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Tuesday, 9 April 2013

Transformers: Basics, Maintenance and Diagnostics

Transformers: Basics, Maintenance and Diagnostics


CONTENTS
1. Introduction......................................................................... 1
2. Introduction to Transformers .............................................. 1
2.1 Principle of Operation................................................ 3
2.2 Transformer Action.................................................... 4
2.3 Transformer Voltage and Current.............................. 8
2.4 The Magnetic Circuit ................................................. 10
2.5 Core Losses................................................................ 13
2.6 Copper Losses............................................................ 13
2.7 Transformer Rating.................................................... 14
2.8 Percent Impedance ..................................................... 14
2.9 Internal Forces ........................................................... 16
2.10 Autotransformers ....................................................... 17
2.11 Instrument Transformers............................................ 18
2.12 Potential Transformers............................................... 19
2.13 Current Transformers................................................. 20
2.14 Transformer Taps....................................................... 22
2.15 Transformer Bushings................................................ 23
2.16 Transformer Polarity.................................................. 24
2.17 Single-Phase Transformer Connections for
Typical Service to Buildings................................... 25
2.18 Parallel Operation of Single-Phase Transformers
for Additional Capacity........................................... 26
2.19 Three-Phase Transformer Connections...................... 28
2.20 Wye and Delta Connections....................................... 28
2.21 Three-Phase Connections Using
Single-Phase Transformers ..................................... 29
2.22 Paralleling Three-Phase Transformers....................... 32
2.23 Methods of Cooling ................................................... 35
2.24 Oil-Filled – Self-Cooled Transformers...................... 36
2.25 Forced-Air and Forced-Oil-Cooled
Transformers ......................................................... 36
2.26 Transformer Oil ......................................................... 37
Contents (continued)
2. Introduction to Transformers (continued)
2.27 Conservator System ................................................... 38
2.28 Oil-Filled, Inert-Gas System...................................... 39
2.29 Indoor Transformers .................................................. 41
3. Routine Maintenance ......................................................... 42
3.1 Introduction to Reclamation Transformers................ 42
3.2 Transformer Cooling Methods Introduction.............. 44
3.3 Dry-Type Transformers ............................................. 45
3.3.1 Potential Problems and Remedial
Actions for Dry-Type Transformer
Cooling Systems ......................................... 47
3.4 Liquid-Immersed Transformers ................................. 48
3.4.1. Liquid-Immersed, Air-Cooled ....................... 48
3.4.2 Liquid-Immersed, Air-Cooled/Forced
Liquid-Cooled ............................................. 50
3.4.3 Liquid-Immersed, Water-Cooled................... 51
3.4.4 Liquid-Immersed, Forced Liquid-Cooled...... 52
3.4.5 Potential Problems and Remedial Actions
for Liquid-Filled Transformer
Cooling Systems ......................................... 52
3.4.5.1 Leaks ............................................... 52
3.4.5.2 Cleaning Radiators.......................... 53
3.4.5.3 Plugged Radiators ........................... 53
3.4.5.4 Sludge Formation............................ 53
3.4.5.5 Valve Problems............................... 54
3.4.5.6 Mineral Deposits............................. 54
3.4.5.7 Low Oil Level ................................. 54
3.4.6 Cooling System Inspections........................... 54
4. Oil-Filled Transformer Inspections..................................... 56
4.1 Transformer Tank ...................................................... 57
4.2 Top Oil Thermometers............................................... 57
Contents (continued)
4. Oil-Filled Transformer Inspections (continued)
4.3 Winding Temperature Thermometers........................ 58
4.3.1 Temperature Indicators Online ...................... 60
4.3.2 Temperature Indicators Offline...................... 60
4.4 Oil Level Indicators ................................................... 61
4.5 Pressure Relief Devices ............................................. 62
4.5.1 Newer Pressure Relief Devices...................... 63
4.5.2 Older Pressure Relief Devices ....................... 66
4.6 Sudden Pressure Relay............................................... 67
4.6.1 Testing Suggestion......................................... 69
4.7 Buchholz Relay (Found Only on Transformers
with Conservators) .................................................. 69
4.8 Transformer Bushings: Testing and
Maintenance of High-Voltage Bushings................. 72
4.9 Oil Preservation Sealing Systems .............................. 76
4.9.1 Sealing Systems Types .................................. 77
4.9.1.1 Free Breathing................................. 77
4.9.1.2 Sealed or Pressurized Breathing ..... 77
4.9.1.3 Pressurized Inert Gas Sealed
System.......................................... 78
4.9.2 Gas Pressure Control Components ................ 81
4.9.2.1 High-Pressure Gauge ...................... 81
4.9.2.2 High-Pressure Regulator................. 81
4.9.2.3 Low-Pressure Regulator.................. 82
4.9.2.4 Bypass Valve Assembly ................. 82
4.9.2.5 Oil Sump......................................... 82
4.9.2.6 Shutoff Valves ................................ 83
4.9.2.7 Sampling and Purge Valve.............. 83
4.9.2.8 Free Breathing Conservator ............ 83
4.9.2.9 Conservator with Bladder or
Diaphragm Design ....................... 84
4.10 Auxiliary Tank Sealing System ................................. 90
Contents (continued)
5. Gaskets................................................................................ 91
5.1 Sealing (Mating) Surface Preparation........................ 92
5.2 Cork-Nitrile................................................................ 93
5.3 Cork-Neoprene........................................................... 94
5.4 Nitrile “NBR” ............................................................ 95
5.4.1 Viton .............................................................. 95
5.5 Gasket Sizing for Standard Groove Depths ............... 97
5.6 Rectangular Nitrile Gaskets ....................................... 99
5.7 Bolting Sequences to Avoid Sealing Problems ......... 103
6. Transformer Oils................................................................. 105
6.1 Transformer Oil Functions......................................... 105
6.1.1 Dissolved Gas Analysis ................................. 105
6.1.2 Key Gas Method ............................................ 109
6.1.2.1 Four-Condition DGA Guide
(IEEE C57-104) ........................... 109
6.1.3 Sampling Intervals and Recommended
Actions ........................................................ 113
6.1.4 Atmospheric Gases ........................................ 117
6.1.5 Dissolved Gas Software................................. 117
6.1.6 Temperature ................................................... 122
6.1.7 Gas Mixing..................................................... 122
6.1.8 Gas Solubility................................................. 123
6.1.9 Diagnosing a Transformer Problem
Using Dissolved Gas Analysis and
the Duval Triangle ...................................... 125
6.1.9.1 Origin of the Duval Triangle........... 125
6.1.9.2 How to Use the Duval Triangle ...... 125
6.1.9.3 Expertise Needed ............................ 131
6.1.9.4 Rogers Ratio Method of DGA........ 131
6.1.10 Carbon Dioxide/Carbon Monoxide Ratio...... 138
6.1.11 Moisture Problems......................................... 141
Contents (continued)
6. Transformer Oils (continued)
6.1 Transformer Oil Functions (continued)
6.1.11 Moisture Problems (continued)
6.1.11.1 Dissolved Moisture in
Transformer Oil ........................... 145
6.1.11.2 Moisture in Transformer
Insulation...................................... 146
7. Transformer Oil Tests that Should Be Completed
Annually with the Dissolved Gas Analysis ............... 150
7.1 Dielectric Strength ..................................................... 150
7.1.1 Interfacial Tension ......................................... 151
7.2 Acid Number.............................................................. 152
7.3 Test for Oxygen Inhibitor .......................................... 153
7.4 Power Factor .............................................................. 154
7.5 Oxygen....................................................................... 155
7.6 Furans......................................................................... 155
7.7 Oil Treatment Specification....................................... 159
7.7.1 Taking Oil Samples for DGA........................ 160
7.7.1.1 DGA Oil Sample Container ............ 162
7.7.1.2 Taking the Sample .......................... 163
8. Silicone Oil-Filled Transformers ........................................ 167
8.1 Background ................................................................ 167
8.2 Carbon Monoxide in Silicone Transformers.............. 170
8.3 Comparison of Silicone Oil and Mineral
Oil Transformers..................................................... 170
8.4 Gas Limits.................................................................. 171
8.5 Physical Test Limits................................................... 175
9. Transformer Testing ......................................................... 176
9.1 DC Winding Resistance Measurement ...................... 176
9.2 Core Insulation Resistance and Inadvertent
Core Ground Test (Megger®) ................................ 178
Contents (continued)
9. Transformer Testing (continued)
9.3 Doble Tests on Insulation .......................................... 180
9.3.1 Insulation Power Factor Test ......................... 180
9.3.2 Capacitance Test ............................................ 180
9.3.3 Excitation Current Test .................................. 181
9.3.4 Bushing Tests................................................. 182
9.3.5 Percent Impedance/Leakage
Reactance Test ............................................ 182
9.3.6 Sweep Frequency Response
Analysis Tests ............................................. 183
9.4 Visual Inspection ....................................................... 186
9.4.1 Background .................................................... 186
9.4.2 Oil Leaks........................................................ 187
9.4.3 Oil Pumps....................................................... 187
9.4.4 Fans and Radiators......................................... 188
9.4.5 Age................................................................. 188
9.4.6 Infrared Temperature Analysis ...................... 189
9.4.7 IR for Transformer Tanks .............................. 189
9.4.8 IR for Surge Arresters.................................... 190
9.4.9 IR for Bushings .............................................. 190
9.4.10 IR for Radiators and Cooling Systems........... 191
9.4.11 Corona Scope Scan ........................................ 193
9.5 Ultrasonic and Sonic Fault Detection ........................ 193
9.5.1 Background .................................................... 193
9.5.2 Process ........................................................... 194
9.6 Vibration Analysis ..................................................... 194
9.6.1 Background .................................................... 194
9.6.2 Process ........................................................... 195
9.7 Turns Ratio Test ......................................................... 195
9.7.1 Background .................................................... 195
9.7.2 Process ........................................................... 195
9.8 Estimate of Paper Deterioration (Online) .................. 196
9.8.1 CO2 and CO Accumulated Total
Gas Values .................................................. 196
9. Transformer Testing (continued)
9.8 Estimate of Paper Deterioration (Online) (continued)
9.8.2 CO2/CO Ratio ................................................ 196
9.9 Estimate of Paper Deterioration (Offline
During Internal Inspection)..................................... 197
9.9.1 Degree of Polymerization (DP) ..................... 197
9.9.1.1 Background ..................................... 197
9.9.1.2 Process ............................................ 198
9.9.2 Internal Inspection ......................................... 198
9.9.2.1 Background ..................................... 198
9.9.2.2 Transformer Borescope................... 200
9.10 Transformer Operating History.................................. 200
9.11 Transformer Diagnostics/Condition
Assessment Summary ............................................. 201
Appendix: Hydroplant Risk Assessment –
Transformer Condition Assessment.................................... 205
References ................................................................................. 231
Acronyms and Abbreviations ................................................... 235
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