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Mehran University
Of Engineering & Technology,
SZAB Khairpur Mirs Campus
TAPPINGS
ENGR. AHSANULLAH MEMON
LECTURER
DEPARTMENT OF ELECTRICAL ENGINEERING MUCET KHAIRPUR MIRS
TAPPINGS
 When a transformer carries load current there is a variation
in output voltage which is known as regulation.
 In order to compensate for this, additional turns are often
made available so that the voltage ratio can be changed
using a switch mechanism known as a tap changer.
 Voltage supplied by transformers can be varied by
changing it s transformation ratio.
 This can be achieved by tappings which are provided on
transformers.
 The tappings are the leads which are connected to various
points on a transformer winding.
 A tap changer is an connection point selection mechanism
along a power transformer winding that allows a variable
number of turns to be selected in discrete steps.
 A transformer with a variable turns ratio is produced,
enabling stepped voltage regulation of the output.
 The tap selection may be made via an automatic or
manual tap changer mechanism.
 tap points are usually made on the high voltage (primary)
or lower current winding of the transformer, to minimize
the current handling requirements of the contacts.
Why Tappings are on High Voltage Side
 A fine voltage regulation is possible with high
voltage winding as it carries large number of
turns.
 The low voltage winding of the transformer
carries large current. So if tappings are provided
on low voltage side then then there are difficulties
encountered in the interruption of high currents
which makes its impracticable.
 For the reasons of requirement of insulation, the
low voltage (l.v.) winding is placed near the core
while the HV winding is placed outside. Hence
practically it is easier and simpler to provide
tappings on high voltage winding.
 In case of step down transformers, it is an added advantage to
provide tappings on h.v. side. At light loads, the l.v. side side
voltage increases. It is required to decrease this voltage by
adjusting the tapping on h.v. side to a position where number
of turns are large. With large number of turns, the flux and flux
density decreases. This results in reduction of core loss which
increases transformer efficiency at light loads.
 If the tappings are provided on the l.v. side then the exact
voltage regulation may not be provided.
 An on-load tap-changer must not break the supply to
the transformer's windings while the transformer is on
load.
 Large network transformers which are provided with
on-load tap changing normally have a much larger
number of taps in smaller steps.
 The principle used is ‘make-before-break’: this means
that the new tap must be connected before the old tap is
broken, otherwise there would be a break in supply and
an interruption of full-load current by the tapping
switch.
 The difficulty with this simple idea is that, during the
transition period while both taps are made, a small
number of turns of the transformer’s HV winding are
short-circuited by the two taps, and a heavy current will
flow through them. Arrangements are therefore made
to insert resistance temporarily into this short-circuited
loop to limit the current until the tap change is complete
and the short-circuit removed.
 A, B and C are adjacent taps on an HV winding. In (a) the tapping is on A, and it
is desired to move it, on load, to B.
 The moving member consists of a main contact M and two ‘transition’ contacts P
and Q which are connected to M each through a resistance. In position (a) M
carries the full load, and P and Q are not in contact.
 In the first part (b) of the transition the main contact M is still on tap A. Contact
Q moves to B and contact P is still on A. Q and M now short-circuit the HV turns
between A and B, but the short-circuit current is limited by the lower half of the
resistance. Meanwhile M is still carrying the load current from tap A.
 At the next stage (c) the moving member has travelled on, and the main contact
M leaves tap A. P and Q now share the load current which passes through both
halves of the resistance. These two halves also limit the current in the shorted
turns between A and B.
 At the next stage (d) the main contact M has moved to tap B, so that it is once
again carrying the load current, but now from the new tap. P however is still on
tap A, so that the current from the shorted turns is limited by the upper half of
the resistance.
 Finally the moving member is at position (e), where the main contact M is on B
and carrying the load, while P and Q are out of contact, as they were in position
(a), but now on the new tap.
 During these transition stages the load current has never been interrupted, nor
has the main contact ever been called upon to break any large
current. Moreover the current in the short-circuited turns is always limited by
one or both halves of the resistance
An off-load tap-changer does not require a make-before-break
arrangement, because it is designed to be operated when the
transformer is disconnected.