ELEN7041: ELECTRIC POWER INVESTIGATIONAL PROJECT
Report: Maintenance and Conditional Monitoring of Power
Transformer’s at Electricity Department
Candidate
Student Name
Student Number
XXXXXX
I hereby declare this is my own unaided work, submitted in partial fulfillment for the
degree in Master of Engineering in Electrical
,ELEN7041: Maintenance and Conditional Monitoring of Power Transformer’s at Electricity
Department
Abstract
Power transformers are electrically tested and maintained by various strategies facing
different goals. Factory tests are followed by start-up tests onsite before commissioning and
it is very crucial to keep the tests records as footprint for future reference. Both tests are to
confirm a span of life for the power transformer of 25-30 years. There is an increasing need
for power utilities to increase the utilisation of their assets while maintaining system
reliability. Asset or equipment reliability not only affects the availability of the power to
customers, but also affect the economic operation of the utility. In addition, substantial
benefits can be realised by maintaining and repairing equipment based on condition and
operating them beyond their expected life. Because of this economic incentive, condition
assessment of plant equipment and associated systems is essential. Key components of
condition assessment are condition monitoring or diagnostic techniques that are employed
as means of verifying the condition for replacement/ refurbishment or identifying failure or
malfunction modes.
Condition assessment and diagnostics of power transformers may be organised as basic
diagnostics, advance diagnostics, on-line monitoring and investigations during repair or post
mortem. The maintenance teams of power transformers mainly perform basic diagnostic
such as oil sampling (off-line DGA) and simple electric tests on a Conditional Based
Maintenance and Time Based Conditional Monitoring or Time Based Maintenance. If these
tests indicate deviations from original conditions or exceeded limits, advanced or specialised
methods are applied. These methods are suited to the detected or assumed failure and
mostly require disconnection of the transformer or an outage. DGA remains the key
assessment technique in determining the condition of power transformers, therefore, On-line
monitoring provides a continuous assessment of the actual transformer condition but cost
restrictions limit this method to important or new, large transformers and after post mortem of
repairs for further monitoring on an Online Conditional Monitoring.
A case study showed that DGA as diagnostic tool in determining the conditions of the oil and
insulation by analysing the moisture content, water, dielectric strength and released gases.
The use of DGA in conjunction with other electrical tests enhances the diagnosis of failure
and can able to locate the failure. The case study showed that there are many factors that
needs to be considered to diagnose the cause of a transformer fault and a careful
consideration needs to be taken in terms of understanding, experience and methods when
utilising the different diagnostic techniques. Performing maintenance task has risk and every
maintenance task requires certain measure of competence, skill and experience. Some
tasks require authorisation (e.g. advanced diagnostic measures), further education
(chemistry for dielectric tests such as DGA, furan analysis) or operator training on special
devices (oil treatment). Therefore, it is crucial to assign each maintenance to maintenance
team according to their skills level and therefore, there needs to be a training program in
place to provide, maintain and develop the skill base in a form of on-the-job training and/or
training programs based in a class room.
Keywords: Dissolved Gas Analysis (DGA), Time Based Maintenance (TBM), Time Based
Conditional Monitoring (TBCM), Online Conditional Monitoring (OLCM), Condition Based
Maintenance (CBM).
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,ELEN7041: Maintenance and Conditional Monitoring of Power Transformer’s at Electricity
Department
Table of Conten
1 Introduction..................................................................................................................... 11
1.1 Background................................................................................................................... 12
1.2 Problem Statement.......................................................................................................12
1.3 Research Objectives....................................................................................................13
1.4 Structure of the Report................................................................................................13
2 Power Transformer Components and Failure Modes...................................................14
2.1 Core and Winding Coils...............................................................................................14
2.2 Insulation...................................................................................................................... 17
2.2.1 Insulation Paper........................................................................................................... 17
2.2.2 Insulation Oil................................................................................................................ 18
2.3 Oil Preservation Systems............................................................................................18
2.3.1 Conservator System....................................................................................................18
2.3.2 Pressure Regulated Gas Blanket.................................................................................19
2.4 Cooling Systems.......................................................................................................... 20
2.5 Auxiliary Equipment.....................................................................................................21
2.5.1 Bushings...................................................................................................................... 21
2.5.2 Tap Changers.............................................................................................................. 22
2.5.3 Gauges, Indicators and Relays....................................................................................23
3 Power Transformer Diagnostic Techniques..................................................................25
3.1 Inspection..................................................................................................................... 25
3.2 Infrared Temperature Analysis....................................................................................26
3.2.1 Infrared (IR) for Transformer Tank...............................................................................26
3.2.2 Infrared (IR) for Radiators and Cooling Systems.........................................................26
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, ELEN7041: Maintenance and Conditional Monitoring of Power Transformer’s at Electricity
Department
3.2.3 Infrared (IR) for Bushings............................................................................................27
3.3 Dissolved Gas Analysis (DGA)....................................................................................28
3.4 Winding or Transformer Turns Ratio..........................................................................31
3.5 Winding Resistance......................................................................................................32
3.6 Magnetisation or Excitation Current...........................................................................33
3.7 Capacitance and Dielectric Dissipation Factor (DDF)/Tan Delta or Power Factor...33
3.8 Leakage Reactance/ Short Circuit Impedance...........................................................35
3.9 Frequency Response of Stray Losses (FRSL)...........................................................36
3.10 Core Insulation Resistance and Inadvertent Ground Test......................................37
3.11 Vibration Analysis......................................................................................................39
3.12 Frequency Response Analysis (FRA).......................................................................39
3.13 Polarization Depolarization Current (PDC) Method.................................................42
3.14 Frequency Domain Spectroscopy (FDS)..................................................................45
3.15 Recovery Voltage Method (RVM)...............................................................................45
3.16 Partial Discharge (PD) Measurement........................................................................46
3.16.1 Electrical PD measurements......................................................................................47
3.16.2 Acoustic Partial Discharge (PD) Measurement..........................................................49
3.16.3 Partial Discharge (PD) Measurement using UHF Technology...................................50
4 Electricity Department’s Maintenance Strategy............................................................50
4.1 Strategic Objectives.....................................................................................................50
4.2 Technical Objectives....................................................................................................52
4.3 Transformer Maintenance Strategy.............................................................................53
4.3.1 Condition Based Maintenance.....................................................................................56
4.3.2 Time Based Maintenance (TBM) and Time Based Condition Monitoring (TBCM).......56
Page |3
XXXXXXXX
Report: Maintenance and Conditional Monitoring of Power
Transformer’s at Electricity Department
Candidate
Student Name
Student Number
XXXXXX
I hereby declare this is my own unaided work, submitted in partial fulfillment for the
degree in Master of Engineering in Electrical
,ELEN7041: Maintenance and Conditional Monitoring of Power Transformer’s at Electricity
Department
Abstract
Power transformers are electrically tested and maintained by various strategies facing
different goals. Factory tests are followed by start-up tests onsite before commissioning and
it is very crucial to keep the tests records as footprint for future reference. Both tests are to
confirm a span of life for the power transformer of 25-30 years. There is an increasing need
for power utilities to increase the utilisation of their assets while maintaining system
reliability. Asset or equipment reliability not only affects the availability of the power to
customers, but also affect the economic operation of the utility. In addition, substantial
benefits can be realised by maintaining and repairing equipment based on condition and
operating them beyond their expected life. Because of this economic incentive, condition
assessment of plant equipment and associated systems is essential. Key components of
condition assessment are condition monitoring or diagnostic techniques that are employed
as means of verifying the condition for replacement/ refurbishment or identifying failure or
malfunction modes.
Condition assessment and diagnostics of power transformers may be organised as basic
diagnostics, advance diagnostics, on-line monitoring and investigations during repair or post
mortem. The maintenance teams of power transformers mainly perform basic diagnostic
such as oil sampling (off-line DGA) and simple electric tests on a Conditional Based
Maintenance and Time Based Conditional Monitoring or Time Based Maintenance. If these
tests indicate deviations from original conditions or exceeded limits, advanced or specialised
methods are applied. These methods are suited to the detected or assumed failure and
mostly require disconnection of the transformer or an outage. DGA remains the key
assessment technique in determining the condition of power transformers, therefore, On-line
monitoring provides a continuous assessment of the actual transformer condition but cost
restrictions limit this method to important or new, large transformers and after post mortem of
repairs for further monitoring on an Online Conditional Monitoring.
A case study showed that DGA as diagnostic tool in determining the conditions of the oil and
insulation by analysing the moisture content, water, dielectric strength and released gases.
The use of DGA in conjunction with other electrical tests enhances the diagnosis of failure
and can able to locate the failure. The case study showed that there are many factors that
needs to be considered to diagnose the cause of a transformer fault and a careful
consideration needs to be taken in terms of understanding, experience and methods when
utilising the different diagnostic techniques. Performing maintenance task has risk and every
maintenance task requires certain measure of competence, skill and experience. Some
tasks require authorisation (e.g. advanced diagnostic measures), further education
(chemistry for dielectric tests such as DGA, furan analysis) or operator training on special
devices (oil treatment). Therefore, it is crucial to assign each maintenance to maintenance
team according to their skills level and therefore, there needs to be a training program in
place to provide, maintain and develop the skill base in a form of on-the-job training and/or
training programs based in a class room.
Keywords: Dissolved Gas Analysis (DGA), Time Based Maintenance (TBM), Time Based
Conditional Monitoring (TBCM), Online Conditional Monitoring (OLCM), Condition Based
Maintenance (CBM).
Page |1
XXXXXXXX
,ELEN7041: Maintenance and Conditional Monitoring of Power Transformer’s at Electricity
Department
Table of Conten
1 Introduction..................................................................................................................... 11
1.1 Background................................................................................................................... 12
1.2 Problem Statement.......................................................................................................12
1.3 Research Objectives....................................................................................................13
1.4 Structure of the Report................................................................................................13
2 Power Transformer Components and Failure Modes...................................................14
2.1 Core and Winding Coils...............................................................................................14
2.2 Insulation...................................................................................................................... 17
2.2.1 Insulation Paper........................................................................................................... 17
2.2.2 Insulation Oil................................................................................................................ 18
2.3 Oil Preservation Systems............................................................................................18
2.3.1 Conservator System....................................................................................................18
2.3.2 Pressure Regulated Gas Blanket.................................................................................19
2.4 Cooling Systems.......................................................................................................... 20
2.5 Auxiliary Equipment.....................................................................................................21
2.5.1 Bushings...................................................................................................................... 21
2.5.2 Tap Changers.............................................................................................................. 22
2.5.3 Gauges, Indicators and Relays....................................................................................23
3 Power Transformer Diagnostic Techniques..................................................................25
3.1 Inspection..................................................................................................................... 25
3.2 Infrared Temperature Analysis....................................................................................26
3.2.1 Infrared (IR) for Transformer Tank...............................................................................26
3.2.2 Infrared (IR) for Radiators and Cooling Systems.........................................................26
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XXXXXXXX
, ELEN7041: Maintenance and Conditional Monitoring of Power Transformer’s at Electricity
Department
3.2.3 Infrared (IR) for Bushings............................................................................................27
3.3 Dissolved Gas Analysis (DGA)....................................................................................28
3.4 Winding or Transformer Turns Ratio..........................................................................31
3.5 Winding Resistance......................................................................................................32
3.6 Magnetisation or Excitation Current...........................................................................33
3.7 Capacitance and Dielectric Dissipation Factor (DDF)/Tan Delta or Power Factor...33
3.8 Leakage Reactance/ Short Circuit Impedance...........................................................35
3.9 Frequency Response of Stray Losses (FRSL)...........................................................36
3.10 Core Insulation Resistance and Inadvertent Ground Test......................................37
3.11 Vibration Analysis......................................................................................................39
3.12 Frequency Response Analysis (FRA).......................................................................39
3.13 Polarization Depolarization Current (PDC) Method.................................................42
3.14 Frequency Domain Spectroscopy (FDS)..................................................................45
3.15 Recovery Voltage Method (RVM)...............................................................................45
3.16 Partial Discharge (PD) Measurement........................................................................46
3.16.1 Electrical PD measurements......................................................................................47
3.16.2 Acoustic Partial Discharge (PD) Measurement..........................................................49
3.16.3 Partial Discharge (PD) Measurement using UHF Technology...................................50
4 Electricity Department’s Maintenance Strategy............................................................50
4.1 Strategic Objectives.....................................................................................................50
4.2 Technical Objectives....................................................................................................52
4.3 Transformer Maintenance Strategy.............................................................................53
4.3.1 Condition Based Maintenance.....................................................................................56
4.3.2 Time Based Maintenance (TBM) and Time Based Condition Monitoring (TBCM).......56
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