Unit 28 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 28 Three-Phase Transformers Objectives: • Discuss different types of three-phase transformer connections. • Show one-line diagram schematics.

Unit 28 Three-Phase Transformers Objectives: • Discuss power quality issues relating to harmonics. • Identify harmonic characteristics. • Discuss solutions to power quality problems associated with harmonics.

Unit 28 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 28 Three-Phase Transformers

Three-phase transformer construction.

Unit 28 Three-Phase Transformers

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

Unit 28 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 28 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 28 Three-Phase Transformers Primary and Secondary Connections This allows four basic connection patterns: • Wye-Wye • Delta-Delta • Wye-Delta • Delta-Wye

Unit 28 Three-Phase Transformers

Wye-delta three-phase connection schematic.

Unit 28 Three-Phase Transformers

Delta-wye three-phase connection schematic.

Unit 28 Three-Phase Transformers

Winding identification.

Unit 28 Three-Phase Transformers

Transformer bank without connections.

Unit 28 Three-Phase Transformers

Transformer bank connected wye-delta.

Unit 28 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 28 Three-Phase Transformers

Industrial one-line power distribution schematic.

Unit 28 Three-Phase Transformers

Safety test prior to closing the delta connection.

Unit 28 Three-Phase Transformers

Example #1 schematic with all values.

Unit 28 Three-Phase Transformers

Example #2 schematic with all values.

Unit 28 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 28 Three-Phase Transformers

Sine wave distortion caused by harmonics.

Unit 28 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 28 Three-Phase Transformers

Pulse-width modulation schematic.

Unit 28 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 28 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 28 Three-Phase Transformers Harmonic Reduction 5. Derate the load capacity of transformers and consider buying harmonic-rated transformers.

Unit 28 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 28 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 28 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 threephase four-wire system. Common voltages produced by this type of connection are 208/120 and 80/277.

Unit 28 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.

Unit 28 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 three-phase four-wire systems.