1. The document discusses parallel operation of transformers and auto transformers. Parallel operation connects multiple transformer primary windings to a common voltage supply and secondary windings to a common load. This allows for continuity of supply, operating at maximum capacity, and adjusting to load requirements. 2. An auto transformer uses a common winding for both the primary and secondary circuits. This makes it cheaper and more efficient than an ordinary two-winding transformer but provides less isolation and can be damaged more easily by faults. 3. Auto transformers are used as starters for induction motors, to provide a small boost to correct voltage drops, and for voltage regulation in power transmission and distribution systems.

1. Transformers
1. Parallel operation
2. Auto transformer

2. Parallel operation:
The Transformer is said to be in Parallel Operation when its primary winding is
connected to a common voltage supply, and the secondary winding is connected to a
common load.

3. Necessity of parallel operation:
1. Continuity of supply
2. Operating them in maximum capacity
3. Easy maintenance
4. Connecting and disconnecting according to load requirements
5. Decreasing iron losses
6. Increasing the life of the units
7. Increasing the capacity of sub-station by adding extra units
8. No stand by units are required

4. Conditions for parallel operation:
1. Same polarity
2. Same voltage ratio and turns ratio
3. Same frequency
4. Same percentage impedance and X/R ratio
5. Same phase sequence (incase of three phase transformers)

5. Case I: Equal voltage ratio

6. Case II: Unequal voltage ratio

7. Auto transformer:
1. A part of the winding is common to both the
primary and secondary circuits.
2. Two windings are electrically connected and it
works on the principle of induction and
conduction.

8. Saving of copper in auto transformer as
compared to ordinary two winding
transformer:

9. Transfer of power in an auto transformer:
Power transferred inductively= Input X (1-K)
Power transferred conductively= Input X K

10. Advantages:
1. Cheaper
2. More efficient, because losses are less as compared to ordinary two winding
transformer of the same rating.
3. Better voltage regulation
Disadvantages:
1. it does not have the primary to secondary winding isolation of a conventional
double wound transformer.
2. If the secondary side winding becomes open-circuited, load current stops flowing
through the primary winding stopping the transformer action resulting in the full
primary voltage being applied to the secondary terminals.
3. If the secondary circuit suffers a short-circuit condition, the resulting primary
current would be much larger than an equivalent double wound transformer due to
the increased flux linkage damaging the autotransformer.
4. Since the neutral connection is common to both the primary and secondary
windings, earthing of the secondary winding automatically Earth’s the primary as
there is no isolation between the two windings. Double wound transformers are
sometimes used to isolate equipment from earth.
5. Used only in the limited places where a slight variation of the output voltage from
input voltage is required.

11. Applications:
1. It is used as a starter to give up to 50 to 60% of full voltage to the stator of a
squirrel cage induction motor during starting.
2. It is used to give a small boost to a distribution cable, to correct the voltage drop.
3. It is also used as a voltage regulator
4. Used in power transmission and distribution system and also in the audio system
and railways.