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