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AUTO-TRANSFORMER
PRESENTED BY
(Group-A)
- Jadeja Pruthvirajsinh(130540111013)
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WHAT IS AUTO-TRANSFORMER?
APPLICATION OF AUTO-TRANSFORMER.
CIRCUIT DIAGRAM.
OPERATION OF AUTO-TRANSFORMER.
AUTO-TRANSFORMER CONNECTION.
SAVINGS OF COPPER IN AUTO-TRANSFORMER.
THIRD (TERTIARY) WINDING IN AUTO-TRANS.
RATING OF TERYIARY WINDING.
T.W. REQUIREMENTS TO CONNECT IN DELTA.
ADVANTAGES AND DISADVANTAGES.
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AUTO TRANSFORMER
The autotransformer is by definition, a transformer consisting of only one winding with a part of
its turns being common to both primary and
secondary circuits i.e. it is a transformer in which
part of the winding is common to both the primary
and secondary circuits. Part of the load in the
receiver circuit is supplied directly from the supply
circuit through the primary winding, the remainder
being supplied indirectly through the secondary
winding by electro-magnetic induction.
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As such, there is no primary or secondary. Any
two points on the winding can be connected to
the supply and likewise a load may be connected
across any two points. A minimum of two voltage
taps are required for an auto-transformer to
perform a useful task. An auto-transformer does
not provide electrical isolation between the input
and output so must not be used in safety critical
applications such as portable tool transformers,
arc welders or car battery chargers. Suitable
applications are in supply voltage matching where
only a small difference exists between input and
output voltages.
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Though the advantages of auto-transformers
have been known, for a long time they were not
used earlier as power units, only much later
interest in the auto-transformers had increased
because of the vast development of electrical
networks and the necessity to produce transformer units of larger capacities. The auto-transformer does not differ from the ordinary transformers
in its fundamental principles. The same laws that
govern the ampere-turn relations in the ordinary
transformer hold good for auto-transformers also.
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But it differs essentially in the manner of connection to the circuits in the primary and secondary
systems.
In the ordinary transformers the primary and
secondary windings are magnetically interconnected but electrically separate.
In the auto-transformers, the windings are both
magnetically and electrically interconnected. The
auto-transformers may have one single continuous winding with one or more taps brought-out or
it may have two or more distinct coils electrically
connected.
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Since the change of voltage in the primary and
secondary windings of transformers go through
their maximum and minimum values at the same
time, the result of connecting the two windings in
series is to produce a voltage which is either the
sum or the difference of the voltages of the
windings, according to the mode of their joints.
K= N1/N2 = V1/V2
APPLICATION OF AUTO-TRANSFORMERS
As stated earlier, autotransformers used frequently in all connections where the ratios of transformation of primary voltage to secondary voltage
do not differ much.
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The more nearly this ratio approach the unity, the
greater the advantage in the use of the autotransformers over the conventional transformers.
They vary widely in rating from those of few units
metering purposes to large sizes for industrial
and power systems.
The major applications of auto-transformers
are:
• Frequently used as boosters to increase the
voltage to a small amount of 10 - 20% in both
single phase and poly-phase circuits.
• For obtaining adjustable output voltages in
testing circuits and laboratories.
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• Frequently used for both potential and current
transformation in connection with poly-phase
metering and relaying.
• As balancers on three-wire, single-phase
system.
• Most common use is for obtaining reduced
voltages for starting induction and synchronous
motors. For small motors only one tap, for larger
motors two or more taps are to be provided.
These starting equipments are usually called
autotransformer starters or compensators.
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• Higher capacity transformers are found in
interconnection high voltage transmission
systems which less than two to one in voltage.
A Y-y (Tertiary) connection is frequently used
for this application.
• Used as regulating transformers to feed
furnace and rectifier transformers.
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In the diagram, a step-down transformer is shown. The
secondary voltage is half the primary, so the current is
doubled. The ampere-turns of the secondary, carrying
20 A, are opposed by the same number of ampereturns of the whole primary.
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In this diagram, the secondary load is shown attached to the primary winding at its mid-point. Now,
the ampere-turns below the connection are opposed
by the ampere-turns above the connection, each
excited by 10 A, but in opposite directions. The
currents add at the node to give 20A in the
secondary load, as required. Conditions are just as
in the case of separate windings, but without the
secondary winding carrying 20 A. This is called an
auto-transformer, and the savings involved in
its use are obvious. The disadvantage, of course, is
that the primary and secondary are no longer
isolated and share a metallic connection. In many
applications, this is no matter, and an auto-transformer can be used.
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This is a "Variac" (a trade name belonging to
General Radio Corporation, but now used like
Kleenex and Vaseline) or variable transformer, a
useful device that provides an adjustable AC
voltage. For safety, the common connection must
be connected to the grounded or white wire of the
AC lines. If, somehow, the connection is
reversed, then everything connected to the Variac
becomes "hot" and hazardous. The common
230/110V converters are auto-transformers, and
present the same hazard.
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In general auto-transformers are used when the
transformation ratio is three or less and
electrical isolation of the two windings is not
required. Auto-transformers are most practical
where a small percentage of voltage raising or
lowering is required.
Auto transformers are used extensively as inter
tie transformers between substations and
generating stations.
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The internal behaviour and the no load operation of
the auto-transformers are not different from those of
an ordinary two winding transformer. Yet there are
specific features as regards to auto - transformers
under load, portion of the load current that is
distributed through common portion is equal to the
difference between the primary and secondary load
currents. Because of this rated (standard) power of
an auto-transformer is less than the throughput
power (so called auto-fraction) and therefore autotransformer derived advantages when compared to
ordinary transformers in several aspects. The degree
of economy depends mainly on the transformation
ratio, K.
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When load is connected, a load current I2 flows
through secondary circuit and a certain fraction of
power P2 = V2 x I2 is consumed. This power
fraction is referred as to throughput power (or the
auto fraction, which is equivalent to the rated
power of the auto-transformer).
Then P1 primary power by the supply network
must be equal to P2 (according to the law of
conservation). The primary power P1 produces a
load current I1 in the primary circuit.
Consequently P1 = V1 x I1 = V2 x I2 =P2.
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OPERATION OF AUTO-TRANSFORMER
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OPERATION OF AUTO-TRANSFORMER
The auto-transformer has a single continuous
winding which is used for the input and output
voltages, as shown in the Fig. A portion BC of the
primary winding AB is used as secondary winding.
It is used where transformation ratio differs a little
from unity. Its theory and operation principles are
similar to that of a two-winding transformer. if N1 is
the primary winding turns and N2 (a portion of N1) is
secondary winding turns, the transformation ratio K
can be represented in the same way as a twowinding transformer
[V2 / V1] = [N2 / N1] = K
When load is connected, a load current I2 flows
through secondary circuit and a certain fraction of
power P2 = V2 x I2 is consumed.
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This power fraction is referred as to throughput
power (or the auto fraction, which is equivalent to the
rated power of the auto-transformer).
Then, P1 primary power by the supply network must
be equal to P2 (according to the law of conservation).
The primary power P1 produces a load current I1 in
the primary circuit.
Consequently P1 = V1 x I1 = V2 x I2 =P2. -------------The current in the secondary winding (may be called
as common winding) is the vector difference of Il and
I. But as the two currents are practically in phase
opposition, the common winding current may be
taken as arithmetical difference of I2 and I1 i.e. (I2 - I1)
where I2 is greater than I1.
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AUTO TRANSFORMER CONNECTIONS:
Autotransformers can have all the connections which the
ordinary transformers have differing from them only by
conductive connection of windings.
Major autotransformer connections are:
Single - phase transformation by means of autotransformers:
Balancer for three-wire phase, single-phase systems;
Step—down autotransformers;
Step—up autotransformers.
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Three - phase transformation by means of
single-phase autotransformers:
(a) Y connection;
(b) D connection;
(c) Extended - delta connection;
(d) Open - delta connection;
(e) Zigzag connection;
(f) Zigzag extended delta or single extended zigzag connection;
(g) Double zigzag connection.
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Phase transformation by means of autotransformers:
(a) Three-phase to two phase, Scott connection,
equal voltage;
(b) Step-down, Scott connection;
(c) Step-up, Scott connection;
(d) Taylor connection (three cases) when it is
necessary to transform three - wire, threephase to four wire, two-phase to equal voltages.
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Savings of Copper in Auto-transformers:
Refering to Fig.,
Weight of copper in AC section (series winding) is
proportional to (N1-N2)I1.
Weight of copper in BC section (common winding) is
proportional to N2(I2-I1).
So, total weight of copper in auto-transformer is
proportional to [(N1-N2)I1 + N2(I2-I1)]
In a two - winding transformer, total weight of copper
is proportional to (N1I1 + N2I2).
Ratio of wts.= Weight of copper in auto-transformer
Weight of copper in two-winding transformer
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= [(N1-N2)I1+N2(I2-I1)] / (N1I1 + N2I2)
= [1] – {[2(N2 / N1)] / [1 + (N2 / N1) (I2 / I1)]}
=[1] – (2K / 2) = 1 – K
Where, (N2 / N1) = (I1 / I2) = K
or weight of copper in auto-transformer (Wa)
= (1 - K) x [weight of copper in ordinary two
winding transformer (W0)].
Hence, saving = W0 - Wa = W0 (I - K)W0 = KW0
or Saving = Transformation ratio (K) x weight of
copper in ordinary two-winding transformer.
Thus, it can be visualized that saving will increase
as K approaches unity.
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Three-winding Transformer --- Purpose
Star/star transformers comprising single-phase
units, or three-phase five-limb core-type units. A
transformer may have additional windings apart
from the two conventional main windings
depending upon the particular application and
type of connection (of the main windings). In
three-winding transformers, the third winding is
normally called as tertiary winding and it is
provided to meet one more of the following
requirements:
(a)For an additional load which for some reason
must be kept isolated from that of secondary.
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(b)To supply phase-compensating devices, such as
condensers, operated at some voltage not equal
to primary or secondary or with some different
connection (e.g. mesh).
(c) In star/star-connected transformers, to allow
sufficient earth fault current (zero-sequence
component current) to flow for operation of
protective gear, to suppress harmonic voltages
and to limit voltage unbalance when the main
load is asymmetrical, the tertiary winding is
delta-connected.
(d) As a voltage coil in a testing transformer.
(e) To load charge split winding generators.
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(f)To inter-connect three supply systems operating at different voltages.
(g)Consequently, when faults occur on the primary or
secondary sides (particularly between lines and
earth), considerable unbalance of phase voltage,
may be produced which is compensated by large
circulating
currents.
In case of single line-to-ground fault, either on
primary or secondary sides, the zero sequence
current flowing through the delta-connected tertiary
winding.
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Thus the reactance of the tertiary winding must be
such as to limit the circulating current to that value,
which can be carried by the copper, other-wise the
tertiary windings may overheat and mechanically
collapse under fault conditions.
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Rating of Tertiary Winding:
Rating of tertiary winding depends upon its use. If it
has to supply additional loads, its winding crosssection and design philosophy is decided as per load
and three-phase dead short-circuit on its terminal
with power flow from both sides of HV and MV.
In case it is to be provided for stabilization purposes
only, its cross-section and design has to be decided
from the thermal and mechanical considerations for
the short duration fault currents during various fault
conditions - single, line-to-ground fault being the
most onerous.
Tertiary windings are mostly delta-connected.
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REQUIREMENT OF DELTA WINDING FOR Y/Y
AND AUTO - TRANSFORMER
In High voltage Power Transmission system a
Star/ Star connection is un popular because of
operating difficulties arising from its natural
instability.
This is due to Three main causes:
(1) Magnetizing currents
(2) Line to neutral unbalanced Loads
(3) Third Harmonic currents.
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The neutral potential of Y-Y Transformers may be
affected by load characteristics and other circuit
conditions so that it becomes hazardous to the
transformers and connected systems and may
interfere with proper transformer operation.--------This connection should be used only where there
are provisions to prevent or reduce neutral
instability. Magnetizing current results from
differences in the quality or quantity of iron, or
differences in core joints. These differences may
cause appreciable magnetizing current variation
in transformers of same design. -------------------The Y - Y connected transformers with the exception of 3 phase core type units can not supply an
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appreciable single phase to neutral load from line
to neutral with out a shift in neutral position. The
excitation of Y-connected Transformers presents a
peculiar case for the third harmonic and oddmultiples magnetizing currents.
The third harmonic current - necessary for single
wave excitation - is completely suppressed, so the
non linear relationship between excitation and flux
induces in each phase a third Harmonic flux and
corresponding third harmonic electromotive force.
The relation between third harmonic and fundamental voltages results in a peaked complex wave
with the maximum crest value of induced voltage
increased by 50 percent.
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The consequence is voltage stress on the insulation;
also 50 percent above its normal value. This third
harmonic phenomena is considerably influenced by
whether the neutral is connected to ground, and the
amount of connected line capacitance. A DELTA
connected tertiary winding is frequently provided to
stabilize the neutral . The delta - connected tertiary
provides a path for the third harmonic currents.
A single phase short circuit to ground on the
Transmission line will cause less voltage drop in the
short circuited phase and consequently less voltage
rise in the remaining two phases.
Conversely a three phase, three leg Y - connected
transformer without a delta tertiary winding provides
very little stabilization of the neutral.
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Increasing the neutral stabilization by DELTA tertiary
winding raises the fault current in the neutral on
single phase short circuit, and this may improve the
system's relay protection efficiency. The third
Harmonic component of the exciting current finds a
relatively low impedance path in the DELTA Tertiary
of a Y - connected transformer and thus less of the
third harmonic exciting current will appear in the
transmission lines, reducing interference with
communication circuits. -------------------------------------Failure to provide a path for third harmonic current in
Y-connected three phase shell type transformers
results in excessive third Harmonic voltage from line
to neutral.
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An outstanding feature of an auto-transformer is
that the standard capacity is always less than the
throughput power, owing to which the auto-transformer of the same capacity as the ordinary two
winding transformer is always less costly.
The ratio of the standard capacity to the throughput power is called the auto-transformer economy
factor Ke = Pst / Pthrough =[1-(1/k)]
(For both V1 > V2 or V2 > V1) where K is the
transformation ratio.
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The auto-transformer economy factor may also
define as the difference between the primary and
secondary voltages related to the higher of the
two. This means that the standard capacity of an
auto-transformer which determines its size and
also weight of its active components may be 1/Ke
times less than the throughput power.
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MULTIWINDING TRANSFORMERS
In general there may be many number (3 and above)
of windings in multi winding transformers. The clear
discussion related for triple wound transformers,
since it is the most important one, in multi winding
transformers. In three winding transformers the core
carries three electrically isolated windings.
Normally, power systems use three-winding transformers to couple electrical systems or networks
operating at different voltages. They may be either
single phase or three phase with standard vector
groups. The three phase winding transformer may be
with one primary and two secondary or vice - versa.
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A three winding transformer does the job of two twowinding transformers one of which connects network (or
system) 1 to network 2 and the other network 1 to
network 3. Economically a three winding transformer is
more attractive than two windings. Taking all other things
equal, it is less expensive to make up less space at the
sub station. On the demerit side, a three winding
transformer is less reliable.
A multi winding transformer must be strong enough to
withstand a three-phase fault at the terminals of any one
winding when the voltage is maintained on the others or
when they are supplied from systems of known fault
level or short circuit impedance.
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Three winding transformers have been installed (in
large numbers) to meet the following conditions:
• Transformers with two primary windings and one
secondary winding are often used in large generating
stations. One generator is being connected to each of
the two primary windings, and the power from the two
being fed to the transmission system, through a common
secondary winding. Such a connection reduces the
rupturing duty of the generator (oil) circuit breaker.
• Transformers with one primary and two secondary
windings are sometimes installed to supply separate
loads at the same voltages or at different voltages.
When used in this manner, it is cheaper and more
efficient than two winding transformers.
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• Transformers with two primary windings having two
voltage ratings may be used to interconnect power
systems of different systems of different voltages and
supply a load through a third winding which has common
secondary.
• Transformers which are to be used for Star-star
connection have one primary winding, one secondary
winding with a tertiary winding with a lower kVA rating to
provide a path to third harmonics current and also to
provide a path for zero-sequence currents which flow
under unbalanced condition. Further for a line to earth
fault, the tertiary permits a flow of sufficient fault current
in order to operate the protective devices.
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• Transformers with four / six windings are sometimes
used in special cases and may be treated in a similar
way.
• Winding of a multi-circuit transformers can be connected to a static capacitor or synchronous condenser for
the purpose of power factor correction or voltage regulation.
• Multi-circuit transformers are essentially used to threephase to six-phase conversion or vice versa.
The rated power of a triple-wound transformer is the
power of the winding with the greatest capacity i.e.
actually the power of its primary windings.
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Some of the advantages of the auto-transformer
over the ordinary transformer having the same
output are:
Lower cost,
Smaller size and weight,
Greater efficiency since the losses are lower,
Smaller exciting current and better regulation.
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ADVANTAGES
a)The comparative cost of an auto-transformer is
approximately a third lower than that of a two
winding transformer due to the need for less
copper, core material and insulation as the
result of fewer windings and use of the
common winding.
b)The weight and overall dimensions of the autotransformer are less than that for a two
winding transformer resulting in less space
being required for the autotransformer and
therefore a reduction in switch yard space
and hence civil foundation costs.
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c)Transport costs are less due to the reduction in
weight and reduced volume of the auto-trans.
d)The efficiency of an auto-transformer is higher than
that of a two winding transformer due to the fewer
windings and lower reactance resulting in lower
losses.
e) The voltage regulation is superior to that of a two
winding transformer due to common winding
producing lower impedance by reduction of
leakage flux and reactance.
f) Self capacity of an auto-transformer is defined as
[(HV- LV)/LV)] MVA. Weight and loss of transformer will be less to that extent. Less loss results
in reduction of maintenance cost.
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Because of these advantages, the autotransformer is extensively used, but one disadvantage of
the auto-transformer is that there is a metallic
connection between the high and low voltage
sides so that electric disturbances on one side
are communicated directly to the other. A ground
on one side is also a ground for the other side.
The low voltage winding and connected apparatus may be subjected to potential stress as high
as the voltage of the high voltage side. Under
short circuit condition, the low impedance of the
auto-transformer may be a disadvantageous.
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DISADVANTAGES
a) The secondary winding of the auto-transformer
will experience high voltage in step down
operation when an open circuit occurs on the
common winding. This occurrence is very rare.
b) The impedance of the auto-transformer due to
the common winding is less than the two
winding transformer resulting in a higher fault
current being available. This can overcome by
specifying the correct transformer impedance to
limit the available fault current to within the
equipment duty ratings.
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c) The auto-transformer provides less of a barrier
to electrical noise than does a comparable two
winding transformer.
d) Auto-transformers share a common ground
return between secondary and primary windings
for earth fault currents which could lead to
increased operating times to allow other devices
to operate first. Directional ground fault relays will
be required to distinguish between primary and
secondary transformer earth faults.
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e) Auto-transformers have the limitation of not
suppressing harmonic currents and acting as
another source of ground fault currents. An
auxiliary Delta winding not connected to the
outside of the tank may be required to absorb
some of the harmonic currents.
These problems are same for auto transformers and star-star connected two winding
transformers.
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CONCLUSIONS AND RECOMMENDATIONS
It would appear from the above comparisons of
the auto-transformer and two winding transformer that overall the auto-transformer with
tertiary unloaded Delta Winding, despite some
of its obvious disadvantages will be the best
economical choice.
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THANQ
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