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Technology Training that works
TRANSFORMERS

What is a transformer?
• An electrical equipment used to transform
ac voltages from one to another value
• Device which works on the principle of
electro-magnetism

Transformer
• Widely used in power systems
• Possible to transmit power at an
economical transmission voltage and
• Utilize power at an economic effective
voltage

> Bulk of generating voltage limited to 15 ~ 25 kV,
though power generated is in hundreds of MW
> Impractical to distribute power at generated
voltage:
> High power loss in transmission/distribution
> Greater cross section of conductors
> Higher voltage drop in distribution system
Why do we need transformers in an
electrical system?

Why do we need transformers in an
electrical system?
• High transmission voltage required -
achieved by using transformers
• Convert AC voltage of any value to any
desired value - Use suitable turns ratio for
transformer windings
• Applicable only for AC circuits, cannot be
used for DC circuits

How can transformers regulate
voltage in electrical power system?
• Maintaining constant voltage for equipment vital
for many power consumers when supply is
availed from utility grid - especially whose
processes are critical
• Simple transformer law - Primary to secondary
voltage ratio equal to its primary to secondary
turns ratio and vice versa
• Change of turns-ratio accomplished by adding/
subtracting required number of turns to either
primary or secondary winding

Transformers for Oil & Gas
Applications
• Transformers for oil & gas applications may range
between 250kVA and 40MVA
• May be air-cooled or water cooled with indoor and
outdoor enclosure
• Generally fall under two categories of construction – Dry
type and liquid immersed type
• Air-insulated and solid insulated constructions are
included under the dry type

Applications

Applications
• Epoxy Cast dry type
transformer
– This type does not use insulation
papers in the windings
– Instead, pure epoxy resin reinforced
with glass fiber rovings are wound
directly with the wire
– Winding processes controlled by
advanced electronics also ensure
even distribution and high levels of
precision

Applications
• Vacuum cast coil dry type transformer

Transformers for Oil & Gas Applications
Maxi cast-resin transformer
 Combines advantages of liquid-filled &
dry type transformers
 Safe, powerful, and environmentally
safe alternative to liquid-filled
transformers
Siemens cast resin power
dry-type transformer

Liquid immersed transformers

Small, medium and large power Transformers
• Small power transformers are distribution transformers with a range of 5 to
30 MVA, and a maximum service voltage of 145 kV.
• Medium power transformers with a power range between 40 and 250 MVA
and a voltage over 72.5 kV are used as network and generator step-up
transformers.
• Depending on onsite requirements, transformers in the power range above
200 MVA can be designed as multi-winding transformers or
autotransformers, in 3-phase or 1-phase versions.
Small Medium Large

Types of transformers
• Generator transformer
• Earthing transformer
• Station transformer
• Unit transformer

Typical scheme for Generator
transformer

Generator Transformer
• Important link between generator in power
generating station and transmission lines
• Normally step-up transformers
• Most present day power generators generate
voltages between 10 kV to 30 kV
• Stator voltage kept as high as possible to keep
current within manageable values
• Generators of around 150 MW generate power at
11 ~ 13.8 kV - Line currents around 9000 amperes
at 0.85 power factor

Earthing Transformer

23.5 kV
Generator
Transformer
Generator
400 kV
Earthing
Transformer
11 kV
11 kV
Scheme for Earthing Transformer
Unit
Transformer

Role of transformers in transmission,
distribution networks
> Reduce transmission, distribution losses
> Reduced size of transmission, distribution line
conductors
> Reduced voltage drops in transmission, distribution

Transformers - Typical Scheme in a
Plant

Transformer Theory

What is magnetism?
• Magnetism is a phenomena by which
materials exert attractive or repulsive forces
on other materials
• A property governed by the atomic
characteristics of the substance

Electricity and magnetism
• Inter-related
• Relationship is often called
electromagnetism
– Physics of electromagnetic field: a field which
exerts a force on particles that possess the
property of electric charge, and is in turn
affected by the presence and motion of those
particles

Electricity & Magnetism
• Current flowing in an electric conductor produces
magnetic field in its vicinity
• Magnetic field linked to the medium in the vicinity
of current varies depending on material of
medium
• Further classified as Magnetic and Non magnetic
depending on ability to retain / allow higher
magnitude of this magnetic field

Electro Magnetic Induction
• Similar to production of magnetic field by flow of
external current, the same magnetic field also
produces an electrical force when it is in vicinity of
a good conductor which is not connected to any
external source
• Called EMF (Electro motive force) induced in the
conductor due to external magnetic field

Electro-magnetic induction
• Induced emf - Use magnetic field to produce electrical
force when magnetic flux is in vicinity of good conductor
• Induced emf classified as:
– Dynamically induced emf
– Statically induced emf

• Statically induced emf - Emf induced in a stationary
conductor due to a change in flux linkage arising from a
change in the magnetic field around the conductor
• Dynamically induced emf - Emf induced in a moving
conductor when the conductor is moved in a stationary
magnetic field
• Statically induced emf is the basic principle used in a
transformer
Basic theory of transformer

Transformer principle
• AC voltage applied between terminals A1 and A2
• Voltage induced between terminals a1 and a2

Magnetic flux links primary and secondary windings

• Applied voltage causes current to flow in first winding
• Current variation in coil creates varying magnetic flux in
core material
• Varying magnetic flux in core induces voltage in second
winding
• Load connected across terminals a1 and a2 causes
current flow in load
• Voltage induced in secondary winding depends on its
number of turns in relation to number of turns in primary
winding
T1 V1 I2
------------- = ------------ = ------------
T2 V2 I1

• Primary current is small under open circuit
conditions so that applied voltage is
almost equal and opposite to the emf
induced in P. Hence,
• Secondary output power is almost equal
to the primary input power and can be
expressed as
V1 I1 Cos 1 = V2 I2 Cos 2
Transformer Principle
1
2
1
2
N
N
V
V


• Secondary voltage can be reduced/
increased by changing turns ratio between
primary and secondary winding
• Primary current increases/ decreases in
accordance with secondary load current
• This balance of ampere-turns in primary
and secondary windings of transformer
holds the key for transformer action and
design

Important points about transformers
• Used to transfer energy from one AC circuit to another
• Frequency remains same in both circuits
• No ideal transformer exists
• Also used in metering, protection applications - Current
transformer (CT), potential transformers (PT)
• Used for isolation of two different circuits (isolation
transformers)
• Transformer capacity expressed in VA (Volt amperes)
• Transformer polarity indicated by dots

• Core Type transformers are the most
common type employed in majority of
transformers in the world
• Shell Types are mostly employed for
furnace transformers
• Windings are normally made of electrolytic
grade Copper and the metallic parts are
basically made of steel

By type of construction:
– Core type: Windings surround a considerable part of
the core
– Shell type: Core surrounds a considerable portion of
the windings

Jeumont-Schneider Shell type Transformer

Types of transformers (By method of
cooling)
– Oil filled self-cooled: Small and medium sized
distribution transformers
– Oil filled water-cooled: High voltage transmission
line outdoor transformers
– Air-Cooled type: Used for low ratings and can be
• Natural air circulation (AN) type
• Forced circulation (AF) type
– Dry type transformer

Types of transformers (By method of
application)
– Power transformer: Large transformers used to change
voltage levels and current levels as per requirement.
Power transformers. Usually used in distribution or a
transmission line
– Potential transformer: Precision voltage step-down
transformers used along with low range voltmeters to
measure high voltages
– Current transformer: Used for measurement of current
where current carrying conductor is treated as primary
transformer. Isolates instruments from high voltage line, as
well as step down current in a known ratio
– Isolation transformer: Used to isolate two different
circuits without changing voltage level or current level

Dry Type Transformers
• No oil in the transformer
• Air cooled and built for
various insulation classes up
to class H
• Insulation between High and
Low voltage windings by FRP
(Fire Retardant Paper)
cylinders
• Epoxy, Cast Resin & VPI are
the most common

Dry Type Transformers
• Normally indoor Type in
suitable enclosures
• Limited to capacities up to 10
MVA and up to 36 KV class
• Lesser thermal efficiency and
lesser over load capacities
compared to oil filled types

Transformer construction
and
main parts

Transformer Parts
Main parts of a transformer may be grouped as
below:
• Transformer Core
• Transformer windings
• Winding Insulation
• Cooling medium
• Transformer Tank/Enclosure
• Transformer accessories for measurements and
protection

Core Laminations
• Transformer cores are not made of solid
steel but are normally built up in the form
of laminations to reduce the eddy current
losses
• Made in the form of E’s and I’s (Also T’s)
to get the closed magnetic circuit of the
core

Transformer Core
• Cold Rolled Grain Oriented (CRGO) steel
– Low reluctance, low loss flux path
– Low hysteresis loss
• Phosphate coated, lacquered laminations -
Reduced eddy current loss
Silicon steel stamping usually shaped like "E" and "I"
Typical Transformer Stampings

Typical Lamination Sections
Cruciform type

Core Laminations
• Circular cross section formed by combining
rectangular laminations of different cross
sections to form into an overall circular shape
• A maximum of 95% fill can be possible by
planning the number of steps and choosing
proper thickness of core laminations

Typical Windows Construction

Mitred Joints of Transformer Core
• Boltless construction normally employs mitred
joints with overlaps of 450.

Comparison of Flux Orientations
• Flux direction follows orientation of grains more smoothly
with least resistance in magnetic core having mitred joints.
Reduces power required to pass magnetic flux in direction
of grain orientation

Hysteresis Loop
• Magnetic Materials
have characteristic
called Hysteresis
• Ability to retain certain
amount of flux (B) even
if magnetizing force (H)
is removed or made
Zero
• Hysteresis loop is
formed when H
completes a full cycle

Three phase transformers
• Large scale power generation - Generally 3 phase
• Requires use of 3 phase step-up and step-down
transformers
• 3 phase transformer - Combination of 3, single-
phase transformers (three primary and three
secondary windings mounted on a core having
three legs)
• Commonly used configurations:
– 3 phase three wire (Delta)
– 3 phase four wire (Star)

Delta connection
• 3-phase windings connected
end-to-end
• 120 degrees apart
electrically
• Generally, delta 3-wire
system used for unbalanced
load systems
• 3-phase voltages remain
constant regardless of load
imbalance

Possible combinations of Star and
Delta
• Primary in delta –
secondary in delta
• Primary in delta –
secondary in star
• Primary in star –
secondary in star
• Primary in star –
secondary in delta
I
V V/a
I/ 3 aI/ 3
aI
(A) Delta-Delta Connection
V
I
I/ 3
V/a
aI/ 3
3V/a
(B) Delta-Star Connection
V
I
V/ 3 V/a 3
aI
V/a
(C ) Star-Star Connection
V/ 3a
aI
aI 3
(D ) Star-Delta Connection
V
I
I V/ 3

Physical connection of Delta (D) -
Star (Y) configuration

3 Phase, 4-wire star connections
• Allows minimum number of
turns/ phase (phase voltage
1/3 of line voltage)
• Higher conductor cross section
• One end of each winding
connected to common end
(neutral point)
• Better to use a star-connected
4-wire source when feeding to
star connected unbalanced
load

Transformer Windings
• Winding materials
– Electrolytic grade Copper and Aluminium conductors are
suitable in transformer windings but Copper is still the most
preferred winding material in the transformer construction
• Brief comparison gives an indication for copper to
be a preferred conductor in transformer windings
Property COPPER ALUMINIUM
Electrical conductivity at 20 deg C 100% 62%
Weight at 20 deg C 100% 33%
Melting point 10830 C 6600 C
Mechanical strength 2250 Kgf/cm2 915 Kgf/cm2
Thermal conductivity 0.941 cal/cm2 0.57 cal/cm2
Specific heat 0.003 cal/gm 0C 0.21 cal/gm 0C

Concentric windings

Design Requirements for Windings
• Design criteria for transformer windings
– Minimum resistance
– To withstand the normal currents and forces associated
with short circuit conditions
– Good mechanical strength without becoming brittle under
operating temperatures
– Good heat transfer capability to the cooling medium,
whether oil or air
• Cost also plays crucial role

Transformer Insulation
• Major materials used as insulation in transformers:
– Mineral oil and Kraft Paper / Press Board / Wood
basically called cellulose products
• Transformers subjected to high temperatures
because of loads, high electrical stresses due to
nature of power source, loads, etc.
• Life of transformer relies mainly on design,
condition of insulation, its ability to withstand
above conditions
• Oil filled Transformers use both solid (cellulose /
Paper) and liquid (oil) as insulation

Winding insulation
• Insulating materials (cellulose) used in a transformer
– Kraft paper
– Cotton cellulose
– Press board
• Characteristics of insulation materials
– High dielectric strength
– Dielectric constant close to transformer oil
– Low power factor
– Freedom from conducting particles

Transformer paper insulation
• Kraft Paper
– Made by sulphate process from Unbleached soft
wood pulp basically removing carbohydrates, waxes,
etc to leave only cellulose fibres
• Cotton Cellulose
– Mixing cotton fibre with wood pulp
• Press Board
– Number of paper layers laid to produce thicker press
boards either by laying papers together at wet stage
or by using bonding adhesives between individual
boards

Winding insulation
Degradation of solid insulation induced by:
– Heat
– Moisture - Free water, suspended water (trapped in oil),
dissolved water and chemically bound water
– Oxygen
– Acids

Transformer paper aging
• Degradation depends upon temperature,
moisture content, oxygen and acids in the
system
• Heat and moisture are the major issues which
affect the life of paper insulation
• Moisture consists of free water, suspended
water (Trapped in oil), dissolved water and
chemically bound water ( used during
manufacture), etc. and hence complete removal
of moisture is impossible

Transformer paper aging
• As transformer ages, cellulose molecular chains
get shorter and produce chemical products such
as furanic derivatives, CO and CO2 which get
dissolved in the oil
• Furanic derivatives ((5 hydroxymethyl-2-furfural,
2 furfuryl alcohol, 2 furfural, 2 acetylfuran, 5-
methyl-2-furfural) dissolved in oil are main cause
for cellulose degradation
• 2-Furaldehyde concentration - major contributing
item in paper degradation

Winding insulation
Transformer oil:
• Can be regularly filtered to keep characteristics intact
• Can be replaced with new oil, if required
However, in transformer paper insulation:
• Degradation of cellulose irreversible
• Solid insulation cellulose products cannot be replaced
• End of insulation life = End of transformer life
• Maintenance very important to ensure long
transformer life

Tap changer - Main functions
To take care of:
– Variations in primary voltages - Mainly large
transformers/ transformers used for critical applications
– Inherent regulation of transformer - Maintain constant
O/P voltage irrespective of load conditions
– Unknown system conditions at time of planning
Electrical system
– Effective control of VAR, mainly in generator
applications

Transformer Tap Changer
• Secondary voltage of transformer varies in line
with input voltage and on load current
• Other factor which decides secondary voltage is
ratio of primary and secondary turns
• Proper operation of many electrical drives
depend on keeping applied voltage close to its
rated voltage
• It is difficult to have control on primary voltage
and hence turns ratio is varied to keep
secondary voltage close to the desired value

Tap changer - Basic principle

Tap changer
3
5
2
4
1
6
7
High Voltage
Low Voltage
Tap
Positions

Tap changer types
• Based on operation mode:
– On load tap changer - Continuity of supply
– Off load tap changer - Supply interruption during change
• Based on location:
– In tank
– External mounted

Tap changer (On Load and Off
circuit)
• Voltage control (or) turns ratio control achieved by using
either Off circuit Tap Changer or On load Tap Changer
(OLTC)
• Switch can be manual operated or motor operated
• Broadly classified as:
– Off Circuit Tap Changer (To be operated in de-
energized condition only)
– On load Tap changer (Can be operated in energized
condition with load)

Off Load Tap Changer
• Universally it is standard practice to have off
load tap switch positioned at +5%, +2.5%, 0%, -
2.5% and -5% so that the LV voltage can be
kept close to its rated value for variation of +/-
5% of High voltage magnitude
• Off load tap changer draw back - Makes it non
operable with transformer in energized
condition, which is unacceptable for utility
substation transformers and transformers used
in continuous process industries

Off Circuit Tap Changers
• Generally all oil filled transformers are provided
with OCTC where brief interruption of service is
not an issue
• Established practice is to have taps at +5%,
+2.5%, 0, -2.5%, -5%
• Users can specify different ranges also but
normally limited to +/- 10% and normally not
below 2.5% steps

On Load Tap Changer
• Continuously monitors secondary voltage,
changes tap position on primary side by using a
motor so that voltage can be kept very close to
desired values continuously either by manual or
auto operation
– Range of OLTC can be anything depending on
system requirements and values like +7.5% to -12.5%
in steps of 1.25% OR + 15% to -15% in steps of
1.25% are common

Need for On load tap Changer
• OCTC can not be operated with transformer in
energized condition
• Unacceptable for utilities feeding many customers and
for continuous process industries
• Includes features to take of arcing currents during
switching operations and normally operated by a motor
which is controlled based on HV magnitude

Basic connections of tap changers
Typical arrangement in 3 phase transformer with
star connection

Typical motor drive of tap changer

In-Tank Tap-changer

Fittings and Accessories
• Bushings
– Main connection between transformer windings and
external source and load. Made of porcelain with
conducting material passing through centre
• Solid type bushings
– Used for applications up to around 30 kV
• Condenser type bushing
– Consists of alternate layers of paper and metal foil
used as condenser surrounding the conductor tube at
centre of porcelain. Paper used can be synthetic
bonded or oil impregnated one

HV Bushings
• Fitted to each phase of HV transformer
• Condenser type
• Oil impregnated paper insulation
• Anti fog sheds
• Shall be required to have impulse voltage withstand
characteristics
• Need to protect against corona discharges and lightning
surges

Solid or Non-Condenser type
bushings
> Used for applications up to around 30 kV
> Voltage not evenly distributed through the
material or along length of bushing
> Bushing dimension increases with increased
rated voltage – Very large dimensions not a
practical proposition
> Partial discharges due to concentration of
stress in insulation, and its surface - Limits
use not beyond 30 kV

Condenser type bushing
• Employed for EHV applications
above 33 KV
• Conducting cylinders inserted into
insulation to divide wall thickness
into number of capacitors
• More uniform voltage distribution in
material and along surface
• Alternate layers of paper and metal
foil surrounding conductor tube at
center of porcelain part
• Paper - Synthetic resin bonded
paper (SRBP) or oil impregnated
paper (OIP)

Transformer cooling
Thermal considerations
• Transformer losses:
– Eddy current losses
– Copper losses
• Necessary to cool windings to keep temperatures below
max. allowable limits
• Transformer insulation also affected by operating
temperature
• Life of paper insulation a function of temperature
• Insulation life halves for every 6°C rise in temperature

Types of transformer cooling
• ONAN – Oil natural Air natural
• ONAF – Oil natural Air forced
• OFAF – Oil forced Air forced
• OFWF – Oil forced Water forced
• ODAF – Oil directed Air forced
• ODWF – Oil directed Water forced

Cooling medium
• Coolant medium ensures that temperature is within limits
of allowable insulation temperature. (rate of aging of
transformer doubles for every 6 deg C increase in
winding temperature)
• Maximum hot spot temperature permissible is around 98
deg C
• Depending upon coolant medium the transformers are
widely classified as below
– Oil cooled transformers
– Air Cooled / Dry Type transformers

Cooling medium
• Temperature of hottest part of winding is arrived by
summing up:
– Ambient temperature
– Temperature rise of winding by resistance.
– Average of temperature readings at top and bottom of oil
– Temperature difference between the maximum and average
gradient of the windings
• To consider a maximum temperature rise of 50/55 deg C
for guaranteed oil temperature rise and 60/65 deg C by
winding resistance method for oil cooled transformers for
an ambient temperature of 45/40 deg C

Transformer oil requirements
• Chemically stable to ensure minimum oxidation
at higher operating temperatures.
• Low water Content to keep its dielectric strength
• High specific heat
• High thermal conductivity
• Low Density
• Non Toxic / Non PCB to avoid pollution
problems.
• Good Arc quenching properties
• Simple to produce and cost is reasonable

Transformer cooling system
(radiators and fans)

Transformer cooling system
(Pump and heat exchanger system)

Role of transformer oil
• Dual role of oil:
– Electrical insulation medium and
– Coolant for removing heat from core, windings
• For large transformers (like generator transformer)
– Oil cooled by oil/ water heat exchanger (OFWF – Oil
forced water forced)
– Forced oil circulation by pumps
– Gravity fed water cooling for oil
– Standby oil cooler and pump provided – Automatic
operation of standby pump in case of pump failure

Conservator
• Reservoir for transformer oil, located at a height above
main transformer tank
• Takes up volumetric changes in oil induced by load
changes
• Sufficient volume to allow for expansion/ contraction of oil
under load conditions
• Designed to withstand full vacuum
• Oil level indicated by:
– Prismatic type oil gauge
– Magnetic, float type gauge

Conservator
• Conservators are so arranged that the
lower part act as a sump in which any
impurities entering the conservator can
collect
• A valve/plug is fitted at lowest point of the
conservator for draining these impurities
along with oil

Oil level gauge and its importance
• Proper oil level vital for transformer safety
• Indicates, monitors oil level in conservator tank
• Magnetically coupled to float arm in oil (no glands used)
• Enables
– Detection of oil leakages
– Preparation for replenishment of oil
– Protection of transformer against low oil level
• Low level oil switch:
– Float actuated, operates if oil falls below predetermined
level

Oil level gauges
Prismatic Oil
level gauge
Float type Oil
level gauge

Moisture in Transformer
• Main cause - Oil subjected to temperature cycles
depending on load and ambient temperature
• Transformers absorb moisture from air when oil leaks
are not arrested properly
• Water solubility increases as oil temperature
increases

Moisture in Transformers
Allowable moisture for transformers in service per ANSI
C57
Aver. Oil <69KV 69-230 KV >230KV
temperature
500 C 27ppm 12ppm 10ppm
Water
saturation 15% 8% 5%
Paper moisture 3% 2% 1.25%

Effects of Moisture on insulation
• Life of insulation reduces by half for each doubling of
water content in oil
• Electrical discharges in high voltage region due to
imbalance in moisture equilibrium - Leads to incipient
faults
• Possibilities of bubble formation with gases
• Rate of thermal deterioration of paper directly proportional
to water content

Moisture affects Dielectric Strength
0
20
40
60
80
100
0 20 40 60 80 100
Relative Humidity of Oil, %
Dielectric
Breakdown
Voltage,
%
Oil w ith 60 micron fibres, approx. 50 g/t
Filtered Oil, approx. 20 g/t particles

Breather (Silica gel type)
• Breather connected to main conservator tank
• Silica gel/ Cobalt chloride mixture used as moisture
absorbing agent
• Oil seal at base prevents moisture ingress when
transformer is not aspirating
• Replace silica gel when colour changes from blue to pink
across mid level of container

Breather (Silica gel type)

Protection devices
• Lightning arrestor
• Pressure release valve/ diaphragm
• Buchholz relay
• Oil level gauge
• Winding temperature monitoring
• Oil and water flow indicators

Lightning arrestor
• Over voltage protection of large transformers
• Protection against steep surge voltages (ex.
Lightning) coming from overhead lines
• Installed as close to transformer as possible
(preferably near bushings)
• Connected to station earthing by shortest way

Pressure release valve/ diaphragm
• Transformer faults can:
– Cause breakdown in cooling oil
– Quickly generate large amounts of gas
– Resulting pressure can rupture transformer tank if not
relieved quickly
• Gas and oil actuated relay does not operate quickly
enough to relieve pressure
• Pressure relief device must be fitted

Pressure release valve/ diaphragm
• Fitted on top of each phase oil tank
• Protection against dangerous internal pressure build up -
Operates within 2 milliseconds
• Pressure released before equipment damage can occur
• Trip initiated
• Mechanical indicator pin must be reset by hand
• Investigate cause in the event of valve operation

Oil and water flow indicators
• Counter balanced vane actuated by flow rate of
transformer oil or cooling water
• Dial chamber completely sealed off from main
body by rigid diaphragm
• Transfer of vane movement to dial chamber by
magnet coupling (No glands employed)
• Second magnet in dial chamber follows vane
movement – Actuates indicator pointer and
mercury switches

Winding temperature indicator
• Generates signals for indication, alarm and trip and cooling
control
• Consists of:
– Compensated bellows system
– Transmission system
– Indicating pointer
– Switches
• Comprises of two detectors:
– Liquid filled thermometer bulb – Reacts to slow
response time of change in oil temperature
– CT – Reacts quickly to change in current flow in phase
winding

Winding temperature indicators
Compensated bellows system comprises:
• Operating bellows, Thermometer bulb, Interconnecting
capillary tubing, CT, Heating coil, adjustable shunt
resistance
• Compensating bellows, Interconnecting capillary tube
– Compensating system enables sensing of winding
temperature alone
– Movement of operating bellows transmitted to indicating
pointer by linkages
– Mercury switches provide alarm, trip signals

Marshalling kiosk
• Houses transformer instrumentation and all
control equipment
• All steel weather proof construction
• Multi compartment with provision for padlocking
• Compartments for incoming supplies, fan and
pump control, winding temperature controllers,
ammeters
• Anti condensation heaters
• Power supply sockets, telephone socket for
maintenance purposes

Transformer Tanks / Enclosures
• Transformer tanks constructed with welded
boiler plates of sufficient thicknesses
depending on internal pressure
requirements
• Tanks shall have removable inspection
cover basically to inspect internal core,
facilitate removal and reinstalling core and
windings in case of any repair jobs

Tank and Painting
• Coating of 6 to 10 mils on tank surface is
recommended to protect against
deterioration due to atmospheric
conditions
• Epoxy paints preferred in corrosive areas

Transformer Operation and
Maintenance

Transformers In Service
• Transformer breakdowns can be avoided by
adopting Standard operating procedures and
recommended maintenance practices.
• Factors causing Failure of a transformer:
– Overload
– Incorrect installation or use
– Faulty design or construction
– Neglect
– Wear and tear and other deterioration
– Accidents

Transformer Inspection
• A rigorous system of inspection and preventive
maintenance ensures long life, trouble-free
service and low maintenance cost
• Maintenance consists of regular inspection,
testing and reconditioning where necessary

Inspection Intervals
• Following table gives preferred schedule
of inspection for a typical transformer
Device Inspection interval Operational interval
Temperature indicators 1 month N.A.
Oil level 3 months Annual samples
Pressure relief device 1 month N.A.
Silica Gel breather 3~ 6 months Reactivation, as
required
Gas actuated relay 1 month 1 year
Tap changer N.A. 1 year
Fans/pumps 6 months 1 year

Tap-changer Maintenance
• Lift out diverter, clean vessel, change oil
• Inspect contacts, change them if required
• Inspect transition resistors
• Check flexible connections for brittleness
• Change main spring, check spring relaxed
• Check drive, molybdenum grease
• Check drive and bevel gears

Electrical tests and oil quality
tests

Objectives of transformer testing
• Verifying configuration (new or repaired
transformers)
• Winding health
• Oil quality verification
• Functional checks of mounted instruments/
relays/tap changer
• Residual life assessment

Winding Resistance Test
• Resistance decides copper losses
• Use Wheatstone bridge or Kelvin bridge
• Apply DC voltage and wait till core saturation
• Ensure windings not in very hot condition during
measurement
• Unequal or infinity values may indicate possibility of
open winding or loose connections
• Winding Resistance values shall be uniform to ensure
healthiness of windings internally

Turns Ratio Test
• Apply around 400 volts AC on primary terminals
(take open circuit voltage readings on
corresponding secondary terminals – ratio
indicates approximate turns ratio)
• Take readings at all tap positions
• Values should not differ by more than 0.5% (of
expected design voltage ratio)
• Portable instruments available to take
measurements

Turns Ratio Test
> Turns ratio measurements shall ensure that transformer meets
application needs
> Results shall be within 0.5% of calculated voltage ratio

Vector Group Test
• Transformers for paralleling must have same
polarity and phase relation to avoid partial or
complete short circuits
• Transformer polarity and phase relation tests
important when two or more transformer are to
be paralleled

Dielectric Tests
Tests applicable
– Applied Potential Test at rated power
frequency for a duration of one minute
– Induced Over voltage test at higher frequency
for reduced duration

Partial Discharge Test
• For transformers rated 220 kV and above
• To check discharges along cavities, cracks, etc
• Caused by improper drying of insulation and
presence of sharp edges
• Requires special equipment. 1.3 times rated
voltage applied for 5 minutes, 1.5 times for 5
seconds and 1.3 times the rated voltage
maintained for 30 minutes
• Should be within 300 pC at 1.3 times and within
500pC at 1.5 times the rated voltage

Bushing testing
• Power factor test or Tan  test basically carried
out to check deterioration and contamination of
bushings
• RIV test done basically to determine corona
discharges (lowers performance and life) in
bushings at rated operating voltage
• Moisture content checked for oil type bushings

Oil Testing
• The condition and safe operation depends on
testing of oil to check the following parameters
and taking corrective steps
– Dielectric Test
– Acid Neutralization Number
– Interfacial Tension Test
– Colour
– Relative Density
– Dielectric Dissipation Factor
– Dissolved Gas Analysis

Moisture in Transformers
• Typical moisture contents of new
transformers
Aver. Oil <69KV >69 KV
temperature
500 C 7ppm 2ppm
600 C 12ppm 4ppm
700 C 20ppm 7ppm
Water saturation 6% 2%
Paper moisture 1% 0.5%

Measurement of Moisture Content
Power factor test or tan  measurement
• Power factor value of 0.5% considered
unacceptable for bigger transformers at high
voltages
• Value of around 1% still accepted for smaller
transformers
• Constant high power factor at various voltages
indicate presence of moisture in transformers

Measurement of Moisture Content
• IR Values - Log-log graph with IR values against
time
– Good insulation gives almost straight-line curve
increasing with time
– Moist/ contaminated insulation gives curve that
raises slowly, flattens out shortly
• Polarization index
– Ratio of 10 minutes IR to 1 minute IR
– Value of 1 totally unacceptable
– For dry type transformers, minimum value of 2
required

Dissolved Gas Analysis
• Content of gases indicate the internal fault
conditions of a transformer
• Following are some established methods.
 Permissible Gas Concentration Limits
 Regression Method
 Combustible Gas Method
 Key Gas Method
 Ratio Method- Rogers & IEC
– Duval’s Triangle method

Main Gases analyzed in DGA
Hydrogen H2
Methane CH4
Ethane C2H6
Ethylene C2H4
Acetylene C2H2
Carbon monoxide CO
Carbon dioxide CO2
Oxygen O2
Nitrogen N2

Duval Triangle Method
• Accurate and trustworthy method using DGA
for deduction of transformer problems
• Based on data base of thousands of
transformers
• About one million DGA analyses performed
every year by more than 400 laboratories
worldwide
• Cause determined based on percentages of
combustible gases evolved

Duval Triangle
PD - Partial Discharge
T1 - Thermal Fault less than 300°C
T2 - Thermal Fault between 300°C and
700°C
T3 - Thermal Fault greater than 700°C
D1 - Low Energy Discharge (Sparking)
D2 - High Energy Discharge (Arcing)
DT - Mix of Thermal and Electrical faults

Symbols and Problem Causes
Symbol Fault Examples
PD Partial Discharge
Corona discharge in voids, gas bubbles with possible formation of X-wax
in paper
D1
Discharges of low
energy
Partial discharges of the sparking type, inducing pinholes, carbonized
punctures in paper
Low energy arcing inducing carbonized perforation or surface tracking of
paper, or the formation of carbon particles in oil
D2
Discharges of high
energy
Discharges in paper or oil, with power follow-through, resulting in
extensive damage to paper or large formation of carbon particles in oil,
metal fusion, tripping of equipment and gas alarms
T1
Thermal Fault
T<300ºC
Evidenced by paper turning brownish (>200ºC) or carbonized (>300ºC)
T2
Thermal Fault,
300<T<700 ºC
Carbonization of paper, formation of carbon particles in oil
T3
Thermal Fault,
T>700ºC
Extensive formation of carbon particles in oil, metal coloration (800ºC) or
metal fusion (> 1000ºC)
DT
Electrical Fault and
Thermal Fault
Development of one type of fault into another type of fault

Oil Dielectric Test
• Collect sample oil and
immerse electrodes with 2.5
mm gap
• Apply high voltage and
increase till flashover - called
Break Down Voltage (BDV)
• Standard value 30 kV but
new oil may have up to 80 kV
• Take sample of five or six
readings

Life Expectancy of
Transformer
• Estimation by deterioration of paper insulation
• Determination of paper insulation condition
– Furan Analysis
– Testing of Kraft paper

Furan Analysis
• Compounds of cellulose decomposition
• Reliable method: Estimation of paper insulation life
• Furans > 250 ppb Paper insulation deterioration
• Range: 100 ppb ~ 70,000 ppb (ppb - parts per billion)
• Important Furans
– 5H2F (5-hydroxymethyl 1-2-furaldehyde) - Oxidation
(aging, heating of paper insulation)
– 2FOL (2-furfurol) - High moisture content in paper
– 2FAL (2-furaldehyde) - Overheating of paper
– 2ACF (2-acetylfuran) - (rarely observed)
– 5M2F (5-methyl-2-furaldehyde) - Local overheating
(hot spots)

DP (Degree of Polymerization)
Test
• Reliable assessment of Paper deterioration
• Cellulose - Long chains of glucose rings
• DP - Average number of rings in molecule
• DP value of new insulation - 1000 ~ 1400
• DP value ≤ 200 - End of insulation life

DP and % of
Remaining Life of paper
New insulation material 1000 DP ~ 1400 DP
60% to 66% remaining
life
500 DP
30% remaining life 300 DP
Zero remaining life 200 DP

Operation of transformers

Conditions for parallel operation of
multiple transformers
Transformers shall have
– Same transformation ratio, rated voltages
– Identical indices – ex. Dy11, Yd11
– Terminals with same polarity (HV and LV side)
connected in parallel
– Tap changers (if provided) in same tap positions
– Rated outputs not deviating more than 1:3
– Same short circuit impedance (within +/- 10%)
• Transformer with lowest short circuit impedance
most heavily loaded

Importance of regular inspection and
maintenance
– Long life of equipment
– Trouble free service – Improved reliability
– Low maintenance expenditure
– Minimum down time of power system
– Improved safety
– Improved morale

Visual external inspection
What it can reveal
• Oil temperature indicator
• Winding temperature indicator
High oil/ winding temperatures – Probable causes
– Blocked radiator, heat exchanger
– Cooling fan, circulation pump failure
– Transformer over loads
– Internal faults

What it can reveal
• Conservator oil level gauge – Low oil level
• Oil stains around transformer
Probable causes
– Leaks from valves, radiators, welded joints etc
– Very low load with very low ambient temperatures
– Insufficient oil - Need for topping up

What it can reveal
Abnormal noise, smell, vibration
Probable causes
• Termination problems – Partial discharge, arcing, sparking
• Bushing problems – Corona, insulator failure
• Internal fault
• Loosening of internals

Sonic and vibration analysis
• Valuable tools for locating internal faults of
transformers
Sonic analysis:
• Low energy arcing/ sparking and partial
discharges emit energy in ultrasonic range
• Ultrasonic emissions detected by sensors placed
outside the tank – Converted to audio signals or
oscilloscope traces
• Fault location can be ascertained before opening
up transformer

Sonic and vibration analysis
Vibration analysis
• Vibrations due to loose components, loose core
and coil segments, defective bearings of fans and
pumps
• Vibrations detected and measured by Vibration
analysers
• Expertise needed to pin point cause of vibration

Life Expectancy of Transformer
• Estimation by deterioration of paper insulation
• Determination of paper insulation condition
– Furan Analysis
– Testing of Kraft paper

Visual inspection
What it can reveal
Silica gel Breather
• Silica gel turned to pink beyond mid level
Probable causes
• Spent silica gel. Need for replacement of silica gel
• Inadequate oil level at bottom of breather

Visual inspection
What it can reveal
Buchholz Relay
• Gas accumulation in relay
• Discolouration of oil
Probable causes
• Air entrapment - Leakages in joints
• Increased moisture level in oil
• Oil deterioration
• Internal fault

Visual inspection
What it can reveal
Pressure Relief Valve - Activation of valve
Cause
• Internal fault
Thoroughly investigate cause for valve operation

Visual inspection
What it can reveal
Rusting, corrosion of tank, transformer parts,
pipe lines
– Check cause for rusting – Water leakage,
ageing, deterioration of protective paint
– Clean the surface and provide corrosion
protection

Internal (borescope) examination
• Specially designed instrument for internal
inspection without opening up oil filled transformer
• Core, windings, connections can be inspected,
photographed
• Can ascertain exact problem to be attended before
opening up transformer
• Proper use can save:
– Lot of time
– Repair expenses

Online gas in oil monitoring
• Comprises of Hydran 201Ti intelligent transmitter
and Hydran controller connected to plant data
network
• Hydran system:
– An intelligent fault monitor
– Reads composite value of gases in ppm
generated by faults
– Warn personnel when diagnostic or remedial
actions required
– Gas in oil information – Can be monitored
locally or remotely

Safety aspects while working near
energized transformers
HAZARD 1:
• Hazardous high voltages – Extreme care must be taken
whilst in vicinity of energized transformers
• With transformer energized, no work may be performed at
transformer tank and cooling system other than following:
– Replenishing oil within conservator
– Collecting gas samples from Buchholz relay
– Not applicable to jobs effected from marshalling kiosk or
motor drive compartment of tap changer
• Isolate fire fighting system if access to compound exceeds
a period longer than few minutes

Transformer compound access
HAZARD 2: EXPOSED HV CONDUCTORS
Precautions:
– Generator Transformer Acoustic Compound is an
enclosed area
• Isolate fire system for Work in this area
• Star point conductor is exposed & may become live
under fault conditions
• Star point is hard earthed. Need not be regarded as
High voltage conductor
– Generator Transformer Compound, a HV Zone
• No climbing or work above ground level except from
installed platforms is permissible
• Long objects e.g. ladders, scaffold poles that would
reduce clearances, should not be taken into compound

Transformer compound access
HAZARD 3: FIRE PROTECTION DELUGE SYSTEM
OPERATION
Precautions:
i) For external work at ground level the fire system may
remain available provided that:
a) Work at heights and hot work is excluded
b) Work in any enclosed area is excluded
c) Control room is informed of compound entry and exit
d) For external work at heights, fire system should be
isolated

Protection of transformers

Why do transformers need protection?
• Abnormal operating conditions
• Increased operating temperatures
• Over loading
• External faults in system
• Internal faults
• System voltage surges (ex. Lightning, switching
surges)

Transformer Short Circuit Currents
• Transformer windings are subjected to mechanical
and thermal stresses during internal short circuits
and also during system short circuits outside
terminals
• Transformer short circuit current during faults
depend on system parameters as well as on its own
impedance
• Transformer windings and construction shall take
care of such short circuits which need to be
considered at design stage itself

• Normally duration of 2 seconds of short circuit
current considered for transformer design
unless client specifically asks to consider
higher/ lower duration depending on his
backup protection
• Short circuit withstand capacity is normally
based on the maximum allowable temperature
rise of the windings under short circuits

• Main reason for failure of transformer due to short
circuits is more because of mechanical forces
produced on windings under short circuits rather
than thermal damage on the insulation
• Electromagnetic force varies directly in line with
square of current, which means 20 times normal
current during short circuit will produce 400 times
the normal stress and the design to take care of
such high forces

Magnetic Stray Field & Forces

Short circuit Effect – Radial
collapse

Types of Stresses
• Stresses are to be faced by transformer
winding insulations during its life:
– Dielectric stress
– Short circuit stress
– Switching surges
– Lightning impulses
– Through faults

Internal problems and external
influence
High winding temperatures:
– Overloading of transformer
– High ambient temperatures
– System fault conditions
– Decreased cooling efficiency

influence
Deterioration of transformer oil:
– Moisture ingress
– High operating temperatures due to overload
– Inadequate maintenance practices:
• Breather maintenance
• Oil inspection and maintenance
• Deteriorated gaskets, seals

influence
Deterioration of winding insulation:
• External system faults
• Over load
• Moisture ingress
• Inadequate maintenance practices:
– Breather maintenance
– Oil inspection and maintenance
– Deteriorated gaskets, seals

Temperature based protection
• Oil temperature monitoring and protection
• Winding temperature monitoring and
protection
• Forced cooling based on temperature rise

Winding Temperature Indicator
• Generates signals for indication, alarm and trip and
cooling control
• Consists of:
– Compensated bellows system
– Transmission system
– Indicating pointer
– Switches
• Comprises of two detectors:
– Liquid filled thermometer bulb – Reacts to slow
response time of change in oil temperature
– CT – Reacts quickly to change in current flow in phase
winding

Winding Temperature Indicators
Compensated bellows system comprises:
• Operating bellows, Thermometer bulb, Interconnecting
capillary tubing, CT, Heating coil, adjustable shunt
resistance
• Compensating bellows, Interconnecting capillary tube
– Compensating system enables sensing of winding
temperature alone
– Movement of operating bellows transmitted to indicating
pointer by linkages
– Mercury switches provide alarm, trip signals

Gas/oil Surge Protection
• Gas/ oil surge protection:
– Buchholz relay
• Oil high pressure protection:
– Pressure relief device (explosion vent)
– Sudden pressure relay

Buchholz Relay

> Located between conservator and main tank
> Detects low oil level, faults within transformer
> Float with mercury switch mechanism
> Two stage device operated by:
> Gas buildup – Initiates Alarm
• Gas created by arcing within transformer tank
• Air ingress
> Oil surge – Initiates Trip
• Due to internal arcing fault
> Buchholz relay settings not adjustable
> Windows permit observation of oil level inside relay
Buchholz relay

Buchholz Relay
• Buchholz relay can detect both gas and oil
surges as it is mounted in pipe to
conservator
• Monitors any such kind of pressure buildup
to avoid explosions
• Buchholz relay is connected between main
tank and conservator in all breathing type
transformers with isolating valves on either
side

Explosion Vent

Sudden Pressure Relay
Transformer faults can:
> Cause breakdown in cooling oil
> Quickly generate large amounts of gas
> Resulting pressure can rupture transformer tank if
not relieved quickly
> Gas and oil actuated relay does not operate quickly
enough to relieve pressure
> Sudden pressure relay very sensitive to variations in
internal pressure, operates quickly

Over voltage protection (by lightning
arrestors)
• Over voltage protection of large transformers
• Protection against steep surge voltages (ex. Lightning)
coming from overhead lines
• Installed as close to transformer as possible (preferably
near bushings)
• Connected to station earthing by shortest way

Differential protection
• Compares currents entering and leaving protected
zone
• Operates when differential current exceeds pre-
determined level
• Types:
– Current balance
– Circulating current scheme

• Relay operation under internal fault conditions
• Detection of unbalance by relay within its
protective zone
• Called UNIT protection - Operates only for
faults on unit it is protecting

Current balance scheme (External fault
conditions)

Current balance scheme (Internal fault conditions)

Restricted Earth Fault Protection
• Simple over-current and earth fault relay
cannot provide adequate protection for
winding earth faults
• Even biased differential relay ineffective
for certain earth faults within winding
• Separate earth fault protection necessary

Restricted Earth Fault Protection
Relay Type: Instantaneous high impedance type

Over Fluxing Relay
Transformer overfluxing due to:
– Over voltage
– Low system frequency
• Transformer cores designed to operated below certain
magnetic flux density
• Excess flux density causes over heating and damage
• Over excitation occurs during
– Startup/ shutdown of generator connected transformers
– Over voltages during load rejection
• Over flux protection does not need high speed tripping –
Instantaneous tripping may cause damage

Electrical protection
Over Current Relay
• Most common protection in a transformer
• Protects against excess withdrawal of current
• Uses IDMTL (Inverse Definite minimum Time)
over-current and earth fault relay on transformer
HV side
• Operating time varies inversely with respect to
current value

Neutral E/F Relay
• Reactance decreases towards the neutral
• Fault current is controlled mainly by leakage
reactance, which varies in a complex manner
• Earth fault current does not vary in a linear
fashion
• Use sustained or sensitive earth fault protection
with CT on star point of winding, or
• Use restricted earth fault protection

Any questions ?

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