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Unit 29 Three-Phase Transformers
Construction
• A three-phase transformer is constructed
by winding three single-phase
transformers on a single core.
• Three-phase transformers are connected
in either wye or delta configurations.
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Unit 29 Three-Phase Transformers
Three-phase transformer construction.
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Unit 29 Three-Phase Transformers
Three-phase transformer (Courtesy of Waukesha Electric Systems).
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Unit 29 Three-Phase Transformers
Transformer Bank
• Two or three single-phase transformers can be connected together to deliver three-phase power. This is referred to as a transformer bank.
• This allows greater maintenance and replacement options.
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Unit 29 Three-Phase Transformers
Primary and Secondary Connections
• The primary (input) side of a three-phase transformer can be connected in a wye or delta configuration.
• The secondary (output) side of a three-phase transformer can also be connected in a wye or delta configuration.
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Unit 29 Three-Phase Transformers
Primary and Secondary Connections
This allows four basic connection patterns:
• Wye-Wye
• Delta-Delta
• Wye-Delta
• Delta-Wye
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Unit 29 Three-Phase Transformers
Wye-delta three-phase connection schematic.
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Unit 29 Three-Phase Transformers
Delta-wye three-phase connection schematic.
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Unit 29 Three-Phase Transformers
Winding identification.
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Unit 29 Three-Phase Transformers
Transformer bank without connections.
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Unit 29 Three-Phase Transformers
Transformer bank connected wye-delta.
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Unit 29 Three-Phase Transformers
One-line Diagram
One-line diagrams are simplified schematics used to show the main power distribution systems for commercial or industrial settings.
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Unit 29 Three-Phase Transformers
Industrial one-line power distribution schematic.
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Unit 29 Three-Phase Transformers
Safety test prior to closing the delta connection.
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Unit 29 Three-Phase Transformers
Example #1 schematic with all values.
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Unit 29 Three-Phase Transformers
Example #2 schematic with all values.
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Unit 29 Three-Phase Transformers
Harmonics
Harmonics are voltages or currents that
operate at frequencies that are multiples
of the fundamental power frequency. The
fundamental frequency is usually 50 or
60 Hz.
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Unit 29 Three-Phase Transformers
Sine wave distortion caused by harmonics.
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Unit 29 Three-Phase Transformers
Pulse Mode Power Supplies
Many power supplies for computers and
office machines use pulse-width
modulation. This can contribute to
harmonic and power quality problems.
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Unit 29 Three-Phase Transformers
Pulse-width modulation schematic.
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Unit 29 Three-Phase Transformers
Harmonic Reduction
1. Reduce the neutral current in three-
phase systems by balancing the currents
in the phase legs.
2. Reduce triplen harmonics on the neutral
by adding filters.
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Unit 29 Three-Phase Transformers
Harmonic Reduction
3. Avoid shared neutrals and provide a separate neutral for each phase conductor.
4. Install larger neutrals, especially on multiphase circuits using a common neutral.
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Unit 29 Three-Phase Transformers
Harmonic Reduction
5. Derate the load capacity of transformers
and consider buying harmonic-rated
transformers.