It is the three phase system which has been
adopted world over to generate, transmit and distribute electrical power.
Therefore to change the level of voltages in the system three phase
transformers should be used.

Three number of identical single phase transformers can be suitably connected for use in a three phase system and such a three phase transformer is called a bank of three phase transformer. Alternatively, a three phase transformer can be constructed as a single unit

Three number of identical single phase transformers can be suitably connected for use in a three phase system and such a three phase transformer is called a bank of three phase transformer. Alternatively, a three phase transformer can be constructed as a single unit

In a single phase transformer, we have only two coils
namely primary and secondary. Primary is energized with single phase supply and
load is connected across the secondary. However, in a 3phase transformer there
will be 3 numbers of primary coils and 3 numbers of secondary coils. So these 3
primary coils and the three secondary coils are to be properly connected so
that the voltage level of a balanced 3-phase supply may be changed to another
3-phase balanced system of different voltage level.Suppose you take three identical transformers
each of rating 10 kVA, 200 V / 100 V, 50 Hz and to distinguish them call them
as A, B and C. For transformer-A, primary terminals are marked as A1A2 and the
secondary terminals are marked as a1a2. The markings are done in such a way
that A1 and a1 represent the dot (•) terminals. Similarly terminals for B and C
transformers are marked

**THREE PHASE TRANSFORMER WINDING DIAGRAM**

It may be noted that individually each
transformer will work following the rules of single phase transformer i.e,
induced voltage in a1a2 will be in phase with applied voltage across A1A2 and
the ratio of magnitude of voltages and currents will be as usual decided by a
where a = N1/N2 = 2/1, the turns ratio. This will be true for transformer-B and
transformer-C as well i.e., induced voltage in b1b2 will be in phase with
applied voltage across BB1B2B and induced voltage in c1c2 will be in phase with
applied voltage across C1C2.

Now let us join the terminals A2, BB2 and C2 of
the 3 primary coils of the transformers and no inter connections are made
between the secondary coils of the transformers. Now to the free terminals A1,
B1B and C1 a balanced 3-phase supply with phase sequence A-B-C is connected

If the line voltage of the supply is V =200*1.73
V

*LL*, the magnitude of the voltage impressed across each of the primary coils will be 3 times less i.e., 200 V. However, the phasors 12*AAV*12BBVand 12CCVwill be have a mutual phase difference of 120º Then from the fundamental principle of single phase transformer we know, secondary coil voltage 12aaVwill be parallel to 12AAV; 12bbVwill be parallel to 12BBVand 12ccVwill be parallel to 12CCV. Thus the secondary induced voltage phasors will have same magnitude i.e., 100 V but are displaced by 120º mutually. The secondary coil voltage phasors 12aaV, 12bbVand 12ccV are shown in figure 26.2. Since the secondary coils are not interconnected, the secondary voltage phasors too have been shown independent without any interconnections between them. In contrast, the terminals A2, B2 and C2 are physically joined forcing them to be equipotential which has been reflected in the primary coil voltage phasors as well where phasor points A2, B2 and C2 are also shown joined. Coming back to secondary, if a voltmeter is connected across any coil i.e., between a1 and a2 or between b1 and b2 or between c1 and c2 it will read 100 V. However, voltmeter will not read anything if connected between a1 and b1 or between b1 and c1 or between c1 and a1 as open circuit exist in the paths due to no physical connections between the coils
Imagine now the secondary coil terminals a2, b2
and c2 are joined together physically

So the secondary coil phasors should not be
shown isolated as a2, b2 and c2 become equipotential due to shorting of these
terminals. Thus, the secondary coil voltage phasors should not only be parallel
to the respective primary coil voltages but also a2, b2 and c2 should be
equipotential. Therefore, shift and place the phasors 12aaV, 12bbVand 12ccVin
such a way that they remain parallel to the respective primary coil voltages
and the points a2, b2 and c2 are superposed.

Here obviously, if a voltmeter is connected between a1 and
b1 or between b1 and c1 or between c1
and a1 it will read corresponding phasor lengths a1b1 or b1 c1 or c1a1 which are
all equal to 200 3V. Thus, Va b11 , b c12V
and 2c a1V are of same magnitude and
displaced mutually by 120º to form a balanced 3-phase voltage system. Primary
3-phase line to line voltage of 200 3V is
therefore stepped down to 3-phase, 100 3V
line to line voltage at the secondary. The junction of A2, BB2 and C2 can be
used as primary neutral and may be denoted by N. Similarly the junction of a2,
b2 and c2 may be denoted by n for secondary neutral.

Star-delta connection

To connect windings in delta, one should be
careful enough to avoid dead short circuit. Suppose we want to carry out star /
delta connection with the help of the above single phase transformers. HV
windings are connected by shorting A2, BB2 and C2 together

As we know, in delta connection, coils are
basically connected in series and from the junction points, connection is made
to supply load. Suppose we connect quite arbitrarily (without paying much
attention to terminal markings and polarity), a1 with b2 and b1 with c1. Should
we now join a2 with c2 by closing the switch S, to complete the delta
connection? As a rule, we should not join (i.e., put short circuit) between any
two terminals if potential difference exists between the two. It is equivalent
to put a short circuit across a voltage source resulting into very large
circulating current. Therefore before closing S, we must calculate the voltage
difference between a2 with c2. To do this, move the secondary voltage phasors such
that a1 and b2 superpose as well as b1 with c1 superpose - this is because a1
and b2 are physically joined to make them equipotential; similarly b1 and c1
are physically joined so as to make them equipotential. The phasor diagram is

. If a voltmeter is connected across S (i.e.,
between a2 and c2), it is going to read the length of the phasorV. By referring
to phasor 2diagram of figure 26.9, it can be easily shown that the voltage across
the switch S, under this condition isV= a c o 100+ 2cos60 100 = 200V . So this
connection is not proper and the switch S should not be closed.