unit 29 three-phase...
TRANSCRIPT
Unit 29 Three-Phase Transformers
Objectives:
• Discuss the construction of three-phase transformers.
• Discuss the formation of a three-phase transformer bank.
• Discuss primary and secondary connections.
Unit 29 Three-Phase Transformers
Objectives:
• 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.
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
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 three-phase 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
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
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
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
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.
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.
Unit 29 Three-Phase Transformers
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.
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:
4. The center connection of a wye is often
tapped to provide a neutral conductor for
three-phase loads. This produces a three-
phase four-wire system. Common
voltages produced by this type of
connection are 208/120 and 80/277.
Unit 29 Three-Phase Transformers
Review:
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.