3 Transformer

1
3? Transformer
BEE2123 ELECTRICAL MACHINES

• Muhamad Zahim Sujod
• Ext 2312
• A1-01-06
• zahim_at_ump.edu.my

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Learning Outcomes

• At the end of the lecture, student should be able
  to
• Understand the principle and the nature of 3
  phase transformer.
• Perform an analysis on 3 phase transformers which
  their principles are basic to the understanding
  of electrical machines.

3
Introduction

• A 3? transformer is essentially 3 single phase
  transformer wound on a common core.
• The geometry of the core is such that the fluxes
  of the phases share common paths as a result,
  the volume of iron in 3? transformer is less than
  the volume of iron of 3 single phase transformer
  of the same rating. Hence, the cost is reduced.
• ? any phase of a 3? transformer behaves exactly
  like a single phase transformer.
• In 3? transformer, the primary and secondary
  windings can be connected either in Y or ?.
• The possible connection are
• Primary Secondary
• ? ?
• ?
  Y
• Y
  ?
• Y
  Y

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Transformer Construction

• 3 Single Phase Transformer

• Used a lot in older transformer
• adv- each unit in the bank can be replaced
• individually

5
Transformer Construction

• 3? Transformer wound a single three legged iron
  core

• more preferred in practice
• adv- lighter, smaller, cheaper, slightly more
• efficient

• iron core must be laminated
• to reduce eddy current loss
• iron must be made of soft iron
• to reduce hysteresis loss
• cooling the transformer - use fan or heat sink

• soft iron must be add with 3-5 silicon - to
  increase the saturation flux in the
  iron
• The amount of silicon must not too high
  otherwise the iron will be brittle
• clamp the lamination - to reduce the humming
  noise

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Delta (?) Delta (?) Connection

• This connection is economical for large,
  low-voltage transformers in which insulation
  problem is not so urgent.

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Delta (?) Delta (?) Connection (Schematic
Diagram)
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Delta (?) Delta (?) Connection (Schematic
Diagram)
Refer to primary,
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Star (Y) Star (Y) Connection

• This connection is the most economical for small,
  high voltage transformers.

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Star (Y) Star (Y) Connection (Schematic Diagram)
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Star (Y) Star (Y) Connection (Schematic Diagram)
Refer to primary,
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3rd harmonic phenomenon and unbalanced loading

• The Y-Y connection has two very serious problems
• If loads on the transformer circuit are
  unbalanced, then the voltages on the phases of
  the transformer can become severely unbalanced
• There is a serious problem with third harmonic
  voltages
• Solution to the problem
• Solidly ground the neutrals of the transformer
• Introduced a third winding connected in ?

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Star (Y) Delta (?)

• This type of connection is mainly used for the
  system where we need to step-down the voltage.

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Star (Y) Delta (?) Connection (Schematic
Diagram)
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Star (Y) Delta (?) Connection (Schematic
Diagram)
Refer to primary,
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Delta (?) Star (Y)

• This type of connection is mainly used for the
  system where we need to step-up the voltage.

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Delta (?) Star (Y) (Schematic Diagram)
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Delta (?) Star (Y) (Schematic Diagram)
Refer to primary,
X01
R01
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Auto-Transformer

• Auto-Transformer is a transformer with only one
  winding.
• That means, the primary and secondary windings
  are not electrically isolated form each other. In
  other words, the primary and the secondary of
  autotransformer are physically connected.
• The theory is almost the same as two winding
  transformer.

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Auto-Transformer (Equivalent Circuits)
Step-up Autotransformer
Step-down Autotransformer
Multiple Tap Autotransformer
Multiple Tap Autotransformer with load
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Auto-Transformer (Analysis for Step-up Autotrans)
V1
N1
V1
N1
Two winding Transformer
Connection as an Auto-Transformer
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Auto-Transformer (Analysis for Step-up Autotrans)
1. Voltage Rating
2. Current Rating
3. Autotrans Turns-Ratio
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Auto-Transformer (Analysis for Step-up Autotrans)
4. kVA Rating
5. Efficiency
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Auto-Transformer (Example)

• A single phase, 100kVA, 2000/200V two-winding
  transformer is connected as an autotransformer as
  shown in Figure 1 such that 2000V is obtained at
  secondary. The portion ab is the 200V winding,
  and the portion bc is the 2000V winding. Compute
  the kVA rating as an autotransformer. Also
  calculate the efficiency for a given losses of
  2800W at 0.866 lagging power factor.

Figure 1
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Auto-Transformer (Example)
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Advantages and Drawback of Auto-Transformer

• Advantages
• The secondary winding is as part of primary
  winding. Hence, autotransformer eliminates the
  need for a separate secondary winding.
• As a results, autotransformers are always
  smaller, lighter, and cheaper.

• Drawback
• Absence of electrically isolation is a serious
  drawbacks in some applications

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Instrument Transformer

• Voltage Transformer (Potential Transformer)
• Is used to measure or monitor the voltage on
  transmission lines and to isolate the metering
  equipment from the lines.

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Instrument Transformer

• Current Transformer
• Is used to measure or monitor the current in
  transmission lines and to isolate the metering
  equipment and relay connected to secondary side.

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Other Types of Transformer

• High Frequency Transformer
• In electronic power supplies, there is always a
  need to isolate the output from the input and to
  reduce the weight and cost of unit.
• The objective are best achieved by using a
  relatively high frequency compared to normal
  frequency of 50Hz / 60Hz.