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

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 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

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 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.