THREE-PHASE
TRANSFORMER
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INTRODUCTION
A three-phase transformer can be obtained in two
different ways.
 Three numbers of identical single-phase transformers
can be suitably connected to make a three-phase
transformer. Such three-phase transformers are called
a bank of three-phase transformer.
 Alternatively, a three-phase transformer can be
constructed as a single unit.
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ADVANTAGES OF SINGLE UNIT
THREE-PHASE TRANSFORMER
1. It occupies less space
2. Its weight is less
3. Its cost is comparatively less
4. It is much easier to transport
5. It is more efficient
6. Core size is comparatively small.
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DISADVANTAGES OF SINGLE UNIT
THREE-PHASE TRANSFORMER
 When one of the phases of a single-unit three-phase
transformer becomes faulty, the entire unit of threephase transformer needs to be removed from the
supply for repair.
 But in the case of a bank of three-phase transformer,
only the faulty transformer is removed from the supply
for repair while the other two transformers remain
connected in the system without interrupting the
supply completely.
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CONSTRUCTION
Shell type
Core type
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Star–Star (Υ–Υ) Connection of
Three-phase Transformer
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Delta–Delta (Δ–Δ) Connection of
Three-phase Transformer
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Delta–Star (Δ –Υ) Connection of
Three-phase Transformer
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Star–Delta (Υ–Δ) Connection of
Three-phase Transformer
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Open Delta (V–V) Connection of Threephase Transformer
Some of the features of open delta connection are
• The power handling capacity of open delta circuit is 0.577 times that of
delta–delta circuit.
• The secondary terminal voltage becomes unbalanced with increase in
load.
• The power factor is 86.6% of the balanced load factor.
• The two transformers in open delta connection will be operated at
different power factor.
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Scott (T–T) Connection of Transformer
It is a type of circuit used to derive two-phase
electric power from a three-phase source, or viceversa. The Scott connection evenly distributes a
balanced load between the phases of the source.
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CONVERSION FROM THREE-PHASE
TO TWO-PHASE
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Double Star Connection
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Double Delta Connection
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Six-phase Star Connection
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Diametric Connection
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Advantages of using tertiary
winding in transformer
i) This transformer has the ability to
redistribute the flow of fault current
ii) It reduces the unbalancing in the primary
due to unbalancing in three-phase load.
iii) Auxiliary load at different voltage levels
from the tertiary winding can be supplied
from this transformer in addition to its main
secondary load.
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