FIELD OF INVENTION:
The present invention relates to an on load tap changer (OLTC), in
high capacity power transformers, using vacuum interrupters,
instead of copper resistance contact, for ' changing the tap
positions without interrupting the load current. The invention
specifically relates to a mechanism by which switching operation
is done in an ordered sequence .for controlling the timing of
operations during on load tap changing.
BACKGROUND:
High voltage transformers, used for example in electrical
substations, are subjected to varying electrical loads depending
upon how much power is being drawn downstream from the
transformers. On-Load tap changers allow for selection of
different turns ratios on a transformer without the need to
interrupt the load current. This makes on-load tap changers useful
for power transformers where interruptions in load current would
be undesirable.

Providing a number of tap positions on a transformer winding
allows the number of turns of the transformer to be selected,
producing a transformer with effectively a variable turn ratio.
This enables voltage regulation of the secondary (output) side of
the transformer to suit different loads.
A contact mechanism of a change-over switch which opens and closes
in insulating oil has heretofore been generally used for on load
tap. changers. With this mechanism, however insulating oil is
subject to contamination by the contacts which open and close.
Recently, therefore, an on load tap changer has been put into
practice by using vacuum switches which have a contact mechanism
and which do not use insulating oil, as current switching elements
of the change over switch. The object of the invention is to
design a diverter switch mechanism using vacuum interrupter.
SUMMARY OF THE INVENTION:
According to the invention an on-load tap-changer switching
mechanism is provided. It comprises of:

Four number of Cams with 3 raised portions and mounted on a
vertical shaft and
Four numbers of Vacuum interrupters placed in horizontal position.
With the rotation of the shaft, cam's raised portion pushes the
roller mounted on the vacuum switches causing operation of the
Vacuum switches. For one tap changing position 72° rotation of cam
shaft is required.
BRIEF DESCRIPTION OF THE ACCOMPAYING DRAWINGS:
The invention can now be described in detail with the help of the
figures of the accompanying drawings in which
Figure 1 shows a circuit diagram of a typical design of an on-load
tap changer;
Figure 2a to 2e shows a schematic view of switching sequence of a
diverter switch for tap changing operation;
Figure 3 shows a 3dimensional view of the newly designed on-load
tap-changer;

Figure 4 shows a schematic plan view of the on-load tap-changer in
first configuration;
Figure 5 shows a schematic plan view of the on-load tap-changer in
second configuration;
Figure 6 shows a schematic plan view of the on-load tap-changer in
third configuration;
Figure 7 shows a schematic plan view of the on-load tap-changer in
fourth configuration;
Figure 8 shows a schematic plan view of the on-load tap-changer in
fifth and final configuration.
DETAILED DESCRIPTION:
The invention will now be described in an exemplary embodiment as
depicted in the accompanying drawing. There can however be other
embodiments of the same invention, all of which are deemed covered
by this description.

A typical circuit design for an on-load tap-changer is shown
schematically in Figure 1. The on-load tap-changer 100 comprises a
transformer coil 150 attached to a first electrical terminal 200,
the transformer coil 150 having a number of tap position switches
110, 112, for selecting different turn ratios on the coil 150. A
diverter switch 130, which includes a rotary switch arm 140, and
diverter impedances 120, 122, connect the tap position switches
110, 112 to a second electrical terminal 300, completing the
circuit.
Tap position switch 110 is shown closed, connecting the associated
tap position to terminal 300 via rotary switch arm 140, which is
shown short circuiting the diverter impedance 120.
The tap charger follows a series of steps to complete an on-load
tap-change between the tap positions associated with the switches
110, 112 outlined as follows.
- Tap switch 110 is closed; -
- Rotary switch arm 140 of the diverter switch 130 rotates
anti clockwise to a position where current passes through
a diverter impedance 120 alone to terminal 300;

- Rotary switch arm 140 turns further to a position where
load passed through diverter impedance 12 0 and 122
simultaneously;
- Rotary switch arm 140 turns further to a position where
load passes through diverter impedance 122 alone to
terminal 300;
- Rotary switch arm 140 of the diverter switch 130 turns
further to a position where diverter impedance 122 is
shorted and the load is supplied through tap position 112
and
- Tap switch 110 is opened.
The above illustrates only one of a number of possible sequences
for tap changing, but shows the general principles involved.
There now follows a description of preferred embodiments of the
invention, by way of non-limiting example, with reference being
made to the accompanying drawings in which:
Figure 2a to 2e shows switching sequence of the diverter switch
for tap changing operation.

Initially shunt-lS and Vacuum switch IV are in contact as shown in
Fig.2a. For changing the tap position, diverter switch follows the
sequence as shown in fig 2a to 2e. In the figure 2e, Vacuum switch
4V and Shunt 2S are in contact, so the tap position in the
transformer is changed.
Figure 3 shows a 3dimensional view of the newly designed on-load
tap-changer. It shows one-third sector of the complete diverter
switch only, as the three sector of the diverter switch is exactly
similar. Hence we will be describing one sector of the diverter
switch only. Opening and closing of Vacuum switch contacts (IV,
2V, 3V and 4V) are done by the Cams (1C, 2C, 3C and 4C) mounted on
a vertical shaft-10 as shown in the figure. And opening and
closing of the moving shunt contacts are done by the Shunt switch
triangles (IT) also mounted on the shaft. Shunt switch triangles
are placed in between cam 2C and 3C. Four vacuum interrupters i.e.
IV, 2V, 3V, and 4V are placed in the horizontal position with two
different elevations. IV and 2V are placed above the moving shunt
contact and 3V and 4V are placed "below the moving shunt contact.
Rollers 1R, 2R, 3R and 4R are mounted on the Vacuum switches IV,
2V, 3V and 4V respectively.

Rollers 1R, 2R, 3R and 4R are in contact with the Cams 1C, 2C, 3C
and 4C respectively. During tap changing operation out of two
Shunt contact and four vacuum switch contact, always two will
remain in contact. In fir.st configuration as shown in fig. 4, 1S
and 1V are in closed position. In this position shunt moving
contact 1N contacts shunt 1S. And cam 1C pressed the roller 1R
which in turn compress the springs in two stages, where in the
first stage of compression vacuum switch contact is established
and in the second stage of compression contact pressure is
applied. In this position remaining Vacuum switches (2V, 3V and
4V) and shunt 2S are in open condition.
With 18° rotation of cam shaft 1V remains in closed position but
1S come in open condition and 2V comes in closed position because
of the position of 2C as shown in fig.5. All the vacuum switches
operate in the similar way. In this position remaining Vacuum
switches (3V and 4V) and shunts (1S and 2S) are in open condition.
With another 18° rotation of cam shaft 2V remains in closed
position but 1V come in open condition and 3V comes in closed

position because of the position of 3C as shown in fig.6. In this
position remaining Vacuum switches (1V and 4V) and shunts (1S and
2S) are in open condition.
With another 18° rotation of cam shaft 3V remains in closed
position but 2V come in open condition and 4V comes in closed
position because of the position of 4C as shown in fig.7. In this
position remaining Vacuum switches (1V and 2V) and shunts (1S and
2S) are in open condition.
With another 18° rotation of cam shaft 4V remains in closed
position but 3V come in open condition and 2S comes in closed
position because of the position of 1T as shown in fig.8. In this
position remaining Vacuum switches (1V, 2V, and 3V) and shunts 1S
are in open condition.

WE CLAIM: -
1. An on-load tap-changer (OLTC) adaptable to a electrical
transformer having a switching mechanism comprising:
- four number of Cams (1C, 2C, 3C and AC)
- four numbers cam rollers (1R, 2R, 3R and 4R)
- two numbers shunt (1S, 2S)
- one pair moving shunt contact (IN)
- one pair shunt switch triangles (1T)
- one vertical shaft (10)
characterized in that 4 no vacuum interrupters (1V, 2V, 3V,
and 4 V) are deployed as diverter switch of the OLTC.
2. The mechanism as claimed in claim 1 wherein the cams are
provided with 3 no raised portion on outer surface of the
cams.
3. The mechanism as claimed in claim 1 wherein the cam rollers
are mounted on the switch assemblies with springs and
brackets.

The mechanism as claimed in claim 1 and claim 2 further
comprising a mounting frame having a number of contact
terminals, the cams and switch assembly.
The mechanism as claimed in claim 4 wherein the cams are
rotatable along with the shaft (10) placed at the center of
the mounting frame.
The mechanism as claimed in claim 4 wherein the switch
assemblies and mounting frame are fixed relative to on
another, and the cams comprises slidable contact with cam
rollers configured to be sequentially engaged with the
number of contact terminals upon rotation of the cam shaft.
The mechanism as claimed in claim 6 wherein the movement of
vacuum switch contact is adjusted by the means of
compression springs.
The mechanism as claimed in claim 6 wherein the slidable
contact is slidable along a planar surface of the mounting
frame.

9. The mechanism as claimed in claim 6 wherein switch contact
shift from one electrical terminal to another electrical
terminal with every 18° rotation of the cam shaft and the
mechanism is designed such that always 2 terminals are in
contact.
10. The mechanism as claimed in claim 9 wherein for one tap
changing in transformer, 72° rotation of cam shaft (10) is
required.
11.The mechanism as claimed in any preceding claims wherein the
electrical switch of an on-load tap-changer comprises a
vacuum switch and cam mounted on a vertical shaft (10)
placed at center of the mounting frame.
12.An electrical transformer comprising the on-load tap-changer
of Claim 1.

The on-load tap changer adaptable to power transformer have
four nos. vacuum interrupters (1V, 2V, 3V and 4 V) which act as
diverter switch. There are two no shunts (1S, 2S) which are
moving and the movement are done by shunt switch triangles (1T)
which is mounted on a vertical shaft (10). 4 no cams (1C, 2C,
3C and 4C) provided with 3 raised portions are mounted on a
vertical shaft and 4 no vacuum interrupters (1V, 2V, 3V and 4V)
are placed in horizontal position. With rotation of shaft,
cam's raised portion pushes the rollers (1R, 2R, 3R and 4R)
mounted on the vacuum switches causing operation of the vacuum
switches in an ordered sequence so that one of the shunt switch
and one of the vacuum interrupters always remain connected in
the circuit throughout the tap changing operation. For one tap
changing position 720 rotation of cam shaft is required.