of
windings
< 40 ppm > 30 KV
At positions other than
neutral end
< 30 ppm > 40 KV
Standard values for transformer oil testing according to
CIGRE 12 – 13 (1982) apply to tap changer oil at
service temperature.

46. OPERATING CONDITIONS
The environment in which a transformer works and the
quality in design and construction play a role on its
performance. A transformer working under normal operating
conditions, in all probability, gives satisfactory performance
throughout its life
.
NORMAL OPERATING CONDITIONS
1. Rated voltage and rated current with permissible margins.
2. Temperatures of oil and windings not exceeding the
prescribed values.
3. Availability of auxiliary and control supply and proper
functioning of accessories and protective devices.
4. Free from external faults such as line breakdowns and
equipment breakdowns.

47. USER SHOULD SPECIFY THE CONDITIONS UNDER WHICH
TRANSFORMER IS EXPECTED TO WORK VIZ. QUALITY
AND NATURE OF LOAD, TEMPERATURE LIMIT, VOLTAGE
CONDITIONS, SHORT CIRCUIT WITHSTAND CAPACITY
CONSIDERING PRESENT AND EXPECTED FAULT LEVELS.
PARAMETERS SPECIFIC TO LOCATIONS ARE TO BE
EVALUATED AND SPECIFIED TO ASSESS THE OPERATING
REQUIREMENT. MANUFACTURERS SHOULD ENSURE
THAT FACTORY TESTS AS REQUIRED UNDER
STANDARDS AND THE USER SPECIFICATIONS ARE DONE
TO VERIFY THE QUALITY AND ABILITY OF THE
TRANSFORMER TO WITHSTAND ALL SERVICE STRESSES
DURING LIFE TIME OF THE TRANSFORMER.

48. Design Basis
• Life-time cost of transformer
= Initial cost of transformer
+
Operational cost for its life period
This is called the
“Capitalized cost of transformer”.

49. DESIGN BASIS - CAPITALIZATION
Rationalized CBIP Capitalization Formula:
Capitalized Cost = Initial Cost (IC) + Capitalized { No-load
Loss (Wn) + Load Loss (Wl) + Auxiliary Losses (Wa) }
Capitalized cost = IC + Xn.Wn +Xl.Wl +
Xa.Wa
Factors affecting Xn; Xl and & Xa
Rate of Interest
Rate of Electrical Energy
Life of Transformer

50. DESIGN BASIS
 The design of a transformer aims at achieving lowest
capitalized cost.
 Low No-load Loss means higher magnetic material cost and
vice-versa
 Low Load Loss means higher copper (material) cost and
vice-versa.
 Several iterations are made to optimize the total cost before
freezing the design and drawings are made.
 Extensive use of CAD programs is needed for finalizing
design.

51. CONSTRUCTION FEATURES

52. Core

53.  Higher the number of steps in cross section, better is space
utilization and smaller is the core diameter.
90 to 95 % utilization factor is optimal.
 Core area (A) is determined by the Flux Density (B) which
in turn depends on many factors - like loss capitalization and
overall design economics.
 As the no load losses attract very high capitalization,
attempts are continuously made to reduce them.
 Improved manufacturing techniques like core building with
2-lamination packets, step-lap joints, v-notched laminations,
bolt-less cores are used.
 Hi-β core steels like M0H, ZDKH, etc are available in which
the specific core losses are lower than normal grades.

54. A A
V ie w A - A
C o n v e n tio n a l S te p la p

55. WINDINGS- L.V WINDING
 L.V Windings in Transformers are either
 Spiral OR layer wound for low current ratings
 Helical Wound with radial cooling ducts
for higher ratings.
 Disc type wound
 Distributed Cross-over (Run-over) coils
 The conductor used is paper insulated rectangular
copper (PICC)
 For higher currents, transposed conductors are used, to
uniformly distribute the current across the cross section
of the wire of coil.

56. SPIRAL/LAYER TYPE WINDING
Mandrel/Press-board cylinder
Cooling Duct
Conductor – Layer 1
Conductor – Layer 2
Conductor – Layer 3

57. Helical Coil (Single layer) Helical coil (Double Layer)
Start Finish

58. TRANSPOSED CONDUCTORS
Transposed conductors (CTC) are used to improve current
distribution in the cross section of the winding wire.
Individual cable can be coated with epoxy so that the cured and
finished conductor is mechanically stronger and withstand short
circuit forces better.

59. H.V WINDING/1
HV winding invariably uses PICC or CTC.
Type of winding used is
- Layer winding or
- Disc winding up to 132 kV and/or
- Interleaved winding or
- Rib shielded winding

60. T em porary O ver-voltage s S w itchin g O ver-voltage s O ver-vo ltages d ue to lightning .
P o w e r S yste m s O ve r vo lta g e s
POWER SYSTEM OVER VOLTAGES

61. TEMPORARY OVER-VOLTAGES
 Typically due to faults
 < 1.2 pu
 ms to tens of second or even minutes
 Not dangerous to insulation

62. SWITCHING OVER-VOLTAGES
 Due to system switching operations
 1.5 pu – 5 pu dépends on system voltage
 mostly damped asymmetric sinusoids
 front time of first peak – tens of µs to a few ms.
 decides external insulation in EHV/UHV systems

63. OVER VOLTAGES DUE TO
LIGHTNING
 Due to ‘direct’ or ‘indirect’ lightning strokes.
 known to contribute to ≅ 50% of system outages in EHV
& UHV systems
 few hundred kV to several tens of MV.
 Few kA to 200 kA
 very short duration : time to front : 1 to few tens of µs
 time to tail : few tens to hundreds of µs.
 Decides line insulation (BIL)
 Severely influences Transformer insulation.

64. Cg
Cs
α = K √ Cg/Cs
IMPULSE VOLTAGE DISTRIBUTION

65. IMPULSE VOLTAGE DISTRIBUTION
α = 0
α = 1 0
α =5
X
V

66. Disc Type Winding
Paper Insulated Conductor
Press-board Cylinder

67. 1234
5 6 7 8
9
1526
3 7 4 8
9
1S23
4 5 S 6
78
Conventional Shielded Interleaved
DISC WINDING CONCEPTS

68. Impulse Voltage
Distribution
1. Plain Disc Winding
2. Rib Shield Winding
3. Inter-leaved Disc Winding
Number of discs from line end
V
O
L
T
A
G
E
G
R
A
D
I
E
N
T
P
u

69. TERTIARY WINDING/1
In Star-Star Connected Transformers and Auto
transformers, Tertiary Winding is used to stabilize
phase to phase voltages in case of unbalanced load
- Suppressing third harmonic currents in earthed
neutral
- reducing zero sequence reactance
- for supplying auxiliary load or for connecting
capacitors.

70. TERTIARY WINDING/2
 Tertiary is required to be designed for a power rating equal to
one-third the rated power, it increases the cost of the transformer
by 10- 12 percent.
 Tertiary winding is known to fail due to transferred surges and
Short circuits
 Present practice is to do away with tertiary up to 100 MVA for 3
phase 3 limbed core transformers.

71. DESIGN PROCESS
 Design should meet
Requirements of customer specification
Relevant National or International standards
Statutory and regulatory requirements
Manufacturer’s Plant Standards
 Optimized design

72. OPTIMIZATION
 Objective of Optimization
 To arrive at a design that yields minimum capitalized cost.
 It is a function of the following:
 Core diameter
 Core height
 Flux Density
 Current Density

73. COMPUTER AIDED DESIGN
 Improve productivity of design personnel
Release of Engineering information may be 25
– 40% of delivery cycle.
 Reduce delivery cycle
 Better analysis and arriving at a most optimum
design
 To solve electro-static, electro-magnetic problems
and to provide a robust structural and thermal
design.

74. WHY IT IN DESIGN
 More precise calculations
 Tailor made designs
 No standard ratings specified above 1 MVA
 Change of specification parameter
 Relative change of material cost
 Ongoing development of technology

75. COMPUTER AIDED DESIGN
 Design optimization
 Design analysis “fem” 2d / 3d engineering
 Analysis
 Electromagnetic
 Electrostatic
 Structural
 Thermal
 Computer aided drafting
 Database and Data Management

76. WHAT IS QUALITY?
 Conformance Quality
 Performance Quality
 Appearance Quality
 Functional Quality
 Esteem Quality
 ‘Ability’ Quality
QUALITY OF
DESIGN/GRADE
FITNESS FOR USE

77. POOR QUALITY RESULTS IN FAILURES.
TYPES OF FAILURES
 Infant failures: Early life failures are the result of
latent defects.
- Latent defects are abnormalities that cause failure,
depending on degree of abnormality and amount of
applied stress.
- Delivered defects are those that escape test /
inspection within the factory
- They are directly proportional to total defects in
the entire processes.

78.  Mid life failures: These are results of –
- Freak system disturbances
- Wrong specifications
- Poor maintenance
 Old age failures: These are results of –
- Ageing of insulation system
- Wear & tear

79. TRANSFORMER
END FRAMES
CORE
INSULATION
WINDINGS
TANK
OIL
RAW MATERIAL
FABRICATION
FITTINGS
MAJOR ASSEMBLIES COMPONENT ELEMENT

80. Electrical
* Power frequency
* Over-voltages
(External & Internal)
* Part winding resonance
* Partial Discharge
contd..
82
MAIN FACTORS CAUSING STRESSES IN
THE WINDING

81. 83
 Mechanical * Core Vibration
* Force due to Short Circuit
or Faults
* Inrush Current
* Over-fluxing
 Thermal * Winding Temperature
* Core loss
* Core
Shorting
* Malfunctioning of Cooling System
* Hot Spot (Local overheat)
* Arcing

82. CHALLENGES IN TRANSFORMER
DESIGN & MANUFACTURING
 Structures design (tank etc.)
To be designed for: Lifting & Jacking
Full or partial vacuum
Internal Pressure
Seismic Load
Tests conducted: Leakage test
Vacuum test
Radiography (if specified)
DPT on load bearing items
Contd..
84

83.  Short-circuit withstand capability
 Adequate radial supports
 Use of pre-compressed press-board to minimize shrinkage in service
 Proper stabilization of coils
 Use of glued conductors
 Springs or hydraulic dampers if required
Contd..
85

84. Stray losses control:
 Stray losses due to linkage of high magnitude of flux with
magnetic materials
 Stray losses form a large part, more than 20% of total load
losses
 These may cause hot spots
 Measures for stray loss control
 Use of laminated material
 By breaking the magnetic path
 By providing non-magnetic shield
 By providing parallel low reluctance
magnetic path contd..
86

85. High Voltage stresses
Design of Insulation system to ensure withstand capability for
 Lightning Impulse and Switching Surges.
 Long duration high voltage system disturbances
 Internal Partial Discharges
This is done by -
 Choosing proper type of windings
 Calculation/plotting of impulse / switching surges and long duration voltage
stress distribution
 Provision of adequate major and minor insulation by using angle rings,
moulded components etc.
 Corona shielding where required
87

86. QUALITY DESIGN
PERFORMANCE
 Following are prerequisites for a long trouble-free service of the
transformers:
 A well designed insulation system.
Good mechanical strength to withstand the inevitable short-
circuit forces.
 Proper design review by a team of engineers from Design,
Quality, Marketing, Production etc to ensure that the design is
meeting customer’s specification.
 Good manufacturing practices to ensure conformation of the
final product to the design documents.
 Proper erection & commissioning and subsequent
maintenance. 88