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NEMA ST 20-2021 Dry Type Transformers for General Applications, 2021
- Foreword
- Scope
- Section 1 Referenced Standards and Definitions [Go to Page]
- 1.1 Referenced Standards
- 1.2 Definitions [Go to Page]
- 1.2.1 Accessible
- 1.2.2 Accessible, Readily
- 1.2.3 Accessories
- 1.2.4 Alternating Current
- 1.2.5 Ampacity
- 1.2.6 Circuit [Go to Page]
- 1.2.6.1 Single-Phase Circuit
- 1.2.6.2 Six-Phase Circuit
- 1.2.6.3 Three-Phase Circuit
- 1.2.6.4 Two-Phase Circuit
- 1.2.7 Combustible Materials
- 1.2.8 Contactor
- 1.2.9 Cooling Systems [Go to Page]
- 1.2.9.1 Dry-Type Self-Cooled Transformer (Class AN)
- 1.2.9.2 Dry-Type Self-Cooled/Forced Air-Cooled Transformer (Class AN/AF)
- 1.2.9.3 Dry-Type Forced-Air-Cooled Transformer (Class AF)
- 1.2.9.4 Dry-Type Self-Cooled/Future-Forced-Air-Cooled Transformer (Class AN/FAF)
- 1.2.10 Crest [Go to Page]
- 1.2.10.1 Crest Value (Peak Value)
- 1.2.10.2 Crest Factor of a Function
- 1.2.11 Current [Go to Page]
- 1.2.11.1 Eddy Currents
- 1.2.11.2 Excitation Current of a Transformer
- 1.2.12 Cycle
- 1.2.13 Diagrams [Go to Page]
- 1.2.13.1 Letter Designations
- 1.2.13.2 Illustrative Diagram
- 1.2.13.3 One-Line Diagram (or Single-Line)
- 1.2.13.4 Schematic Diagram or Elementary Diagram
- 1.2.13.5 Connection Diagram
- 1.2.14 Dielectric Testing [Go to Page]
- 1.2.14.1 General
- 1.2.14.2 Applied Dielectric Test
- 1.2.14.3 Induced Dielectric Test
- 1.2.14.4 Impulse Tests
- 1.2.15 Distance [Go to Page]
- 1.2.15.1 Creepage Distance
- 1.2.15.2 Striking Distance
- 1.2.16 Distribution Center (Secondary Distribution)
- 1.2.17 Duty [Go to Page]
- 1.2.17.1 Continuous Duty
- 1.2.17.2 Intermittent Duty
- 1.2.17.3 Periodic Duty
- 1.2.17.4 Short-Time Duty
- 1.2.17.5 Varying Duty
- 1.2.18 Enclosures
- 1.2.19 Equipment
- 1.2.20 FCAN
- 1.2.21 FCBN
- 1.2.22 Frequency
- 1.2.23 Grounded
- 1.2.24 Hertz
- 1.2.25 Impedance [Go to Page]
- 1.2.25.1 Impedance Voltage of a Transformer
- 1.2.25.2 Impedance kVA
- 1.2.26 Insulation [Go to Page]
- 1.2.26.1 Insulating Material
- 1.2.26.2 Insulation System
- 1.2.27 Interlock
- 1.2.28 IR-Drop Compensation
- 1.2.29 K-Factor
- 1.2.30 Load (Output)
- 1.2.31 Losses [Go to Page]
- 1.2.31.1 Total Losses
- 1.2.31.2 No-Load (Excitation) Losses
- 1.2.31.3 Load Losses
- 1.2.32 Nonenclosed
- 1.2.33 Overcurrent Protection
- 1.2.34 Part [Go to Page]
- 1.2.34.1 Dead-Metal Part
- 1.2.34.2 Live-Metal Part
- 1.2.35 Rating [Go to Page]
- 1.2.35.1 Rating of a Transformer
- 1.2.35.2 Continuous Rating
- 1.2.35.3 Primary Voltage Rating
- 1.2.35.4 Secondary Voltage Rating
- 1.2.35.5 Short-Time Rating
- 1.2.35.6 Voltage Rating of a Grounding Transformer
- 1.2.35.7 Winding Voltage Rating
- 1.2.35.8 Withstand Voltage Rating
- 1.2.36 Ratio [Go to Page]
- 1.2.36.1 Turn Ratio of a Transformer
- 1.2.36.2 Voltage Ratio of a Transformer
- 1.2.37 Reactor [Go to Page]
- 1.2.37.1 Current-Limiting Reactor
- 1.2.38 Regulation of a Transformer
- 1.2.39 Resistant (Used as a Suffix)
- 1.2.40 Taps [Go to Page]
- 1.2.40.1 Tap (in a Transformer)
- 1.2.40.2 Rated kVA Tap (in a Transformer)
- 1.2.40.3 Reduced kVA Tap (in a Transformer)
- 1.2.41 Temperatures [Go to Page]
- 1.2.41.1 Ambient Temperature
- 1.2.41.2 Inside Top Air Temperature
- 1.2.41.3 Hottest-Spot Temperature
- 1.2.41.4 Hot Spot Differential
- 1.2.41.5 Limiting Temperature
- 1.2.41.6 Limiting Insulation System Temperature (Limiting Hottest-Spot Temperature)
- 1.2.42 Transformer [Go to Page]
- 1.2.42.1 Autotransformer
- 1.2.42.2 Buck-Boost Transformer
- 1.2.42.3 Compound-Filled Transformer
- 1.2.42.4 Dry-Type Transformers
- 1.2.42.5 Gas-Filled Transformer
- 1.2.42.6 Grounding Autotransformer
- 1.2.42.7 Indoor Transformer
- 1.2.42.8 Insulating Transformer
- 1.2.42.9 Isolating Transformer
- 1.2.42.10 Main Transformer
- 1.2.42.11 Nonventilated Transformer
- 1.2.42.12 Outdoor Transformer
- 1.2.42.13 Sealed Transformer
- 1.2.42.14 Shielded Transformer
- 1.2.42.15 Step-Down Transformer
- 1.2.42.16 Step-Up Transformer
- 1.2.42.17 Teaser Transformer
- 1.2.42.18 Ventilated Dry-Type Transformer
- 1.2.43 Knockout
- 1.2.44 Twistout
- 1.2.45 Multiple Knockout
- 1.2.46 Windings [Go to Page]
- 1.2.46.1 Common, or Shunt, Winding of an Autotransformer
- 1.2.46.2 High-Voltage and Low-Voltage Windings
- 1.2.46.3 Primary Winding
- 1.2.46.4 Secondary Winding
- 1.2.46.5 Series Windings of an Autotransformer
- 1.2.46.6 Stabilizing Windings
- 1.2.47 Wiring or Busing Terminal, Screw, and/or Lead
- Section 2 STANDARD RATINGS [Go to Page]
- 2.1 Ratings and Primary Taps
- 2.2 Voltage Ranges
- Section 3 DESIGN AND MARKING [Go to Page]
- 3.1 Design and Construction
- 3.2 Means of Handling
- 3.3 Transformer Enclosure [Go to Page]
- 3.3.1 General
- 3.3.2 Corrosion Resistance
- 3.3.3 Outdoor
- 3.4 Openings [Go to Page]
- 3.4.1 Enclosure
- 3.4.2 Wiring and/or Terminal Compartment [Go to Page]
- 3.4.2.1
- 3.4.2.2
- 3.4.2.3
- 3.4.3 Conduit Entrance Threaded
- 3.4.4 Conduit Entrance Knockouts
- 3.4.5 Cord and Wire Entry Holes
- 3.4.6 Covers, Bottoms, or Panels
- 3.5 Wall-Mounted Enclosures
- 3.6 Testing a Knockout Surrounded by Multiple Twistouts
- 3.7 Thermal Rating
- 3.8 Classification of Insulating Materials
- 3.9 Classification of Insulation Systems [Go to Page]
- 3.9.1 Systems Testing
- 3.9.2 System Limiting Temperature
- 3.9.3 Typical Materials Used in 105 C Insulation Systems
- 3.9.4 Typical Materials Used in 130 C Insulation Systems
- 3.9.5 Typical Materials Used in 155 C Insulation Systems
- 3.9.6 Typical Materials Used in 180 C Insulation Systems
- 3.9.7 Typical Materials Used in 200 C Insulation Systems
- 3.9.8 Typical Materials Used in 220 C Insulation Systems
- 3.9.9 Typical Materials Used in Insulation Systems Rated Above 220 C
- 3.10 Coil Insulation
- 3.11 Internal Wire, Cable, and Bus
- 3.12 Corrosive Materials
- 3.13 Flexible Cords
- 3.14 Switches, Circuit Breakers, Control Devices, etc.
- 3.15 Capacitors
- 3.16 Limits of Winding Temperature Rise for Continuously Rated Transformers
- 3.17 Maximum Allowable Temperature Rise for Materials and Component Parts
- 3.18 Reference Temperature for Efficiency, Losses, Impedance, and Regulation
- 3.19 Transformer Surface Temperatures [Go to Page]
- 3.19.1 Transformer Surface Temperature of 90 C (194 F) or Less
- 3.19.2 Transformer Surface Temperature Greater than 90 C (194 F)
- 3.19.3 Marking
- 3.20 Temperature Rise of Transformer Terminations for Field Connections [Go to Page]
- 3.20.1
- 3.20.2
- 3.20.3
- 3.20.4
- 3.21 Clearance Between Terminals for Field Connections
- 3.22 Bonding of Core
- 3.23 Contact Surfaces of Grounding Terminals
- 3.24 Marking
- 3.25 Nameplates
- 3.26 Designation of Voltage Ratings of Windings [Go to Page]
- 3.26.1 General
- 3.26.2 Single-Phase
- 3.26.3 Three-Phase
- 3.27 Autotransformers
- 3.28 Tap
- 3.29 Terminal Markings [Go to Page]
- 3.29.1 Scope and Purpose
- 3.29.2 Precautions
- 3.29.3 Location of Markings
- 3.29.4 Markings
- 3.29.5 Significance of the Terminal Letter
- 3.29.6 Significance of the Numeral 0 (Zero)
- 3.29.7 Significance of Numerals on Terminals
- 3.29.8 Definition of Phase Sequence
- 3.29.9 Direction of Rotation of Vectors
- 3.29.10 Markings of Terminals and Identification of Windings
- 3.29.11 Single-Phase Transformers
- 3.29.12 Three-Phase Transformers
- 3.29.13 Three-Phase to Six-Phase Transformers
- 3.30 Field Connections
- 3.31 Connection of Transformers for Shipment
- 3.32 Audible Sound Levels
- Section 4 TESTING [Go to Page]
- 4.1 Classification of Tests [Go to Page]
- 4.1.1 Routine Tests
- 4.1.2 Design Tests
- 4.1.3 Other Tests
- 4.1.4 Tests for Dry-Type Transformers
- 4.2 Routine, Design, and Prototype Tests [Go to Page]
- 4.2.1 Ratio Tests [Go to Page]
- 4.2.1.1 General
- 4.2.1.2 Voltage Tolerances [Go to Page]
- 4.2.1.2.1 Voltage Tolerance for Uncompensated Turn Ratio Transformers
- 4.2.1.2.2 Voltage Tolerance for IR-Compensated Turn Ratio Transformers
- 4.2.1.3 Ratio Test Methods
- 4.2.1.4 Ratio by Voltmeter Method
- 4.2.1.5 Ratio by Comparison
- 4.2.1.6 Alternate Comparison Method
- 4.2.1.7 Ratio by Ratio Bridge
- 4.2.2 Polarity and Phase-Relation Tests
- 4.2.3 Method of Polarity Test [Go to Page]
- 4.2.3.1 Polarity by Comparison
- 4.2.3.2 Polarity by Inductive Kick
- 4.2.3.3 Polarity by Alternating Voltage Test
- 4.2.3.4 Polarity by Ratio Bridge
- 4.2.4 Phase-Relation Tests [Go to Page]
- 4.2.4.1 Test for Phasor Diagram for Transformers Having a Ratio of Transformation of 30 to 1 or Less
- 4.2.4.2 Phase-Sequence Test
- 4.2.5 Total Losses [Go to Page]
- 4.2.5.1 Tolerances on Measured Losses
- 4.2.6 No-Load Losses (Excitation Losses) [Go to Page]
- 4.2.6.1 General
- 4.2.6.2 Average-Voltage Voltmeter Method for Single-Phase Transformers
- 4.2.6.3 Test Method for Three-Phase Transformers
- 4.2.7 Excitation Current (No-Load Current) [Go to Page]
- 4.2.7.1 Excitation Current Test
- 4.2.7.2 Excitation by Average Method
- 4.2.8 Impedance and Load Loss [Go to Page]
- 4.2.8.1 Impedance Voltage
- 4.2.8.2 Interlacing Impedance Voltage of a Scott-Connected Transformer
- 4.2.8.3 Tolerances for Transformer Impedance
- 4.2.8.4 Impedance and Load-Loss Tests [Go to Page]
- 4.2.8.4.1 Test Method for Transformer
- 4.2.8.4.2 Test Method for Autotransformer
- 4.2.8.4.3 Test Method for Three-Phase Transformer with Single-Phase Voltage
- 4.2.8.4.4 Test Method of a Three-Winding Transformer
- 4.2.8.4.5 Impedance Bridge Method
- 4.2.8.5 Load Loss on a ‘K’-Rated Transformer
- 4.2.9 Dielectric Tests [Go to Page]
- 4.2.9.1 General
- 4.2.9.2 Applied-Potential Test
- 4.2.9.3 Induced-Potential Tests
- 4.2.10 Audible Sound Level Tests [Go to Page]
- 4.2.10.1 Test Conditions
- 4.2.10.2 Measurements
- 4.2.11 Resistance Measurements [Go to Page]
- 4.2.11.1 Importance of Measurements
- 4.2.11.2 Determination of Cold Temperature
- 4.2.11.3 Conversion of Resistance Measurements
- 4.2.11.4 Resistance Measurement Methods
- 4.2.12 Temperature Tests [Go to Page]
- 4.2.12.1 General
- 4.2.12.2 General Requirements for Temperature Rise Tests
- 4.2.12.3 Temperature Rise of Single-Phase Transformers by the Loading-Back Method
- 4.2.12.4 Temperature Rise Test of Three-Phase Transformers by the Loading-Back Method
- 4.2.12.5 Temperature Rise Tests of Three-Phase Transformers or Three Single-Phase Transformers by the Impedance kVA Method
- 4.2.12.6 Short-Circuit Test (Separate Load-Loss and Excitation Tests)
- 4.2.12.7 Load-Loss Test Plus Additive Factor
- 4.2.12.8 Determination of Average Measured Winding Temperature and Average Winding Temperature Rise by the Hot-Resistance Method
- 4.2.12.9 Correction of Observed Temperature Rise for Variation in Altitude
- 4.2.12.10 Correction to Be Applied When Temperature Test Must Be Corrected for a Difference in Total Loss, Including K-Factor
- 4.2.12.11 Correction Back to Shutdown, Cooling Curve Method
- 4.2.12.12 Alcove Construction for Temperature Rise Test
- 4.2.12.13 Temperature Rise Test on a K-Factor Transformer
- 4.2.13 Short-Circuit Capability [Go to Page]
- 4.2.13.1 Short-Circuit Test
- 4.2.13.2 Short-Circuit Thermal Requirements
- 4.3 Regulation and Efficiency [Go to Page]
- 4.3.1 Regulation of a Transformer [Go to Page]
- 4.3.1.1 Impedance Watts and Impedance Volts
- 4.3.1.2 Regulation Computation
- 4.3.1.3 Tolerance for Regulation
- 4.3.2 Efficiency
- 4.4 Enclosure Compression Test [Go to Page]
- 4.4.1
- 4.4.2
- Section 5 APPLICATION [Go to Page]
- 5.1 Selection of Rated Transformer kVA
- 5.2 Usual Temperature and Altitude Service Conditions
- 5.3 Unusual Temperature, Altitude, and Environmental Service Conditions [Go to Page]
- 5.3.1 Higher or Lower Ambient Temperatures
- 5.3.2 Application at Altitudes Greater than 3300 Feet
- 5.3.3 Environmental Conditions
- 5.3.4 Impact Loading
- 5.3.5 Overexcitation
- 5.4 Loading [Go to Page]
- 5.4.1
- 5.4.2
- 5.5 Parallel Operation
- 5.6 Autotransformers [Go to Page]
- 5.6.1 Advantages and Disadvantages
- 5.6.2 Autotransformer Delta Systems Grounding
- 5.6.3 Autotransformers in Wye Systems
- 5.6.4 Precautions in Connecting Autotransformers
- 5.7 Single-Phase Transformer Connections for Three-Phase Banks
- 5.8 Use of Transformers in Connecting Systems of Various Phase Displacements [Go to Page]
- 5.8.1 0-Degree Phase Displacement—Delta-Delta or Y-Y
- 5.8.2 30-Degree Phase Displacement—Delta-Y or Y-Delta
- 5.8.3 60-Degree Phase Displacement—Delta-Y or Y-Delta
- 5.8.4 90-Degree Phase Displacement
- Section 6 INSTALLATION [Go to Page]
- 6.1 Inspection Prior to Energization [Go to Page]
- 6.1.1
- 6.1.2
- 6.1.3
- 6.1.4
- 6.2 Handling [Go to Page]
- 6.2.1 Handling in Inclement Weather
- 6.2.2 Precautions in Lifting
- 6.2.3 Skidding or Rolling
- 6.2.4 Protection of Accessories
- 6.3 Grounding
- 6.4 Room Requirements [Go to Page]
- 6.4.1
- 6.4.2
- 6.4.3
- 6.5 Outdoor Applications [Go to Page]
- 6.5.1
- 6.5.2
- 6.5.3
- 6.6 Maintenance Accessibility [Go to Page]
- 6.6.1
- 6.6.2
- 6.6.3
- 6.7 Personnel and Public Safety [Go to Page]
- 6.7.1
- Section 7 MAINTENANCE [Go to Page]
- 7.1 Maintenance
- Appendix A LOADING [Go to Page]
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