Presented byTravis Johnson

Manager, Electric Distribution StandardsManager, Design Strategy and Performance

HOW TO BUILD/REVISE A TRANSFORMER SPECIF ICATION

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THE BASICS

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• Overhead/Padmount• Primary Voltage• Secondary Voltage• Wye/Delta• kVA• Bushings: 1 or 2 (or more..)• To Tap or Not to Tap

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PRIMARY VOLTAGE

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• Remember the square root of 3? 1.73• System Voltage: 15kV, 25kV, 35kV• Wye (15 kV system examples)

– Phase to Phase: 12.47 kV, 13.2 kV, 13.8 kV– Phase to Ground: 7.2 kV, 7.62 kV, 7.97 kV

• Delta– Phase to Phase: 2.4 kV, 13.2 kV

• How did we get so many voltages…

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SECONDARY VOLTAGES

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• Single Phase – 120/240 V, 240/480 V– 240/120 V, 480/240 V– 240 V, 480 V

• Three Phase– 120/208 V, 277/480 V (Wye)– 120/240 V, 240/480 V (Delta)

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INDUSTRY SPECIFICATIONS

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• IEEE C57– C57.12.00-2015 - IEEE Standard for General Requirements for Liquid-

Immersed Distribution, Power, and Regulating Transformers– C57.12.20 - IEEE Standard for Overhead-Type Distribution Transformers

500 kVA and Smaller; High Voltage, 34 500 V and Below; Low Voltage, 7970/13 800Y V and Below

– C57.12.35-2013 - IEEE Standard Bar Coding for Distribution Transformers and Step-Voltage Regulators

– C57.12.37 - IEEE Standard for the Electronic Reporting of Distribution Transformer Test Data

• ANSI (publishes, doesn’t make specifications)• NEMA• ASTM• These make your life easier. (Thank your

colleagues)

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PRIMARY BUSHINGS

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• Bushing Definition: An electrically insulating lining for a hole to protect a through conductor

• Usually Porcelain• Pole Mount Transformers

– Typically mounted on lid of tank– Can be mounted on Side of tank

• BIL(Basic Insulation/Impulse Level)– (95kV BIL, 125kV BIL, etc)– Voltage withstand– Lightning withstand

• Creepage

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SECONDARY BUSHINGS

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• Bushing Definition: An electrically insulating lining for a hole to protect a through conductor

• Usually Polymer in North America• Usually Porcelain in South America• Pole Mount Transformers

– Typically mounted on Side of tank– Can be rewired for different voltages

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WHY USE TAPPED TRANSFORMERS?

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• Flexibility• Cover 5 voltages with 1 transformer• Type (2) 2.5% up, (2) 2.5% down• But can be any 5 you want

– e.g.,8001, 7811, 7620,7430, 7200

• Downsides– Reliability (small impact)– Cost

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ARRESTERS

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• Rated to protect your transformer from voltage surges, lightning strikes, etc. Apply appropriately for your type of construction and BIL of the transformer.

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INSULATION – PAPER AND WOOD

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• Paper (and paper-like alternatives)– Degrade over time– Water– Overload

• Wood spacers to make cooling channels for oil circulation

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INSULATION

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• Insulating Fluid– Mineral Oil (petroleum based)

• Inexpensive (type), stays liquid at low temp– Natural Ester coolant

• Higher flash point than Mineral Oil• Operational Concerns (taps)

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ELECTRIC – KVA, IMPEDANCE, LOSSES

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• kVA – Power rating of Transformer• Impedance

– Limit fault current – Limit losses– Department of Energy impacts

• Load Losses (C57.12.00)• No-Load Losses (C57.12.00)

– Temperature, Total losses (in Watts)

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TANK CONSTRUCTION

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• Bracket (ANSI calls for 11-1/4”)• Welds• Gaskets• Make sure appropriate for your

insulation fluid and running temperature

Q U E S T I O N S ?