• Practical transformer design requires knowledge of electrical
principles, materials, and economics.
• For a transformer using a sine or square wave, one needs to know the
incoming line voltage, the operating frequency, the secondary
voltage(s), the secondary current(s), the permissible temperature rise, the
target efficiency etc.
• Specifications for transformer design varies with transformer
application, tolerance level and cost limitations.
• Data in next slide gives an idea of technical parameter’s specified for
designing a 500KVA , 3 Phase transformer
FOR A 500 KVA, 60HZ, K-
FACTOR, THREE PHASE DRY
Input voltage 3 x 480V, delta
Output voltage 3 x 208/120V, star
Output power 500kVA, K-
Ambient temperature 40°C, in a cabinet
Temperature rise Max. 120°K, insulation
°Short-circuit voltage 4-5%
Steel&core M6, not annealed, strips
for alternated stacking
• The designer first starts with the primary voltage and frequency
• After that power of each secondary winding is calculated by multiplying the
voltage by the current of each coil. These are added together to get the
total power the transformer must provide to the load.
• The transformer losses are estimated and added to this sum to give a total
power the primary coil must supply.
• The losses arise from :
Winding resistance :
LOSSES IN TRANSFORMER
Mechanical losses : The alternating magnetic field causes fluctuating
electromagnetic forces between the coils of wire, the core and any nearby
metalwork, causing vibrations and noise which consume power.
Stray losses : A portion of the leakage flux may induce eddy currents within
nearby conductive objects such as the transformer's support structure, and
be converted to heat.
Cooling system : The power used to operate the cooling system
• Power transformers are used in transmission network of higher voltages for
step-up and step down application (400 kV, 200 kV, 110 kV, 66 kV, 33kV) and
are generally rated above 200MVA.
• Distribution transformers are used for lower voltage distribution networks as a
means to end user connectivity. (11kV, 6.6 kV, 3.3 kV, 440V, 230V) and are
generally rated less than 200 MVA.
• Transformer Size / Insulation Level:
Power transformer : big in size as compare to distribution transformer, high
Distribution transformer : small size, easy in installation and low insulation
• Iron Losses and Copper Losses :
Power Transformers :
1. used in Transmission network so they do not directly connect to the
consumers, so load fluctuations are very less.
2. These are loaded fully during 24 hr’s a day, so Cu losses & Fe losses takes
place throughout day.
Distribution Transformer :
1. directly connected to the consumer so load fluctuations are very high.
2. not loaded fully at all time so iron losses takes place 24hr a day and cu
losses takes place based on load cycle.
Power transformer generally operated at full load. Hence, it is designed such
that copper losses are minimal. However, a distribution transformer is always
online and operated at loads less than full load for most of time. Hence, it is
designed such that core losses are minimal.
• Maximum Efficiency :
Distribution transformer : designed for maximum efficiency at 60% to 70%
load as normally doesn’t operate at full load all the time. Its load depends
on distribution demand.
Power transformer :designed for maximum efficiency at 100% load as it
always runs at 100% load being near to generating station.
In A Nutshell
• Core material should have a high permeability and a high flux density
• A steel core's magnetic hysteresis means that it retains a static magnetic
field when power is removed.
• Certain types have gaps inserted in the magnetic path to prevent magnetic
saturation. These gaps may be used to limit the current on a short-circuit.
• Distribution transformers can achieve low off-load losses by using cores
made with amorphous (non-crystalline) steel having very large permeability .
• Various type of cores are used : Steel cores, Solid cores, Air cores , Toroidal
core etc. depending upon the frequency of operation
• Ring-shaped core, which is made from a long strip of silicon steel
• Strip construction ensures that all the grain boundaries are pointing in the
optimum direction, making the transformer more efficient by reducing the
• The ring shape eliminates the air gaps in core
• The primary and secondary coils are wound concentrically to cover the
entire surface of the core.
• This minimizes the length of wire needed, and also provides screening to
prevent the core's magnetic field from generating electromagnetic
• Total amount of the steel laminations needed to produce the correct core
area for the power in watts, or volt amperes that the transformer is required
• Core can be stacked in two ways :
Interleaved fashion : each lamination is staggered opposite to the other
(turned 180 degrees opposite the other)
provides for the least amount of air gap in the core, and the highest
Butt Stacked fashion : all the E type lams are stacked on one side, and all the I
type lams are stacked on the other.
This however creates an air gap thus increasing the losses.
However, when a DC current is superimposed on an AC current as in an
audio transformer or filter choke, the air gap can stop the core from
• Primary and secondary conductors are coils of conducting wire because
each turn of the coil contributes to the magnetic field, creating a higher
magnetic flux density than would a single conductor.
• Larger power transformers are wounded with wire, copper or aluminum
rectangular conductors, or strip conductors for very heavy currents.
• High frequency transformers operating in the tens to hundreds of kilohertz
uses windings made of Litz wire, to minimize the skin effect losses in the
• Windings on both primary and secondary of a power transformer may have
external connections (called taps) to intermediate points on the winding to
allow adjustment of the voltage ratio
• Thickness of winding also plays a major part in designing process.
• The voltage that each winding sees determine the wires insulation thickness.
• Once this voltage is known, the diameter of the selected insulated wire can
• By knowing the wire diameter, the number of turns per layer is calculated
• Number of layers are calculated using the window height and winding
• The windows are the openings on either side of the core. The window area =
window width * height
• The thickness of the insulation paper for the layers of each winding is
adjusted in accordance to the voltage between the coils.
• This thickness is added to the total coil thickness by multiplying the paper
thickness by the number of layers
• Finally the bobbin thickness is added.
• Bobbin is a permanent container for the wire, acting to form the shape of
• This total thickness is compared to the window dimensions for a fit
• The design should not exceed 80-85% of available opening to allow for
THROUGH FAULT - SHORT CIRCUIT
• The windings are subject to both radial and axial forces related to the
current and flux interactions.
• Radial forces in the inner winding (normally the LV winding) are in
compression while the outer winding (normally the HV winding) forces are in
• Design of the windings and bracing must consider the magnitude of these
forces and provide adequate strength to withstand them without significant
mechanical deformation which could result in a dielectric failure.
• Flux fields are dependent of the balance of the ampere turn distribution of
the HV and LV windings.
• When the ampere turns of the HV and LV windings are equal and
balanced, the only forces are radial.
• If the HV and LV windings are not aligned axially or one winding is physically
shorter than the other, ampere turn balance is significantly affected and
axial forces are magnified.
CORE FORM AND SHELL FORM
• When windings surround the core, the transformer is core form; when
windings are surrounded by the core, the transformer is shell form.
• Shell form design are more prevalent than core form design for distribution
transformer applications due to the relative ease in stacking the core around
• Core form design tends to be more economical, and therefore more
prevalent, than shell form design for high voltage power transformer
• The conductor material must have insulation to ensure the current travels
around the core, and not through a turn-to-turn short-circuit.
• In power transformers, the voltage difference between parts of the primary
and secondary windings can be quite large.
• So insulation is inserted between layers of windings to prevent arcing.
• Transformer may also be immersed in transformer oil that provides further
• To ensure that the insulating capability of the transformer oil does not
deteriorate, the transformer casing is completely sealed against moisture
• The main purpose of the varnish, besides increasing the electrical insulation,
is to keep any form of moisture from effecting the coil
• Varnish dip is done in a vacuum chamber.
• Accomplished by preheating the conductor coils and then, when heated,
dipping them in varnish at an elevated temperature.
• The coils are then baked to cure the varnish.
• To remove the waste heat produced by losses.
• The windings of high-power or high-voltage transformers are immersed in
transformer oil - a highly-refined mineral oil that is stable at high temperatures
• Earlier, polychlorinated biphenyl (PCB) was used as it has very high ignition
temperature and is highly stable.
• Now a day, nontoxic, stable silicone-based oils or fluorinated hydrocarbons
• To improve cooling of large power transformers, the oil-filled tank may have
external radiators through which the oil circulates by natural convection
Oil transformer with air convection cooled heat exchangers
Cutaway view of liquid-immersed construction transformer.
The conservator (reservoir) at top provides liquid-to-atmosphere
isolation as coolant level and temperature changes.
• An electrical bushing is an insulated device that allows the safe passage of
electrical energy through an earth field.
• Larger transformers are provided with high-voltage insulated bushings made
of polymers or porcelain
• A basic porcelain bushing is a hollow porcelain shape that fits through a hole
in a wall or metal case, allowing a conductor to pass through its center, and
connect at both ends to other equipment..
• Outside surfaces have a series of skirts to increase the leakage path distance
to the grounded metal case
• The inside of the porcelain bushing is often filled with oil to provide additional
PERCENT IMPEDANCE VOLTAGE
• The percent impedance is the percent voltage required to circulate rated
current flow through one transformer winding when another winding is short-
circuited at the rated voltage tap at rated frequency.
• This drop in voltage is due to the winding resistance and leakage
• The percentage impedance of the transformer is calculated as
Z % = (Impedance Voltage/Rated Voltage)*100
• So for a two winding transformer with a 5% impedance, it would require 5%
input voltage applied on the high voltage winding to draw 100% rated
current on the secondary winding when the secondary winding is short-
• The primary and secondary windings of a transformer can be connected in
different configuration to meet practically any requirement.
• In the case of three phase transformer windings, three forms of connection
are possible: "star" (wye), "delta" (mesh) and "interconnected-star" (zig-zag).
• The combinations of the three windings may be with the primary delta-
connected and the secondary star-connected, or star-delta, star-star or
delta-delta, depending on the transformers use.
• The standard method for marking three phase transformer windings is to
label the three primary windings with capital (upper case) letters A, B and C,
used to represent the three-phases of RED, YELLOW and BLUE. The
secondary windings are labelled with small (lower case) letters a, b and c
• Symbols are generally used on a three phase transformer to indicate the
type or types of connections used.
Connection Primary Winding Secondary Winding
Delta D d
Star Y y
Interconnected Z z