Transformers & its Characteristics

2. 3.21 3-Phase Transformer Construction, Principal,
Working, Operation Advantages Over 1-Phase
Transformer
• Introduction
• Advantages
• Construction
• Principal
• Working

3. Introduction
• The generation of an electrical power is usually three
phase and at higher voltages like 13.2 KV, 22 KV or some
what higher.
• Similarly transmission of an electrical power is also at very
high voltages like 110 KV, 132 KV, 400 KV.
• To step up the generated voltages for transmission
purposes it is necessary to have three phase transformers.

4. Advantages
• Less space
• Weight Less
• Cost is Less
• Transported easily
• Core will be smaller size
• More efficient
• Structure, switchgear and installation of single three phase
unit is simpler

5. Principal of Operation

6. • The three cores are arranged at 120° from each other. Only
primary windings are shown on the cores for simplicity.
• The primaries are connected to the three phase supply.
• The three fluxes is also zero at any instant.

7. • Hence the centre leg does not carry any flux.
• So if centre leg is removed, any two legs provide the return
path for the current and hence the flux in the third leg.
• This is the general principal used in the design of three
phase core type transformers.

8. Three Phase Transformer Connections
• The primary and secondary winding of three phase
transformers as three phase winding can be connected in
different ways such as in star or in delta. With suitable
connection the voltage can be raised or lowered.
• In this section some commonly used connections for three
phase transformers are discussed.

9. • Star-Star connection
• Delta-Delta connection
• Star-Delta connection
• Delta-Star connection

13. Parallel Operation of Three Phase Transformer
• The transformers are connected in parallel when load on
one of the transformers is more then it capacity
• The reliability is increased with parallel operation than to
have single larger unit.

14. • The Transformers connected in parallel must have same
polarity so that the resultant voltage around the local
loop is zero. With improper polarities there are chances of
dead short circuit.
• The relative phase displacements on the secondary sides
of the three phase transformers to be connected in
parallel must be zero. The transformers with same phase
group can be connected in parallel

15. • As the phase shift between the secondary voltages of a
star/delta and delta/star transformers is 30°, They cannot
be connected in parallel.
• But transformers with +30° and -30° phase shift can be
connected in parallel by reversing phase sequence of one
of them

16. • The voltage ratio of the two transformers must be same.
This prevents no load circulating current when the
transformers are in parallel on primary and secondary
sides.
• As the leakage impedance is less, with a small voltage
difference no load circulating current is high resulting in
large I2R losses.

17. Chap 1: Transformer 17
3.15 Transformer applications
1. Voltage level adjustment (step-up and step-down transformers).
2. Voltage and current measurement.
3. Isolation for safety (isolation transformers)
4. Impedance matching (for maximum power transfer from the source
to the load)
The resistance of the load, as seen from the primary-side of the transformer by the source, equal to the
internal source resistance. In other words, the objective is to realize: Rin = Rs.

18. 3.14 Name Plate Rating
• Used to specify the voltages and current
In transformer, both voltages in the primary
and secondary side should be in the range of
maximum safe voltages.
If current varies, the temperature in the coil of
the conductor changes, which may lead to
damage. Hence, the current should be in the
maximum range.

19. 3.16 ACCOUNTING FOR FINITE
PERMEABILITY AND LOSSES

20. The Magnetization Current in a Real Transformer
Io
Ic
IM
E1
f
qo
When an ac power source is connected to the primary of a transformer, a
current flows in its primary circuit, even when there is no current in the
secondary. The transformer is said to be on no-load. If the secondary current is
zero, the primary current should be zero too. However, when the transformer
is on no-load, excitation current flows in the primary because of the core
losses and the finite permeability of the core.
Magnetization current IM
(current required to produce flux
in the core)
Core-loss current Ih+e
(current required to make up for
hysteresis and eddy current losses)
Excitation current, Io
IM is proportional to the flux f
Ic = Ih+e = Core loss/E1

21. The losses of a Transformer
The losses that occur in transformers have to be accounted for in any
accurate model of transformer behavior.
1. Copper (I2R) losses. Copper losses are the resistive heating losses in the
primary and secondary windings of the transformer. They are proportional
to the square of the current in the windings.
2. Eddy current losses. Eddy current losses are resistive heating losses in
the core of the transformer. They are proportional to the square of the
voltage applied to the transformer.
3. Hysteresis losses. Hysteresis losses are associated with the rearrangement
of the magnetic domains in the core during each half-cycle. They are a
complex, nonlinear function of the voltage applied to the transformer.
4. Leakage flux. The fluxes which escape the core and pass through only
one of the transformer windings are leakage fluxes. These escaped fluxes
produce a self-inductance in the primary and secondary coils, and the
effects of this inductance must be accounted for.

22. Modeling the copper losses: resistive losses in the primary and secondary
windings of the core, represented in the equivalent circuit by RP and RS.
Modeling the leakage fluxes: primary leakage flux is proportional to the
primary current IP and secondary leakage flux is proportional to the
secondary current IS, represented in the equivalent circuit by XP (=fLP/IP) and
XS (=fLS/IS).
Modeling the core excitation: Im is proportional to the voltage applied to the
core and lags the applied voltage by 90o. It is modeled by XM.
Modeling the core loss current: Ih+e is proportional to the voltage applied to
the core and in phase with the applied voltage. It is modeled by RC.

23. Effect of Finite Core Permeability
m
1 1 2 2 m
m 2
1 2
1 1
1 m
Finite core permeability means a non-zero mmf
is required to maintain in the core
,
where is the reluctance.
This effect is usually modeled as a magnetizing current
N i N i R
R
R N
i i
N N
N
i i
f
f
f
 
 
  2 m
2 m
1 1
where ,
modeled by resistance and inductance.
R
i i
N N
f

23