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.
Unit 29 Three-Phase Transformers
Three-phase transformer construction.
Unit 29 Three-Phase Transformers
Three-phase transformer (Courtesy of Waukesha Electric Systems).
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.
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 threephase transformer can also be connected
in a wye or delta configuration.
Unit 29 Three-Phase Transformers
Primary and Secondary Connections
This allows four basic connection patterns:
• Wye-Wye
• Delta-Delta
• Wye-Delta
• Delta-Wye
Unit 29 Three-Phase Transformers
Wye-delta three-phase connection schematic.
Unit 29 Three-Phase Transformers
Delta-wye three-phase connection schematic.
Unit 29 Three-Phase Transformers
Winding identification.
Unit 29 Three-Phase Transformers
Transformer bank without connections.
Unit 29 Three-Phase Transformers
Transformer bank connected wye-delta.
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.
Unit 29 Three-Phase Transformers
Industrial one-line power distribution schematic.
Unit 29 Three-Phase Transformers
Safety test prior to closing the delta connection.
Unit 29 Three-Phase Transformers
Example #1 schematic with all values.
Unit 29 Three-Phase Transformers
Example #2 schematic with all values.
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.
Unit 29 Three-Phase Transformers
Sine wave distortion caused by harmonics.
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.
Unit 29 Three-Phase Transformers
Pulse-width modulation schematic.
Unit 29 Three-Phase Transformers
Harmonic Reduction
1. Reduce the neutral current in threephase systems by balancing the currents
in the phase legs.
2. Reduce triplen harmonics on the neutral
by adding filters.
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.
Unit 29 Three-Phase Transformers
Harmonic Reduction
5. Derate the load capacity of transformers
and consider buying harmonic-rated
transformers.