FIELD OF INVENTION
The present invention relates to a fixture to regulate rotational lags in on-load
tap changer fitted in a transformer and a method for the same where the on-
load tap changing is involved; in generator transformers, network transformers
and also in phase-shifting transformers.
BACKGROUND OF INVENTION
On-load tap changers (OLTC) serve for the voltage regulation of power
transformers by varying transformer turns ratio under load without power
interruption. Operation is carried out in steps. Therefore, the transformer
winding for regulation purposes is provided with taps which are connected to the
tap selector of the tap changer.
OLTC operates on principle of ‘make before break’. For no instance, the current
carrying path shall be broken. If there is discontinuity in current path during tap
change operation, arcing occurs and the entire power transformer gets
damaged. To maintain the continuous path of current during tap change
operation, the sequence of operation of both the tap selector sub-assembly and
diverter switch sub-assembly is important. Tap selector sub-assembly need to
operate prior to the diverter switch operation.

As the operation of OLTC is carried out in multiple steps, the transformer winding
for regulation purposes is therefore provided with taps, which are connected to
the tap selector of the tap changer.
The following sub- assemblies primarily comprise the OLTC:
1. Diverter Switch Assembly
2. Tap Selector Assembly
3. Driving Mechanism
One of the most important aspects of any manufacturing process is to ensure
reliability of the product. In generation and transmission unit, the role of tap
changer is critical, since it is the only moving part in transformers. If there
remain open circuit in tap changer or transformer winding there could be havoc.
Arcing, the expanding gases and the subsequent pressure rise inside the
transformer could be avoided with proper checking implantation with fixtures. It
is, therefore, very important that at no point, there would be breaking of current
path outside diverter switch.
In entire tap changer operation, the sequence of operation is the most
vulnerable and may get disturbed for component defect. In summary, this
invention is to cater the above mentioned issue.

Normal operation of OLTC:
Tap windings of transformer are connected one to one with the fixed contact
available on the Tap selector sub assembly. Tap selector sub assembly has two
sets of moving contact. At any instance, current path is established through any
one moving contact and other moving contact is dummy that is no actual current
is being carried by that path. When instruction is given, the two moving contacts
inter change their role. Thus instruction (tap change signal) converts the dummy
moving contact into current carrying path and vice versa. This is known as
operation of tap selector sub assembly.
Once tap selector assembly has operated, diverter switch assembly operates.
This sub assembly diverts the current path from earlier selected (before tap
change signal) moving contact to the dummy moving contact. Thereby the
dummy contact becomes the current carrying path.
Suppose transformer output voltage need to be raised and command is given to
move the tap position from tap 6 to 7. The moving contacts of tap selector
assembly must be holding the two consecutive tap positions: 6 and 7 but current
path is through either of them. In the considered case current flowing through
tap 6 and tap 7 is connected with dummy moving contact. Once command is
given, tap 7 become ready to take role of current carrying path. Thereafter, the

diverter switch sub assembly switches the current path from 6 to 7 through
sequence of making and breaking of auxiliary contacts. Diverter Switch sub
assembly is inside oil chamber which isolates the transformer oil from diverter
switch oil. Arcing occurs inside oil compartment but because of physical isolation
this arcing does not contaminate the transformer oil.
Abnormal operation of OLTC and need of invention:
As mentioned above for safe operation of tap changer and transformer, tap
selector must operate before diverter switch.
For any reason if diverter switch sub assembly operate before or simultaneously
with tap selector sub assembly, it could cause discontinuity in current path and
resultant arcing may damage entire transformer. Power supply will be therefore,
interrupted.
With the development of fixture, ‘Fixture for rotational lags in OLTC”: the
sequence of operation of tap selector sub assembly and diverter switch sub
assembly could be checked and corrected before assembling the two.
Rotational lag:
In OLTC moving contacts can rotate clockwise and anti-clockwise depending
upon the command. During normal operation number of rotation of driving shaft

is nearly equal for clockwise and anti-clockwise rotation of OLTC. Further in this
rotation, delay in operation of diverter switch sub assembly from tap selector sub
assembly must be at least 3 number of rotation of driving shaft. If there is no
delay (simultaneous operation of tap selector sub assembly and diverter switch
sub assembly) or delay is less than 3 number of rotation of driving shaft, the
situation is called rotational lag. This lag in respective operation of diverter
switch sub assembly and tap selector sub assembly if get disturbed, can result
discontinuity of current path.
As the tap Changer operates on the principle of make before break phenomenon
hence, tap selector sub assembly designed for carrying current, must operate
before the diverter switch sub assembly operation which is designed to switch
(break and make) the current path. Minute deviation in the selector assembly
and diverter switch results in improper rotational lag, which could cause havoc at
commissioned site. In this context, the present invention relates to the design
and construction of jigs and fixtures that simplifies the production of components
which has bearing on rotational lag.
Further, the selector sub-assembly has two sets of moving contact; current path
is established by one contact at a time and other one is idle. In operational
phase idle contact will move first and thereafter diverter switch swipe the current

path from previous path to the next path. In any situation diverter switch cannot
operate before or along with tap selector sub assy. Otherwise there would be
arcing and entire transformer may catch fire. Components play important role in
rotational lag are identified and fixtures developed for assembly checking. In this
context, the fixture designed and developed in this invention can verify all the
components contributing in problem.
OBJECTS OF THE INVENTION
Therefore it is an object of the invention to propose a fixture to regulate
rotational lags in on-load tap changer fitted in a transformer which is capable of
controlling rotational lag in on-load tap changer (OLTC) before assembling it.
Another objection of the invention is to propose a fixture to regulate rotational
lags in on-load tap changer fitted in a transformer which is able to integrate
multiple components which are checked in conjunction with other components in
semi-assembled form and also individually with primary focus on a multipoint
checking of all the factors contributing to rotational lag between selector
assembly and diverter switch and further to identify the most critical
components.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Figure 1: Is a isometric view of checking of gear II individually, according to
the invention.
Figure 2: Is a indicate direction of the flow of power in the fixture, as per the
invention.
Figure 3: Is an isometric view of checking angular motion triggered by gear
II, according to the invention.
Figure 4: Shows a line diagram of different components including a
transformer.
Figure 5: Shows a photograph of actual assembly with driving mechanism.
Figure 6: Shows a photograph of actual assembly with energy transmission
gears.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
This invention describes a method to control rotational lag in OLTC before
assembling it. For this purpose, a single fixture has been designed and
developed for arresting the frequent problem of improper rotational lag. Hence,

more specifically, the present invention relates to the design and construction of
jigs and fixtures in a manner that eliminates the problems associated to
conventional rotational lag. The invention also simplifies the production process
of components that reduced the manufacturing time as well as the duration of
routine testing, which has bearing on rotational lag.
The invention actually integrates multiple components which are checked in
conjunction with other components in semi-assembled form and also individually.
The primary focus is on a multipoint checking of all the factors contributing to
rotational lag between selector assembly and diverter switch, in which ‘Gear I’
and ‘Gear II’ are identified as most critical components and the fixture checks the
gear’s static profile and dynamic motion.
The phenomenon of rotational lag in tap changer (11) is observed for defective
gear assembly and components of energy releasing unit. This fixture meets the
requirement of checking of components individually and in tandem without
actually undergoing assembly.
Two gears, gear I (5A) and gear II (5B fixed on a base plate (9) maintaining
exact center distance of the said gears for verifying the profile of gear II;

The figure 1 refers to checking fixture assembly (F) embodying multiple
attachments aimed to serve different purpose. As shown in fig.2 the driving
linkage of the fixture has coupling shaft (18), bevel gear box (7) and hand crank
(6). Hand crank or handle shaft (6) is rotated which in turn rotate the gear I
(5A). The hand crank (6) drives bevel gear (7) which drives coupling shaft (18)
and gear I (5A) subsequently. Gear I (5A) transmit power to gear II. At this
juncture the mutual play between gear I and gear II is checked.
The figure 2 depicts the checking of profile of Gear II (5B). Gear II (5B) has
angular sector (3) of fixed degree. This sector (3) drives the tap selector sub
assembly (13) and which has role in timing sequence. Apart from angle (110
degree), the sector (3) must be equi position with respect to center line of gear I
(5A) and gear II (5B). First gear II (5B) is place (a) on base plate (9) at location
shown in fig 2. Two dowels are adjusted. The sectorial fixture component which
complements the angular profile of gear II (5B) is then placed which verifies the
profile of gear II (5B).
Figure 3 depicts the checking of eccentric component. The replaceable
attachment gauges the pitch clearance.

Figure 3
In figure 3, symmetry of operation in clockwise and anti-clockwise is verified.
Clockwise rotation at 6 (handle shaft) induce clockwise rotation of drive shaft
(12). Dial plate (8) has marking divided into 360 degree. Dial plate is read for
both clockwise and anti-clockwise rotation. For Non symmetry, Gear II (5B) is
rotated in the direction to which driving shaft rotation is more.
Without this fixture (F), Gear I (5A), Gear II (5B) and driving shaft (12) are
assembled in respective sub-assemblies and complete tap changer (OLTC) is
mounted on testing structure. Tap Change undergo routine test and problem of
rotational lag is observed. From clockwise movement (suppose from tap 5 to 6)
operation is completed in 7 +1/2 Rotation and in anti -clockwise movement it
requires only 5+1/5 rotation. In this case then two sub-assemblies are
dismantled and gear II (5B) position is corrected that is rotated by 1 degree
towards anticlockwise direction so that both direction rotation is equal , After
reassembly, test has to be performed again. This entire process consume 2 man
days.
With fixture, rotational lag among transmission system of tap changer (OLTC)
(11) could be identified before actual assembly and hence avoided at the actual
testing of the final product that is tap changer.

By moving Gear I (5A) antic lock wise with the help of bevel gears inside the box
(7) connected to the shaft (18) attached to Gear I (5A) which imparts a
clockwise movement to Gear II (5B).
1) The fixture assembly comprising of attachments for gear I and gear II and
driving linkage which enable to check the gears individually as well as their
mutual compatibility and transmission of power. This fixture allow to check
the cause of rotational lag which otherwise could be detected only after
complete assembly. This fixture has distinction of checking the static and
dynamic character of the component.
2) The driving linkage is configured to produce a defined motion of the Gear
wheel upon a defined angular rotation of the handle.
3) The angular attachment for gear II is adjustable which could predict the
direction of lag even before actual assembly.

WE CLAIM
1. A fixture (F) to regulate rotational lags in on-load tap changer fitted in a
transformer comprising:
two gears, gear I (5A) and gear II (5B) fixed on a base plate (9)
maintaining exact center distance of the said gears for verifying the profile of
gear II;
a handle shaft (6) connected to the bevel gears (7) for driving the shaft
(18) and the gear I (5A) for imparting rotational movement to gear II (5B)
and driving shaft (12);
a sector (3) attached to the gear II (5B) for driving the tap selector sub
assembly (13) and for maintaining the timing sequence of the movement of
the said gears (5A, 5B), the said sector disposed at equi distance with respect
to gear I (5A) and gear II (5B);
a driving mechanism having coupling shaft (18), bevel gear box (7) and a
hand crank for imparting rotation to gear I (5A) through said bevel gears
disposed in the bevel gear box (7) and said shaft (18);
Characterized in that,
a dial plate (8) is disposed for indicating rotation of drive shaft (12) in
clockwise as well as anticlockwise direction for identifying correction needed

before actual assembly for the rotational lag among transmission system of
tap changer (OLTC) avoiding finding of correction required with complete tap
changer (13) being mounted on testing structure.
2. A method to regulate rotational lags in on-load tap changer adapted in a
transformer comprising;
Imparting clockwise rotation to a hand crank (6) to induce clockwise
rotation to gear II (5B) and to a drive shaft (12);
imparting anticlockwise rotation to the said hand crank (6) to induce
anticlockwise rotation to gear II (5B) and to the said drive shaft (12);
measuring the rotation for both clockwise and anticlockwise direction by a
dial plate (8), to identify rotational lag of tap changer (11), wherein, the
difference in the rotational degrees between clockwise and anticlockwise
movement is corrected by rotating gear II (5B) to the required degrees of
correction identified earlier by the fixture (F) to eliminate rotational lag of said
tap changer (11).