FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
As amended by the Patents (Amendment) Act, 2005
AND
The Patents Rules, 2003
As amended by the Patents (Amendment) Rules, 2006
COMPLETE SPECIFICATION
(See section 10 and rule 13)
TITLE OF THE INVENTION
A cascaded tap changer mechanism for a transformer.
APPLICANT(S)
Crompton Greaves Limited, CG House, 6th Floor, Dr. Annie Besant Road,
Worli, Mumbai - 400030, Maharashtra, India; an Indian Company.
INVENTOR(S)
Hassan Hafiz Imtiaz, Saha Raja and Wachasundar Shripad; all of Crompton Greaves Limited, CG Global R&D, Aryabhatta Building, Kanjur Marg (East), Mumbai - 400042, Maharashtra, India; all Indian Nationals.
PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the nature of this invention and the manner in which it is to be performed:

FIELD OF THE INVENTION
This invention relates to the field of electrical and electronics engineering.
Particularly, this invention relates to the field of transformers.
More particularly, this invention relates to tap changers for transformers.
Specifically, this invention relates to a cascaded tap changer mechanism for a transformer.
BACKGROUND OF THE INVENTION
Transformer is an electrical device that transfers energy from one circuit to another by magnetic coupling without any moving parts. It works on the Principle of Faradays Law of Electromagnetic Induction together with other laws of electricity. When a magnetic flux linked with an electric circuit varies, an electro motive force (voltage) is generated in the electric circuit, proportional to the rate of variation of flux with time.
The basic functional components of the transformer are a core made of magnetic material, a primary winding, and a secondary winding. The primary winding is connected to a source of an alternating current and the secondary winding is connected to the load. As the alternating current flows through the primary winding, the change in current in the primary windings creates a time-varying magnetic flux in the core, which induces a voltage in the secondary windings.

The formula for working of an ideal transformer is:
Vp /Vs = Np/Ns where, Vp is the voltage in the primary side, Vs is the voltage in the secondary side, Np is the number of turns of winding on the primary side, Ns is the number of turns on the secondary side.
Upon varying, the number of turns on any side, subsequent changes of voltages occurs. Thus, it is possible to have a step-up transformer, wherein the formula executes an output power which is higher than the input power. Conversely, it is possible to have a step-down transformer, wherein the formula executes an input power which is higher than the output power.
This selection of turns may be automated by the provisioning of a central tap. Both the primary and secondary windings on power transformers may have external connections, called taps, to intermediate points on the winding to allow selection of the voltage ratio.
For many power transformer applications, a supply interruption during a tap change is unacceptable, and the transformer is often fitted with a more expensive and complex on-load tap-changing (OLTC) mechanism. On-load tap changers may be generally classified as mechanical, electronically assisted, or fully electronic.
The on-load tap changing (OLTC) regulators have been widely used since the introduction of electric energy. They ensure a good regulation of the output voltage in presence of large variations of the input voltage with typical response time from 100 ms to several seconds. The continuous growth of power semiconductor devices, such as the insulated gate bipolar transistor, allows the development of

fast OLTC regulators being able to fix other problems in the ac mains, like sags and flicker.
The common practice in Transformer industry for voltage regulation is by using an OLTC (on load tap changer). Taps can be taken out either from primary (High voltage side) or secondary (Low voltage) side. By changing the taps output voltage of the transformer can be regulated.
With the advent of fast semiconductor devices and fast control techniques, 'Electronics Tap changers' are gaining more popularity over mechanical ones. The prime advantageous of electronics based OLTC over mechanical ones are fast response time, arc-less operation, and being maintenance free.
The obvious practice is to use tapping either in primary or in secondary side of the main transformer. The main problems of using the electronics switching in primary side are requirement of switching of high PIV (peak inverse voltage), their isolation and insulation. Whereas in other case, i.e. tapping from secondary, would require switches of high current rating, which will introduce extra losses in the system.
PRIOR ART
To cope with these problems, the common practice is to use a Compensation transformer in series with secondary side, which will reduce the current stress on the devices. However, the extraction of tapping from secondary is quite difficult in a step-down transformer, as the numbers of turns are very less compared to

primary. And using another transformer in form of compensating transformer would contribute in extra loss, size and cost of the system.
US 1537927 discloses improved arrangement of tapped transformer windings with suitable means for so adjusting the interconnections between tap leads. Tap connections of a transformer winding may be varied and ratio of transformation of transformer conveniently adjusted without interrupting the current in the load circuit.
CN201689773 discloses OLTC tap changing. The tap-changer has positive and negative modulation switches whose ends are connected with tail end of main coil and pressure regulating coil respectively. The ends of through switch are connected to tapping point of regulating coil and edge coil of transformer. The modulation switches and through switch are provided with corresponding light-operated trigger circuits. A control unit is connected with the trigger circuits through optical fiber. Switching of tapping points of the transformer can be realized easily and electric arc generation can be reduced, also switching speed can be increased. However, rating of the switches will be high which will impact in isolation cost.
OBJECTS OF THE INVENTION
An object of the invention is to provide a tap changer mechanism with reduced number of switches.
Another object of the invention is to provide a cascaded tap changer mechanism with reduced number of switches.

Yet another object of the invention is to provide a cascaded tap changer mechanism so as to reduce losses.
Still another object of the invention is to provide a tap changer with reduced number of drivers.
SUMMARY OF THE INVENTION
According to this invention, there is provided a tap changer mechanism for a
transformer with primary windings and extra windings connected to said primary
windings,
said tap changer mechanism comprises:
• a switching topology at said primary windings, said switching topology
comprisng a first set of turns in series with a second set of turns, said
switching topology further comprising a first switch in parallel to said first
set of turns and a second switch in parallel to said second set of turns, and
said switching topology still further comprising an additional third switch,
having at least a first terminal and at least a second terminal, said third
switch connected between said first switch and said second switch at said
first terminal and said first set of turns and at said second set of turns at said
second terminal.
According to this invention, there is also provided a cascaded tap changer mechanism for a transformer with primary windings and extra windings connected to said primary windings, said tap changer mechanism comprises:
• a switching topology at said primary windings, said switching topology
comprisng a first set of turns in series with a second set of turns, said

switching topology further comprising a first switch in parallel to said first set of turns and a second switch in parallel to said second set of turns, and said switching topology still further comprising an additional third switch, having at least a first terminal and at least a second terminal, said third switch connected between said first switch and said second switch at said first terminal and said first set of turns and at said second set of turns at said second terminal,
characterised, in that, said switching topologies are connected in series wih each
other and to said main windings to provide multiple taps.
Typically, said first switch and said second switch are in series with each other.
Typically, closing said first switch provides positive voltage across turns.
Typically, closing said second switch provides negative voltage across turns.
Typically, closing said third switch provides zero voltage across turns.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 illustrates a switching topology, of the prior art, for tap changing.
The invention will now be described in relation to the accompanying drawings, in which:
Figure 2 illustrates a schematic electrical configuration of the invention.

Figure 3 illustrates a cascaded arrangement of the schematic electrical configuration of Figure 2.
DETAILED DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 illustrates a switching topology, of the prior art, for tap changing.
In this topology, switches Al and A4 are in series, switches A3 and A2 are in series, switches Al and A3 are in parallel, and switches A4 and A2 are in parallel Secondary turns, which provide voltage V2, are in parallel to these switches. An additional switch connects between switches Al and A4 and between switches A3 and A2. This additional switch AA is connected to extra windings EW of a transformer.
According to this invention, there is provided a cascaded tap changer mechanism for a transformer.
Figure 2 illustrates a schematic electrical configuration of the invention.
In accordance with an embodiment of this invention, there is provided a switching topology at the primary windings or high voltage side of a transformer. The switching topology comprises a first set of turns V21 in series with a second set of turns V22. Reference numeral EW refers to extra windings of the transformer. A first switch SI is provided in parallel to the first set of turns V21. A second switch S2 is provided in parallel to the second set of turns V22. Typically, the first switch SW1 and the second switch SW2 are in series with each other. An additional thrid switch SW3 connects between the first switch SW1 and second switchSW2 at its

first terminal and first set of turns V21 and second set of turns V22 at its second terminal.
By performing different switching configurations, three sets of voltage, i.e. V2, 0, -V2 are obtained.
In prior art of Figure 1;
a) + V2 was achieved by closing Al, A2;
b) 0 was achieved by closing either Al, A3 or A4, A2 ; and
c) -V2 was achieved by closing A3, A4
In the current invention of Figure 2;
a) +V21 (=+V2) is achieved by closing switch SI;
b) 0 is achieved by closing switch S3; and
c) -V21 (=-V2) is achieved by closing switch S2.
Hence same functions can be obtained by both the topologies, but the current topology is offered with less number of switches (3 in compared to 4). Therefore, there will be less cost and increased reliability apart from higher response time.
The range of voltage by this configuration can be varied from (VI ± V2 * (3n -l)/2). And the effecting tappings are 3.
Figure 3 illustrates a cascaded arrangement of the schematic electrical configuration of Figure 2.

Reference numeral MW refers to main windings. Reference numeral SW refers to secondary windings. Reference numeral EW1 refers to first set of extra windings connected to main windings MW. Reference numeral EW2 refers to second set of extra windings connected to main windings MW. Reference numeral EW3 refers to third set of extra windings connected to main windings MW. These extra sets of windings are in series with each other.
According to a non-limiting exemplary embodiment, 9 effective tappings can be achieved by using two extra windings and 27 effective tappings are achieved by employing three extra winding. Hence it can be said that maximum number of tappings will be 3n (n = number of extra windings) provided the voltage ration among the windings are 3. Following table compare the prior art technologies and proposed topology for 27 effective tappings.
Table 1, below, discloses parameters in relation to prior art as compared with the current invention, and advantages, thereof.

PARAMETER PRIOR ART CURRENT INVENTION
Number of extra windings 3 3
Number of switches 12 9
Losses (considering 20A phase current and on-state voltage drop) 120W 60W
Number of gate drivers 12 9
Reliability Less High
Table 1

The INVENTIVE STEP of the current invention lies in employing relatively lesser number of switches for achieving same tap configuration of the prior art, thereby increasing reliability, increasing response time, and reducing stress on switches. The employment of a single extra winding in primary side results in total three effective tappings, whereas the number of effective tappings can be increased by using the extra windings and connected them in cascaded mode.
While this detailed description has disclosed certain specific embodiments of the present invention for illustrative purposes, various modifications will be apparent to those skilled in the art which do not constitute departures from the spirit and scope of the invention as defined in the following claims, and it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the invention and not as a limitation.

We claim,
1. A tap changer mechanism for a transformer with primary windings and extra
windings connected to said primary windings,
said tap changer mechanism comprising:
• a switching topology at said primary windings, said switching topology comprisng a first set of turns in series with a second set of turns, said switching topology further comprising a first switch in parallel to said first set of turns and a second switch in parallel to said second set of turns, and said switching topology still further comprising an additional third switch, having at least a first terminal and at least a second terminal, said third switch connected between said first switch and said second switch at said first terminal and said first set of turns and at said second set of turns at said second terminal.
2. The tap changer mechanism for a transformer with primary windings and extra windings connected to said primary windings as claimed in claim 1, wherein said first switch and said second switch are in series with each other.
3. The tap changer mechanism for a transformer with primary windings and extra windings connected to said primary windings as claimed in claim 1, wherein closing said first switch provides positive voltage across turns.
4. The tap changer mechanism for a transformer with primary windings and extra windings connected to said primary windings as claimed in claim 1, wherein closing said second switch provides negative voltage across turns.

5. The tap changer mechanism for a transformer with primary windings and extra windings connected to said primary windings as claimed in claim 1, wherein closing said third switch provides zero voltage across turns.
6. The cascaded tap changer mechanism for a transformer with primary windings and extra windings connected to said primary windings,
said tap changer mechanism comprising:
• a switching topology at said primary windings, said switching topology comprisng a first set of turns in series with a second set of turns, said switching topology further comprising a first switch in parallel to said first set of turns and a second switch in parallel to said second set of turns, and said switching topology still further comprising an additional third switch, having at least a first terminal and at least a second terminal, said third switch connected between said first switch and said second switch at said first terminal and said first set of turns and at said second set of turns at said second terminal,
characterised, in that, said switching topologies are connected in series wih
each other and to said main windings to provide multiple taps.
7. The cascaded tap changer mechanism for a transformer with primary windings and extra windings connected to said primary windings as claimed in claim 6, wherein said first switch and said second switch are in "series with each other.
8. The cascaded tap changer mechanism for a transformer with primary windings and extra windings connected to said primary windings as claimed

in claim 6, wherein closing said first switch provides positive voltage across turns.
9. The cascaded tap changer mechanism for a transformer with primary windings and extra windings connected to said primary windings as claimed in claim 6, wherein closing said second switch provides negative voltage across turns.
10. The cascaded tap changer mechanism for a transformer with primary windings and extra windings connected to said primary windings as claimed in claim 6, wherein closing said third switch provides zero voltage across turns.