Distribution transformers are used to reduce high primary voltages to lower utilization voltages for consumers. They come in various types including large distribution transformers used to receive energy from high voltage levels and distribute to substations or industries, and single-phase pole mounted transformers used for residential overhead distribution. Voltage regulation is the percentage difference between no-load and full-load voltages, and is affected by the voltage drop due to current flowing through the transformer windings. Losses in distribution transformers include core losses, copper losses from winding resistance, and stray losses from stray fluxes.

1. Instructor
Engr. Muhammad Yaseen
Reviewed By
Engr Muhammad Imran

2. Introduction of Distribution Transformer
 In general distribution transformers are used to reduce primary
system voltages (2.4-34.5kV) to utilization voltages (120-
600V).
 IEC (International electrotechnical comission) standards do
not distinguish between distribution transformers and power
transformer. They are all power transformers in the sense that
their purpose is to transmit power from one voltage level to
another.
 Traditionally, transformers that transform the voltage down to
the domestic consumer voltage (usually 400V or less) are
called distribution transformers.

3.  Common for most transformers regardless of size and
application is the basic physics and the dominant materials, like:
 Special types of thin magnetic steel plates in the core, which provides the necessary
strong magnetic field because of the unique magnetic properties of iron;
 Copper or aluminum as conductor materials in the windings;
 Cellulose products like high density paper and pressboard as solid insulation materials
has been and with few exceptions still is dominant;
 Mineral oil is the dominant insulating fluid, which also has cooling function.
 In practical transformer design the manufacturer has the choice
between two different basic concepts:
 Core type transformer
 Shell type transformer
 In a nutshell we can say that while the winding of core –type
enclose the core, the core of shell-type encloses the windings.
 Looking at the active part (i-e the core with the windings) of a
core type, the windings are well visible, but they hide the core
limbs. Only the upper and lower yoke of the core are visible.

4.  Looking at the active part (i-e the core with the windings) of a
core type, the windings are well visible, but they hide the core
limbs. Only the upper and lower yoke of the core are visible.
 In shell-type the core hides the major part of the windings.
 Another difference is that the axis of the core-type windings is
normally vertical while it can be horizontal or vertical in shell-
type.
 Today much more core-type transformers than shell-type
transformers are manufactured in the world.

6. A wound type of core used in single-phase distribution transformers is shown in
below

7. Distribution Transformers
Large Distribution Transformers, LDT
Transformers of this type are used for receiving the energy from higher
voltage levels and to transform and distribute the energy to lower voltage level
substation or directly to large industrial consumers.
The core is constructed of grain oriented steel laminations . The windings are
made of paper insulated rectangular wire in the form of multi-layer disc or
helical windings . And the conductor material are either copper or aluminum.
The tanks typically have radiators.

8. Distribution Transformers
Small Distribution Transformers, SDT
Transformers of this type are used to step down three-phase high voltage to
low voltage for energy distribution , mainly in the countryside or low-density
populated areas.
The transformers are three phase oil immersed hermetically(air tight) sealed,
adaptable for pole mounting or assembly in substations.
On customer request, the transformer can be equipped with an oil conservator.
Hot dip zinc coating is often the preferred surface treatment for outdoor
applications.

9. Distribution Transformers
Single phase pole mounted
Transformers of this type are generally oil immersed and suitable for pole
mounting. They represent an economical option for certain networks,
particularly those with low population densities.
Depending on customer requirements, transformers may be connected
between two phases of three phase system (two HV bushings) or from one phase
to ground (single HV bushing).
They are suitable for residential overhead distribution loads, as well as light
commercial or industrial loads.

10. Distribution Transformer
Types of Distribution Transformer
 There are several types of transformer used in the distribution
system. Such as single phase transformer, three phase
transformer, pole mounted transformer, pad mounted
transformer, and underground transformer.
What is Voltage Regulation?
 The voltage regulation is the percentage of voltage difference
between no load and full load voltages of a transformer with
respect to its full load voltage.

11. Distribution Transformer
Explanation of voltage regulation of transformer
 If an Electrical power transformer is an open circuited, means load
is not connected with secondary terminals. In this situation, the
secondary terminal voltage of the transformer will be its
secondary induced emf E2.
 Whenever full load is connected to the secondary terminals of the
transformer, rated current I2 flows through the secondary circuit
and voltage drop comes into picture. At this situation, primary
winding will also draw equivalent full load current from source.
 The voltage drop in the secondary is I2Z2 where Z2 is the
secondary impedance of transformer.

12. Distribution Transformer
 Now if at this loading condition, any one measures the voltage
between secondary terminals, he or she will get voltage V2 across
load terminals which is obviously less than no load secondary
voltage E2 and this is because of I2Z2 voltage drop in the
transformer.
Expression of voltage regulation of transformer
 Expression of Voltage Regulation of Transformer, represented in
percentage, is

13. Tap Changer of Transformer

14. Distribution Transformer
Losses in distribution transformer
 As the electrical transformer is a static device, mechanical loss in
transformer normally does not come into picture. We generally
consider only electrical losses in transformer. Loss in any
machine is broadly defined as difference between input power and
output power.
 When input power is supplied to the primary of transformer, some
portion of that power is used to compensate core losses in
transformer i.e. Hysteresis loss in transformer and Eddy current
loss in transformer core and some portion of the input power is
lost as I2R loss and dissipated as heat in the primary and
secondary windings, because these windings have some internal
resistance in them.

15. Distribution Transformer
 he first one is called core loss or iron loss in transformer and the
later is known as ohmic loss or copper loss in transformer.
 Another loss occurs in transformer, known as Stray Loss, due to
Stray fluxes link with the mechanical structure and winding
conductors.
Copper Loss in Transformer
 Copper loss is I2R loss, in primary side it is I1
2R1 and in secondary
side it is I2
2R2 loss, where I1 & I2 are primary & secondary current
of transformer and R1 & R2 are resistances of primary &
secondary winding. As the both primary & secondary currents
depend upon load of transformer, copper loss in transformer vary
with load.

16. Distribution Transformer
Core Losses in Transformer
 Hysteresis loss and eddy current loss, both depend upon magnetic
properties of the materials used to construct the core of
transformer and its design. So these losses in transformer are
fixed and do not depend upon the load current. So core losses in
transformer which is alternatively known as iron loss in
transformer can be considered as constant for all range of load.
Hysteresis loss in transformer is denoted as,

17. Distribution Transformer

18. Distribution Transformer
Efficiency of Distribution Transformer
 The efficiency of transformer is defined as the ratio between
output power to input power of the transformer at full load
condition, but in case of a distribution transformer, the concept is a
little bit different as the possibility of running a distribution
transformer at its full load condition is nearly nil.
 In this concept, we use the ratio of total energy delivered by the
transformer to the total energy fed to the transformer, during a 24
hrs span of time instead of ratio of power output and input of the
transformer. Hence, all day efficiency is determined as, total KWh
at the secondary of the total KWh at the primary of the
transformer for a long specific period preferably 24 hrs. i.e,

19. Distribution Transformer
 This is very much use full to judge the performance of a
distribution transformer, whose primary is connected to the system
forever, but secondary load varies tremendously throughout the
day.

20. Distribution Transformer
Construction of Silica Gel Breather
 The silica gel breather of transformer is very
simple in the aspect of design. It is nothing
but a pot of silica gel through which, air
passes during breathing of transformer. The
silica gel is a very good absorber of
moisture. Freshly regenerated gel is very
efficient, it may dry down air to a dew point
of below − 40°C. A well maintained silica
gel breather will generally operate with a
dew point of − 35°C as long as a large
enough quantity of gel has been used. The
picture shows a silica gel breather of
transformer.