Note: The source of the technical material in this volume is the Professional
Engineering Development Program (PEDP) of Engineering Services.
Warning: The material contained in this document was developed for Saudi
Aramco and is intended for the exclusive use of Saudi Aramco’s
employees. Any material contained in this document which is not already in the public domain may not be copied, reproduced, sold, given, or disclosed to third parties, or otherwise used in whole, or in part, without the written permission of the Vice President, Engineering Services, Saudi Aramco.
Engineering Encyclopedia
Saudi Aramco DeskTop Standards
CONTENTS PAGES
INFORMATION
Purpose and Usage Within Saudi Aramco ... 1
Applicable Standards... 1
Saudi Aramco Engineering Standards ... 1
Saudi Aramco Material System Specifications... 4
Saudi Aramco Design Practices... 9
Industry Standards ... 12
CONSTRUCTION... 15
Core ... 15
Core Material ... 16
Core Assembly... 20
Types of Transformer Core Construction ... 22
Grounding of Core ... 28
Coil or Winding Assembly... 28
Core-Type Coils... 29
Shell-Type Coils ... 29
Coil Stress ... 29
Core and Coil Assembly Clamping Construction ... 32
Coil Material ... 38
INSULATION SYSTEM... 40
Coordination of Insulation... 40
Types of Insulating Materials ... 43
Coil Assembly Insulation... 44
Functions of Solid Insulation... 45
Classification of Solid Insulation... 46
Solid Insulation and Moisture ... 47
Insulating System and Temperature ... 49
Insulating Mineral Oil ... 49
Specification for Transformer Oil... 50
Importance of Insulating Oil ... 51
Insulation Coordination of Transformer Oil ... 51
"Synthetic" Insulating Fluids ... 52
Engineering Encyclopedia Electrical Power Transformers
POWER TRANSFORMER ENCLOSURES... 55
Enclosure Types for Power Transformers ... 55
Sealed Tank ... 55
Conservator or Expansion Tank... 57
Inert Gas Preservation System ... 59
Gas Liquid Seal System ... 60
Control Cabinets... 61
Fill and Drain Valves... 62
COOLING SYSTEMS... 64
Cooling Circuits... 64
Temperature Gradient... 65
Methods Used for Cooling ... 66
Cooling Classes ... 72
Self-Cooled ... 73
Self-Cooled and Forced-Air Cooled (OA/FA)... 73
Forced-Oil Circulation Cooling (Pumps)... 74
Forced-Oil-Cooled Process... 74
TRANSFORMER ACCESSORIES... 78
Pressure Relief Devices... 78
Mechanical Relief ... 78
Diaphragm Relief... 80
Fault Gas Detector Relays ... 83
Sudden Pressure Relays ... 84
Gas in Oil Detector Relay (Buchholz) ... 86
Indicators... 86
Liquid Temperature (Top-Oil) Gauge... 86
Hot-Spot Temperature Indicator ... 89
Liquid-Level Indicator (Dial-type)... 99
Pressure/Vacuum Indicator... 102
Pressure-Vacuum Bleeder and Regulator ... 107
Bushings ... 108
Ratings for Bushings... 108
Types of Bushings ... 108
Bushing Features... 112
Bushing Current Transformers (BCT)... 117
No-load Tap Changers (NLTC)... 120
Operation ... 120
POWER TRANSFORMER LOAD TAP CHANGER (LTC)... 129
Purpose of LTC Voltage Regulating ... 129
Tap Changer Compartment Construction... 132
Load Tap Changer Mechanism ... 134
Transfer Switches... 138
Selector Switches... 138
Reversing Switch ... 139
Motor Mechanism... 140
LTC Operation ... 140
Electronic Control System... 142
Load Tap Changer (LTC) Operating Methods ... 145
Automatic Operation... 145
Remote Operation ... 148
Manual Operation ... 148
Parallel Operation of Two ALTC's ... 149
Parallel Operation of Electrical Control Scheme ... 151
POWER TRANSFORMER NAMEPLATE DATA... 154
Transformer Nameplate with NLTC... 154
Transformer Nameplate with LTC ... 157
RECEIPT OR ACCEPTANCE INSPECTION ... 160
Transformer Acceptance from Manufacturer/Vendor ... 160
Manufacture Test Results... 160
Transformer Specifications and Nameplate Data Verification... 160
Transformer Receipt Inspection ... 161
General Precautions ... 161
Exterior Tank Inspection (Oil-Filled) ... 161
Accessories Inspection... 162
TRANSFORMER RECEIPT TESTING... 167
TRANSFORMER INSTALLATION INSPECTION... 168
Transformer Location Verification... 168
Engineering Encyclopedia Electrical Power Transformers
TRANSFORMER INSTALLATION TESTING ... 172
Turns-Ratio Tests ... 175
TTR Testing Indications ... 179
Expected Test Results ... 180
Interpretation of Data... 180
Review of Sample Data ... 181
Winding-to-Winding Polarity Test... 185
Polarity... 185
Three-Phase Polarity and Phase Sequence ... 185
Voltmeter Flicks-Method Polarity Test... 185
Winding Resistance Test ... 190
Three Phase Transformer (Wye With a Neutral Bushing) ... 194
Three Phase Transformer (Wye Without a Neutral Bushing)... 194
Three Phase Transformer (Delta Connected)... 194
Winding Resistance (Second Method)... 194
Winding Insulation Testing (Megger Test) ... 195
Winding Insulation-Resistance Test ... 196
Core-Ground Inspection and Test... 203
Transformer Tank Ground Test... 204
Tap Setting Verification ... 206
Applied Voltage (Hi-Pot) Test... 206
Insulation Power-Factor Test... 207
Definition of Power Factor of Insulation per ANSI C57 ... 209
Winding Insulation Test... 215
Temperature Correction ... 215
Instruments and Testing Procedure... 220
Power Factor Values... 220
Insulating Oil Testing ... 221
Testing Categories ... 221
Oil Specifications... 222
Types of Transformer Oil Test... 223
Visual Examination... 232
Fluid Sampling Method ... 234
Comparing Oil-Test Data... 236
Gas Analysis of Operating Transformers ... 237
Major Causes of Gases in Oil-Filled Transformers... 237
Analysis of Transformer Combustible Gases... 238
Methods for Analyzing Combustible Gas... 239
Solubility of Gases in Transformer Oil ... 241
TRANSFORMER SYSTEM PRE-OPERATIONAL CHECKOUT ... 253
Performance Testing... 253
Installation Checklist ... 254
Electrical External Connections ... 256
Transformer Accessory Component Checkout... 256
Pressure-Vacuum Gauge Test... 257
Pressure Relief Test ... 259
Oil-Level Inspection and Test... 260
Top-Oil Temperature Inspection and Test ... 262
Cooling-Fan Inspection and Test ... 262
Power Cable Termination Checkout ... 263
TRANSFORMER OPERATIONAL TESTING ... 264
Operational Test ... 264
Complete System Functional Test... 264
Types of Transformer Operational Testing ... 265
Exciting Current Check... 265
No-Load Voltage Output (Secondary) Check... 265
Voltage Phasing (or Rotation)... 266
Synchronizing for Parallel Operation ... 266
Transformer Noise Level ... 266
TRANSFORMER OPERATIONAL OBSERVATION PERIOD CHECKS AND INSPECTIONS... 269
Transformer and System Temperature Checks... 269
Transformer Operational Inspection... 269
Transformer Operational Problem Inductors... 270
Oil Leaks... 270
Pressure (Over/Under) ... 270
Overheating... 271
Load Voltage, Current, and Temperature Relationship Checks... 271
PREVENTIVE MAINTENANCE AND FAILURE MODE ANALYSIS... 272
In-Service Inspections for Power Transformers ... 272
Current and Voltage Readings ... 274
Temperature Readings ... 275
Engineering Encyclopedia Electrical Power Transformers
Visual Inspection ... 276
Analyzing Failure Modes... 277
Transformer Failure ... 277
Out-of-Service Inspections for Power Transformers... 280
Insulation Testing... 282
Gauges and Alarms... 282
Tap Changer... 283
Analyzing Oil Test Data in the Transformer Maintenance Records... 283
Dielectric Test... 284
Oil Power Factor... 284
Interfacial Tension Test ... 285
Visual (Color) Examination ... 285
Neutralization Number (Acidity) ... 285
Water Content Test ... 286
Gas-in-Oil Analysis... 286
Range of Combustible Gases (ppm) ... 286
Combustible Gas Test ... 287
Dissolved Combustible Gas Testing ... 288
Dissolved Combustible Gas Analysis ... 288
Analyzing Electrical Test Data in the Transformer Maintenance Records... 290
Insulation Resistance ... 290
Insulation Power-Factor on Power Transformers ... 291
Transformer Turns-Ratio ... 291
ANSI C57 TRANSFORMER FAILURE MODE ANALYSIS METHOD... 292
Determination and Investigation of a Failure Occurrence... 292
Following a Suspected Failure... 292
Investigation Flow Chart... 292
Failure Mode Data Collection ... 293
General Approach ... 293
On-Site Investigation ... 293
Electrical Tests... 298
Sampling and Tests of Gas and Insulating Fluid ... 299
WORK AID 1: PRE-OPERATIONAL FIELD INSTALLATION
CHECKLIST POWER TRANSFORMERS (OIL-IMMERSED) ... 309
WORK AID 2: FIELD TESTING... 310
WORK AID 3: TABLE OF FIELD INSPECTION ... 311
WORK AID 10: TRANSFORMER TEST METHOD DATA... 312
Work Aid 10A: Turns Ratio Test (TTR) ... 312
Work Aid 10B: Polarity Test (3 Methods)... 314
Work Aid 10C: Winding Resistance... 316
Work Aid 10D: Winding Insulation Resistance (Four Types of Tests) .. 317
Work Aid 10E: Core Ground Test ... 322
Work Aid 10F: Transformer Tank Ground Test... 323
Work Aid 10G: Insulation Power-Factor Test ... 324
Work Aid 10J: Oil Test Results Comparison... 328
Engineering Encyclopedia Electrical Power Transformers
LIST OF FIGURES
Figure 1. SAES-P-121 Table of Contents... 2
Figure 2. Sample Information from SAES-P-121... 3
Figure 3. 14-SAMSS-531 Table of Contents... 5
Figure 3. 14-SAMSS-531 Table of Contents (cont’d)... 6
Figure 4. Sample Information from 14-SAMSS-531... 8
Figure 5. SADP-P-121 Table of Contents ... 10
Figure 6. Hysteresis Effects on Magnetic Domains within the Core ... 17
Figure 7. Eddy Currents in a Solid Core... 19
Figure 8. Eddy Currents in Insulated Core ... 19
Figure 9. Typical Core Construction and Lamination Configuration ... 21
Figure 10. Core-Type Construction and Shell-Type Construction ... 23
Figure 11. Shell-Type Unit ("Pancake" Coils)... 24
Figure 12. Conventional 3-Phase Core for the Rectangular-Pancake-Interleaved-Coil Sructure (Shell Type) ... 25
Figure 13. Cruciform-Type Core ... 26
Figure 14. Wound Core ... 27
Figure 15. Vertical (Axial) Forces ... 30
Figure 16. Horizontal Repulsion (Axial) Force ... 31
Figure 17. A Vertical Cross-Section of Major Transformer Components ... 33
Figure 18. Hydraulic Dashpot,... 34
Figure 19. Typical Old Type Construction of Shell-Type Transformer ... 35
Figure 20. Typical Oil Type Construction of Core-Type Transformer... 36
Figure 21. Typical Bolted Clamping Structure ... 37
Figure 22. Typical Simplified Boltless Clamping Structure ... 37
Figure 23. Continuously Transposed Multi-Strip Conductor... 39
Figure 24. Insulation Structure for a Core-Form Type Transformer ... 41
Figure 25. Oil-Filled Cellulose System... 42
Figure 26. Impulse Strength of Paper Insulation ... 48
Figure 28. Sealed Tank Oil Preservation Method... 56
Figure 29. Expansion Tank Oil Preservation Method... 57
Figure 30. Constant Oil Pressure System... 58
Figure 31. Inert-Gas Preservation Method... 59
Figure 32. Gas-Liquid Seal System ... 60
Figure 33. Typical Combination Drain Filter Sampling Valve... 63
Figure 34. Transformer Cooling Circuit ... 65
Figure 35. Cooling of Core and Coils by Natural Oil Circulation or Thermosiphon Flow ... 67
Figure 36. Oil-Immersed, Self-Cooled ... 68
Figure 37. Liquid Filled Transformer with Radiators ... 69
Figure 38. Oil-Immersed, Self-Cooled Forced-Air Cooled ... 70
Figure 39. Liquid Filled Transformer with Radiators and Fans... 71
Figure 40. Liquid Filled Transformer with Top Mounted Cooling Fans ... 75
Figure 41. Oil-immersed, Self-Cooled Air Cooled and 2-Stage Forced-Oil-Cooled Transformer ... 76
Figure 42. Oil Circulating Pump... 77
Figure 43. Mechanical Relief Device ... 79
Figure 44. Diaphragm Relief Device (Sealed Position)... 80
Figure 45. Typical Pressure Relief Device ... 81
Figure 46. Diaphragm Relief with Alarm Device (Venting Position)... 83
Figure 47. Typical Sudden Pressure Relay ... 85
Figure 48. Dial Type Liquid Temperature Indicator... 88
Figure 49. Components for Winding Temperature Indicator... 90
Figure 50. Typical Mounting Arrangement of Winding Temperature Indicator and Accessories ... 91
Figure 51. Temperature Gradient between Top-Oil, Heated-Sensor, ... 93
Figure 52. “Hot-Spot” Indicating Circuit... 94
Figure 53. Connection Diagram for Current Transformer and Heating Coil (Used with Winding Temperature Indicator)... 95
Engineering Encyclopedia Electrical Power Transformers
Figure 55. Hot-Spot Temperature Indicator... 98
Figure 56. Dial-Type Magnetic Liquid-Level Indicator ... 100
Figure 57. Typical Transformer Nameplate... 101
Figure 58. Pressure-Vacuum Gauge ... 104
Figure 58A. Typical Pressure/Vacuum Indicator ... 105
Figure 58B. Pressure/Vacuum Gauge and Bleeder Valve Installation ... 106
Figure 59. Pressure-Vacuum Bleeder ... 107
Figure 60. Bushing with Draw-Through Cable Leads ... 109
Figure 61. Bushing with Hollow-Core Conductor... 111
Figure 62. Oil-Filled Bushing ... 113
Figure 63. Core of Condenser Type Bushing without Porcelain Cover and Skirts... 114
Figure 64. Typical Oil-Filled and Condenser Type Bushing for 66 kV Transformer ... 115
Figure 65. Condenser Bushing - Oil Impregnated ... 116
Figure 66. Bushing Current Transformer Mounting ... 119
Figure 67. Bushing-Type Current Transformer ... 119
Figure 68. Wedge-Type Tap Changing Mechanism... 121
Figure 69. Operating Mechanisms... 122
Figure 70. Operating Handle for No-Load Tap Changer Set to Position #5... 123
Figure 71. Transformer Nameplate with No-load Tap Changer Voltages ... 125
Figure 72. Typical Automatic Load Tap Changer Installation ... 131
Figure 73. Position Indicator for Load Tap Changer ... 133
Figure 74. Load Tap Changer Mounted on a Transformer ... 135
Figure 75. UTT-B Load Tap Changer (Internal View) ... 136
Figure 76. Phase Assembly... 137
Figure 77. UTT-B Load Tap Changer (External View)... 138
Figure 78. Typical UTT-B Load Tap Changer Schematic Connection Diagram with Sequence Chart... 139
Figure 79. Control System Block Diagram... 142
Figure 80. Regulator Control Functions ... 144
Figure 81B. Typical Transformer Tap Changer Control Circuit ... 147
Figure 82. Installation Using Parallel Transformers ... 150
Figure 83. Typical Paralleling Control Scheme... 152
Figure 84. Typical Transformer Nameplate... 156
Figure 85A. Typical Transformer Nameplate... 158
Figure 85B. Typical Transformer Nameplate... 159
Figure 86. Transformer Turn-Ratio (TTR) Test Set... 176
Figure 87. Schematic Diagram for Transformer Turns-Ratio (TTR) Test Set ... 177
Figure 88. TTR Polyphase Transformer Connections ... 178
Figure 89. TTR Set Indications when Balanced ... 179
Figure 90. Sample TTR Readings (Set One) ... 182
Figure 91. Sample TTR Readings (Set Two)... 184
Figure 92. LV/HV Winding Markings/ Polarity Voltage Reading Method Polarity Test ... 186
Figure 93. Voltmeter Flick-Method Polarity Test... 188
Figure 94. Angular Displacements ... 189
Figure 95. Bridge Network Connections ... 191
Figure 96. Wheatstone Bridge ... 192
Figure 97. Digital Low-Resistance Ohmmeter ... 193
Figure 98. Connections for Measuring Transformer Winding Resistance... 195
Figure 99. Schematic Diagram for Measuring the Insulation-Resistance of a Typical Single-Phase (Two-Winding) Transformer ... 200
Figure 100. Schematic Diagram for Measuring the Insulation-Resistance of a Typical Three-Phase Delta-Wye Transformer... 201
Figure 101. Digital Low-Resistance Ohmmeter ... 205
Figure 102. Dielectric Loss of Each Capacitor Divided by Capacitive Volt-Amperes is Equal to Power Factor ... 208
Figure 103. Typical Insulation Power Factor Test Data ... 210
Figure 104. Typical Two-Winding Transformer Simplified Diagram... 211
Engineering Encyclopedia Electrical Power Transformers
Figure 107. Schematic Diagram for Measuring the Capacitance and Insulation Power Factor from the High-Voltage Winding to the Low-Voltage
Winding and Ground for a Three-Phase Delta-Wye Transformer... 217
Figure 108. Insulating Fluid Dielectric Test Set ... 225
Figure 109. Effects of Water in Insulating Oil... 226
Figure 110. Field Test Kit for Insulating Fluid Acidity ... 228
Figure 111. Electric Strength of Transformer Oil vs. Water Content ... 231
Figure 112. Insulating Fluid Sample Kit... 235
Figure 113. Comparing Oil-Test Results ... 236
Figure 114. Comparative Rates of Evolution of Gases from Oil as a Function of Decomposition Energy ... 242
Figure 115. Gas Sample Kit... 248
Figure 116. Typical Sampling Kit for Gas-in-Fluid Analysis... 250
Figure 117. Sampling Kit for PCB Analysis... 252
Figure 118. Pressure-Vacuum Gauge Calibration and Press-Relief Device Test ... 258
Figure 119. Typical Transformer Nameplate... 261
Figure 120. Transformer In-Service Inspection Report for Dry-Type and Liquid-Filled Transformers ... 273
Figure 121. Typical Out-of-Service Inspection Report for Oil-Filled and Pad-Mounted Transformers ... 281
Figure 122. Suggested Investigation Flowchart that Forms the Basic for this Guide ... 294
Figure 126. Form P-025 (7/94) Sheet 1 of 8... 338
Figure 127. Form P-025 (7/94) Sheet 2 of 8... 339
Figure 128. Form P-025 (7/94) Sheet 3 of 8... 340
Figure 129. Form P-025 (7/94) Sheet 4 of 8... 341
Figure 130. Form P-025 (7/94) Sheet 5 of 8... 342
Figure 132. Form P-025 (7/94) Sheet 6 of 8... 343
Figure 135. Form P-025 (7/94) Sheet 7 of 8... 344
Figure 136. Form P-025 (7/94) Sheet 8 of 8... 345
Figure C1. Ungrounded Specimen Test on Transformer Bushings... 362
Figure C2. Typical Field Test Data for a Large Transformer Bushing, Undergrounded Specimen Test (UST) ... 363
Figure C3. Hot-Collar Test Method for Testing of Bushing Insulators... 364
Figure C4. Typical Field Test Data–Bushing Hot-Collar Tests... 365
Figure C5. Liquid Insulation Cell Connected for Ungrounded Specimen Testing... 367
Figure C6. Measurement of Ie in a Single-Phase Transformer... 369
Figure C7. Measurement of Ie in a Wye-Connected Transformer Winding... 370
Figure C8. Measurement of Ie in a Delta-Connected Transformer Winding... 371
Figure C9. Variation of Power Factor with the Moisture Content of Oil-Impregnated Pressboard ... 372
Engineering Encyclopedia Electrical Power Transformers
PURPOSE AND USAGE WITHIN SAUDI ARAMCO
Power transformer are used in Saudi Aramco as the main power supply from distribution systems all the way up to transmission systems. The power rating of these devices can range from 750 kVA up to 1,000 MVA. All Saudi Aramco Plant facilities use power transformers with voltage levels from 230 kV primary and transformed down through several steps to the secondary distribution system at 480 V. The construction and application of these devices have rigid standards (ANSI) because of their importance, the amount of power they handle, and cost.
Applicable Standards
The engineer must consult these types of standards for specifications concerning Power transformers:
• Saudi Aramco Standards, Specifications, Practices and Form – Saudi Aramco Engineering standards (SAES)
– Saudi Aramco Material System Specifications (SAMSS) – Saudi Aramco Design Practices (SADP)
– Electrical Pre-Commissioning Form (Form P-025) • Industry Standards
– National Electrical Code (NEC)
– National Electrical Safety Code (NESC)
– American National Standards Institute (ANSI) standards
Saudi Aramco Engineering Standards
For specifications on the power transformer, the engineer consults these Saudi Aramco standards:
SAES-P-121 - “Transformers, Reactors, Voltage Regulators,” contains the minimum
requirements for the design and installation of transformers, reactors, voltage regulating transformers, and instrument transformers. Do not deviate from the requirements of this standard. Any deviations that reduce the requirements must have written approval from the Saudi Aramco Chief Engineer, Dhahran. User/specifier requirements that exceed the minimum requirements need no waiver approval. The Saudi Aramco Chief Engineer also must resolve any conflicts between this standard and other SAESs, SAMSSs, codes, forms, and SADPs.
The major topics discussed in this standard are as follows: • Material Requirements
• kVA Rating of Power and Voltage Regulating Transformers • Installation
This standard has six chapters. Figure 1 shows the table of contents.
CHAPTER SUBJECT PAGE
1 SCOPE 2
2 CONFLICTS AND DEVIATIONS 2
3 APPLICABLE CODES AND STANDARDS 2
4 MATERIAL REQUIREMENTS 3
5 kVA RATINGS OF POWER AND VOLTAGE
REGULATING TRANSFORMERS 5
6 INSTALLATION 7
Figure 1. SAES-P-121 Table of Contents
Figure 2 shows a sample of the type of information contained in SAES-P-121.
*NOTE: 5.4 - This is intended for the lone transformer carrying both loads not to exceed manufacture temperature limit for transformer insulation damage.
Engineering Encyclopedia Electrical Power Transformers
5 kVA RATING OF POWER AND VOLTAGE REGULATING
TRANSFORMERS
5.1 Transformers shall be supplied with ANSI Standard preferred kVA Ratings at normal service condition, unless otherwise specified on SAMSS Data Schedule-1
5.2 The minimum OA self-cooled kVA rating of each OA/FA transformers shall be equal to the maximum operating load, plus projected future load 5.3 For transformers that are self-cooled only, a 10 percent load growth factor
shall be added to the calculated load )maximum operating load plus projected future load).
5.4 The forced-cooled FA site rating of each transformer serving a double-ended substation shall be capable of feeding the entire load of both buses with the bus-tie breaker closed.
5.5 Forced-air cooling fans and controls shall be provided on all transformers rated 2500 kVA or larger. On transformers smaller than 2500 kVA, forced-air cooling shall not be provided.
5.6 Two stages of forced cooling shall be allowed for transformers with OA ratings of 90 MVA or larger. The forced cooling may be forced air (FA) and/or forced-oil-air (FOA).
Figure 2. Sample Information from SAES-P-121
SAES-P-119 - This standard prescribes the minimum mandatory requirements for the design
and installation of on-shore power substations. This standard provides the definition for a substation and the type and size of transformer required. It provides the orientation of the transformer within the substation yard. This also states when to use lightning arrester on transformers.
Table of Contents
1 Table of Contents
2 Conflicts and Deviations
3 Applicable Codes and Standards
4 General
5 Substation Buildings 6 Substation Yards
SAES-P-114 - This standard prescribes the minimum mandatory requirements for the design
and installation of protective relaying for power systems and equipment. Chapter 7 deals with transformer protective devices and schemes for power and distribution transformers. The major interests are the sections dealing with pad-mounted distribution and pole-mounted distribution transformers, as well as the inherent devices built as part of the transformer such as:
• Pressure-rise relay (device 63T, 63GT) • Overtemperature devices (device 49T) • Low oil level indicator
• Lockout relays (device 86T1, 86T2, 87T3)
Chapter 7 Table of Contents
7.1 General
7.2 Transformer protection schemes
7.3 Protection device application requirements 7.4 Fuse protection of transformers
Saudi Aramco Material System Specifications
The 14-SAMSS-Series specifications contain the minimum technical requirements for power transformers used in Saudi Aramco electrical systems. Engineers should use these documents when specifying new power transformers. As with the SAES’s, any deviations that reduce the requirements must have written approval from the Saudi Aramco Consulting Services Division (CSD), Dhahran. User/specifier requirements that exceed the minimum requirements need no waiver approval.
SAMSS’s do not directly state all of the specifications for new power transformers. Saudi Aramco’s practice is to adopt the ANSI standard specifications for transformers, and then modify the ANSI specifications to meet the specific requirements of Saudi Aramco installations. The modifications consist of exceptions, deletions, and additions to the ANSI standards.
The ANSI standard that is the base document for the transformer specifications is referenced in chapter D of each applicable SAMSS. The modifications to the base document are listed through use of numbers that refer to the sections of the ANSI standard to be changed. The type of modification is listed in parenthesis next to the ANSI standard section number.
Engineering Encyclopedia Electrical Power Transformers
14-SAMSS-531 (Power Transformers) - Figure 3 shows the table of contents. Chapter D in
the table of contents shows that the base documents for this standard are ANSI C57.12.10-1977 and ANSI C57.12.30-C57.12.10-1977. 14-SAMSS-531 is more difficult to interpret than most Saudi Aramco standards because ANSI C57.12.30-1977 no longer exists in the latest revision of ANSI C57. A previous revision of the ANSI standards combined ANSI C57.12.10 and ANSI C57.12.30 into a single standard. The new standard is ANSI C57.12.10-1988.
C
HAPTERT
ABLE OFC
ONTENTSP
AGEA SCOPE 3
B REFERENCES 3
C FIELD EXPERIENCE 3
D MODIFICATIONS TO ANSI C57.12.10-1977
(INDICATED BY (10) & C57.12.30-1977 (INDICATED
BY (30) 4
SECTION A C57.12.10-1977 & C57.12.30-1977 4
PART I. BASIC ELECTRICAL AND
MECHANICAL REQUIREMENTS 4 4 Ratings 4 5 Insulation Level 5 6 Impedance Voltage 5 8 Routine Test 6 9 Construction 6
PART II. OTHER REQUIREMENTS THAT MAY BE
SPECIFIED FOR SOME APPLICATIONS 9
10 Other Ratings 9
11 Other Tests 9
12 Other Construction 9
C
HAPTERT
ABLE OFC
ONTENTSP
AGE SECTION B C57.12.10-1977 & C57.12.30-1977 13PART I. BASIC ELECTRICAL AND MECHANICAL
REQUIREMENTS 13 14 Ratings 13 15 Insulation Level 13 16 Impedance Voltage 14 18 Routine Tests 14 19 Construction 14
PART II. OTHER REQUIREMENTS THAT MAY BE
SPECIFIED FOR SOME APPLICATIONS 15
20 Other Ratings 15
21 Other Tests 15
22 Other Construction 15
E ADDITIONAL REQUIREMENTS 16
ATTACHMENTS
Data Schedule-1 (Data to be supplied by Buyer) Data Schedule-2 (Data to be supplied by each Bidder)
Engineering Encyclopedia Electrical Power Transformers
The section numbers and required modifications contained in 14-SAMSS-531 refer to specific sections of ANSI C57.12.10-1977 and ANSI C57.12.30-1977 that must be changed to meet the needs of Saudi Aramco installations. A direct correlation of the required modifications no longer exists between the section numbers in the revised version of ANSI C57.12.10-1988 and the section numbers in 14-SAMSS-531. However, the engineer must still incorporate the modifications contained in 14-SAMSS-531 when developing specifications for new power transformer installations.
The engineer should use the most recent revision of the applicable ANSI standard (ANSI C57.12.10-1988) as the base document for developing new power transformer specifications. The specifications in the base document should then be modified as required by SAMSS-531. The engineer will have to carefully read and study ANSI C57.12.10-1988 and 14-SAMSS-531 to determine the specific sections of ANSI C57.12.10-1988 to which the modifications in 14-SAMSS-531 apply. Conflicts or questions concerning the requirements contained in ANSI C57.12.10-1988 and 14-SAMSS-531 should be resolved in the same fashion as any other conflict between a Saudi Aramco and an Industry Standard. Deviations that reduce the requirements of 14-SAMSS-531 must have written approval from the Saudi Aramco Consulting Services Division (CSD), Dhahran. ANSI C57.12.10-1988 requirements that exceed the minimum requirements of 14-SAMSS-531 need no waiver approval.
9.1 (10) (Addition) The minimum BIL Levels of winding bushings shall be as shown in the following table, unless specified otherwise in Data Schedule-1:
Nominal Voltage (kV) Bushing BIL Level (kV)
2.4 60 4.16 75 13.8 110 34.5 200 69 350 115 650 9.1.2 (10) 9.1.2 (30)
(Exception) Outdoor bushing shall have minimum creepage of 40 mm per kV Line-to-Line of nominal system voltage. 9.1.3 (10)
9.1.3 (30)
(Exception) Type and location of bushings shall be as specified on Data Schedule-1
9.1.4 (10) 9.1.4 (30)
(Exception) Type and location of bushing for wye-connected low voltage windings shall be as specified on the Data Schedule-1 9.2 (10)
9.2 (30) (Exception) All accessories listed in Figure 2 shall be furnished forall transformers 9.2.1 (20)
9.2.1 (30) (Addition) The type of tap changer for de-energized or for load tapchanging operation shall be as specified on Data Schedule-1
9.2.2 (10) (Addition) A magnetic liquid level gauge shall be provided with all transformers. The alarm contacts shall be rated and wired in accordance with Section 12.3.6 of the ANSI Standard.
9.2.3 (10) (Addition) A magnetic liquid level gauge shall be provided with all transformers. The two electrically separate sets of contacts shall be rated and wired in accordance with Section 12.3.6 of this ANSI Standard. 9.2.4 (10) (Addition) A pressure-vacuum gauge shall be provided on all
transformers equipped with a sealed tank oil preservation system. The two electrically separate sets of contacts shall be rated and wired in accordance with Section 12.3.6.1 of this ANSI Standard.
9.2.5 (10)
9.4.5 (30) (Addition) Valves shall be provided on each transformer equipped withdetachable radiators to enable these radiators to be removed without affecting the liquid in the tank. The tank drain valves shall be padlocked.
Engineering Encyclopedia Electrical Power Transformers
SAMSS-534 (Overhead-Type Distribution Transformers) - The base document for
14-SAMSS-534 is ANSI C57.12.20-1981. The most recent version of this ANSI standard is ANSI C57.12.20-1988. The engineer should use ANSI C57.12.20-1988 as the base document for developing new overhead-type distribution transformer specifications. The specifications in the base document should then be modified as required by 14-SAMSS-534.
14-SAMSS-534 is also less difficult to interpret than 14-SAMSS-531 because a direct correlation still exists between the section numbers in the revised version of ANSI C57.12.20-1988 and the section numbers of the modifications required by 14-SAMSS-534. Conflicts or questions concerning the requirements contained in ANSI C57.12.20-1988 and 14-SAMSS-534 should be resolved in the same fashion as any other conflict between a Saudi Aramco and an Industry Standard. Deviations that reduce the requirements of 14-SAMSS-534 must have written approval from the Saudi Aramco Consulting Services Division (CSD), Dhahran. ANSI C57.12.20-1988 requirements that exceed the minimum requirements of 14-SAMSS-534 need no waiver approval.
14-SAMSS-534 has the same type of information as 14-SAMSS-531. The information simply applies to a different type of transformer.
Saudi Aramco Design Practices
SADP’s give the background information needed to explain, amplify, and apply the mandatory requirements of the SAES’s and SAMSS’s. The information in the SADP’s is not mandatory and not necessarily up-to-date. Written approval is not needed to deviate from the SADP’s. In case of conflict between an SADP and an SAES/SAMSS, the SAES/SAMSS govern. Capital letters are used in some statements of the text in the SADP’s. These statements are mandatory because they come from the SAES’s, SAMSS’s, and SAMD’s. Reference the SADP’s when tutorial or background information is needed on the selection, specification, or troubleshooting of transformers. These are the SADP’S the engineer refers to concerning power transformers
• SADP-P-121
• SADP-P-431
• SADP-P-434
SADP-P-121 (Transformers) - This design practice has two parts. Part one has a single
page. The statements on the page give the rationale for the technical requirements in SAES-P-121 that are not obvious. The basis of the rationale is many years of Aramco's experience.
Part two contains twelve chapters of information. Figure 5 shows the Table of Contents. This part of the SADP has tutorial information. Use this information to clarify the technical requirements given in the SAES’s and SAMSS’s. The sections that follow give the scope of each chapter.
CHAPTER TABLE OF CONTENTS PAGE
1 TRANSFORMER-INTRODUCTION 13
2 POWER AND DISTRIBUTION TRANSFORMERS 31
3 TRANSFORMER THERMAL RATING 41
4 TRANSFORMERS IMPEDANCES AND VOLTAGES 53
5 TRANSFORMER INSULATION 71
6 TRANSFORMER ENCLOSURE 80
7 TRANSFORMER CONNECTIONS AND TERMINATIONS 84
8 TRANSFORMER AUXILIARY EQUIPMENT 95
9 TRANSFORMER PERFORMANCE REQUIREMENTS 102 10 TRANSFORMER TESTING AND DELIVERY 109
11 INSTRUMENT TRANSFORMERS 115
12 SPECIAL TRANSFORMERS 129
Engineering Encyclopedia Electrical Power Transformers
These are the subjects of the listed chapters:
1. Selection, application, and general specifications of transformers for use in Saudi Aramco installations
2. Requirements and general guidelines for these types of transformers used by Saudi Aramco:
– Power
– Distribution – Auxiliary Power
– Non-Flammable Insulating Liquid Filled – Conventional Dry-Type
– Cast-Resin Type
3. Thermal aspects of transformers, including temperature rise, service conditions, and methods of cooling
4. Selection of transformer impedance values and tap ranges
5. Selection of transformer withstand levels, voltage surge suppression, insulating liquids, and oil preservation techniques
6. Selection of enclosures for transformers
7. General requirements and conventions of terminal connections for all types of transformers, except instrument transformers, also covers the operation of transformers in parallel and disconnecting facilities
8. Selection of transformer accessories, including tap changers, and monitoring and protection equipment
9. Requirements for tolerances, losses, and noise levels, and how to capitalize losses
10. General requirements and policies adopted with regard to factory testing and inspections
11. Requirements of current transformers and inductively coupled voltage transformers
SADP-P-431 - gives the rationale for the technical requirements of 14-SAMSS-531 that are
not obvious.
SADP-P-434 - gives the rationale for the technical requirements of 14-SAMSS-534 that are
not obvious.
Industry Standards
Saudi Aramco Standards often reference Industry Standards. This practice eliminates the need to rewrite all the applicable Industry Standards into Saudi Aramco Standards. Instead, the Saudi Aramco Standards give exceptions, additions, or deletions to the industry standards. The following Industry Standards contain information that pertains to selection, specification, and troubleshooting of transformers:
• National Electrical Code (NEC)
• National Electrical Safety Code (NESC)
• International Electrical and Electronic Engineers (IEEE) • American National Standards Institute (ANSI) standards
• National Electrical Manufacturers Association (NEMA) standards
National Electrical Code - The purpose of the NEC is the practical safeguarding of persons
and property from the hazards of using electricity. The NEC only contains provisions needed for safety and does not guarantee an efficient, convenient, expandable installation.
The specific section of the NEC that contains information pertinent to the selection and specification of transformers is Article 450. The title of Article 450 is “Transformers and Transformer Vaults”. This article applies to all transformers except the following:
• Current transformers
• Power transformers that constitute a component part of other apparatus and comply with the requirements for such apparatus as motor control centers or potential transformers (PT's)
• Transformers that are an integral part of an X-ray, high-frequency, or electrostatic-coating apparatus
• Transformers for use with Class 2 or Class 3 circuits that comply with Article 725-3(b) (such as communications small transformers)
Engineering Encyclopedia Electrical Power Transformers
• Transformers for use with power-limited fire protective signaling circuits that comply with Part C of Article 760
• Liquid-filled or dry-type transformers for use in research, development, or testing, where effective arrangements are provided to safeguard unqualified persons from contacting high-voltage terminals or energized conductors
Article 450 is divided into three major sections: part A, part B, and part C. Part A contains the general provisions for all covered transformers. Part B contains specific provisions applicable to different types of transformers. Part C contains the provisions for transformer vaults.
National Electrical Safety Code - The purpose of the rules in the NESC is the practical
safeguarding of persons during the installation, operation, or maintenance of electric supply and communication lines, and associated equipment. The rules contain the basic provisions needed for the safety of employees and the public under the specified conditions.
Part 1, Section 15 has specific information on transformers. General Safety rules related to troubleshooting and maintaining industrial/utility type electrical equipment are found throughout the text.
ANSI/IEEE Standards and Guidelines - The Industry Standards used most often in
selection, specification, and troubleshooting of power transformers are ANSI/IEEE Standards. IEEE standards give information on how to produce, test, measure, and buy equipment. This information is the consensus opinion of a group of subject matter experts. The requirements and procedures given in the standards are useful when selecting, specifying, and troubleshooting power transformers.
ANSI does not write standards. ANSI adopts standards written by other organizations. ANSI standards give a uniform method of manufacturing, marketing, purchasing, and using a given piece of equipment. This information is useful when selecting, specifying, or troubleshooting transformers.
ANSI has adopted most of the IEEE standards that relate to transformers. All the applicable standards are available in a single book titled “C57,” which is the name of this collective group of standards. C57 contains information on distribution, power, and regulating transformers. These standards should be used when Saudi Aramco Standards reference them.
• C57.12.00 is “General Requirements for Liquid-Immersed Distribution, Power
and Regulating Transformers.” This standard gives the basis for the establishment of performance, limited electrical and mechanical interchangeability, and safety requirements. C57.12.00 also gives assistance in selecting the right equipment.
• C57.12.10 is “230 kV and Below 833/958 through 8333/10,417 kVA,
Single-Phase, and 750/862 through 60,000/80,000/100,000 kVA, Three-Phase Without Load Tap Changing; and 3750/4687 through 60,000/80,000/100,000 kVA with Load Tap Changing-Safety Requirements.” This standard covers certain electrical, dimensional, and mechanical characteristics. C57.12.10 considers certain safety features of 60-HZ, two-winding, liquid-immersed transformers of the ratings covered.
• C57.12.90 is “Standard Test Code for Liquid-Immersed Distribution, Power,
and Regulating Transformers and Guide for Short-Circuit Testing of Distribution and Power Transformers.” This standard describes the methods for performing tests specified in C57.12.00. C57.12.90 also describes other standards applicable to liquid-immersed distribution, power, and regulating transformers. C57.12.90 is intended for use as a basis for performance, safety, and proper testing of transformers.
• C57.13 is “Standard Requirements for Instrument Transformers.” This
standard is intended for use as a basis for performance, interchangeability, and safety of the equipment covered. C57.13 also helps in selecting the right instrument transformers.
• C57.92 is “Guide for Loading Mineral-Oil Immersed Power Transformers Up
to and Including 100 MVA with 55°C or 65°C Average Winding Rise.” This standard covers the general recommendations for loading mineral-oil immersed power transformers.
The ANSI/IEEE standard the engineer uses to find information on the power transformers is C57.12.10, “230 kV and Below 833/958 through 8333/10,417 kVA, Single-Phase, and 750/862 through 60,000/80,000/100,000 kVA, Three-Phase without Load Tap-Changing; and 3750/4687 through 60,000/80,000/100,000 kVA with Load Tap Changing-Safety Requirements.” This standard is intended for use as a basis for determining performance, interchangeability, and safety of the equipment covered. C57.12.10 also helps in selecting the right equipment.
Engineering Encyclopedia Electrical Power Transformers
CONSTRUCTION
The construction of a power transformer deals with these components or assemblies of components:
• Core
• Coil or Winding Assembly • Insulation System
• Enclosure Types (Tank) – Control Cabinet – Fill and Drain Valves • Cooling System
• Accessories
– Faults Gas Detector Relays – Indicators
– Pressure Relief Devices
– Bushings
– No-load Tap Changers – Load Tap Changers
– Bushing Current Transformers
Core
The construction of a power transformer starts with the core. In its simplest form, the transformer consists of two coils which are mutually coupled. When the coupling is provided through a ferromagnetic ring (circular or otherwise), the transformer is called an iron-core transformer. When there is no ferromagnetic material but only air, the device is described as an air-core transformer. The air-core type transformers are usually very small type transformers used for small electrical and electronic circuits. This module will only pay attention exclusively to iron-core type transformers.
Core Material
The core is made of steel alloy because its molecular structure allows the material to carry and hold a magnetic flow 10,000 times greater than air. Steel alloy is a much better conductor of magnetic flux than air.
The material used for the transformer core is selected to afford its molecules the greatest ease in reversing their position as the AC magnetic field reverses its direction. As they reverse themselves, the friction developed between these magnetic molecular particles creates heat. This action causes a core or iron loss known as hysteresis (Figure 6). This hysteresis loss is minimized by using a special grade of heat-treated and cold-rolled grain-oriented silicon steel alloy sheet. The two important parameters that affect the core are the core design and the core material. Both the design and material are chosen to reduce the reluctance of the flux path. This reduces the amount of excitation current required to induce flux into the core and the amount of power lost due to circulating currents, eddy currents and hysteresis.
Engineering Encyclopedia Electrical Power Transformers
A B
C D
Random molecular structure of core before energized
+ Polarized molecular structure of core during first positive part of sine wave
Residual magnetizism of core during zero crossing of sine wave
- Polarized molecular structure of core during first negative part of sine wave
N S S S S S S S S S S S S S S S S S S S S S S S S N N N N N N N N N N N N N N N N N N N N N N N N
Eddy currents are core or iron losses that are due to a current flow in the core from the primary induced voltage. The current flow results in a power loss because it represents a portion of the primary flux that is not transferred to the secondary windings. This current flow in the core is like small circular paths through the material. These swirling currents get their name from their resemblance to eddies in a pond of water (Figure 7). Eddy currents are minimized in transformer design by constructing the core from a number of insulated, laminated sections. This reduces the magnitude of eddy currents since current cannot flow across the insulation between laminations where the eddy currents are confined to smaller areas. This limits the total eddy current losses of the transformer (Figure 8).
The metallic composition of the transformer core is made of special high grade silicon sheet steel. A typical sheet of steel is 0.3 mm (0.014 inch) thick. These sheets of steel are laminated into sections that are several inches wide. The core laminations are provided to help reduce eddy currents or currents induced into the iron parts of the unit. Each of the laminations are coated with an insulating material. This coating helps to prevent magnetic losses and reduces heating losses.
The core is the major part of the magnetic circuit along with a clamping structure. It is part of the transformer magnetic field that oscillates.
Engineering Encyclopedia Electrical Power Transformers
Figure 7. Eddy Currents in a Solid Core
Core Assembly
The core assembly is constructed by stacking individual laminations in accordance with the pattern and configuration selected by the designer. Some of the basic types of core construction and lamination configurations are shown in Figure 9. In all types the layers of laminations are placed so that the air gaps between lamination ends of one layer are overlapped by the laminations in the next layer. For any interleaved joint it is important to minimize the gap (and thus reduce possible eddy currents) between abutting plates.
Example: If the gap in a joint were only 1/1000 of an inch, the magnetizing current to push the flux across each gap would be equivalent to 10,000 times or 10 inches of steel for each gap. The gap would materially increase the exciting current. This is why the test for the no-load excitation current is valuable.
Example: If a unit is moved from one location to another, shipped over land by rail or truck, or is rewound, the no-load excitation current will verify a good transformer or damaged in shipment.
For small distribution transformers the cores are built from strip wound loops (Figure 10). The wound core is spirally constructed from a continuous strip of cold rolled steel and is cut at every other turn to permit assembly. This type core is primarily used for smaller core construction.
Engineering Encyclopedia Electrical Power Transformers
Types of Transformer Core Construction
That transformer core design has four different classes:
• Core type
• Shell type • Cruciform type
• Wound type
Core Type – The core type is one of two types of transformer core constructions. This refers
to the arrangements of the steel core with reference to the windings. In the core-type transformer, the magnetic circuit takes the form of a single ring encircled by two or more groups of primary and secondary windings distributed around the periphery of the ring (Figure 10). Core type means (in the USA) coils are cylindrical and concentric (the outer winding over the inner). This type of design has an inherently simpler insulation structure and are easier to build. Core form designs are typically used for most medium and small power transformers.
Shell Type – The shell type is the second major type of transformer core construction. This
also refers to the arrangement of the steel core with reference to the windings. The shell-type transformer core has the primary and secondary windings take the form of a common ring which is encircled by two or more rings of magnetic material distributed around its periphery (Figure 11). The shell type denotes large pancake coils which are stacked or interleaved to make primary and secondary groups (Figure 12). Primary-secondary-primary (PSP) grouping is common but primary-secondary-primary-secondary-primary (PSPSP) is also often used. In actual practice the cylindrical-concentric coil structure is sometimes used with an enclosing (shell-form) core in single-phase or with a five-legged core in three-phase to reduce overall height. Figure 12 show the conventional three-phase shell form core with the coils in section. Generally, shell-form provides greater mechanical strength, good voltage impulse distribution and better conductor cooling, but the shell-form construction is more complex and costly. Because of these, shell-form designs are usually used on large power transformers ³ (100 MVA) where high strength for through faults and good voltage distribution of voltage impulses are important.
Cruciform Type – The cruciform-type of transformer core is made like a larger plus sign (
+).
This type of core is often used for economy and is common on very small sizes of transformers. This type transformer has coils that are made cylindrical in shape, which enables the insulation to have a higher factor of safety since there are no sharp bends in the insulating material and a better opportunity for the radiation of heat. Both of these factors permit the use of less material for a given output (Figure 13).Engineering Encyclopedia Electrical Power Transformers
Wound Type – The wound-type transformer core construction is used from small distribution
transformers. These cores are built from strip wound loops (Figure 14). The wound core is spirally constructed from a continuous strip of cold rolled steel and is cut at every other turn to permit assembly. Nevertheless, its practical use has been generally limited to smaller core construction. For smaller transformers where wound cores are used, the manufacturer does stress annealing after cutting the core. Using stacked cores, the steel laminations are frequently given an additional layer of this organic polymer coating.
The two major type of transformer core construction are the core type and the shell type. These two will be the only forms discussed further in this module on power transformers.
Laminated Cores
L. V. Winding H. V. Winding
L. V. Winding H. V. Winding
Core Type Shell Type
Insulating Tube
Primary Coil Secondary Coil Spacers Insulation Ventilation Ducts Core
Engineering Encyclopedia Electrical Power Transformers Primary Coil Secondary Coil Primary Coil Phase 1 Phase 2 Phase 3 Core Insulation
Figure 12. Conventional 3-Phase Core for the Rectangular-Pancake-Interleaved-Coil Sructure (Shell Type)
Primary HV Winding Insulation Cruciform Core Secondary LV Winding
Engineering Encyclopedia Electrical Power Transformers
Rolling Direction and Flux Path
Core Area
Grounding of Core
The laminated magnetic core of a liquid filled is insulated from ground, and then by design grounded at one point to maintain its potential at ground level. This prevents the flow of circulating currents which causes additional heating. Then, by design, the core is grounded at one point only for the following reasons:
• Place core at ground potential. • Remove static charges.
• Prevent core voltage from electrically varing.
• Provide protection during a winding to core contact or short.
When more than one ground on the core occurs circulating currents will flow which adds heat to the transformer and increase the ambient noise level.
Lack of a ground will cause the core to float at elevated potential because of induced voltage. Most power transformers have a flexible bolted connection at top near the top cover of the tank.
The insulation between the laminations is only a few ohms resistance but is sufficiently high to prevent damaging eddy currents within the core and at the same time is sufficiently low to permit the entire core to be effectively grounded by a connection to only one of the laminations.
The core connection is usually located at the top of the transformer on same designs, this connection is not solid but instead is made through a heavy-duty resistor in the 250 to 1000 ohm range. A resistor in this range still accomplishes the effect grounding of the core, and at the same time limits circulating currents. This ground connected is usually conveniently mounted under a manhole at the top of the transformer.
Shell-form transformers may have more than one ground because it is not important that the laminations be grounded in only one spot, since the flux distribution in this type of unit differ from core-form transformers.
Coil or Winding Assembly
Transformer coils are designed to get the required number of turns into a minimum of space. Additionally, the cross-section of the conductor must be large enough to carry the current without overheating and sufficient space must be provided for the insulation and for cooling paths, if any. These coils may be made of copper or aluminum, the choice depends on the cost to achieve the low resistance and small space requirements.
Engineering Encyclopedia Electrical Power Transformers
For small units, the coil size may be round, insulated with cotton, enamel, shellac, varnish, paper, or a combination of these. For larger units, the wire may be square or rectangular ribbon, usually insulated with oil impregnated paper. Where transformer operation at high temperatures is desired, special glass or asbestos insulation may be used. The insulation should be provided not only for normal operating voltages, but also for surges of high voltage resulting from lightning or switching.
Core-Type Coils
In core-type transformers, the low voltage coils are usually placed next to the core and the high voltage coils external and concentric with them. This reduces the insulation requirements of both coils. If the high voltage winding was placed next to the core, two layers of high voltage insulation would be required, one next to the core and the other between the two windings (Figure 10). Sometimes, where large and heavy connections are involved, this arrangement may be reversed.
The high voltage windings may be separated from the low voltage windings by insulating cylinders. The high voltage may be composed of several disc shaped coils, each disc insulated from others by insulating strips. If the windings were placed on separate legs of the core, a relatively large amount of the flux produced by the primary windings would fail to link the secondary winding, resulting in a large loss in effectiveness of the flux.
Shell-Type Coils
In shell-type transformer, "pancake construction" is often used (Figure 11). Here, the high voltage and low voltage coils are alternately place around the core, with the required insulation between them, each coil having the rough appearance of a pancake. Often, space is left between coils for cooling purposes. Such an arrangement of coils reduces the reactance between the coils and improves the operation of the transformer, particularly the large-size transformers, where heavy currents are experienced. The shell-type transformers also make arrangements for air cooling paths simpler and easier to provide.
Coil Stress
The coils in transformers that are energized and loaded have mechanical and electrical stresses at all times. All coils which carry currents in the same direction attract one another and coils which carry currents in opposite directions repel one another. Hence, all the coils of the primary attract one another as do all the coils of the secondary. However, primary coils repel the secondary coils and vice-versa. Under normal operating conditions, these forces are relatively small. In case of short circuit or the carrying of very large currents, these forces may become great enough to damage the transformer if the coils are not adequately supported.
The short circuit stresses that interact between windings result in both horizontal and vertical electromagnetic forces.
The vertical or axial forces causes the low voltage and high voltage windings to shift with respect to each other, a condition called telescoping (Figure 15). These forces make the windings take positions that will increase the magnetic flux of the system. If two windings are in series, the electromagnetic varies as the square of current. Example: A short circuit current 20 times normal will produce (20)2, or 400 times, the normal stress.
The vertical force between primary and secondary windings results because it is impossible to exactly balance the low- and high-voltage electrical-center lines. This vertical force is the hardest to design for.
Figure 15. Vertical (Axial) Forces
(Between High- and Low-Voltage Coils in Core-Form Transformers in Through Short Circuit)
Engineering Encyclopedia Electrical Power Transformers
The horizontal force (radial or hoop) is the major force (Figure 16). The principal component of leakage flux is axial and its interaction with the circumferential winding currents produces radial forces acting in the outward direction on the outer winding and in the inward direction on the inner winding.
Figure 16. Horizontal Repulsion (Axial) Force (Between High- and Low-Voltage Coils in Core-Form
Transformers in Through Short Circuit)
Coils and core must be mechanically capable of withstanding these short-circuit stresses. Where the coils are concentrically placed, the forces produced are radial which may tend to distort the shapes of the coils. If interleaved coils are not exactly balanced, axial forces develop, also tending to distort the coils. These are usually so interleaved that forces between coils are balanced, except at the ends. Hence, when assembled, coils must be carefully centered on the cores and rigidly blocked to prevent any movement, bending or distortion from normal positions under the stresses caused by heavy currents. The extra bracing and blocking also tend to reduce the noise emanating from the vibration of several elements, brought about by the effects of the alternating magnetic fields.
Precaution is always taken so that a failure of the high voltage insulation to the low voltage side will not impose the high voltage on the winding of the low voltage coil. Additional insulation and barriers are often placed between the primary and secondary coils to lessen the chances of such occurrences.
Core and Coil Assembly Clamping Construction
The final step of core and coil assembly construction is the proper clamping. If laminations are not properly secured, vibration will be the result, contributing to increased hum of the transformer and unit failure may result.
The clamping structure is part of the core assembly (and magnetic circuit). Its purpose is to hold the core together and the coils in place with the pressure applied to each coil assembly. As with core construction, clamping of the windings minimizes these forces and in large units provides a means for taking up insulation shrinkage.
During manufacture of the transformer (the step just before putting the coil over the core leg) all windings of transformer are progressively tighten during the vapor phase (or other method) of drying, as well as when the completed core and coil assembly is finally mated together and clamping device installed and compressed. A locking device may be fitted into the adjusting screw to prevent any loosening.
The transformer insulation (cellulosic material) is heated in an oven at a temperature to dry the insulation and remove water. The heating causes the insulation to loose the water and become drier. The drier the insulation becomes the more the insulation shrinks. The shrinking allows the jack screw to tighten, compressing the coils closer and tighter together, and raising the insulation dielectric level because of the moisture loss. The predetermined design has estimated how much water will be lost and the amount of shrinkage that will occur. The more pressure the coil and core are compressed, the less chance of coil and core stress from abnormal conditions losing the clamping structure (Figure 17). Prior to final assembly (putting core and coil assembly in tank) RTE-ASEA manufacture relies on pre-compressing four times the coils and spaces in a hydraulic press to pressures exceeding maximum forces the unit will experience.
The Dyna-Comp adjustable clamping system illustrates one of the most recent types of spring loaded dashpots (Figure 18). This design assures a tight coil throughout the assembly, shipping and service life of the transformer. The coil springs on the dashpots, by providing a constant follow-up pressure, prevent any loosening of the windings.
The final step of core and coil assembly construction is the proper clamping. If laminations are not properly secured, vibration will be the result, contributing to increased hum of the transformer and unit failure may result.
Engineering Encyclopedia Electrical Power Transformers
The older transformers had core legs and yokes clamped together by means of insulated steel bolts passing through holes punched in the laminations (Figure 19, shell-type and Figure 20, core-type transformers and Figure 22). This provided good clamping but had disadvantages. Newer transformers have the leg laminations held tightly together by strong tape applied on smaller cores, or by suitably spaced high-strength resin-glass beads applied to the periphery of larger cores. The yoke laminations are secured by fabricated steel clamps (Figure 21). These clamps are lined with resilient packing to obtain uniform pressures and minimize the transmission of sound and vibrations. When the clamping is complete, vertical tie bolts hold together the steel frames clamping the top and bottom yokes. Consequently, electrical and mechanical stresses are minimized and core bolt failure is eliminated.
Figure 17. A Vertical Cross-Section of Major Transformer Components (focusing on jack screw clamping of insulated copper conductors)
Figure 18. Hydraulic Dashpot,
(Part of the new Dyna-Comp™ adjustable coil clamping system, assures a tight winding structure and prevents winding movement under short circuits)
Engineering Encyclopedia Electrical Power Transformers Insulated Yoke Bolt Insulating Pressure Collar H.V. Winding L.V. Winding Insulating Barrier Locking Angle Laminated Core Insulated Yoke Bolt Lifting Loop Tie Plate Adjustable Pressure Plate with Insulation Cooling Ducts Insulating Tubes Insulated Core Bolt Non-Adjustable Pressure Plate with Insulation End Frame Centering Pin
Tie Plate Lifting Loop Adjustable Pressure Plate with Insulation Cooling Ducts Insulation Barrier Between Coils Insulating Pressure Collar Static Plate H.V. Winding L.V. Winding Insulating Tubes Adjustable Pressure Plate with Insulation
Centering Channel
Insulated Core Bolt Centering Pin End Frame Insulated Yoke Bolt Laminated Core
Engineering Encyclopedia Electrical Power Transformers
Figure 21. Typical Bolted Clamping Structure (Used in Older Transformers)
Figure 22. Typical Simplified Boltless Clamping Structure (Used in Large Modern Transformers)
Coil Material
The coil materials mainly used are copper and aluminum, with copper being the more preferred at Saudi Aramco. The design of the coil material must be chosen from the following characteristics of conductor material.
Copper coil advantages:
• Mechanical strength
• Electrical conductivity (smaller coils) Aluminum coil advantages:
• Lower cost
• Efficient heat dissipation for sheet wound (small capacities) • Reduction in weight
Wire is normally found only in the high voltage coils of distribution units where current requirements are fairly low. The wire size increases as the current values of transformer loading go up. Eddy currents can be reduced by the use of rectangular wire as it has less surface area than a wound wire of the same cross-sectional area.
The cross-sectional area of the turns is adjusted so as the current is increased, cross-sectional area is increased. This helps to keep resistance loss to a minimum. This also helps to reduce eddy current losses. There has to be uniform current distribution to obtain maximum efficiency. Transposition of the coils help make each wire in the turn enclose the same amount of flux leakage producing the same volts per turn or ampere-turns (Figure 23).
Sheets and foils are used mainly on distribution and small transformers where current levels are low. Aluminum sheet windings have more uniform electrical conductance, greater short-circuit withstand voltage and better heat conduction.
Engineering Encyclopedia Electrical Power Transformers
Figure 23. Continuously Transposed Multi-Strip Conductor (Courtesy of Feinberg, Modern Power Transformer Practice)
INSULATION SYSTEM
An insulation system is an assembly of fabricated, processed and in-place combinations of component insulating materials with related structural parts as used in liquid-filled transformers, in this case power units as referenced in ANSI C57.12.00-1987.
Coordination of Insulation
Solid insulation is required in a transformer whenever a difference in potential exists between two points. The selection of insulation is generally made in proportion to the anticipated overvoltages and with a safety margin to compensate for decreases due to normal service aging. Various components are designed to work best together and achieve what is called "coordination of insulation" within the insulation system.
A conclusion that should have been reached by now is that insulation is one of the most important, if not the most important, component in a transformer. The internal insulation of the transformer are a number of critical areas that must be adequately insulated to assure that the transformer will operate properly and provide a long service life (Figures 24 and 25). These areas are:
• Turn-to-turn insulation
• High-voltage to low-voltage insulation • Low-voltage to core insulation
• Phase-to-phase insulation • Core-to-ground insulation
These insulation areas must have proper types and combinations of insulation selected and in place to have a transformer that will operate during normal and abnormal conditions and provide a long service life.
Engineering Encyclopedia Electrical Power Transformers
High Density Organic Winding Sticks (B) High Density Cellulose Spacers (B) High Density Kraft Paper Tube Betwween Primary & Secondary Windings (C) High Density Organic Winding Sticks (C) High Voltage Windig Paper Insulation (A) Laminated Magnetic Steel Core Paper Insulation (A) Low Voltage Winding Copper High Density Kraft Paper Tube (B)
Heavy Cellulose Phase-to-Phase Insulation (D)
Rule of Thumb:
0.3 x KVA Rating = Weight of cellulose paper in pounds Example: 0.3 x 1500 KVA = 450 pounds cellulose insulation Phase A Phase B Phase C Phase Insulation
Figure 25. Oil-Filled Cellulose System
[Basic insulation system of a core type power transformer where, (A) is insulation on wire; (B) is insulation to ground; (C) is insulation between windings; and (D) is
Engineering Encyclopedia Electrical Power Transformers
Types of Insulating Materials
Transformers use various types of insulating materials, which together, form an insulation system.
• Pressboard (1/8"-1/2" thick) • Kraft paper (5-20 mils thick) • Manila and hemp paper • High-density particle-board
• Pressboard collars and end insulation • Laminated (plywood-type particle-board)
• Enamels
• Inorganic and organic core • Lamination coatings
• Porcelain
• Epoxy power coatings • Maple wood structural forms • Vulcanized fiber
• Cotton
• Plastics and cements, adhesive tapes, glass-fiber bands, etc. • Liquid dielectric fluid
The transformer insulation system materials isolate the windings from each other and from ground to "insulate" the current-carrying parts of the transformer from the magnetic-iron and structural-steel parts. The insulation is more than just a mechanical means for keeping the wires or turns apart.
