Unit 29 Three-Phase Transformers
Unit 29
Three-Phase Transformers
Construction
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Unit 29 Three-Phase Transformers
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
Objectives:
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Discuss the construction of three-phase
transformers.
Discuss the formation of a three-phase
transformer bank.
Discuss primary and secondary connections.
Three-phase transformer construction.
Unit 29 Three-Phase Transformers
Unit 29 Three-Phase Transformers
Objectives:
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Discuss different types of three-phase transformer
connections.
Show one-line diagram schematics.
Discuss power quality issues relating to harmonics.
Identify harmonic characteristics.
Discuss solutions to power quality problems
associated with harmonics.
Three-phase transformer (Courtesy of Waukesha Electric Systems).
Unit 29 Three-Phase Transformers
Unit 29 Three-Phase Transformers
Transformer Bank
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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.
Wye-delta three-phase connection
schematic.
Unit 29 Three-Phase Transformers
Unit 29 Three-Phase Transformers
Primary and Secondary Connections
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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.
Delta-wye three-phase connection schematic.
Unit 29 Three-Phase Transformers
Unit 29 Three-Phase Transformers
Primary and Secondary Connections
This allows four basic connection patterns:
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Wye-Wye
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Delta-Delta
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Wye-Delta
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Delta-Wye
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
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
One-line Diagram
One-line diagrams are simplified schematics
used to show the main power distribution
systems for commercial or industrial settings.
Example #1 schematic with all values.
Unit 29 Three-Phase Transformers
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.
Example #2 schematic with all values.
Unit 29 Three-Phase Transformers
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.
Pulse-width modulation schematic.
Unit 29 Three-Phase Transformers
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.
Sine wave distortion caused by harmonics.
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
Unit 29 Three-Phase Transformers
Review:
3. It is common practice to center-tap one of the
transformers in a delta connection to provide
power for single-phase loads. When this is
done, the remaining phase connection
becomes a high leg.
Unit 29 Three-Phase Transformers
Review:
Harmonic Reduction
5. Derate the load capacity of transformers and
consider buying harmonic-rated
transformers.
Unit 29 Three-Phase Transformers
4. The center connection of a wye is often
tapped to provide a neutral conductor for
three-phase loads. This produces a threephase four-wire system. Common voltages
produced by this type of connection are
208/120 and 80/277.
Unit 29 Three-Phase Transformers
Review:
Review:
1. Three-phase transformers are constructed by
winding three separate transformers on the
same core material.
2. Single-phase transformers can be used as a
three-phase transformer bank by connecting
their primary and secondary windings as either
wyes or deltas.
5. Harmonics are generally caused by loads that
pulse the powerline.
6. Harmonic distortion on a single-phase line is
often caused by computer power supplies,
copy machines, fax machines, and light
dimmers.
Unit 29 Three-Phase Transformers
Review:
7. Harmonic distortion on three-phase power
lines is generally caused by variable frequency
drives and electronic DC drives.
8. Triplen harmonics generally cause
overheating of the neutral conductor on threephase four-wire systems.