Transformer rating specifies in (kVA)because
of copper loss of a transformer relies upon current and iron loss on voltage.
So, total transformers relies upon volt ampere (VA) and not on phase angle
between voltage and current. It is independent of load power factor. That is
the reason rating of transformer in (kVA) and not in KW(kilowatt).
5. Why transformer
rating specified in KVA?
The four basic configurations of a three-phase transformer,
we can list the transformers secondary voltages and currents with respect to the
primary line voltage, VL and its primary line current IL as shown in the following table. Where: n equals
the transformers “turns ratio” (T.R.) of the number of secondary windings NS, divided by the number of primary windings NP. (NS/NP) and VL is the line-to-line voltage with VP being the phase-to-neutral voltage.
transformer line and voltage current.
Where again, VL is
the line-to-line voltage, and VP is
the phase-to-neutral voltage on either the primary or the
Three phase voltage and current.
connection- Two transformers are used in this type of connection. One
of the transformers has centre taps on both primary and secondary windings. Scott-
connection can also be used for three phase to two phase conversion. The
connection is made as shown in the figure above.
(v) Open delta connection- Open
delta connection can be utilized when one of the transformers in delta-delta is
disabled and the service is to be proceeded until the faulty transformers is
repaired or supplanted. It can likewise be utilized for small three phase loads
where installation of full three transformer bank is un-essential. The total
load conveying capacity of open delta connection is 57.7% than that would be
for delta-delta connection.
delta star turns ratio.
connection- The primary winding is associated in
delta and the secondary winding is associated in star with neutral grounded.
This kind of association is basically utilized in step up transformer toward the
start of transmission line. The proportion of secondary to primary line voltage
is ?3 times the transformation ratio. There is 30 degree shift between the
primary and secondary line voltages.
Formulae shows star delta turns ratio.
(iii) Star-delta connection-
The essential winding is star-star associated with grounded neutral and the
secondary winding is delta associated. This connection is mainly utilized as a
part of step down transformer at the substation end of transmission line. The
proportion of secondary to primary line is 1/?3 times the transformation
proportion. There is 30 degree shift between the primary and secondary line
connection –Generally, this connection is use by large,low voltage transformer.
Number of required turns is moderately greater than that for star-star
connection. The transformation ratio of the transformer is equal to the ratio
of line voltages on the primary and the secondary side. This connection can be utilized
even for unbalanced loading. Even if one transformer is disable, system can
continue to operate in open delta connection is one of the advantage of this
Usually small, high voltage transformers
use Star-star connection. Number of
required turn is reduced because of star connection. Thus, reduction of the
amount of insulation also occur. If the connected load is balanced, this
connection can be utilized.
or delta can be connected in the winding of primary or secondary windings. It
will provide a total of 4 possible connections for 3 phase transformer.
phase transformer connection
In a shell type three phase
transformer, three phases are more independent than they are in core type. Each
phase has its individual magnetic circuit. The construction of shell type three
phase transformer is illustrated in the figure at right. The construction is
similar to that of three single phase shell type transformers kept on the top
of each other.
The construction of a core type three
phase transformer is as shown in the figure. The core consists of three legs or
limbs. As usual, the core is made up of thin laminated sheets to reduce eddy current losses.
Each limb has primary and secondary windings in cylindrical shape arranged
phase ac system present in almost all major generation and distribution systems
in the world. It constructed from 3 single phase transformers. A three-phase
transformer is set up by three sets of primary and secondary windings, each set
wound around one leg of an iron core assembly. Basically it looks like three
single-phase transformers sharing a joined core as in figure. Three phase
transformer core contains three sets of windings. Those sets of primary and
secondary windings will be connected in either ? or Y configurations to shape
an entire unit. This winding can be associated together in different
Three phase transformer (operation
So the saving in copper material depends on the value
of 1/K. The lower the value of K,more saving in copper material.
of copper as step up transformer.
Therefore, the saving in copper material depends on
the value of K (voltage transformation ratio).The higher the value of K the
more saving in copper material.
The weight of winding material(copper) is proportional
to the length and area of across section of the conductor.
of copper in auto transformer as step down transformer.
How copper is saving in autotransformer?
Absent of galvanic isolation.
2. Used only in the
limited places where a slight variation of the output voltage from input
voltage is required.
If an auto transformer is used to
supply low voltage from a high voltage and there is a break in the secondary
winding, the full primary voltage comes across the secondary terminal which is
dangerous to the operator and the equipment. So the auto transformer should not
be used to for interconnecting high voltage and low voltage system.
1. The secondary winding is not
insulated from the primary winding.
Disadvantages of auto transformer.
3. A auto transformer
has higher efficiency than two winding transformer. This is because of
less ohm loss and core loss due to reduction of transformer material.
2. Auto transformer has better voltage
regulation as voltage drop in resistance and reactance of the single
winding is less.
For an case, if transformation ratio = 2, the size of the auto transformer would be
approximately half of the corresponding size of two winding transformer (50%).
For transformation ratio say 20 however the size would be 95 %. The saving in
cost of the material is of course not in the same ratio. The saving of cost is
appreciable when the ratio of transformer is low, that is lower than 2. Thus
auto transformer is smaller in size and cheaper.
of auto transformer
of step down transformer. The picture shows that number of primary higher than
number of secondary coil.
Step Down Transformer- A transformer
in which Np >Ns is called a step down transformer. A step down transformer
is a transformer which converts high alternating voltage to low alternating
of step up transformer. The picture shows that Ns>Np.
Step Up Transformer –At the point when
the number of secondary coil is higher than the primary coil, it will be called
as step up transformer. It converts low alternative voltage to high alternative
The currents of
the primary and secondary windings are steaming on the opposite directions, so
the total current flowing through the basic part of the winding is equivalent to
the difference between the current on the low-voltage winding and the current on the high-voltage winding. The
two windings should have the same winding sense in order for an autotransformer
to work properly.
operation of the transformer and common transformer is the same, then the connection
amongst input and output voltages and input and
output currents and the ratio of number of turns
between the primary and the secondary winding is the same.
of working operation.
is done by comparing the amount of copper saved in the winding. If the ratio
‘n’ characterized as the ratio of the lower voltage to the higher voltage, at
that point it can be demonstrated that the saving in copper is: n100%
to conventional double wound transformers of the same VA rating,
autotransformers are much cheaper. When choosing upon using an autotransformer
it is common to contrast with that of an equivalent double wound type.
any case, the regulation, leakage inductance and physical size of an
autotransformer for a given VA or Kva rating are less than for a double wound
these lines, the part of the windings passes on the contrast between the two
currents cab be made from a smaller conductor size, since the currents are much
smaller saving on the cost of an equivalent double wound transformer.
An autotransformer is used principally for the
adjustments of line voltages to either change its value or to keep it
consistent. If the voltage modification is by a small amount, either up or
down, at that point the transformer ratio is small as Vp and Vs are almost
equal. Currents Ip and Is, are also nearly equivalent.
Auto transformer terminal
The standard procedure for denoting an auto transformer
winding is to name it with capital letters. Essentially the normal unbiased
association is set apart as N or (n). For the secondary tapping’s, suffix
numbers are utilized for all tapping points along the auto transformer
secondary winding. These number generally begin at number1 and continue in
ascending order for all tapping points as shown.
Autotransformer with multiple tapping
The autotransformer can
also be constructed with more than one single tapping point. Auto-transformers
can be utilized to provide different voltage points along its winding or
increase its supply voltage concerning its supply voltage VP as shown.
the point when the essential current Ip, is moving through the single winding
toward the arrow as shown up, the secondary current, Is, streams in the
opposite direction. Along these lines, in the part of the winding that
generates the secondary voltage, Vs the present streaming out of the winding is
the difference of Ip and Is.
The type with programmed
voltage alteration can be utilized as programmed voltage regulator, to keep up
an steady voltage at the clients administration during a wide range of line and
load conditions. Another application is a lighting dimmer that doesn’t create
EMI typical of most thyristor dimmers.
The yield voltage alteration can
be manual or automatic. The manual kind is relevant just for moderately low
voltage and it known as variable AC transformer. These are regularly utilized
as a part of repair look for testing gadgets under various voltages or to
reproduce irregular line voltages.
A constant variable turns of
ratio can be observed by uncovering some part of the winding coils and making
the secondary connection through a sliding brush. It is allowing for very
smooth control of output voltage. Discrete voltage represented by actual number
of turns does not effects the output voltage. The voltage can be smoothly
varied between turns as the brush has a relatively high resistance (compared
with a metal contact) and the actual output voltage is a function of the
relative area of brush in contact with adjacent windings. The relatively high
resistance of the brush also prevents it from acting as a short circuited turn
when it contacts two adjacent turns. Typically, the primary connection connects
to only a part of the winding allowing the output voltage to be varied smoothly
from zero to above the input voltage and thus allowing the device to be used
for testing electrical equipment at the limits of its specified voltage range.
A variable auto
Auto transformer is a device that with only one winding wound on a laminated
core. An auto transformer is similar to a two winding transformer but differ in
the way the primary and secondary winding are interrelated. Both primary side
and secondary side is common to a part of the winding. On load condition, a
part of the load current is obtained directly from the supply and the remaining
part is obtained by transformer action. An Auto transformer works as a voltage regulator.