Three Phase Transformer
Three-phase transformer consists of three primary coils and three secondary coils and is represented as 3-phase
or 3ɸ. A three-phase system can be constructed using three individual identical single-phase transformers, and
such a 3-phase transformer is known as the bank of three transformers. On the other hand, the three-phase
transformer can be built on a single core. The windings of a transformer can be connected in either delta or wye
configurations. The working of the 3-phase system is similar to a single-phase transformer, and they are
normally employed in power generation plants.
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Disadvantages of this system are:
This system is more expensive than a single unit of three phase transformer of equivalent
capacity.
This system is less efficient than a single unit of three phase transformer of equivalent capacity.
This system is occupying more space than a single unit of three phase transformer of equivalent
capacity.
Some advantages of this system are:
Where single unit of three phase transformer is very large for transportation to the site in the
hilly region, three units of single-phase transformer would be easier for transportation.
If it is required to have a stand by unit for better reliability of supply continuity during the
maintenance period, only one unit of single-phase transformer can be installed as a spare unit.
Whereas in case of three phase transformer, the whole three phase unit has to be installed as
spare unit. Hence, the investment on spare will be less in this system.
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Advantages of Three Phase Transformer over Bank of Three Units
It requires smaller quantity of iron and copper. Hence, its cost is nearly 15% lesser than a bank
of three single phase transformers of equal rating.
It has smaller size and can be accommodated in smaller tank and hence needs smaller quantity
of oil for cooling.
Because of smaller size, it occupies less space; moreover, it has less weight.
It needs a smaller number of bushings.
It operates at slightly better efficiency and regulation.
These transformers suffer from the following disadvantage.
It is more difficult and costly to repair three-phase transformers.
It is difficult to transport single large unit of three-phase transformer than to transport three
single phase transformers individually.
Three Phase Transformer Connections
1. Star-Star Connection
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2. Delta-Delta Connection
Advantages of D-D Connection
3. Star-Delta Connection
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4. Delta Star Connection
Advantages:
Due to D connection in primary side, winding
cross section required is less.
On secondary side, neutral is available, due to
which it can be used for 3-phase and 4-wire
system
There is no distortion due to third harmonic
component
Windings connected in star make it economical
due to saving in insulation required
Large unbalanced loads can be handled without
any difficulty
Disadvantages
Due to phase shift between primary and secondary,
difficulty in parallel connection with other
combination of T/Fs
Summary:
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Disadvantages of Open D Connection
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Power and Distribution Transformer
Distribution Transformer
An electrical transformer which is employed in the power system near the load points for
stepping-down the high voltage to the safest rated utilization voltage is known as distribution
transformer. Therefore, the distribution transformer is a step-down transformer which reduces
the supply voltage to the voltage level demanded by the consumer’s load (both 1-phase and 3-
phase). In actual practice, the voltage ratings of the distribution transformers are 11 kV, 6.6 kV,
3.3 kV, 440 V, 120 V and 110 V. Distribution transformers are designed in such a way that they
have maximum efficiency at 60% to 70% of load.
Power Transformer
An electrical transformer having very high MVA rating and is used to transmit high electric
power at high voltage level through the power lines to the distribution centres is known as
power transformer. Generally, a power transformer is rated over 200 MVA with voltage rating
of 33 kV, 66 kV, 132 kV, 220 kV, 440 kV, and so on. The power transformer are mainly used
for increasing the low voltage level (at generating end) to a high voltage level for the
transmission purpose. The power transformers are designed in such a way that they have
maximum efficiency at or near the full load.
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