Transformers: Basics, Maintenance, and Diagnostics U. Department of the Interior Bureau of Reclamation April 2005 Transformers: Basics, Maintenance, and Diagnostics U. Department of the Interior Bureau of Reclamation Technical Service Center Infrastructure Services Division Hydroelectric Research and Technical Services Group Denver, Colorado April 2005 PREFACE Transformers have been used at powerplants since the inception of alternating-current generation, a century ago. While operating principles of transformers remain the same, the challenges of maintaining and testing transformers have evolved along with transformer design and construction.
Modern transformers are designed to closer tolerances than transformers in the past. Thus, effective, regular maintenance and testing is even more essential to continued operation when traditional “overdesign” cannot be relied on to overcome abnormal conditions. The utility engineer must be familiar with all aspects of maintenance and testing and make use of state-of-the-art tools and techniques for evaluating transformer condition. While on-line diagnostic systems and computerized testing methods are very helpful, they are not a substitute for sound engineering judgment and expertise.
This volume provides timely, practical advice to those seeking to better understand how transformers work, how they are best maintained, and how to test and evaluate their condition. It has been developed with the assistance of Bureau of Reclamation engineers responsible for operating and maintaining transformers at important powerplants in the Western States. Support and funding was provided through the Reclamation Power Resources Office in Denver and via the Manuals and Standards development program. The authors gratefully acknowledge the assistance of all who contributed.
Hydroelectric Research and Technical Services Group Denver, Colorado April 2005 iii Transformers: Basics, Maintenance, and Diagnostics Contents Page 1. Introduction to Transformers .1 Principle of Operation.3 Transformer Voltage and Current.4 The Magnetic Circuit .17 Single-Phase Transformer Connections for Typical Service to Buildings.18 Parallel Operation of Single-Phase Transformers for Additional Capacity.19 Three-Phase Transformer Connections.20 Wye and Delta Connections.21 Three-Phase Connections Using Single-Phase Transformers .22 Paralleling Three-Phase Transformers.23 Methods of Cooling .24 Oil-Filled – Self-Cooled Transformers.25 Forced-Air and Forced-Oil-Cooled Transformers. 37 v Transformers: Basics, Maintenance, and Diagnostics Contents (continued) Page 2. Introduction to Transformers (continued) 2.28 Oil-Filled, Inert-Gas System.1 Introduction to Reclamation Transformers.2 Transformer Cooling Methods Introduction.3 Dry-Type Transformers .1 Potential Problems and Remedial Actions for Dry-Type Transformer Cooling Systems .4 Liquid-Immersed Transformers.
Liquid-Immersed, Air-Cooled .2 Liquid-Immersed, Air-Cooled/Forced Liquid-Cooled .3 Liquid-Immersed, Water-Cooled.4 Liquid-Immersed, Forced Liquid-Cooled.5 Potential Problems and Remedial Actions for Liquid-Filled Transformer Cooling Systems .7 Low Oil Level .6 Cooling System Inspections. Oil-Filled Transformer Inspections.2 Top Oil Thermometers. 57 vi Transformers: Basics, Maintenance, and Diagnostics Contents (continued) Page 4. Oil-Filled Transformer Inspections (continued) 4.3 Winding Temperature Thermometers.1 Temperature Indicators Online .2 Temperature Indicators Offline.4 Oil Level Indicators .5 Pressure Relief Devices .1 Newer Pressure Relief Devices.2 Older Pressure Relief Devices .6 Sudden Pressure Relay.8 Transformer Bushings: Testing and Maintenance of High-Voltage Bushings.9 Oil Preservation Sealing Systems .1 Sealing Systems Types .2 Sealed or Pressurized Breathing .3 Pressurized Inert Gas Sealed System.2 Gas Pressure Control Components .1 High-Pressure Gauge .2 High-Pressure Regulator.3 Low-Pressure Regulator.4 Bypass Valve Assembly .7 Sampling and Purge Valve.8 Free Breathing Conservator .9 Conservator with Bladder or Diaphragm Design .10 Auxiliary Tank Sealing System.
90 vii Transformers: Basics, Maintenance, and Diagnostics Contents (continued) Page 5.1 Sealing (Mating) Surface Preparation.5 Gasket Sizing for Standard Groove Depths .6 Rectangular Nitrile Gaskets .7 Bolting Sequences to Avoid Sealing Problems .1 Transformer Oil Functions.1 Dissolved Gas Analysis .2 Key Gas Method .1 Four-Condition DGA Guide (IEEE C57-104) .3 Sampling Intervals and Recommended Actions .5 Dissolved Gas Software.9 Diagnosing a Transformer Problem Using Dissolved Gas Analysis and the Duval Triangle .1 Origin of the Duval Triangle.2 How to Use the Duval Triangle .4 Rogers Ratio Method of DGA .10 Carbon Dioxide/Carbon Monoxide Ratio. 141 viii Transformers: Basics, Maintenance, and Diagnostics Contents (continued) Page 6.1 Transformer Oil Functions (continued) 6.1 Dissolved Moisture in Transformer Oil .2 Moisture in Transformer Insulation. Transformer Oil Tests that Should Be Completed Annually with the Dissolved Gas Analysis .3 Test for Oxygen Inhibitor .7 Oil Treatment Specification.1 Taking Oil Samples for DGA .1 DGA Oil Sample Container .2 Taking the Sample. Silicone Oil-Filled Transformers .2 Carbon Monoxide in Silicone Transformers.3 Comparison of Silicone Oil and Mineral Oil Transformers.5 Physical Test Limits.1 DC Winding Resistance Measurement .2 Core Insulation Resistance and Inadvertent Core Ground Test (Megger®).
178 ix Transformers: Basics, Maintenance, and Diagnostics Contents (continued) Page 9.3 Doble Tests on Insulation .1 Insulation Power Factor Test .3 Excitation Current Test .5 Percent Impedance/Leakage Reactance Test .6 Sweep Frequency Response Analysis Tests .4 Fans and Radiators.6 Infrared Temperature Analysis .7 IR for Transformer Tanks .8 IR for Surge Arresters.9 IR for Bushings .10 IR for Radiators and Cooling Systems.11 Corona Scope Scan .5 Ultrasonic and Sonic Fault Detection .7 Turns Ratio Test .8 Estimate of Paper Deterioration (Online) .1 CO2 and CO Accumulated Total Gas Values. 196 x Transformers: Basics, Maintenance, and Diagnostics Contents (continued) Page 9.8 Estimate of Paper Deterioration (Online) (continued) 9.2 CO2/CO Ratio .9 Estimate of Paper Deterioration (Offline During Internal Inspection).1 Degree of Polymerization (DP) .10 Transformer Operating History.11 Transformer Diagnostics/Condition Assessment Summary. 201 Appendix: Hydroplant Risk Assessment – Transformer Condition Assessment. 231 Acronyms and Abbreviations.
235 Tables Table No. Page 1 Operative Parallel Connections of Three-Phase Transformers. 34 2 Inoperative Parallel Connections of Three-Phase Transformers. 34 3 Transformer Gasket Application Summary.
96 4 Vertical Groove Compression for Circular Nitrile Gasket. 97 xi Transformers: Basics, Maintenance, and Diagnostics Tables (continued) Table No. Page 5 Vertical Groove Compression for Rectangular Nitrile Gaskets. 103 6 Transformer DGA Condition Summary Table.
107 7 345-kV Transformer Example. 108 8 Dissolved Key Gas Concentration Limits. 110 9 Actions Based on Dissolved Combustible Gas. 112 10 TOA L1 Limits and Generation Rate Per Month Alarm Limits.
120 12 Dissolved Gas Solubility in Transformer Oil Accurate Only at STP, 0 °C (32 °F) and 14.93 Inches of Mercury). 124 13 L1 Limits and Generation Rate Per Month Limits. 126 14 Dissolved Gas Analysis Detection Limits. 132 15 Rogers Ratios for Key Gases.
135 16 Typical Faults in Power Transformers. 140 17 Comparison of Water Distribution in Oil and Paper. 142 18 Furans, DP, Percent of Life Used of Paper Insulation. 156 19 Doble Limits for Inservice Oils.
158 20 Additional Guidelines for Inservice Oils. 159 21 Comparison of Gas Limits. 172 22 Suggested Levels of Concern (Limits). 173 23 Doble and IEEE Physical Test Limits for Service-Aged Silicone Fluid.
175 24 Paper Status Conditions Using CO2 and CO. 197 25 DP Values for Estimating Remaining Paper Life. 198 26 Reclamation Transformer Condition Assessment Summary. 201 xii Transformers: Basics, Maintenance, and Diagnostics Figures Figure No.
Page 1 Typical GSU Three-Phase Transformer. 7 5 Step-Up and Step-Down Transformers. 11 7 Three-Phase Core Form and Three-Phase Shell Form Transformer Units. 12 8 Transformer Internal Forces.
17 10 Connections of Instrument Transformers. 21 13 Photograph of Current Transformers. 25 15 Single-Phase Transformer. 26 16 Single-Phase Paralleling.
27 17 Three-Phase Connections. 29 18 Delta-Delta Connections, Single-Phase Transformers for Three-Phase Operation. 30 19 Wye-Wye Connections, Using Single-Phase Transformers for Three-Phase Operation. 31 20 Delta-Wye and Wye-Delta Connections Using Single-Phase Transformers for Three-Phase Operation.
36 22 Forced-Air/Oil/Water-Cooled Transformers. 37 23 Conservator with Bladder. 38 24 Typical Transformer Nitrogen System. 40 25 Transformer Diagnostics Flowchart.
43 26 Typical Oil Flow. 51 27 Oil Level Indicator. 62 28 Conservator Oil Level. 62 xiii Transformers: Basics, Maintenance, and Diagnostics Figures (continued) Figure No.
Page 29 Pressure Relief Device. 64 30 Photograph of a Pressure Relief Device. 64 31 Sudden Pressure Relay, Section. 68 32 Photograph of a Sudden Pressure Relay.
69 33 Buchholz Relay, Section. 70 34 Photograph of a Buchholz Relay. 70 35 Pressurized Breathing Transformer. 78 36 Pressurized Inert Gas Transformer.
79 37 Gas Pressure Control Components. 80 38 Free Breathing Conservator. 84 39 Conservator with Bladder. 87 41 Photograph of a Bladder Failure Relay.
87 42 Bladder Failure Relay. 88 43 Auxiliary Sealing System. 91 44 Cross Section of Circular Gasket in Groove. 98 45 Cross Section of Gasket Remains Constant Before Tightening and After.
101 46 Bowing at Flanges. 104 47 Bolt Tightening Sequences. 104 48 Combustible Gas Generation Versus Temperature. 115 49 The Duval Triangle.
126 50 Duval Triangle Diagnostic Example of a Reclamation Transformer. 128 51 Maximum Amount of Water Dissolved in Mineral Oil Versus Temperature. 145 52 Transformer Oil Percent Saturation Curves. 146 53 Water Distribution in Transformer Insulation.
147 54 Myers Multiplier Versus Temperature. 148 55 Water Content of Paper and Oil Nomogram. 149 56 Interfacial Tension, Acid Number, Years in Service. 152 xiv Transformers: Basics, Maintenance, and Diagnostics Figures (continued) Figure No.
Page 57 Oil Sampling Piping. 165 60 Sample Syringe Bubble Removal. 166 61 Relationship of Oxygen to Carbon Dioxide and Carbon Monoxide as Transformer Ages. 169 62 SFRA Test Traces of a New Transformer.
184 63 SFRA Test Traces of a Defective New Transformer. 186 64 Normal Transformer IR Pattern. 190 65 IR Image of Defective Arrester. 191 66 IR Image Showing Blocked Radiators.
192 67 IR Image of Defective Bushing. 192 68 Transformer Diagnostic Test Chart (Adapted from IEEE 62-1995™). 204 xv Transformers: Basics, Maintenance, and Diagnostics 1. Introduction This document was created to provide guidance to Bureau of Reclamation (Reclamation) powerplant personnel in maintenance, diagnostics, and testing of transformers and associated equipment.
This document applies primarily to the maintenance and diagnostics of oil-filled power transformers (500 kilovoltamperes [kVA] and larger), owned and operated by Reclamation, although routine maintenance of other transformer types is addressed as well. Specific technical details are included in other documents and are referenced in this document. Guidance and recommendations herein are based on industry standards and experience gained at Reclamation facilities. However, equipment and situations vary greatly, and sound engineering and management judgment must be exercised when applying these diagnostics.
All available information must be considered (e., manufacturer=s and transformer experts’ recommendations, unusual operating conditions, personal experience with the equipment, etc.) in conjunction with this document. Introduction to Transformers Generator step-up (GSU) transformers represent the second largest capital investment in Reclamation power production—second only to generators. Reclamation has hundreds, perhaps thousands, of transformers, in addition to hundreds of large GSU transformers. Reclamation has transformers as small as a camera battery charger, about one-half the size of a coffee cup, to huge generator step-up transformers near the size of a small house.
The total investment in transformers may well exceed generator investment. Transformers are extremely important to Reclamation, and it is necessary to understand their basic functions. 1 Transformers: Basics, Maintenance, and Diagnostics A transformer has no internal moving parts, and it transfers energy from one circuit to another by electromagnetic induction. External cooling may include heat exchangers, radiators, fans, and oil pumps.
Radiators and fans are evident in figure 1. The large horizontal tank at the top is a conservator. Transformers are typically used because a change in voltage is needed. Power transformers are defined as transformers rated 500 kVA and larger.
Larger transformers are oil-filled for insulation and cooling; a typical GSU transformer may contain several thousand gallons of oil. One must always be aware of the possibility of spills, leaks, fires, and environmental risks this oil poses. Figure 1 – Typical GSU Three-Phase Transformers smaller Transformer. than 500 kVA are generally called distribution transformers.
Pole-top and small, pad-mounted transformers that serve residences and small businesses are typically distribution transformers. Generator step-up transformers, used in Reclamation powerplants, receive electrical energy at generator voltage and increase it to a higher voltage for transmission lines. Conversely, a step-down transformer receives energy at a higher voltage and delivers it at a lower voltage for distribution to various loads. All electrical devices using coils (in this case, transformers) are constant wattage devices.