2. A distribution transformer is also known as a typical
kind of isolation transformer. The main function of this
transformer is to alter the high voltage to the normal
voltage to use in electric power distribution.
A distribution transformer has the same structure
as an ordinary transformer. The main parts of this
transformer are mainly composed by Oil Tank,
Conservator, Buchholz Relay, Breather Unit, Oil
Indicator, Temperature Detector, Pressure Relief
Device, Thermal Relay, Radiator, and Bushing.
4. A. Hand Hole –serves as an access point of a technician to tap
changers/mechanisms located inside the tank without the need of
opening the cover. Most modern transformers have tap changers located
outside the tank for convenience.
B. Lifting Lugs – This is used, where the hook/rope is connected, for
lifting, either using a hoist or a crane.
C. Terminal Markings (Secondary) – The markings provide identification
about the terminals of a transformer.
D. LV Bushing – The bushing is made of porcelain. It serves as the output
and is the low voltage side of the transformer, which usually supplies
power on residential/commercial establishments.
E. Ground Tap – As the name implies, this is where the grounding of the
transformer is connected.
5. F. Transformer Markings – Are markings which indicate the capacity (in
Kilovolt-amperes) of a transformer and its voltage output.
G. Radiator Fins –This is the cooling mechanism of a transformer. The cooling
mechanism of a transformer depends on the size or rating of the transformer.
The bigger the rating, the more cooling mechanisms are used.
H. Casing (Tank) – It is generally made of steel. It encloses the core-coil and
is the container for the liquid coolant/insulant.
I. HV Bushing – This is the bushing for the high voltage side of the
transformer. It is usually made of solid porcelain. This is the input of the
transformer.
J. Pressure Relief Device – This is a spring loaded device which releases
excess pressure.
K. Mounting Lugs – This is usually used if the transformer is mounted on
poles. It is connected on a transformer cluster which is connected on a pole.
6. L. Tap Changer – This sets the ratio of the HV and LV windings. Most
distribution transformers have taps ± 5%, ± 2.5% and 0.
M. Nameplate – Contains all data about the distribution transformer it is
connected to.
N. Core – Part of a transformer that serves as a path for the flow of magnetic
flux. There are two types of transformer core, the Shell type and the Core
type.
• Core Type – The core is in the form of a rectangular frame with coils
placed on two vertical sides. They are divided, part of each primary and
secondary on each of the two vertical legs.
• Shell Type – The core surrounds the coils, instead of the coils
surrounding the core.
O. Windings – Arrangement of conductors wound on an insulating form with
each turn insulated from all the other turns. This usually determines the
rating of the transformer as the winding is designed by the amount of current
it can carry.
7. TYPES OF DISTRIBUTION
TRANSFORMERS
Based on the application or requirement, these transformers are categorized into
different types like single phase, three phases, underground, pad-mounted, pole-mounted
transformers.
• Single Phase
These transformers are specially used for
networks wherever a three-phase supply is not
required. Usually, these are used for repairing
overhead distribution loads in residential. These
are also applicable in industrial lighting, light
commercial loads & power applications.
8. TYPES OF DISTRIBUTION
TRANSFORMERS
• Three Phase
This kind of transformer is used to hold electrical
energy from the main distribution circuit to a minor
distribution circuit. This type of transformer transmits
the current to a secondary distribution circuit and also
reduces the voltage of the primary distribution circuit.
These transformers reduce the voltage supply for the
primary circuit based on the consumer requirement.
• Pad-Mounted
This type of transformer includes a locked
steel cupboard that is arranged on a concrete pad.
This type of transformer is installed in places where
they do not have space for a fenced enclosure. This
transformer is used with electric power distribution
lines at an overhead electrical line for reducing the
primary voltage to supply for the customers.
9. TYPES OF DISTRIBUTION
TRANSFORMERS
• Pole Mounted
These transformers are mounted on an
electrical service pole at the height of the
overhead cables. These are used for changing high
distribution voltage to low like 120/240 volt
power. These types of transformers are used in a
wide rural area, ranges from 16 kVA to 100kVA.
10. This transformer changes from high voltage electricity to low voltage
electricity, used in homes & businesses.
The main function of this is to step down the voltage to provide
isolation between two windings like primary & secondary
This transformer distributes the power to remote areas which are
generated from the power plants
Generally, this transformer distributes the electrical energy to
industries with less voltage under 33KV and 440volts to 220volts
for domestic purposes.
The uses of the distribution transformer include the following:
1
2
3
USES/APPLICATIONS
4
11. Three Phase Transformer Connections
Windings of a three phase transformer can be connected in various
configurations as (i) star-star, (ii) delta-delta, (iii) star-delta, (iv) delta-star, (v)
open delta and (vi) Scott connection.
Star-Star (Y-Y)
• Generally used for small, high-voltage
transformers.
• The ratio of line voltages on the primary
side and the secondary side is equal to
the transformation ratio of the
transformers.
• Line voltages on both sides are in phase
with each other.
12. Delta-Delta (Δ-Δ)
• This connection is generally used for large, low-
voltage transformers. Number of required
phase/turns is relatively greater than that for
star-star connection.
• The ratio of line voltages on the primary and
the secondary side is equal to the
transformation ratio of the transformers.
• This connection can be used even for
unbalanced loading.
13. Star-Delta OR Wye-Delta (Y-Δ)
• The primary winding is star star (Y) connected with grounded
neutral and the secondary winding is delta connected.
• This connection is mainly used in step down transformer at the
substation end of the transmission line.
• The ratio of secondary to primary line voltage is 1/√3 times the
transformation ratio.
• There is 30° shift between the primary and secondary line voltages.
Delta-Star OR Delta-Wye (Δ-Y)
• The primary winding is connected in delta and the secondary
winding is connected in star with neutral grounded. Thus it can be
used to provide 3-phase 4-wire service.
• This type of connection is mainly used in step-up transformer at the
beginning of transmission line.
• The ratio of secodary to primary line voltage is √3 times the
transformation ratio.
• There is 30° shift between the primary and secondary line voltages.
14. Open Delta (V-V) Connection
Two transformers are used and primary and secondary
connections are made as shown in the figure below. Open delta
connection can be used when one of the transformers in Δ-Δ bank is
disabled and the service is to be continued until the faulty transformer
is repaired or replaced. It can also be used for small three phase loads
where installation of full three transformer bank is un-necessary.
Scott (T-T) Connection
Two transformers are used in this type of connection. One of the
transformers has centre taps on both primary and secondary windings
(which is called as main transformer). The other transormer is called as
teaser transformer. Scott connection can also be used for three phase
to two phase conversion.
16. Under the specified operating environment and operating conditions,
transformer nameplates are typically marked with the main technical data.
Transformer Nameplate Information
A. Rated capacity (kVA): For example, the capacity specified on the transformer nameplate is the
rated capacity, which means that the tap changer is located in the main tap, and is the product of
the rated no-load voltage, rated current and the corresponding phase coefficient. For three-phase
transformers, rated capacity = rated no-load line voltage x rated line current, and the rated
capacity is generally expressed in kVA or MVA.
B. Rated voltage (kV): The maximum voltage a transformer can withstand for an extended period
of time. To meet the changing needs of the grid, the high-voltage windings of the transformer are
tapped, and the output voltage of the low-voltage winding is adjusted by adjusting the number of
turns of the high-voltage winding.
C. Rated current (A): The current allowed to flow continuously through the transformer for a long
period of time.
17. 1
D. No-load loss (kW): The active power absorbed when one winding is connected to the
terminals of the rated voltage while the remaining windings are open-circuited. There is a
relationship between the performance and manufacturing process of the silicon steel sheet,
as well as the applied voltage.
E. No-load current (%): When the secondary side of the transformer has no load at rated
voltage, the current flowing through the primary winding is zero. Typically expressed as a
percentage.
F. Load loss (kW): Short-circuit the secondary winding of the transformer and pass the rated
current through the primary winding at the rated tap position, as this is the power consumed
by the transformer at this time.
G. Impedance voltage (%): The primary winding of the transformer should be gradually
increased in voltage by short-circuiting the secondary winding. The voltage on the primary
side is equal to the short-circuit current of the secondary winding when the secondary short
circuit current is equal to the rated current. The rated voltage is usually used. This is
expressed in percentages.
18. 1
H. The number of phases and frequency: The difference in temperature between the winding
or oil surface of the transformer and its surrounding environment is the temperature rise. For
oil-immersed transformers, the maximum temperature rise limit is 65°C, and the maximum
temperature rise for oil surfaces is 55°C.
I. Temperature rise and cooling: The difference in temperature between the winding or oil
surface of the transformer and its surrounding environment is the temperature rise.
J. Insulation level: There are insulation grade standards. Using the formula
LI200AC85/LI75AC35, a 35kV transformer with a low voltage rating of 10kV can have a high
voltage rating of 200kV and a power frequency rating of 200kV. The lightning impulse
withstand voltage is 85 kV, the low voltage withstand voltage is 75 kV, and the power
frequency withstand voltage is 35 kV. Although the withstand voltage is 35kV, it can be
ignored due to the low voltage of 400V.
K.Connection group label: Transformer windings can be connected differently depending on
the phase relationship between first and secondary windings. Such combinations of
transformer windings are known as transformer winding connection groups.
