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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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