unit 29 three-phase...

Unit 29

Three-Phase Transformers

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.


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.


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.


Three-phase transformer construction.


Three-phase transformer (Courtesy of Waukesha Electric Systems).


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.


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.


Primary and Secondary Connections

This allows four basic connection patterns:

• Wye-Wye

• Delta-Delta

• Wye-Delta

• Delta-Wye


Wye-delta three-phase connection schematic.


Delta-wye three-phase connection schematic.


Winding identification.


Transformer bank without connections.


Transformer bank connected wye-delta.


One-line Diagram

One-line diagrams are simplified schematics used to show the main power distribution systems for commercial or industrial settings.


Industrial one-line power distribution schematic.


Safety test prior to closing the delta connection.


Example #1 schematic with all values.


Example #2 schematic with all values.


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.


Sine wave distortion caused by harmonics.


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.


Pulse-width modulation schematic.


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.


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.


Harmonic Reduction

5. Derate the load capacity of transformers

and consider buying harmonic-rated

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.


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.


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.


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.


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

three-phase four-wire systems.

unit 29 three-phase...

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