POWER TRANSFORMER SENSIBLE MAINTENANCE PLANNING
BASED ON FIELD TESTING AND FAILURE ANALYSIS
Waleed Abdullah Al-Saadi ¹ * and Mukhtar Ahmed Fazal Karim ² *
Saudi Electricity Company (SEC)
Kingdom Of Saudi Arabia
changer operation, load profile, environmental
conditions and its location criticality.
SUMMARY
The paper discusses the concepts of sensible
maintenance planning focused on diagnostic
electrical testing for power transformers
selected on the bases of certain risk factors like
age, load, and environments in SEC-COA
Transmission network.
Various scenarios of transformer failures are
presented here based on the testing analysis of
the large power transformers.
Using
the
experience
gained
through
investigation, the paper attempts to address the
problem areas assessed in previous failures
and tries to put recommendation to improve the
existing maintenance planning effective and
result
oriented
towards
continuous
improvement.
Samples from groups containing identical
transformers are selected as study cases to find
transformer deterioration trends.
Proposals for advance testing with diagnostic
tools are furnished.

Maintenance procedures based on diagnostic
testing to address the problem areas of power
transformer such as tap change maintenance,
bushing change and oil reprocessing.

Comprehensive life extension program with the
goal to guarantee a reliable and prolonged
service.

Proposing
maintenance
steps
through
extensive diagnostic testing on sample
transformer selected in a group of identical
characteristics.
COMPLEX PROBLEMS TO BE ADDRESSED

How to deal with particular equipment showing
a symptom of abnormality?

How to assess
transformers?

How to prevent sudden failures particularly
catastrophic ones?

The goal of this paper is to reply on abovementioned questions and set the maintenance
policy directions to achieve best results.
KEYWORDS
Sensible Management Planning, Diagnostic
Testing, Transformer Structure, Frequency
Response Test, Recovery Voltage Method,
Polarization Spectrum, Dynamic Impedance
Test. Data Management System, Data
Analyzing Tools
1.
INTRODUCTION
The existing maintenance policy for HV power
apparatus in SEC-network is time based
executed during winter season from October to
April every year. The existing maintenance
policy shall be impressed further based on
following concepts for life management of
power transformers.
 Time
based management based on
condition of transformers such as age, load tap
*¹΄²
2.
and
extend
the
life
of
TRANSFORMER LIFE CYCLE
CONDITION
Four key properties determine the transformer
functional serviceability.

Ability to transfer energy at specified condition
including permissible over voltage and over
loaded conditions with out over heating, over
Saudi Electricity Company - Protection and Testing Division – Central, P. O. Box 57- Riyadh - 11411, KSA
losses, gassing, over vibrations and sound
levels.




Gas generation.
Ability to carry load current without developing
hot spots such as high resistance and over
heating of current path like bushings, windings,
neutral path, selector switch & diverter switch
contacts etc.

PD activity.

Oil deterioration.

Increase in sound level and vibration.
Ability to withstand strength of insulating
materials such as oil, paper, pressboards and
core and winding clamps against high voltages
high temperature stresses.

Increase of no load losses and magnetizing
current.

Increase in tap changer contact resistance
causing local over heating, oil breakdown and
gas generation.

Oil over heating, oil break down, bubble
formation, gas generation.

Surface contamination with conducting particles
and oil aging products leading to flashover.

Formation of sludge in oil shows trend of
accelerated degradations.

Loosening of clamping, distortion of winding
geometry, further deformation by switching
surge and through fault current leading to
ultimate failure
Ability to maintain mechanical integrity under
the influence of through fault current, without
winding displacements and disturbance of
internal components physical geometry.
If any one of the above-mentioned conditions is
exceeded, the transformer tends to fail or
shorten its life span.
3.
TRANSFORMER STRUCTURE
Transformer is considered to have following
structural components.

Electro-magnetic circuit.

Current carrying path.

Insulation
boards).

Mechanical tap selection system

What to do if transformer is gassing in service?

Cooling system.


Bushings and internal connecting leads.
What to do if transformer has low insulation
resistance and low polarization index (PI) value
from its reference value found during
maintenance test?

Tap changer leads.

Oil preservation and expansion system.

What to do if transformer has high dissipation
factor (%PF)?

Protection and monitoring system.
system
(Oil,
Paper,
Press
5. ANALYSIS TO TRANSFORMERS
HAVING PROBLEMS (HISTORY CASES)

What to do if transformer has high ratio error?
Any defect in all above-mentioned structures
can affect the functional serviceability of
transformer.

What to do if transformer has high winding
resistance?
4.

What to do if transformer has high magnetizing
current?

What to do if transformer has high winding and
oil temperature?

POSSIBLE DEFECTS/FAULTS IN
THE TRANSFORMER STRUCTURAL
COMPONENTS
Rise of temperature.

What to do if transformer has high vibration
and sound level?
In reply to above-mentioned questions few
study cases are presented below showing
actual measurements and real defects found.
5.1 STUDY CASE NO 1
IN SERVICE GAS ALARM WHEN
TRANSFORMER LOAD
INCREASES
.
A three-phase generator transformer (GT31) at
power plant # 5 in Riyadh with rating 69MVA,
132000/13800, Ynd1, was in the service since
1978. ON 13 April 2004 gas relay alarm
operated. Gas relay was found full of gas.
Transformer was shut down and thorough
investigation launched by on site testing.
Winding resistance test at LV side winding
revealed the actual defect shown in table 1.
Measure
ments
Between
phases
R-Y
R-B
Y-B
Measured
values (m Ω
at 35ºC
ambient
temperature
7.76
7.75
9.45
5.2 STUDY CASE NO 2
TRANSFORMER FAILED DUE TO
REPEATED SWITCHING ON FAULTY
DISTRIBUTION CABLE FOR
LOCALIZATION OF FAULTY
SECTION
A three-phase distribution transformer (T12) at
substation # 7119 in Riyadh with rating 20MVA,
33000/13800, Ynyn0, was in the service since
1983. ON 27 December 2004 transformer
tripped when distribution staff was localizing the
faulty section of one of the 13.8kV under ground
cable feeder being fed by this transformer by
live switch on to fault.
Site tests were performed and the analysis
results are as under
Readings after
Time Elapsed
HV To
Ground
LV To
Ground
HV To
LV
1 Minute
1.0 G Ώ
150 K Ώ
10 Minute
1.5 G Ώ
100 K Ώ
1.5 G Ώ
PI value
1.5
0.67
1.5
1.0 G Ώ
Remarks
Normal
Normal
High resistance in current
path
Table 1: LV Winding Resistance Measurements
by Transformer Ohmmeter.
Exciting current
HV side (mA)
R
Y
B
Ph
Ph
Ph
Measured ratio
Table 3: Ratio and Exciting Current Test Results
Winding
under test
mA
Watts
C (pF)
%
POWER
FACTOR
LV to HV+
Ground
108
1002
10814
246
Table 4: Power Factor Test at 5kV ac
From the table 3 and 4, it is clear from the test
results that the winding displacement has
occurred resulting drastic changes in internal
geometry of physical components.
6.91
6.82
2.3902
B
Ph
287
Y
Ph
2.3902
R
Ph
2.4274
The reason of Gassing transformer in service
was due to dangerous overheating of the
contact of interconnecting lead between Y-B of
LV windings at higher loads.
This problem was revealed by precision winding
resistance test
TAP NO
Computed
Ratio
2.3913
5.1.1 ANALYSIS REMARKS
LV side
defective
Table 2: Polarization Index (PI) Test Results
5
All other tests like turns ratio, insulation
resistance, magnetizing current, dissipation
factor were found normal.
Gas was released and transformer energized
again. When load reached 50MW, gas alarm
initiated again confirming the loose joint.
It was decided to carry out inspections after untanking the windings.
A clear burnt-joint was found in the LV side Y-B
interconnecting lead.
Burnt contact was properly repaired and tested.
Transformer
was
re-energized
after
reassembling without any further problem.
Remarks
5.2.1 ANALYSIS REMARKS




Loosening its clamping system has caused
winding displacement. Loosening of
clamping can’t happen suddenly. It is a
slow process in stages developed by
mechanical forces due to through fault
currents passing repetitively.
Voltage ratio, magnetizing and Cap & DF
measurement tests are important for
predictive maintenance. In order to prevent
sudden failures, a predictive approach can
be developed by maintaining the history
record of all above-mentioned test results.
Once a method is adopted with certain test
equipment the same shall be repeated for
next planned maintenance turn.
Any change in ratio, magnetizing current
and capacitance will indicate winding
movement. Once the movement is
detected, steps could be taken prevent it by
particular service arrangements.
Instructions were issued to use alternate
methods for fault isolation instead of live
switch on to faulty cable.
5.3 CASE STUDY NO 3
POLARIZATION INDEX (PI) VALUE
FOUND LOW WHEN MEASURED
DURING INVESTIGATION TESTS
AFTER TRIPPING.
A three-phase generator transformer (GTR 5) at
power plant 7 Riyadh with ratings 125MVA
132kV/13.1kV/13.1kV, Ynd1d1 was in service
since 1978. On 18 December 2004, the
transformer tripped on differential protection
operated.
After necessary isolation the transformer was
thoroughly checked and tested with following
predictive test results.
Reading
After Time
Elapsed
1 minute
10
minutes
PI value
Insulation resistance (IR)----M Ώ
HVLV1LV2Ground
Ground
Ground
960
332
2800
2160
308
5700
2.25
0.927
2.037
Remarks
LV1 is
suspected
Table 5: Insulation Resistance Test with (PI)
Measurement by 5kv Tester

It was decided to isolate LV1 bushings from its
windings. After isolation, the same PI test was
repeated on LV1 windings and bushings
separately with results indicated in table 5
analyzed as above.




LV1 windings found normal.
LV1 red and yellow phase bushings found
normal with PI value >2.
LV1 blue phase bushing found abnormal with
PI value< 1
Blue phase bushing was replaced with new one
and same test repeated with good results.
Transformer was put back into service without
any further problem.
5.3.1




ANALYSIS REMARKS
Polarization index (PI) i.e. [10 minute IR value/1
minutes IR value] is an important predictive test
as experienced above.
It can assess the degree of deterioration.
PI value less than 1 indicates serious problem.
Timely change of defective parts like bushings
can save the transformer from total failure in
future.
6. MAINTENANCE PLANNING STRATEGY
Before proceeding to transformer maintenance
planning, one should understand following points.

What are the factors affecting the
transformer life?

What are the steps for planning focused
maintenance

How to manage electrical testing to predict
future failures?

How to make decisions for preventive
actions?
7. FACTORS AFFECTING TRANSFORMER
LIFE
a.
Voltage Transients
b.
Heat and overloading
c.
Moisture and dirt.
d.
Insulating oil Decay
e.
External Short Circuits.
f.
Poor manufacturing

9.
Figure 1: Transformer Life Cycle Stress
levels
8.
PLANNING FOCUSED
TRANSFORMER MAINTENANCE
When setting up a maintenance program the
key is to get the most efficient use of your
maintenance resources based on the risk
factors and importance of the transformer.
Therefore planning a maintenance program
consideration should be given to the following
key questions.




How important is this transformer in its
present location and what is affected if it
is lost?
If a transformer has sensitive location
feeding important consumers and directly
affects a key process and has no backup
available then it should be considered a
high priority
How old is the transformer?
Old transformers are more subject to
failures of external components and
gaskets because of their age.
What is the shape of the load profile?
If the load is steady state, then there will
be less internal mechanical stresses. A
load profile that has drastic fluctuations will
cause heavy mechanical stress on the coil
assembly. .
What is the surrounding environment
like?
Units that operate inside closed transformer
bay tend to suffer more from overheating
because of poor airflow. They may also be
exposed to harsh environments depending
of the process being operated at the facility.
Outdoor units tend to be less sensitive to
the environment but are more at risk
because of things like vandalism and
weather conditions.
Start the planning stage by identifying the
most important transformers in the network
system. These transformers will be easy to
find since they are the ones that if lost, will
cause the greatest effect on consumer on
going process.
TRANSFORMER SELECTION
CRITERIA FOR STUDY CASES
SEC-COA existing maintenance policy should
be impressed further to include study cases in
order to diagnose trends towards deteriorations
through electrical testing.
Certain selection criteria are needed to choose
such samples.
To find such study samples groups containing
transformers with same manufacturer and
ratings are separated. Out of these groups,
transformers as study case are selected based
on following risk factors.




Importance
Age
Loading
Environment
10. MAINTENANCE ACTION PLANS
Maintenance plan consists of four key actions
[4]




Outline Inspections
Preventive Actions
Predictive Actions
Corrective Actions
10.1 OUTLINE INSPECTIONS
10.1.1 VISUAL INSPECTION IN SERVICE
THROUGH SPECIFIED CHECKLIST
SUMMARIZED AS UNDER.








Abnormal Sound levels in transformer bay
Winding and oil temperature
On line DGA device readings.
HV bushing pressure gauge readings
Oil level gauge readings.
Oil and Gas sampling device checking for
any gas present.
Oil leakages
Discoloring of transformer tank surface.
Visual inspection in service should be
performed every month for all transformers
especially during the peak load season.
and Recovery Voltage Measurement test
(RVM) [2]

10.1.2 DETAILED INSPECTIONS &
TESTS DURING OUTAGE PERIOD
WITHOUT DISMANTLING ANY
EQUIPMENT





Windings
and
bushings
insulation
resistance test with polarization index (PI)
measurements
Dielectric
loss
(Tan-Delta
Test)
measurement on HV bushings through test
tap
Winding resistance test on all taps and on
all windings
Voltage ratio and magnetizing current
measurement test.
Tap
changer
motor
input
current
measurement during tap change cycle [1]
Detailed inspections should be performed
every 5th year for all transformers.
10.2




OLTC Dynamic Resistance Measurement
test [1]

Voltage ratio and magnetizing current
measurement test
Short circuit impedance measurement tests


Tap changer driving motor input power
measurement test during tap change cycle
[1]

Sweep Frequency Response Analysis test
(SFRA) [3]
PREVENTIVE ACTIONS
DGA for Insulating oil samples performed
on regular bases
Repair or replacement of aged parts,
assessed
from
outline
inspections
mentioned above, as leading to future
troubles
10.3 PREDICTIVE ACTIONS FOR
STUDY CASES
For thorough investigation on a particular
transformer selected as study case or bad
history of alarms or trips initiated in service,
following diagnostic tests are recommended.
Suitable planned outages shall be arranged for
suspected equipment based on above
mentioned inspections and previous history of
gas alarm generation
Certain part like cables, jumpers should be
dismantled. Only transformer with its bushings
left for detailed tests.
Detailed testing is performed as under
Windings
and
bushings
insulation
resistance test with Polarization Index (PI)


Core and core clamp insulation resistance
test.
Dielectric
loss
(Tan-Delta
Test)
measurement for windings and HV
bushings through test tap.
Winding resistance test on all taps and on
all windings with assessment of diverter
switch condition during tap change
operation.
10.4. CORRECTIVE ACTIONS
Decision should be taken by well-experienced
senior engineers on the bases of abovementioned test record and its evaluation to
perform internal inspections through inspection
holes.
Major overhaul should be launched in case of
the defects are repairable.
11. DATA MANAGEMENT SYSTEM [6]
Gathering correct data from field-testing and
maintaining records in a proper way is key to
successful diagnosing problem much earlier
than it becomes a disaster for the transformer.
Field-testing without comparison to previous
records has no meaning but wastage of time
and money. In order to manage records of field
tests, keep all test records in the database
especially
available
with
special
test
equipments listed in appendix 3 for particular
transformers under maintenance.
Three steps are explained as under:



Data Management
Test Conditions
Analysis Techniques
11.1 DATA MANAGEMENT
12. TEST CONDITIONS
All modern test equipments listed in appendix 3
have their-own data management and
analyzing software, which can be efficiently
used to diagnose faults.
If the test equipment is not associated by any
data handling software then data can be
arranged in normal MS Excel Spreadsheets as

Sheet 1 as transformer identity and name
plate data

Sheet 2 as Visual inspection in service
through specified checklist.

Sheet 3 as Windings and bushings
insulation resistance test with Polarization
Index
(PI)
and
Recovery
Voltage
Measurement test (RVM)

Sheet 4 as Core and core clamp insulation
resistance test.
Measurements are sensitive to ambient
conditions such as temperature humidity etc.
Transformer maintenance shall be planned at
same date and time when it was tested in
previous maintenance turn so that the ambient
conditions do not vary too much.
Note down all test conditions in the test data
sheet with date and time.
Transformer condition should be same as
tested before such as cables and jumpers
removed. Only transformer own bushings shall
be used as test points.
Try to use the same test equipment as used
before in previous tests on the same
transformer under study.
Use latest model test equipment listed in
appendix 3 with data management and analysis
software.






Sheet 5 as Dielectric loss (Tan-Delta Test)
measurement on HV bushings through test
tap
Sheet 6 as Winding resistance test on all
taps and on all windings with assessment
of diverter switch condition during tap
change operation
Sheet 7 as OLTC Dynamic Resistance
Measurement test
Sheet 8 as Voltage ratio and magnetizing
current measurement test.
Sheet 9 as Short circuit impedance
measurement tests
Sheet 10 as Tap changer driving motor
input power measurement test during tap
change cycle.

Sheet 11 as Sweep Frequency Response
Analysis test (SFRA)

Sheet 12 as Transformer problems and
corrective measures history record

Rename all sheets with suitable names.
13. DATA ANALYZING TECHNIQUES
Two analyzing techniques are used for the
purpose of graphical interpretation. The
methods are signature and comparison.
13.1 SIGNATURE TECHNIQUE
This technique takes the measurement from all
the three phases of a transformer and creates a
single value to represent the condition of the
transformer at the testing time as under.
Key equation:
A = (R + Y +B) / 3
Where,
R Ξ Red phase measurement value
Y Ξ Yellow phase measurement value
B Ξ Blue phase measurement value
A Ξ Average value of red, yellow and blue
phases for particular test.
S = √ [(R – A) ² + (Y – A) ² + (B – A) ²
11.2 GRAPHICAL DISPLAY OF TEST
RECORDS
Where,
All the test data should be kept in columns of
spreadsheets.
S Ξ Single value to represent transformer
condition.
MS Excel charts generate graphical displays of
test records and comparing them with one
another or with footprints will assess the
transformer condition trends.
S = Zero means all phases ideally identical.
S > 0 indicates dissimilarity in phase’s
condition.
A single value created out of measured values from
all the three phases to show the similarity of
windings at the testing time. This technique requires
transformer characteristics in healthy condition.
Measurements acquired from factory can be
regarded as footprint or reference value. How ever if
factory test records are not available then historical
test data can be considered as fingerprints.
mentioned in this paper can diagnose failures
of power transformers much before these
actually happened.

Though real testing at site is limited due time
constraints. The schedules are being made
very tight not enough time given for proper
testing. This hasty test has no use. It is better to
select a group of transformers with same
characteristics and select one sample out of
those to perform representative detailed study.
Result can be applied to all. Sensible
maintenance plans can save lot of time, money
and enhance the efficient use of manpower.

In order to promote result oriented and updated
the maintenance testing techniques, this paper
tries to introduce some of the dynamic testing
techniques in addition to conventional test
methods.

Managing test data and then analyzing it for
further preventive action is primary task of this
paper.
13.2 COMPARISON TECHNIQUE
This technique compares a reading from one phase
of a transformer to a reference set of data. The
difference between the two readings represents the
condition of transformer winding as compared to
reference. Reference data can be obtained from the
healthy phase of the same transformer or the
identical transformer near by in same location.
It is to be highlighted here that the signature of the
healthy reference phase must be obtained from the
same winding that is either high voltage or low
voltage as phase under study
D = M – Ref
Where
REFERENCES
M Ξ Measured value
Ref Ξ Reference data
[1]
J.J. Smith+, “ Decision Making Experience
With Maintenance Diagnosis Of High
Voltage Equipment”, Kema T & D Power
[2]
Volker Karius +, “ Practical Foundation Of
The Recovery Voltage Method (RVM)”,
Hefley Trench AG Switzerland.
[3]
Muhammad
Aziz
Abdul
Rahman,
Halimatun,
Ps
Ghosh,
“Frequency
Response
Analysis
Of
A
Power
Transformer” Universiti Tenaga Nasional.
[4]
M.H. Rahman, “ Maintenance Of Electrical
Equipment” IEP-SAC Journal 2003-2004.
[5]
V.V Sokolov, “ Consideration On Power
Transformer Condition Based Maintenance”
EPRI
Substation
Equip
Diagnostic
Conference VIII Feb 20-23-2000, New
Orleans, LA
[6]
Robert Houbaer+,’ Power Transformer
Asset Management’ APWA International
Public Works Congress, NRCC/CPWA
Seminar Series 2000.
D Ξ Difference between measured and reference
data.
D = Zero means all phases ideally identical.
D > OR < 0 indicates dissimilarity in phases
condition.
This technique needs reference data. It will be very
useful if historical data is available so that the
difference can be analyzed.
From the above analysis, ones the clear trend is
established for particular test, and the trend line
reach to unacceptable limits set by the appendix 1,
the problem source can be identified explained in
appendix 2 for further preventive or corrective
measures.
14. CONCLUSIONS

Power transformer is the heart of electrical
utility. In order to insure it’s healthy operation,
steps are required to take preventive and
predictive measures. Diagnostic techniques
APPENDIX 1
Test Criteria and Defect Analysis
Type Of Tests
Test Criteria and defect analysis
Windings and bushings insulation resistance
test with polarization index (PI) measurements
PI <1-- Equipment defective
1 < PI > 1.5 --satisfactory
1.5 <PI >2.5 --good
PI > 2.5 –excellent
Recovery voltage test
On polarization spectra curve less dominant
time constant (Td) indicates more moisture
contents. Curve peak shifting towards left
side with steep rise indicates high moisture
contents in the transformer.
Refer to figure 3.
Dielectric loss (Tan-Delta Test) measurement
on HV bushings through test tap
%DF (PF) at 20ºC
0.25 to 1 for new P/TFs
0.75 to 1.5 for 15 year old P/TFs
More than 5.0 is problem for investigated
Winding resistance test on all taps and on all
windings. Condition of diverter switch for
transition time between tap change process
Resistance shall increase or decrease
gradually. Any abrupt change from tap to
tap indicate tap changer contacts bad
condition. If the transformer ohmmeter trips
during tap change operation then there is
problem in diverter switch diverting
resistances.
OLTC Dynamic Resistance measurement test
Magnitude of current transients captured on
each tap change indicates the condition of
tap position. Comparing the transients
captured on several tap positions it can be
concluded as high resistance and action for
further check up.
Turns
ratio
and
measurement test
magnetizing
current
Measured turns ratio should be within 0.5% of
computed value. Especially at principal tap,
turns ratio outside this limit indicates winding
defects. Any abrupt change in magnetizing
current from tap to tap indicates turn-turn short.
Tap changer motor input current measurement
during tap change cycle.
Power measured at the time of commissioning
shall be taken as fingerprints. If new record
shows any noticeable change then there could
be mechanical problem in driving mechanism.
Short circuit impedance measurement tests.
Fingerprints shall be developed for short
circuit impedance when transformer is new
and normal. Noticeable change in
impedance value when measured under
same
condition
indicate
mechanical
distortion and dislocation of transformer
windings due passage of fault currents.
Sweep Frequency Response Analysis (SFRA)
test
Refer to figure 2
For a given transformer new frequency
response plot is compared with previous
records taken at different time during
maintenance. Clear differences from
previous records indicate core movement,
winding deformation, faulty core grounds,
partial winding collapse, broken or loosened
clamping structures and short or open
windings.
Figure 2- Sample of Sweep Frequency Response (SFRA) Test Record
Dominant time
constant (Td)
Figure 3: Sample Of Recovery Voltage Method (RVM) Test Record
Newly energized
After 15years In Service.
Figure 4 - RVM Polarization Spectrum For Transformers
Shifting of the curve
peak (Td) towards
right side indicates
more moisture
contents in
transformer under
test.
APPENDIX 2
Power Transformer Component Defects [5]
System, Components
Defect
Fault & Symptoms
Electromagnetic
Circuit
Loose Clamping
Short Circuit (Opencircuit) in grounding
circuit
Abnormal circulating
current
Floating potential
Aging lamination
Insulation degradation
General overheating
Localized hot spot
Sparking/dischargers
Gassing
Winding strands, leads
Poor joint
Localized hot spot
Connection, joins,
contacts
Poor contacts, Contact
deterioration
Open-circuit, Short-circuit
Major insulation
Excessive water, Oil
contamination
Destructive PD
Minor insulation
Surface contamination
Localized tracking
Leads insulation
Abnormal aged oil
Creeping discharge
Electrostatic shields
Abnormal cellulose
ageing
PD of low energy
Excessive
aged/overheated
Cellulose Flashover
Loose clamping
Winding distortion
Core
Structure Insulation
Clamping Structure
Magnetic Shields
Grounding Circuit
Windings (Turns,
parallels)
Failure
Current carrying
circuit
Dielectric system
Mechanical
Windings
Clamping
Radial buckling
Leads support
Axial Twisting
Failure of insulation
Cooling system
Heat exchanger
Contamination
Cooling deficiency
Pumps
Wrong rotation, bearings
General overheating
Fans
Overheating, failure,
Malfunction
Piping external and
internal
Poor oil flow
Penetration of decay
product Into oil
Bushings
Local defect: moisture,
air
Ionization
Condenser core
Overstressing,
Dielectric overheating
Ageing, oil instability
Thermal run away
Core surface, oil
Conductor
Over-saturation
Moisture,
Flashover
Aging
Explosion
Poor contact,
Overheating
OLTC
Motor driver, shaft
Couplings, fixing
Selector & reverser
Diverter switch
Mechanical damage
Cooking Discharges
Contacts overheating
Malfunction
Contamination
Dielectric breakdown
Contacts wearing out
Contamination
Mechanical deficiency
Mal-function
Oil preservation &
expansion
Tank, conservator,
Low oil level
Preserving components,
Poor sealing
Piping
APPENDIX 3
Recommended Test Equipment For Power Transformer Diagnostic Testing.
Diagnostic Test
Suitable Test Equipment
Windings and bushings insulation resistance test
with Polarization Index (PI) and Recovery Voltage
Measurement test (RVM)
Recovery Voltage & Insulation
Measuring Unit Type ETP-2
Recovery Voltage Meter type RVM
5462
Core and core clamp insulation resistance test.
5kv Insulation Tester
Type BM21
Dielectric loss (Tan-Delta Test) measurement on HV
bushings through test tap
10kv Automated Insulation Test Set
Type Delta-2000
Winding resistance test on all taps and on all
windings with assessment of diverter switch
condition during tap change operation.
Winding Resistance Measurement
Unit
Type ETP-3
OLTC Dynamic Resistance Measurement test.
Transient Recorder
Type IDMT1
Voltage ratio and magnetizing current measurement
test.
Transformer Turns Ratio
Measurement Unit
Type ETP-1
Short circuit impedance measurement tests.
Short Circuit Impedance
Measurement Unit Type ETP-4
Tap changer driving motor input power
measurement test during tap change cycle.
Power Multi-Meter Type PMM-1 VER
2.5
Frequency Response Analysis test (FRA)
Sweep Frequency Response
Analyzer type M5100,