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A tap changer is a device fitted to power
transformers for regulation of the output
voltage to required levels.
On-load tap-changers (OLTCs) are
indispensable in regulating power
transformers used in electrical energy
networks and industrial applications
Tap changer alters the turn
ratios of the transformer on the
system by altering the number
of turns in one winding of the
Tap changers offer variable
control to keep the supply
voltage within these limits.
Tap changers are of two types
◦ Off load
◦ On load
However since the offload tap
changer causes interruption in
the supply on load tap
changers are more preferred
in in today power system.
By the transformation
From the above equation
by altering the turn ratio
to suitable values the
secondary voltage can be
adjusted desired level.
Lets – N1=2750,
N2=100, turn ratio=
why we choose this turn
ratio - V1=11 KV,
Each tap corresponds to different
transformation ratio. For Initial
installation we need to choose a
suitable tap as per our voltage levels.
Tap changers provides 5 to 20 %
voltage range for voltage regulation.
An on load tap changer
has 3 major steps
a. Tap selector- it selects
the appropriate tap
b. Fixed contact- These
contacts hold a fixed
c. Moving contact- This
contact moves to open/
close the appropriate
Based on the AVR
feedback the suitable
tap is selected fig (a) to
After the tap is selected
the moving contact
moves and it closes both
taps for a while. It
moves further and finally
completely opens to
earlier tap and closes to
the next tap fig (d) to (i)
The moving contact closes two taps
for a while during the tap changing,
due to which a circulating current
starts to flow. This current needs to
be minimized. For limiting this
current resistor or reactors are used.
The moving contact is operated by
charged spring tension and it
completes the process within 30 to
70ms. The spring is charged by
While opening and closing the
contacts sparks are produced due to
arising recovery voltage. The
contacts are kept in oil or SF6 gas ,
which work as insulation medium as
well as arc quenching.
switch / Oil
Motor drive unit (MDU)- This unit has got a
motor diver and other necessary controls.
Motor charges the spring to rapidly perform
the switching. Power is transferred via shaft &
Diverter switch /Oil compartment- This has
the diverter switch, which has moving and
fixed contacts. It is filled with oil or SF6 gas
to avoid the sparking & quenching during
opening and closing of the contacts.
Tap selector- It has different tap
combinations as specified by the customer
requirements and different standards.
Due to opening closing of contacts the sparks
produce and the oil degrades over the time. The oil
test & analysis needs to be done in lab at regular
The contacts wear over the time after number of
The oil seals damage over the time and leakage
happens. Maintaining the oil level is must. For
condition monitoring its must to check for oil leak
and level regularly.
Oil dripping is dangerous for environment as well.
OLTC includes a lot of gear, spring and mechanical
parts which have a usual wear and tear, that need to
be taken care as an when required.
Vacuum type OLTC use the vacuum interrupters
which has some advantages over oil / SF6.
It is hermetically sealed and has no interaction
with surrounding medium despite the arc.
Contacts wear is lower than OIL/SF6.
No aging or quenching medium is required.
Its more environment friendly.
Vacuum interrupters perform up to 600000
operation with out replacement.
Has low maintenance cost and greater reliability.
Vacuum OLTC Vacuum interrupter
A dual voltage transformer can be defined as
the one which is capable of providing two
types of voltage.
A dual voltage transformer has two windings
in primary and two other windings in
By connecting the both windings either of
primary or secondary in series or parallel two
different voltages are produced.
From the figure two
primary windings and
connected in series
resulting in 240 volt
input and 24 volt out
From the adjacent
figure the primary and
secondary windings are
connected in parallel,
resulting 120 volt
input and 12 volt out
There can be either
type of arrangements
as per requirement.
The details in the
adjacent figure shows
the winding arrangement
of a dual transformer. It
produces 33/7.1 and
33/11 kV voltages
arrangement of winding.
The unit produces
different outputs, based
on the connections as
advised in the name plate
Upon simplifying the
they turn out to be series
connected winding for
33/11 kv output and
combination of series
and parallel for 33/7.1