Download Condition Monitoring Transformers

Working principle of a
transformer
A transformer is static (or stationary) piece of
apparatus which:
1.Transfers electric power from one circuit to
another.
2.It does so without a change in frequency.
3.The principle is based on mutual induction
between two circuits linked by a common
magnetic flux.

Basic parts of a transformer
Basically a transformer consists o f a :
1.A primary coil or winding.
2.A secondary coil or winding.
3.A core that supports the coils or the
windings

Transformer construction
Main constructional elements of Transformers are A) Magnetic circuit
Core & clamping structure
B) Electric Circuit
Winding,Insulation, Bracing devices.
C) Terminals
Tapping, Tapping switches, Terminal
Insulator, Leads , Bushings
D) Tank
Oil, Cooling devices, conservator, piping,
Breather
E) Protective Circuit
& Monitoring
Buchholz relay, WTI, OTI, Oil surge relay
pressure relief device, MOG
Transformer construction
A)Magnetic Circuit:
The core provides closed path for flux. It is made up of CRGO
insulated laminations. (CRGO has iron loss of about 1.3 W / Kg at
1.6 Tesla )
B)Electric Circuit:
Winding, insulation & bracing are constructional parts of electrical
circuit of transformers. This is the most vulnerable part of
transformer because of direct association with power system. Must
be designed to withstand voltage stress resulting from system
fault, transient over voltage and thermal stresses (lightening or
switching surges)
Transformer construction

Insulation:
Commonly used material are Paper or press board
Oil is used as insulating medium
Insulating varnish applied to make coil mechanically
strong.
C)
Terminal:
Leads :
Connection of winding are (copper rod or bus bar) taken to
bushing.
Bushing:
Up to 33 kV porcelain bushing are used. Above 33 kV,
condenser & oil filled terminal bushings are used.
Transformer construction
D) Transformer tank

Cooling:
Small transformers are air cooled whereas large transformers
are provided with oil or oil & air cooling .
The transformer tank is designed to withstand full vacuum.
Types of Cooling:
Air insulated air cooled
AN
Air Natural
AF
Forced Air Cooling
ANAN
Natural Air cooling inside and outside transformer
Transformer construction
Oil immersed air cooled ONAN
ONAF
OFAN
OFAF
Natural Oil Circulation. Natural Air flow
Natural oil and forced air flow
Forced Oil & Natural Air Circulation
Forced Oil and Forced Air Circulation
Oil immersed & water cooled
ONWF
OFWF
Natural Oil, water (internal) cooler
Forced oil, water (external) cooler
Transformer construction
E) Protective devices :
Various protective devices mounted on transformer are as
follows:
Bucchholz Relay
Gas actuated relay, Transformer
Internal fault.
WTI/OTI
Provided for alarm and / OR trip
against over load
PR Device
To release internal pressure
generated in the transformer
during fault.
MOG
Alarm when oil level is low
Oil Surge Relay
To release actual pressure
generated during fault in OLTC
Accessories & their functions
Terminal & Bushing : Type of Bushing, terminal is selected
depending on voltage, currents & operating conditions. Porcelain,
condenser type of oil filled bushings are used as per requirement.

Cable Boxes : Cable boxes are primarily designed for receiving
and protecting cable ends and ensure effective sealing of cable
against ingress of moisture.

Conservator : It is provided to accommodate change in oil
volume caused due to change in loads or ambient conditions.
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Breather :Whenever there is change in ambient temperature or
load , there is a change in oil temperature and hence the volume of
oil. Increase in volume causes the air above oil level in conservator
to be pushed out and decrease causes air to be drawn in. Thus
the transformer breathes.
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Accessories & their functions
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When air is breathed in, moisture and dust from
atmosphere is sucked in .
Silica gel crystals absorb moisture. Color of silica gel
is blue when dry and turns pink when absorbs
moisture.
Oil cup at the bottom is filled with oil which acts as
coarse filter and removes dust form outside air.
Magnetic Oil Level gauge:
This is a dial instrument operated by magnetic
coupling from a float on oil surface.
It is normally fitted with contact to give alarm for low
oil level.
Accessories & their functions

Oil temperature indicator:
Bourdon tube with a pointer arrangement mounted in a case
comprising of a reading dial and a glass cover. There is a
temperature sensing bulb which communicates to the bourdon
tube through the armored capillary.

Winding temperature indicator:
It comprises of following:
WTI pot :mounted at top of transformer tank. Oil in pot is temp. of top
oil.
Imae coil:Heater coil and develops additional hear raising temperature of
oil incide heater coil.
WTI CT:
Mounted on one of the line lead with secondary connected to image coil
WTI: The bulb of the WTI is immersed in oil inside image col
.Temperature of this oil is dependent on top oil temperature and load on
transformer.
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Accessories & their functions
Buchholz Relay : Gas & oil operated relay detects formation of
gas or development of sudden pressure inside the oil of transformer.
Any electrical fault inside the transformer is accompanied by
evolution of gas.
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Pressure Relief Device: This is provided to relieve the internal
pressure in the event of major fault within the transformer.
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Tapping Switch: To maintain secondary voltages reasonably
constant at load end when incoming voltage and/or load on
transformer changes, it is necessary to change the voltage ratio (I.e,
turns ratio of the winding) of the transformer.
This is achieved by changing the number of turns ( HT Side) by
operating a switch called as tapping switch.
Depending on the requirement, off circuit or on load tap
changer is installed in the transformer.

Accessories & their functions
Radiators :
The function of radiator is to limit the temperature of oil and winding by
dissipating heat that is generated due to losses within transformer while in
service.
 When transformer is in operation warm oil rises and enters the radiator
from the top valve cools and then descend to enter the bottom of the
tank.

Other Accessories: Inspection cover, jacking lugs, Earthing terminals,
Rating Plate, Filter valve, Drain Valve, Terminal marking, Rollers etc.

DGA
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Insulating materials within transformer breakdown to
liberate gases.
The identity of these gases indicate the type of fault
and the rate of gas generation indicate the severity
of fault.
Causes of fault gases can be divided into three
categories:
Corona or partial discharge
DGA
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Pyrolisis or thermal heating
Arcing
Arcing is the most severe fault (intensity of energy
that is dissipated per unit time per unit volume of
the fault) ,less with heating, least with corona.
One of the most important technique to indicate the
health of a transformer.
Major (Minor) fault gases under
various fault conditions
Sampling And Labeling Procedure
Sampling
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Dry Weather, avoid contamination
Clean, dry, leak proof glass or stainless steel
container.
Equipment operating normally
Take safety precautions.
Sample bottle must be full without any air trap
completely sealed and should be properly labeled.
FREQUENCY OF SAMPLING
NEW TRANSFORMERS
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FIRST
DGA TEST : BEFORE HEAT RUN TEST ON
TRANSFORMER
SECOND DGA TEST : AFTER HEAT RUN TEST ON
TRANSFORMER (2&24 HRS)
THIRD
DGA TEST : BEFORE ENERGIZING
TRANSFORMER (B.M.)
FOURTH DGA TEST : WITHIN THREE MONTHS OF
SERVICE
FREQUENCY OF SAMPLING
IN-SERVICE TRANSFORMERS
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ANNUALLY AS A ROUTINE CKECKING
BEFORE (LATEST DATA) & AFTER FILTRATION
/TOPPING UP
TRANSFORMERS AFTER OVERHAULING,
REPAIRS, MAINTANANCE
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BEFORE ENERGISING TRANSFORMER
WITHIN THREE MONTHS OF SERVICE
Interpretation of Results
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Ensure that gas concentrations are high
enough to warrant further investigation.
Gas being present even in normal operating
condition without any fault being present.
Gases might have formed on the occasion of
previous faults or during repairs by brazing,
welding etc., and not completely removed.
Since gases are produced in normal ageing
also, the service duration of the oil has to be
taken into account.
PERMISSIBLE LIMITS OF DISSOLVED GASES IN
OIL OF A HEALTHY TRANSFORMER
GAS <4YRS IN PPM) 4-10 YRS (PPM) >10 YRS (PPM)
H2
100-150
200-300
200-300
100-150
200-300
GAS <4YRS IN PPM) 4-10 YRS (PPM) >10 YRS (PPM)
H2
100-150 50-70200-300
CH4
CH4
50-70
100-150
C2H2 20-3030-50
C2H2 20-30
C2H4 100-150
150-200
C2H4 100-150
C2H6 30-50
100-150
CO
200-300
C2H6
30-50400-500
CO2
3000-3500
CO
4000-5000
200-300
CO2 3000-3500
200-300
200-300
30-50
100-150
200-400
150-200
800-1000
600-700
100-150
200-400
100-150
800-1000
400-500
600-700
4000-5000
9000-12000
9000-12000
Condition monitoring of
transformer oil
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Role of transformer oil:
It is used as coolant.
It is used as insulating material.
Reasons for deterioration of transformer oil
Physical contamination:
Release of fibrous impurities by paper, pressboard, wood and
cotton tapes in contact with oil for longer period at elevated
temperatures.
Due to dissolution of varnish .
Due to foreign matters like dust, metallic particles and other
solid impurities
Due to moisture
Condition monitoring of
transformer oil
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Effects:
Life is reduced by high sludge formation.
Electrical properties of insulating oil get disturbed due
to conductivity of suspended particles.
Chemical deterioration:
It is due to oxidation.
Effects of oxidation:
Results in acids, sludge .Acid attack solid insulation
and metal. Sludge causes poor thermal conduction
and mechanical hindrance to proper oil circulation.
Condition monitoring of
transformer oil
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Contamination of gases:
Gases are present in oil due to following:
Those which dissolve in the oil from atmosphere.
Those which are generated inside due to thermal
decomposition of oil, decomposition of oil by arcing.
Effects of gases:
The ignition of inflammable gases can be causes by
corona occurring in th air space or arcing.
Transformer oil testing
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Transformer oil testing is carried out to detect
abnormalities in transformer and based on test results
corrective actins can be taken before actual failure takes
place.
To evaluate quality of oil
To decide periodic maintenance (filtration. reclamation
etc)
To know health of transformer (by DGA)
To estimate remaining life of transformer
Oil Testing
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New oil (IS-335)- 15 tests
Oil in service (IS-1866)- 8 tests
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Dielectric strength (BDV)
Dielectric dissipation factor (Tan delta)
Resistivity
Neutralisation value (Acidity)
Flash point
Water content
Sludge
Interfacial tension
Transformer oil testing
1.Physical condition of the oil:
Color ,clarity and odour gives information regarding
quality of oil and presence of certain contaminants in
oil.
2.Electrical strength:
Important parameter as used as insulating medium.
This test gives conductive contaminants and moisture
present in oil.
3.Water content:
Reveals total water content ,leak or cellulosic
deterioration
Transformer oil testing
4.Specific resistance (Resistivity):
This test provides a measure of the total soluble contaminants and
ageing products .It is numerically equal to the resistance between
opposite faces of a centimeter cue of the oil and is expressed as
ohm com.
5.Dissipation factor:
This test provides a measure of the total soluble
and ageing products.
6.Neutralisation value:
contaminants
This test gives acid present in the oil. It is the no. of milligrams of
potassium hydroxide required to neutralise completely the acids
produced in one gram of oil.
Transformer oil testing
7.Interfacial tension test:
This test provides a measure of sludge and
polar component present in oil. It is
expressed as molecular attractive force
between the molecules of water and oil at
oil-water interface.
8.Flash point:
Sudden drop in flashpoint is indicative of
unsafe working condition of transformer.
Scheduled of oil characteristics for transformer
in service as per IS : 1866-2000
Property
Highest Voltage of equipment, kV
< 72.5
72.5 to 170
> 170
Breakdown voltage (kV),
Min.
More than
30
More than 40
More than
50
Water content (ppm),
Max.
Max. 95
Max. 40
Max. 20
Neutralization value (mg
KOH/g), Max.
Max. 0.3
Max. 0.3
Max. .0.3
Sediment & Sludge, %
by mass
ND
ND
ND
0.1 x 1012
0.1 x 1012
Resistivity @ 90°C x 1012 0.1 x 1012
(ohm-cm), Min.
Property
Highest Voltage of equipment, kV
< 72.5
72.5 to 170
> 170
Dielectric dissipation
factor @ 90°C, Max.
1.0
1.0
0.2
Interfacial tension
(mN/m), Min.
15
15
15
Flash Point, (°C), Min.
125
125
125
Condition Monitoring - Oil testing
Sr. No.
Test
Remedial Action for
Deviations from
permissible limits
1
Electric strength 2.5
Oil filtration
mm gap (Break down
voltage)
2
Water content
Oil filtration
3
Sediment and / or
Oil filtration
precipitable sludge
Condition Monitoring - Oil testing
Sr.
Test
No.
4
5
Remedial Action for Deviations
from permissible limits
Specific resistance
Oil filtration if the tan  value
(Resistivity at 90 deg.C
permits other wise replace the oil
Dielectric dissipation factor (
Replace the oil
tan  )
6
Neutralisation value (total
Replace the oil
acidity) of the oil
7
Inter facial tension of the oil
against water at room
temperature
Replace the oil
Furanic compound test
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Life of transformer is dependent on life of solid
insulation and life limit is determined by thermal
degradation of winding paper.
Kraft insulation paper is used as solid insulation.
Furanic compound test
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When oil soaked paper is damaged by heat, some oil
soluble compound are released into oil called furans.
Paper is made of cellulose consisting of log chains of
glucose rings joined by glycosidic bonds.
During degradation bonds are broken and glucose
rings are opened. Glucose is unstable which further
degrades which are more stable and oil soluble called
furans.
Furanic Compounds
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The most commonly found furanic derivative is 2furfuraldehyde(2 FAL) and other derivatives are
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2- furfuryl alcohol (2 FOL) ,
2- Acetyl furan (2 ACF),
5- hydroxymethyl furfuraldehyde,(5HMF)
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5 methyl furfuraldehyde(5MEF).
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FURANS AND GASES
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Cellulose
degradation=Glucose+H2O+CO+CO2+Organic acids.
With DGA and furan test extent of paper damage can
be seen.
CO and CO2 are determined by DGA and are
considered as level indicator for cellulosic
degradation.
In case of severe localised paper damage ,high
furans and high gas content can be seen.
In case general heating slow building of furans
without necessarily seeing an increase of gas
content.
Degree of polymerization (DP)

Degree of polymerization (DP) is another way of
expressing the molecular weight. Physical properties
of the paper depends on the degree of
polymerization of paper
M = Dp x m
Where
M= molecular weight of the polymer
Dp = the degree of polymerization
and m = the molecular weight of monomer.
Degree of polymerization (DP)
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DP value 1000 to 1500 i.e,1000 to 1500
glucose units are present in cellulose
molecule.
Degradation of paper is due to temperature,
water ad oxygen.
When DP value<300,paer becomes brittle
and more suspectible to failure.
DP relates directly to cellulosic degradation
Correlation between Dp and Furan
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The absolute correlation of Furan to DP is
difficult, but can be related fairly accurately
extent through an empirical formula.
Dp = - 100 ln (2 FAL) +709
INSULATION RESISTANCE AND
POLARISATION INDEX TESTS
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These tests are performed to verify state of
dryness if insulation.
These tests are intended to check overall
cleanliness ,dryness ,localized defects and general
condition of insulation system.
When DC voltage is applied across insulation, the
current flows is the resultant of three currents:
Capacitive charging current
Absorption current
Leakage current
S
C
DC Voltage
source
RL
Absorption current
RA
Conduction or leakage
current
Capacitance charging current
INSULATION RESISTANCE AND
POLARISATION INDEX TESTS

Capacitance leakage current:
The current lasts for a few seconds as DC voltage is
applied and drops out after the insulation is charged
to its full voltage. The time depends on the size and
capacitance of the test object. Larger time for larger
capacitance objects.
INSULATION RESISTANCE AND
POLARISATION INDEX TESTS
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Absorption leakage current:
It is caused by polarisation of molecules within
dielectric material.
In low capacitance equipments the current is high for
first few seconds and decrease slowly to nearly zero.
In high capacitance equipment or wet and
contaminated insulation ,there will be no decrease of
absorption current for long time.
Conduction or leakage
current:
Conduction or leakage current
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This is the current that normally flows through the
insulation ,between conductors or conductors to
ground.
It increases quickly and becomes stable.
This current increases as insulation deteriorates and
becomes predominant after absorption current
vanishes.
It is steady and time independent .Hence the
important current for measuring insulation resistance.
Interpretation of results:
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If IR value shows a decreasing trend it shows
gradual deterioration of insulation quality due to
humidity ,dust accumulation etc..
A very sharp drop indicates insulation failure.
IR MEASUREMENTS

THE TEST VOLTAGES FOR IR MEASUREMENT OF
TRANSFORMER ARE AS UNDER.
RATING
TEST VOLTAGE
415/440 V
500 V
3.3 kV
1000 V
6.6 kV
2500 V
11 kV & ABOVE

5000 V
THE MINIMUM ACCEPTABLE LOWER LIMIT FOR
IR VALUES IS GIVEN IN STANDARDS AS
V L-L + 1 M
Polarisation index
measurements
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Polarisation Index=600sec IR value/60 sec IR value
PI<1 , for bad insulation and PI in the range of 1.2 to
2 can be considered as an indication of good
insulation.
A very high value of PI is also not advisable since it
shows the brittleness of insulation .
STEP VOLTAGE TEST
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The DC voltage is applied in various steps and in
each step the leakage current is noted. Step
duration:60 seconds.
The variation of this leakage current (or IR) with test
voltage gives the condition of insulation.
If insulation is dry, clean and with out physical
damages shall show the same value at all voltage
levels.
If insulation value decreases at higher voltage levels
,may be due to dirt, moisture, cracking, aging etc.
The application of increased voltage creates electrical
stresses on internal insulation cracks. This can reveal
aging and physical damage in relatively clean and dry
insulation which would have not been apparent at
lower voltages.
CAPACITANCE MEASUREMENTS
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THE CAPACITANCE VALUE IS DEPENDENT ON
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THE CHARACTERISTICS OF THE DIELECTRIC MATERIAL
THE PHYSICAL CONFIGURATION OF THE ELECTRODES
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 A
C = ----
d
Void /impurities may discharge partially during a voltage apication
and the effective distance between eectrodes ncreases.
HENCE C WILL INCREASE WITH INCREASE IN VOLTAGE,
WHICH INDICATES PRESENCE OF PD AND THE DETERIORATION
OF INSULATION
A
ELECTRODE
DIELECTRIC
d
ELECTRODE
TAN  MEASUREMENTS
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THE TAN  VALUE DIRECTLY INDICATES THE POWER
DISSIPATED BY THE INSULATION
Ic
Ic

90
v
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v
IT INCREASES WITH INSULATION DETERIORATION AND
SERVES AS AN EARLY INDICATOR OF FAILURE HAZARDS
Concept of Tan 

In an ideal capcitor the voltage and current
are phase shifted by 90 deg.and current
through insulation is capacitive.
If there are impurities in insulation,the
resistance of insulation decrease resulting in
increase of resistive current.
Thus the total current I = Ic + Ir which leads
the voltage by a phase angle < 90°. And
lags the Ic by an angle .
20.4
30.6
45.9
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The tangent of this angle directly
indicates the heat dissipation that takes
place inside the dielectric material.
The values obtained on new insulation
forms the reference value for periodic
measurements.
Partial discharge:
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It is an electrical discharge that occurs across
a portion of the insulation between two
conducting electrodes without completely
bridging the gap.
This results in localized, nearly instantaneous
release of energy.
The most convention unit for quantifying the
PD quantity is Pico coulomb
Interprtation of results:
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High value of tan-delta at low voltage gives an
indication of contamination and presence of moisture
content.
Tan-delta tip up gives an indication of void content
(variation of tan delta with applied voltage).
Generally tan delta values shall not increase as
applied voltage increase.
A higher tan delta tip up at a applied voltage
indicates presence of voids/moistures and the
inception of partial discharges at this voltage.
Increase in tan delta above passing of time also
indicates deterioration of insulation
Transformer protection
Transformer - Protections
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Over load Capability:
Working life of transformer – dependent on life of insulation
Rate of deterioration of insulation – increases with increasing
winding temperature  W
Winding temperature – dependent on loading
Transformer has substantial over load capability
IF  W < 80 deg.c – use of life negligible
If transformer is operated @ 104 deg. For every hour of operation
= 2 hrs of life is lost.
If transformer is operated @ 116 deg. For every hour of operation
8 hrs of life is lost
Rate of using transformer life doubled for every temperature
increase of 6 deg.c
Transformer - Protections
.
Relative Rate of using
Life in Hours
Transformer Life Vs Temperature
100
10
98
1
104
2
110
4
Accelerated Ageing
1
Normal Ageing
0.1
80
92
104
116
128
Winding Temperature in Deg C
140
Transformer - Protections
Period1
Period2
Period3
Total Life
Lost
Loss
Hrs./Day
24 Hrs
@80oC
24 X 0.125
3
24 Hrs
@98oC
24 X 1
24
9 Hrs @80oC 7 Hrs @80oC 8 Hrs @80oC 9 X 0.125 +
7 X1 +8 X 2
24
24 Hrs
@104oC
48
24 X 2
Transformer - Protections
Criteria for overload operation
For normal duty cycle, current shall not exceed 150% I RAT
 For emergency duty, current can exceed 150% I RAT provided
associated cables, switch gear, tap changers, bushings etc. are
suitable rated.
Under no circumstances,
  Winding shall exceed 140 deg.c
  Oil shall exceed 115 deg.c.
Transformer - Protections
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Buchholz Protection:
Relay installed in the pipe line between transformer tank and
conservator
Two Floats
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Upper float = responds to slow accumulation of gas due to mild or
incipient faults – for alarm
Lower float (Vane) – responds to oil surge caused by major internal
faults – for trip
Relay mounting precautions
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Gas shall freely pass up the pipe work
Extra turbulence shall not be induced in oil stream
Relay shall be mounted on straight run of pipe line which should
slope from transformer to conservator at an angle of 5 deg.
Operating time: 100 to 200 milliseconds
Petcock provided on top of housing to draw accumulated gas for
analysis
Transformer - Protections
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Buchholz Protection:
Gas actuated relay
Popularly used in all countries except USA
Used to detect incipient faults which may lead to major damage if
allowed to continue
Some Examples:
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Hot spots on the core due to short in lamination insulation
Core bolt insulation failure
Faulty joints
Inter turn faults
Loss of oil due to leakage
Depends for its operation on the fact that most internal faults
generate gases.
Maintenance Schedule
Frequency of
Inspection
Daily
Half yearly
Item to be inspected
Remarks
Amb. Temp.
Oil temp.
Winding temp.
Load current
Voltage
Oil level / leakage
Bushing
Cable boxes
Breather
Tap changer operation
WTI/OTI
IR value
Check whether
temp. rise is
reasonable
Check against rated
values
Take corrective action
if abnormality is
noticed
Maintenance Schedule
Frequency of
Inspection
Yearly
Item to be inspected Remarks
Yearly
DGA analysis
Five yearly or
condition
monitoring
report based
Over hauling of
transformer
Oil testing
Take corrective
action (oil filtration
/ oil replacement)
as per the test
report
Over haul the
transformer if
abnormality is
indicated in the report
Take corrective action
if abnormality is
noticed