Unit 29 Three-Phase Transformers

Transcription

1 Unit 29 Three-Phase Transformers

2 Objectives: Discuss the construction of three-phase transformers. Discuss the formation of a three-phase transformer bank. Discuss primary and secondary connections.

3 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.

4 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.

5 Three-phase transformer construction.

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

7 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.

8 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.

9 Primary and Secondary Connections This allows four basic connection patterns: Wye-Wye Delta-Delta Wye-Delta Delta-Wye

10 Wye-delta three-phase connection schematic.

11 Delta-wye three-phase connection schematic.

12 Winding identification.

13 Transformer bank without connections.

14 Transformer bank connected wye-delta.

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

16 Industrial one-line power distribution schematic.

17 Safety test prior to closing the delta connection.

18 Example #1 schematic with all values.

19 Example #2 schematic with all values.

20 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.

21 Sine wave distortion caused by harmonics.

22 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.

23 Pulse-width modulation schematic.

24 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.

25 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.

26 Harmonic Reduction 5. Derate the load capacity of transformers and consider buying harmonic-rated transformers.

27 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.

28 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.

29 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.

30 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.

31 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.