Distribution Transformer
Reference Handbook
FEBR UARY 202 6
Forward From electric utilities to governmental facilities to heavy industrial applications such as mining: electric distribution transformers have widely varying specifications and applications to meet these needs. This guide aims to provide an overview of transformers and their common applications. Also included are “good to know” transformer information and useful calculations. If you need an application not covered in this handbook, please contact Wasion Americas, Inc. near Raleigh, at wasionamericas.com or reach out directly to our team at WAI-Sales@wasion.com.
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Acknowledgement As we created this handbook, the authors drew not only from their own personal electric utility experience, but from a variety of industry sources, including IEEE Std C57.105™-2019, RUS specifications, and numerous manufacturer handbooks, spanning at least the last 70 years. We further discussed this effort with industry professionals who work daily to provide reliable, cost-effective electric service to their customers/members. We deeply appreciate all the contributors who helped capture the wealth of industry knowledge provided in this guide.
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Table of Contents Topic
Page
Wasion Americas, Inc. Overview
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Single-Phase Pole-Mounted Transformer Single-Phase Pad-Mounted Transformer Three-Phase Pad-Mounted Transformers Transformer Reference Information Transformer Basic Operating Principles Transformer Nameplate Explained 1. Maximum Winding Temperature Above Ambient at Rated Load 3. High and Low Voltage Basic Insulation Level (BIL) Ratings 4. Transformer Winding Relationship 5. Secondary Voltage and Current Ratings 6. Primary Winding Tap Information 7. Percent Impedance at 85° C With Base kVA and Voltage 8. Primary and Secondary Voltage Ratings 2. Oil Type
10 11 13 14 15 16 18 20 22 23 24 25 29 31 32 34 36
9. Full Rated Load 10. Cooling Type
More “Good to Know” Transformer Information and Calculations
A. Polarity
B. Polarity Testing C. Altitude Derating
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Topic
Page
D. Short Circuit Current
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E. Short Circuit Current Calculation Examples
F. Operating Costs
G. No Load and Full Load Losses
H. Total Cost of Ownership (TCO): The Effects of No Load and Full Load Losses on Operating Cost / Lifecycle Transformer Connection Diagrams A. Single-Phase: 2 or 3-Wire Service B. WYE – DELTA: 3-Wire, 3-Phase Service C. DELTA – DELTA: 3-Wire, 3-Phase Service D. WYE – DELTA: 4-Wire, 3-Phase Service E. DELTA – DELTA: 4-Wire, 3-Phase Service F. WYE – WYE: 4-Wire, 3-Phase Service G. DELTA - WYE: 4-Wire, 3-Phase Service H. OPEN WYE – OPEN DELTA: 4-Wire, 3- Phase Service I. OPEN DELTA – OPEN DELTA: 4-Wire, 3-Phase Service
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About Wasion Americas, Inc. Wasion Americas partners with utilities, industries, and government agencies across North America to deliver advanced, standards- compliant hardware platforms that enhance grid reliability, efficiency, and safety.
Headquartered near Raleigh, NC Manufacturing in Silao, Mexico; USMCA Compliant Compliant with all ANSI/IEEE, IEC, RUS, DOE, CSA, and UL Deep industry experience with over 250 years of combined utility experienced staff
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Single-Phase Pole-Mounted Transformer
*Completely Self-Protected or CSP means the transformer contains a re- settable secondary circuit breaker to clear low voltage faults, primary isolation link(s) to isolate the transformer core in the event of an internal failure, and appropriate lightning arrestors. Conventional transformers require external fusing and lightning arrestors.
Key Features •
Voltage Class: 15 kV, 25 kV, and 35 kV kVA: 10, 15, 25, 37.5, 50, 100, 167 Optional Taps: Two 2.5% steps, raise and lower
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Bushings: Single or Double
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CSP* or Conventional
Mineral or FR3 dielectric oil
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Single-Phase Pad-Mounted Transformer
Key Features •
Voltage Class: 2.4 kV to 34.5 kV kVA: Industry Standard sizes 10 to 167 kVA
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Radial or Loop
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Mineral or FR3 dielectric oil
Customer Specific Designs: o Tap Changer, Fusing, Switches, Accessories
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Three-Phase Pad-Mounted Transformers
Key Features •
Voltage Class: 2.4 kV to 34.5 kV kVA: Industry standard sizes 45 to 2500 kVA KVA: Industry Standard sizes 45 to
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Radial or Loop
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Mineral or FR3 dielectric oil
Customer Specific Designs: o Tap Changer, Fusing, Switches, Accessories Tap Changer, Fusing, Switches, Accessories
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Notes:
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Transformer Reference Information
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Transformer Basic Operating Principles
Simplified Transformer
Key Features • Magnetic core facilitates energy transfer. Core materials are typically: o Grain-oriented electrical steel (GOES) o Amorphous • Two coils: Primary (input) and Secondary (output) o The coil windings may be constructed from either copper or aluminum • Transformers operate on the following ratios:
Where: Vp and Vs are the primary and secondary voltages; Np and Ns are the number of turns of conductor on the primary and secondary side; and Is and Ip are the primary and secondary currents.
Each transformer will have a characteristic impedance which is stated in %Z.
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Transformer Nameplate Explained Three-Phase Pad-Mounted Nameplate
Single-Phase Pole-Mounted Nameplate
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1. Maximum Winding Temperature Above Ambient at Rated Load This rating is the highest temperature rise of transformer windings above the surrounding ambient temperature when operating at rated load. •
Rated Load is the maximum continuous load (in kVA) that the transformer is designed to handle under standard conditions without exceeding temperature limits.
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Typical Limits o
Average winding rise: 65°C above ambient Hot-spot rise: 85°C above ambient Ambient temperature design is typically 30°C
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Standard Conditions o
Within designed cooling method parameters
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Total Hot-Spot Temperature permissible is Ambient (30°C) + Hot-spot rise (85°C) = 115°C maximum Standard Reference is IEEE C57.12.00 and C57.12.90 for liquid-filled transformers. Higher temperatures accelerate insulation aging Maintaining limits ensures expected service life and reliability under normal operating conditions.
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2. Oil Type Oil acts as an insulating medium and coolant for the transformer windings and core. • FR3 Fluid - biodegradable, natural ester- based insulating oil used in transformers, offering high fire safety (high flash point), excellent moisture tolerance, and extended insulation life - D6871 / IEEE C57147 standard.
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Mineral Oil - petroleum-based insulating and cooling fluid most common transformers, offering good dielectric strength and thermal performance but is flammable and less environmentally friendly - D3487 / IEEE C57106 standard.
FR3 Fluid
Mineral Oil (Traditional Choice)
Flash Point Fire Point
330ºC 360ºC
155ºC 165ºC
Costs
$$$
$
Enhanced insulation life
Potential for insulation degradation Less heat tolerant
Comparisons
Increased loading capacity Reduced risk of failure – moisture & contaminant resistant
Degrades over time – moisture & contaminants intrusion
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3. High and Low Voltage Basic Insulation Level (BIL) Ratings
BIL represents the maximum voltage a piece of equipment can withstand without insulation breakdown. Defined by ANSI, UL, IEC, and NEMA standards.
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High Voltage (HV) BIL Ratings
BIL for HV system’s maximum voltage a piece of equipment can withstand lightning or switching surges without insulation breakdown. Typical Range is from 60 kV to over 1800kV depending on system voltage and application. Testing Method applies impulse tests using simulated lightning strikes (e.g., 1.2/50 µs waveform). Ensures reliability and safety of equipment under transient overvoltage conditions. Higher BIL requires more insulation and physical clearance.
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Low Voltage (LV) BIL Ratings •
BIL for LV systems is lower and typically refers to surge withstand capability of the transformer secondary.
BIL for 1kV and under is 30 kV.
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Testing Method applies surge tests or dielectric withstand tests. Protects against switching transients and nearby lightning strikes.
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4. Transformer Winding Relationship Vector Diagrams also referred to as Vector Groups. Vector groups describe the phase displacement and winding configuration between primary and secondary sides of a transformer. • They follow IEC 60076-1 notation (e.g., Dyn11, Yyn0, Yd1). • Common Winding Configurations: Common Winding Config Symbol Config Description Y Wye (Star) Neutral available D Delta
No neutral, closed loop Used for grounding or harmonic suppression
Z
Zigzag
• Clock Notation (Phase Shift)
o Indicates phase displacement between HV and LV windings o Clock positions: 0 = 0° shift 1 = 30° lead (voltage leads current)
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11 = 30° lag (voltage lags current)
• Example: Dyn1
o D – Delta Primary o Y – Wye Secondary o N – Neutral o 1 – Voltage is Leading Current by 30°
• Vector Groups
o System compatibility - Ensures correct phase alignment when paralleling transformers o Load balancing - Supports proper distribution of three-phase loads o Harmonic mitigation- Zigzag and delta configurations help suppress harmonics o Grounding strategy - Determines neutral availability and fault current paths.
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5. Secondary Voltage and Current Ratings • Common secondary voltages – o 120V, 208V, 240V, 277V, 480V, 600V • Often configured as single-phase or three- phase systems. • Full Load Current Calculation Formula –
𝐼𝐼 = kVA×1000 √ 3× 𝑉𝑉 LL (for three-phase) 𝐼𝐼 = kVA× 𝑉𝑉 1000 LL (for single-phase) o 75 kVA, 480 V (three-phase): 𝐼𝐼 = 75,000 √ 3×480 ≈ 90 A
• Typical Example:
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Three-Phase Current Where Voltage is Line- Line
kVA Rating
120V
208V
240V
Amperage
45 50 75
216.5 240.6 360.9 541.3 721.7 1082.6 1443.4 2405.7 3608.6 4811.4
124.9 138.8 208.2 312.3 416.4 624.6 832.7 1387.9 2081.9 2775.8
108.3 120.3 180.4 270.6 360.9 541.3 721.7 1202.9 1804.3 2405.7
112.5
150 225 300 500
750 1000
kVA Rating
277V
480V
600V
Amperage
45 50 75
93.8 104.2 156.3 234.5 312.7 469.0 625.3 1042.2 1563.3 2084.4
54.1 60.1 90.2
43.3 48.1 72.2
112.5
135.3 180.4 270.6 360.9 601.4 902.1 1202.9
108.3 144.3 216.5 288.7 481.1 721.7 962.3
150 225 300 500
750 1000
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6. Primary Winding Tap Information A device that adjusts the transformer's turns ratio to regulate output voltage by selecting different tap points on the winding. • Typically, ±2 × 2.5% Tap Changer – Four Taps from +5% to -5% • Manually or mechanically adjusted (e.g., via a dial or switch) when the transformer is de- energized (disconnected from power)
Lower cost than on-load tap changers Requires transformers to be offline, causing service interruptions - Not suited for frequent or dynamic voltage adjustments - Manual operation requires trained personnel maintenance - Reliable for stable load applications Works without connection changes. Adjusts primary voltage input internally reducing or increasing top for turns ration No direct impact on LV bushings (2, 3, or 4), which determine secondary voltage output configuration (e.g., 120/240 V for 3 LV bushings)
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7. Percent Impedance at 85°C With Base kVA and Voltage Transformer Impedance is the opposition to current flow caused by the transformer's internal resistance and leakage reactance. •
Expressed as a percentage of rated voltage at 85 o C, typically 2% to 10% for distribution transformers 85°C matters since impedance is temperature- dependent: It’s standardized at 85°C, the typical operating temperature of a liquid-filled transformer
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Ensures consistent comparison across manufacturers and models and reflects real-world performance under load conditions It’s important because it is used in design and operation of the transformer • Short-circuit protection: Higher impedance limits fault current, protecting downstream equipment •
Voltage regulation:Affects how much voltage drops under load. Lower impedance = tighter regulation Parallel operation: Matching impedance is critical when connecting multiple transformers together as in a three-phase bank
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Used to: •
Used in coordination studies, fault analysis, and system design Transformer sizing, breaker selection, and load balancing
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8. Primary and Secondary Voltage Ratings •
ANSI C84.1 defines acceptable voltage ranges for electrical systems in North America for all Classes o Primary = Medium Voltage o Secondary = Low Voltage Most common Primary Voltages per ANSI C84.1 o Wye 4 wire - 12.47 kV, 13.2 kV, 24.94 kV,
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34.5 kV systems
Delta 3 wire – 4.16 kV & 13.8 kV
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Most common Secondary voltages per ANSI C84.1
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120/240V – Residential split-phase 208Y/120V – Muti residential premises (apartments, boat docks, etc.) 208Y/120V – Commercial three-phase wye
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480Y/277V – Industrial three-phase wye
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9. Full Rated Load The maximum continuous load (in kVA or MVA) the transformer can carry at its rated voltage and frequency without exceeding temperature limits. • Determined by: o Nameplate Rating specified by manufacturer o Cooling Class – ONAN (Oil-Air) for transformers using mineral oil KNAN (Oil-Air) for transformers utilizing FR3 oil FA (Forced Air – not typical in Distribution Transformers) FOA (Forced Oil-Air – not typical in Distribution Transformers) o Temperature Limits based on insulation class (e.g., 65°C rise for mineral oil- filled units) • Includes Losses, accounting for core losses (no-load) and winding losses (load losses)
Voltage Regulation to maintain rated secondary voltage under full load conditions Reductions: o Higher ambient temperatures may reduce allowable load
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o Short-term overloads possible if thermal limits are not exceeded o Transformer life calculations from IEEE Efficiencies: o DOE Standards 2016 and 2029 o DOE 2016: Applies to low-voltage and medium-voltage dry-type and liquid- filled distribution transformers covering single-phase (10–500 kVA) and three- phase (15–2500 kVA) units with primary voltage ≤345 kV and secondary voltage ≤600 Volts o DOE 2029: Maintains the same general scope but introduces stricter efficiency targets and core material requirements, with a focus on reducing energy losses by 10% to 30% depending on transformer type and size. Increases the transformer's efficiency to 98.95%
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Single-Phase Transformers Liquid-Immersed Efficiency by kVA Rating kVA
DOE 2016 Efficiency
DOE 2029 Efficiency
10 kVA 15 kVA 25 kVA
98.50% 98.60% 98.70% 98.75% 98.80% 98.85% 98.90% 99.33% 99.39%
98.75% 98.85% 98.95% 99.00% 99.05% 99.10% 99.15% 99.46% 99.51%
37.5 kVA 50 kVA 75 kVA 100 kVA 167 kVA 250 kVA
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Three-Phase Transformers Liquid-Immersed Efficiency by kVA Rating kVA
DOE 2016 Efficiency
DOE 2029 Efficiency
45 kVA 75 kVA
98.92% 99.03% 99.11% 99.16% 99.23% 99.42% 99.49% 99.43% 99.46% 99.51% 99.53% 99.55%
99.14% 99.22% 99.29% 99.33% 99.38% 99.51% 99.59% 99.62% 99.64% 99.66% 99.68% 99.70%
112.5 kVA 150 kVA 225 kVA 300 kVA 500 kVA 750 kVA 1000 kVA 1500 kVA 2000 kVA 2500 kVA
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10. Cooling Type ONAN (Oil Natural Air Natural) •
Transformer oil circulates naturally (by convection) inside the tank, transferring heat from windings to tank walls; air cools the tank naturally. No fans or pumps required; it relies on natural convection and radiation. Standard cooling method Distribution transformers. Simple and reliable, but limited cooling capacity compared to forced methods.
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• Designation - “O” = Oil, “N” = Natural circulation, “A” = Air cooling, “N” = Natural air flow. KNAN (K-Class Oil Natural Air Natural) • Same cooling principle as ONAN, but uses K-class fluid (such as FR3 natural ester) instead of mineral oil. • Designation - “K” = K-class fluid, “N” = Natural circulation, “A” = Air cooling, “N” = Natural air flow.
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Notes:
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More “Good to Know” Transformer Information and Calculations
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A. Polarity Polarity refers to the relative direction of voltage between the primary and secondary windings when terminals are connected and energized. It affects how voltages combine or oppose during testing and connection. + Additive Polarity • Voltages across the primary and
secondary terminals add together during testing (see Polarity Testing diagram on next page). Typically affect transformers rated below 200 kVA and with primary voltage windings under 8,660V.
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Applied to single- phase units. For a pole-mounted transformer, the X1 terminal is opposite H1 (see example in image)
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Subtractive Polarity •
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Voltages across the primary and secondary terminals subtract during testing (shown in diagram on next page) Standard for transformers rated above 200 kVA or with primary voltage windings above 8,660V. Found in larger capacity pole-mounted and pad-mounted transformers on higher-voltage distribution systems
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For a pole-mounted transformer, the X1 terminal is aligned with the H1 terminal (see image on previous page)
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Polarity Testing Diagram
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B. Polarity Testing IEEE 57.12.90 lays out a clear way to test transformers polarity. As shown in the diagram below, add a jumper to connect the H1 bushing to the adjacent low voltage terminal A convenient low voltage AC source such as 120VAC is applied to the high voltage terminals. WARNING – DO NOT CONNECT TEST VOLTAGE TO THE LOW VOLTAGE TERMINALS OF THE TRANSFORMER AS THIS WILL CREATE PRIMARY LEVEL VOLTAGES ON THE HIGH VOLTAGE TERMINALS. While energized with the test voltage, read the voltage between the H2 terminal and the adjacent low voltage terminal as shown below. If the resulting voltage is higher than the test voltage, the transformer polarity is additive. If the resulting voltage is lower, the transformer is subtractive polarity.
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WARNING: Connect Test Voltage To High Side Terminals Only!
120VAC Test Voltage
120.0
V
Additive Subtractive
115.8
124.2
V
V
H1 H2
Jumper H1 to the
adjacent low side terminal
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C. Altitude Derating For transformers located at higher altitudes, they must be derated to accommodate the decrease in cooling capability due to the thinning atmosphere. The rule of thumb for transformers installed above 1,000 meters (~3,200 feet), is the transformer should be derated 0.5% for every 100 meters above 1,000 meters. 𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝑫 𝒌𝒌𝒌𝒌𝒌𝒌 = 𝑹𝑹 𝑹𝑹𝑹𝑹𝑹𝑹𝑹𝑹 𝒌𝒌𝒌𝒌𝒌𝒌 𝒙𝒙 𝟏𝟏𝟏𝟏𝟏𝟏 − ඌ𝑨𝑨𝑨𝑨𝑨𝑨𝑨𝑨𝑨𝑨𝑨𝑨 𝑨𝑨𝑨𝑨 𝑰𝑰𝑰𝑰 𝑰𝑰 𝑰𝑰𝑰𝑰𝑰𝑰𝑰𝑰𝑰𝑰 𝑰𝑰 −𝟏𝟏 , 𝟎𝟎𝟎𝟎𝟎𝟎 𝟏𝟏𝟏𝟏𝟏𝟏 𝒙𝒙 𝟎𝟎 . 𝟓𝟓 % ඐ ൨ Where: Derated kVA = Maximum kVA loading at installed altitude Rated kVA = Nameplate full load rating Altitude Installed = the installation altitude in meters when over 1,000m
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Example: Breckenridge, CO is located at approximately 2,900m altitude. For a 1000 kVA transformer, the derated value would be: 𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝑫 𝒌𝒌𝒌𝒌𝒌𝒌 = 𝟏𝟏 𝟏𝟏 𝟏𝟏𝟏𝟏 𝒌𝒌𝒌𝒌𝒌𝒌 𝒙𝒙 𝟏𝟏𝟏𝟏𝟏𝟏 − ඌ𝟐𝟐𝟐𝟐𝟐𝟐𝟐𝟐 − 𝟏𝟏 , 𝟎𝟎 𝟎𝟎 𝟎𝟎 𝟏𝟏 𝟏𝟏 𝟏𝟏 𝒙𝒙 𝟎𝟎 . 𝟓𝟓 % ඐ ൨ 𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝑫 𝒌𝒌𝒌𝒌𝒌𝒌 = 𝟏𝟏𝟏𝟏𝟏𝟏𝟏𝟏 𝒌𝒌𝒌𝒌𝒌𝒌 𝒙𝒙 [ 𝟏𝟏𝟏𝟏𝟏𝟏 − ⌊𝟏𝟏𝟏𝟏 𝒙𝒙 𝟎𝟎 . 𝟓𝟓 % ⌋ ] 𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝑫 𝒌𝒌𝒌𝒌𝒌𝒌 = 𝟏𝟏𝟏𝟏𝟏𝟏𝟏𝟏 𝒌𝒌𝒌𝒌𝒌𝒌 𝒙𝒙 [ 𝟏𝟏𝟏𝟏𝟏𝟏 − ⌊𝟗𝟗 . 𝟓𝟓 % ⌋ ] 𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝑫 𝒌𝒌𝒌𝒌𝒌𝒌 = 𝟏𝟏𝟏𝟏𝟏𝟏𝟏𝟏 𝒌𝒌𝒌𝒌𝒌𝒌 𝒙𝒙 [ 𝟗𝟗𝟗𝟗 . 𝟓𝟓 %] 𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝑫 𝑲𝑲𝑲𝑲𝑲𝑲 = 𝟗𝟗𝟗𝟗𝟗𝟗 𝒌𝒌𝒌𝒌𝒌𝒌
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D. Short Circuit Current Calculating fault current at the transformer secondary ignoring motor contribution. Data Needed:
Nameplate kVA Rating
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Nameplate Secondary Voltage
Nameplate %Z First, calculate per phase nameplate current: For three-phase transformers this would be:
For single-phase transformers:
Then simply divide the “Nameplate Current” by the %Z divided by 100.
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E. Short Circuit Current Calculation Examples (ignoring motor contribution) 3 – Phase Transformer Example kVA = 1500 Secondary Voltage = 480V LL Impedance = 4.5% 𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁 𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶 = 1500 𝑘𝑘𝑘𝑘 𝑘𝑘 𝑥𝑥 1,000 √ 3 𝑥𝑥 480 𝑉𝑉 𝐿𝐿𝐿𝐿 ≅ 1,806 𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴 𝑆𝑆ℎ𝑜𝑜𝑜𝑜𝑜𝑜 𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶 𝐶𝐶𝐶𝐶𝐶𝐶 𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶 = 1,806 𝐴𝐴 𝑥𝑥 4.5% 100 𝑍𝑍 ≅ 4,190 𝐴𝐴 Single-Phase Transformer Example Transformer kVA = 37.5 kVA Voltage = 240V LL Percent Impedance = 1.8% 𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁 𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶 = 37.5 𝑘𝑘𝑘𝑘 𝑘𝑘 𝑥𝑥 240 1,000 𝑉𝑉 𝐿𝐿𝐿𝐿 ≅ 156 𝐴𝐴 𝑆𝑆ℎ𝑜𝑜𝑜𝑜𝑜𝑜 𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶 𝐶𝐶𝐶𝐶𝐶𝐶 𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶 = 156 𝐴𝐴 𝑥𝑥 1.8% 100 ≅ 8,667 𝐴𝐴
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F. Operating Costs Sources of Loss No-Load Loss (Core Loss) •
Occurs when the transformer is energized but not supplying load No-load losses occur 24/7, even when the transformer isn’t supplying load Mostly constant regardless of load
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Full-Load Loss (Load Loss) • Occurs when the transformer is supplying rated current • Increases with load and temperature • Full-load losses scale with usage and temperature •
Utilities use their Load Factor (profile) data to calculate per day loss calculations
Why 65°C Matters •
Losses are temperature-corrected to 65°C for oil- filled units to reflect real-world operating conditions Higher temperature = higher resistance = more load loss Transformers with lower losses may cost more upfront but save thousands in energy costs. For example, reducing no-load loss by 100 W can save $3,000–$10,000 over the life of the transformer, depending on energy prices.
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Higher Efficiency = Lower Lifetime Cost •
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G. No-Load and Full Load Losses No-Load Losses Losses occur when the transformer is energized, even if it is not supplying load. No load losses are due to the following: • Hysteresis Loss - magnetization and demagnetization of the core. •
Eddy Current Loss - induced circulating currents in the core laminations. These are influenced by lamination thickness and flux density. Stray Eddy Losses - occur in metallic parts outside the core due to the leakage of magnetic flux. Dielectric Losses - minor losses in insulation materials. These are typically negligible in distribution transformers.
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No-Load and Full-Load Losses (continued) Full-Load Losses
Typical Utility Load Profile •
Load losses occur anytime the transformer is supplying current to a load. Load losses for a transformer are generally expressed as full load losses. These losses include winding (I²R) losses and stray losses which increases with load and temperature. Stray Load Losses - caused by leakage flux inducing currents in structural components; influenced by geometry, shielding, magnetic design and manufacturing practices. Total losses = No load losses plus load losses. Application Load Profile determine the per day loss.
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H. Total Cost of Ownership (TCO)
The Effects of No Load and Full Load Losses on Operating Cost / Lifecycle Cost TCO Formula (Simplified) • TCO=Purchase Price+(A × No Load Loss)+ (B × Full-Load Loss) • A and B are cost factors ($/W) based on energy price, load profile, and expected lifetime. Losses are temperature-corrected to 65°C for oil-filled units to reflect real-world operating conditions. o Higher temperature = higher resistance = more load loss. • Higher Efficiency = Lower Lifetime Cost o Transformers with lower losses may cost •
more upfront but save thousands in energy costs over the transformer life. For example, reducing no-load loss by 100 W can save $3,000–$10,000 over the life of the transformer, depending on energy prices.
o
Calculation Example We are to evaluate an amorphous core 500 kVA three-phase pad-mount transformer. The manufacturer has supplied both the no-load and full-load losses. As follows: • No-load losses at 100% rated voltage = 250W.
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Full-load losses at 100% rated current = 4000W.
•
Further, in our example, the manufacturer has bid $24,000 as the delivered price of the transformer. The utility evaluating the transformer has provided the annual cost of losses to be:
No-Load Losses = $4.50/Watt Full-Load Losses = $2.00/Watt
The utility’s practice is to evaluate their total cost of ownership (TCO) over a three-year period. Using
this information, we can determine TCO. From above we have the cost equation: 𝑇𝑇 𝑇𝑇 𝑇𝑇 = 𝑇𝑇 𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇 𝑇𝑇𝑇𝑇 𝑇𝑇
𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝ℎ𝑎𝑎𝑎𝑎𝑎𝑎 𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 + ൣ 𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸 𝐸𝐸𝐸𝐸 𝐸𝐸𝐸𝐸𝐸𝐸 𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃 𝑃𝑃𝑃𝑃 𝑌𝑌𝑌𝑌𝑌𝑌𝑌𝑌𝑌𝑌 ∗ ൫ ( 𝑁𝑁𝑁𝑁𝑁𝑁 ∗ 𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁 ) +( 𝐹𝐹𝐹𝐹𝐹𝐹 ∗ 𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹 ) ൯൧
Where: 𝑇𝑇 𝑇𝑇 𝑇𝑇
is the total cost of ownership over a specific
number of years. 𝑇𝑇 𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇 𝑇𝑇𝑇𝑇 𝑇𝑇
𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝ℎ𝑎𝑎𝑎𝑎𝑎𝑎 𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 is the initial transformer
purchase price including any shipping cost.
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EvaluationPeriod Years is the number of years over which the utility will evaluate TCO NLL and NLLC are the No-Load Losses and No- Load Loss Cost respectively
FLL and FLLC are the Full-Load Losses and Full- Load Loss Cost respectively
By substituting each variable into the TCO equation we have: 𝑇𝑇 𝑇𝑇 𝑇𝑇 =$24,000+ 3 𝑦𝑦𝑦𝑦𝑦𝑦𝑦𝑦𝑦𝑦 𝑥𝑥 ൫ (250 𝑊𝑊 ∗ $4.50) +(4000 𝑊𝑊 ∗ $2.00) ൯൧ 𝑇𝑇 𝑇𝑇 𝑇𝑇 =$24,000+ 3 𝑦𝑦𝑦𝑦𝑦𝑦𝑦𝑦𝑦𝑦 𝑥𝑥 ൫ ($1,125) +($8,000) ൯൧ 𝑇𝑇 𝑇𝑇 𝑇𝑇 =$24,000+ [3 𝑦𝑦𝑦𝑦𝑦𝑦𝑦𝑦 𝑦𝑦 𝑥𝑥 $9,125] 𝑇𝑇 𝑇𝑇 𝑇𝑇 = $24,000 + $27,375 𝑇𝑇 𝑇𝑇 𝑇𝑇 = $24,000 + $27,375 The evaluated 3-year TCO for this transformer is $51,375.
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Notes:
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Transformer Connection Diagrams
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A. Single-Phase: 2 or 3-Wire Service
A B N
H1
H2
X3
X1
Tank GND
X2
n a b
This is the most common distribution transformer connection used in North America. While the diagram above depicts a single-phase conventional transformer with two high voltage bushings, single bushing transformers in both conventional and CSP type are also common. Also in the above diagram, the high voltage side of the transformer is connected phase to neutral. Some utilities utilize transformers with high voltage ratings to connect between two high voltage phases rather than phase to neutral.
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Low voltage secondary services from this transformer connection are: • 2 - wire 120V • 3 - wire 120/240V • 3 - wire 240/480V In most cases, a 240/480 transformer will have two high side bushings. In each of these service types, the X2 terminal is connected to the transformer tank ground, system neutral, and earth ground. For a 2-wire 120V service, the service neutral is connected to X2 with the energized conductor being connected to either X1 or X3. For 3 – wire services, the service neutral is connected to X2 with one energized service conductor being connected to X1 and the other to X3. kVA Rating The kVA ratings for this transformer is simply equal to the total 1-phase load the transformer is intended to support.
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B. WYE – DELTA: 3-Wire, 3-Phase Service
A B C
H1 H2
H1 H2
H1 H2
X1
X3
X1
X3
X1
X3
Tank GND
Tank GND
Tank GND
X2
X2
X2
a b c
FOR A CORNER GROUNDED SERVICE, GROUND ONE OF THE PHASES. LEAVE FLOATING IF UNGROUNDED SERVICE IS REQUIRED.
For this type of transformer bank, two bushing conventional transformers of equal kVA rating are to be used. Primary Wiring: With a WYE connected high- voltage side, the H1 terminal of each transformer is typically connected to a different phase of primary. The H2 terminals are connected and allowed to form a “floating neutral”. NOTE: Do not connect the H2 terminals to ground . Grounding the floating neutral can create over-voltages during a fault event that can damage the transformers. Also note, the H2 bus is considered an energized conductor. Secondary Wiring: This secondary connection is a straight delta and is utilized for electric services that are strictly three-phase with no single-phase requirements. Typical services are 240V or 480V straight delta used for industrial loads, pumping stations, and other motor loads. For a delta
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connected secondary, the X2 terminals are connected to the X1 terminals on the adjacent transformers to form a “circle.” NOTE: Some utilities require a “corner grounded” secondary to reference secondary voltages to ground. Other utilities allow ungrounded delta secondary. It does not matter which phase is grounded, just ensure only one secondary phase is grounded and that any loads are grounded on the same phase. kVA Rating: The kVA rating for this type of transformer bank is three
times the individual transformer rating. Example. 3 – 25 kVA
CAUTION: Hazardous voltages exist between all ungrounded X2 transformer bushings, service conductors, the neutral, and/or ground. Exercise caution when working near these transformer bushings.
transformer will have a 75 kVA bank rating. Note: To minimize circulating currents, the three
transformers used for this service should have matching impedances as much a practical. Mismatched impedances may lead to overheating due to circulating current and pre-mature failure of the transformers.
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C. DELTA – DELTA: 3-Wire, 3-Phase Service
A B C
H1 H2
H1 H2
H1 H2
X1
X3
X1
X3
X1
X3
Tank GND
Tank GND
Tank GND
X2
X2
X2
a b c
FOR A CORNER GROUNDED SERVICE, GROUND ONE OF THE PHASES. LEAVE FLOATING IF UNGROUNDED SERVICE IS REQUIRED.
For this type transformer bank, two bushing conventional transformers of equal kVA rating are to be used. Primary Wiring: With a Delta connected high- voltage side, the H1 terminal of each transformer is connected to the H2 terminal of the adjacent transformer in circular fashion. These H1/H2 pairs are then connected to the primary phases. Secondary Wiring: This secondary connection is a straight delta and is utilized for electric services that are strictly Three-phase with no single-phase requirements. Typical services are 240V or 480V straight delta used for industrial loads, pumping stations, and other motor loads. For a delta connected secondary, the X2 terminals are connected to the X1 terminals on the adjacent transformers to form a “circle.”
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NOTE: Some utilities require a “corner grounded” secondary to reference secondary voltages to ground. Other utilities allow ungrounded delta secondary. It does not matter which phase is grounded, just ensure only one secondary phase is grounded and that any loads are grounded on the same phase. kVA Rating: The kVA rating for this type of transformer bank is three times the individual transformer rating. Example. 3 – 25 kVA transformer will have a 75 kVA bank rating. Note: To minimize circulating currents, the three transformers used for this service should have matching
impedances as much as practical. Mismatched impedances may lead to overheating
due to circulating current and pre- mature failure of the transformers.
CAUTION: Hazardous voltages exist between all ungrounded X2 transformer bushings, service conductors, the neutral, and/or ground. Exercise caution when working near these transformer bushings.
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D. WYE – DELTA: 4-Wire, 3-Phase Service
A B C N
H1 H2
H1 H2
H1 H2
X3
X1
X1
X3
X1
X3
Tank GND
Tank GND
Tank GND
X2
X2
X2
a b c n
Primary Wiring: With a WYE connected high- voltage side, the H1 terminal of each transformer is typically connected to a different phase of primary. The H2 terminals are connected and allowed to form a “floating neutral”. NOTE: Do not connect the H2 terminals to ground. Grounding the floating neutral can create over-voltages during a fault event that can damage the transformers. Also note, the H2 bus is considered an energized conductor. Secondary Wiring: This secondary connection supports both single-phase and three-phase loads. 240∆ /120 banks are popular due to their ability to serve multiple customer types from the same transformer bank: single-phase for customers with lighting and small motor loads, and three-phase for other customers who have
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both single-phase lighting and three-phase motor loads. The bank wiring is similar to that of the three-phase, 3-wire Delta banks shown in the prior examples. The exception is the center transformer in this diagram is grounded at its center tap and connected to the service neutral to form a 4-wire service. Voltage ratings for this type of bank may be 240∆/120 or 480∆/240 . As mentioned previously, 240∆/120 are very common where a mix of 120/240 3-wire single-phase loads are present along with 240∆ three -phase loads. 480∆/240 services are used for agricultural irrigation as well as oil-field and other industrial motor load. kVA Rating: The kVA ratings of each of the transformers in this bank may be conservatively derived from the following:
𝑇𝑇 𝑙𝑙𝑙𝑙𝑙𝑙ℎ 𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡 = 1 𝜙𝜙 𝑘𝑘𝑘𝑘𝑘𝑘 + 1 3 3 𝜙𝜙 𝑘𝑘𝑘𝑘 𝑘𝑘 𝑇𝑇 𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 = 1 3 1 𝜙𝜙 𝑘𝑘𝑘𝑘 𝑘𝑘 + 1 3 3 𝜙𝜙 𝑘𝑘𝑘𝑘𝑘𝑘
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Where: T lighting = center tapped transformer servicing the single-phase load. This transformer is commonly referred to as the Lighting transformer. T power = two remaining transformers serving transformers. 1 𝜙𝜙 𝑘𝑘𝑘𝑘 𝑘𝑘 = the total estimated single-phase load. 3 𝜙𝜙 𝑘𝑘𝑘𝑘 𝑘𝑘 =the total estimated three-phase load. Notes: 1) In the above equations, many utilities will three-phase load. These transformers are commonly referred to as the Power size the Lighting transformer for 2/3 the single-phase load plus 1/3 the three-phase load. Sizing the Lighting transformer for full single-phase load plus 1/3 three-phase load allows margin for unbalanced single-phase loading. 2) When single-phase loads are larger, the Lighting transformer may be larger than the power transformers. However, due to circulating currents, the Lighting transformer kVA rating should not be more than twice (2x) the Power transformer kVA rating.
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CAUTION: Hazardous voltages exist between all ungrounded X2 transformer bushings, service conductors, the neutral, and/or ground. Exercise caution when working near these transformer bushings.
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E. DELTA – DELTA: 4-Wire, 3-Phase Service
A B C N
H1 H2
H1 H2
H1 H2
X3
X1
X1
X3
X1
X3
X2
Tank GND
Tank GND
Tank GND
X2
X2
a b c n
For this type of transformer bank, two bushing conventional transformers of equal kVA rating are to be used. Primary Wiring: With a Delta connected high- voltage side, the H1 terminal of each transformer is connected to the H2 terminal of the adjacent transformer in circular fashion. These H1/H2 pairs are then connected to the primary phases. Secondary Wiring: This secondary connection supports both single-phase and three-phase loads. 240∆/120 banks are popular due to their ability to serve multiple customer types from the same transformer bank: single-phase for customers with lighting and small motor loads, and three-phase for other customers who have both single-phase lighting and three-phase motor loads. The bank wiring is similar to that of
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the three-phase, 3-wire Delta banks shown in the prior examples. The exception is the center transformer in this diagram is grounded at its center tap and connected to the service neutral to form a 4-wire service. Voltage ratings for this type of bank may be 240∆/120 or 480∆/240 . As mentioned previously, 240∆/120 are very common where a mix of 120/240 3-wire single-phase loads are present along with 240∆ three -phase loads. 480∆/240 services are used for agricultural irrigation as well as oil-field and other industrial motor load. kVA Rating: The kVA ratings of each of the transformers in this bank may be conservatively derived from the following:
𝑇𝑇 𝑙𝑙𝑙𝑙𝑙𝑙ℎ 𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡 = 1 𝜙𝜙 𝑘𝑘𝑘𝑘𝑘𝑘 + 1 3 3 𝜙𝜙 𝑘𝑘𝑘𝑘 𝑘𝑘 𝑇𝑇 𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 = 1 3 1 𝜙𝜙 𝑘𝑘𝑘𝑘 𝑘𝑘 + 1 3 3 𝜙𝜙 𝑘𝑘𝑘𝑘𝑘𝑘
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Where: T lighting = center tapped transformer servicing the single-phase load. This transformer is commonly referred to as the Lighting transformer. T power = two remaining transformers serving three-phase load. These transformers are commonly referred to as the Power transformers. 1 𝜙𝜙 𝑘𝑘𝑘𝑘 𝑘𝑘 = the total estimated single-phase load. 3 𝜙𝜙 𝑘𝑘𝑘𝑘 𝑘𝑘 =the total estimated three-phase load. Notes: 1) In the above equations, many utilities will size the Lighting transformer for 2/3 the single-phase load plus 1/3 the three-phase load. Sizing the Lighting transformer for full single-phase load plus 1/3 three-phase load allows margin for unbalanced single-phase loading. 2) When single-phase loads are larger, the Lighting transformer may be larger than the power transformers. However, due to circulating currents, the Lighting transformer kVA rating should not be more than twice (2x) the Power transformer kVA rating.
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F. WYE – WYE: 4-Wire, 3-Phase Service
A B C N
H1 H2
H1 H2
H1 H2
X3
X1
X3
X1
X3
X1
Tank GND
Tank GND
Tank GND
X2
X2
X2
a b c n
Primary Wiring - WYE: With a WYE connected high-voltage side, the H1 terminal of each transformer is typically connected to a different phase of primary. The H2 terminals are connected together and must be solidly grounded. Secondary Wiring – WYE : Transformers banks with this secondary configuration are widespread and used to service both single-phase and three- phase loads. Common service voltages are 208Y/120V and 480Y/277V. Occasionally, 416Y/240V and 830Y/480V services will be seen and can be accommodated with this bank configuration. 120/240V transformers used for the 208Y/120V service must be “split” by opening the transformer top, splitting the winding leads and connecting them in parallel. Failure to split and parallel the transformer windings will result in halving the kVA rating of the transformer.
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For 480Y/277V services, the transformers must be purchased with a single 277V winding. For the 416Y/240V service, three un-split 240V transformers will be used. Likewise, for the 830Y/480V service, three un-split 480V transformers will be utilized. NOTE: For three-wire self-contained metered services pulled from a WYE connected secondary transformer bank, standard 3-wire, Form 2S meters must not be used as they will register less than actual use. Rather for these type services, utilize a Form 12S or Form 25S meter. Both require a 5- terminal socket, not a 4-terminal socket. kVA Rating: This bank, especially with the 208Y/120V, serves both single and three-phase loads. The assumption is any single-phase load is equally divided across each transformer. With this assumption, the kVA rating of the three individual transformers can be determined by the following equation: 𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇 𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇 𝑘𝑘𝑘𝑘𝑘𝑘 = 1𝜙𝜙 𝑘𝑘𝑘𝑘𝑘𝑘+3𝜙𝜙 𝑘𝑘𝑘𝑘𝑘𝑘 3 For this transformer bank, individual transformer impedances do not need to be matched.
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Connection 208Y/120 480Y/277 416Y/240 830Y/480 a - b 208 480 416 830 b - c 208 480 416 830 c- a 208 480 416 830 a - n 120 277 240 480 b - n 120 277 240 480 c - n 120 277 240 480 3-Phase, 4-Wire WYE Voltages
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G. DELTA - WYE: 4-Wire, 3-Phase Service
A B C N
H1 H2
H1 H2
H1 H2
X3
X1
X3
X1
X3
X1
Tank GND
Tank GND
Tank GND
X2
X2
X2
a b c n
Primary Wiring - DELTA: With a Delta connected high-voltage side, the H1 terminal of each transformer is connected to the H2 terminal of the adjacent transformer in circular fashion. These H1/H2 pairs are then connected to the primary phases. System neutral connected to the transformer tank and secondary neutral/ ground. Secondary Wiring – WYE : Transformers banks with this secondary configuration are widespread and used to service both single-phase and three- phase loads. Common service voltages are 208Y/120V and 480Y/277V. Occasionally, 416Y/240V and 830Y/480V services will be seen and can be accommodated with this bank configuration. Transformers used for the 208Y/120V service must be “split” by opening the transformer top, splitting the winding leads and connecting them in parallel.
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Failure to split and parallel the transformer windings will result in halving the kVA rating of the transformer. For 480Y/277V services, the transformers must be purchased with a single 277V winding. For the 416Y/240V service, three un-split 240V transformers will be used. Likewise, for the 830Y/480V service, three un-split 480V transformers will be utilized. NOTE: For three-wire self-contained metered services pulled from a WYE connected secondary transformer bank, standard 3-wire, Form 2S meters must not be used as they will register less than actual use. Rather for these type services, utilize a Form 12S or Form 25S meter. Both require a 5- terminal socket, not a 4-terminal socket. kVA Rating: This bank, especially with the 208Y/120V, serves both single and three-phase loads. The assumption is that any single-phase load is equally divided across each transformer. With this assumption, the kVA rating of the three individual transformers can be determined by the following equation: 𝑇𝑇 𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇 𝑇𝑇𝑇𝑇 𝑇𝑇 𝑘𝑘𝑘𝑘𝑘𝑘 = 1𝜙𝜙 𝑘𝑘𝑘𝑘𝑘𝑘+3𝜙𝜙 𝑘𝑘𝑘𝑘𝑘𝑘 3 For this transformer bank, individual transformer impedances do not need to be matched.
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