FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
As amended by the Patents (Amendment) Act, 2005
&
The Patents Rules, 2003 As amended by the Patents
(Amendment) Rules, 2003
COMPLETE SPECIFICATION
(See section 10 and rule 13)
TITLE OF THE INVENTION
Tapping arrangement in Power Transformer
APPLICANTS
Crompton Greaves Limited. CG House, Dr Annie Besant Road, Worli, Mumbai 400
030, Maharashtra, India, an Indian Company
INVENTORS
Bhatia Anilkumar Brijlal, of Crompton Greaves Ltd, Transformers Division, Kanjurmarg (E), Mumbai - 400042, Maharashtra, India, an Indian National
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
The present invention relates to power transformers and more specifically to power transformers having tapped winding arrangements for providing variable voltage ratings.
BACKGROUND OF THE INVENTION
Existing power transformers have tapped windings which may be interconnected in different arrangements for changing voltage rating of a power transformer. With the help of tapped windings, the voltage rating of a main voltage winding may be decreased or increased by a certain voltage rating value.
The tapped windings include a plurality of tapped terminals, each connected to a conductor turn. The plurality of tapped terminals is operated by a tap changer, which interconnect the tap terminals according to the desired voltage rating of respective transformer.
For example, in order to provide a voltage rating of 20% variation, a tapped winding may include 16 equal steps of 1.25% each. This means that the tapped winding may include total 17 tapped terminals, where conductor turns between any two adjacent tap terminals contribute 1.25% of the voltage rating.
For a low voltage power transformer, the number of conductor turns in each step of 1.25% becomes very low and for such low voltage, the current becomes very high and it is difficult to make a tapped winding of a disc winding type. Further, in a disc winding, the number of turns per disc could be as low as 1, 2, or 3 and to provide voltage rating of 1.25% in each step, the tapped terminals may be required to be connected to innermost conductor turns of discs of the disc winding. However, connecting tap terminals to

innermost conductor turns of a disc winding increases complexity in the1 system and makes it cumbersome.
Alternatively, tapped windings may be made using edge winding. However, edge windings have their own set of disadvantages, such as higher eddy losses and increase in conductor size.
Hence, there is a need for a tapped winding arrangement for a low voltage transformer, which is of disc winding type, but gives the same tap range'and tapping increment with a smaller number.of tap terminals. There is also a need for a tapped winding arrangement in which no taps are made on the innermost turns of discs of the disc winding.
DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION
Various embodiments of the present invention provide a power transformer which
comprises a main winding section comprising a plurality of interconnected discs, a
plurality of conductor turns in each disc, a first tap terminal being connected to an inner
turn of a last disc and a second tap terminal being connected to an outer turn of the last
disc; and a tap winding section comprising a plurality of interconnected discs, a plurality
of conductor turns in each disc, and a plurality of tap terminals connected to outermost
conductor turns thereof, such that two adjacent tap terminals select twice the number of
conductor turns selected between the first and second tap terminals.

Preferably, the power transformer comprises a first tap changing mechanism for selecting one of the first and second tap terminals of the main winding section for connecting to a first tap terminal of the tap winding section.
Preferably, the power transformer comprises a second tap changing mechanism for selecting a tap terminal from the plurality of tap terminals,
Preferably, the number of conductor turns selected between the first and second tap terminals of the main winding section provides a voltage rating equal to 1.25 percent of overall voltage rating of the power transformer.
Preferably, the number of conductor turns between two adjacent tap terminals of the tap winding section provide a voltage rating equal to 2.5 percent of overall voltage rating of the power transformer..
These and other aspects, features and advantages of the invention will be better understood with reference to the following detailed description, accompanying drawings and appended claims, in which,
Fig.l illustrates a tapping arrangement in a power transformer in accordance with an embodiment of the present invention;
Fig.2 illustrates an exemplary tapping arrangement in a power transformer in accordance with an embodiment of the present invention; and

FIG. 3 is a table illustrating some of the possible tap terminal interconnections which may be used to change the voltage rating of the transformer.
Referring now to the drawings, and more particularly to FIG. 1, there is shown a power transformer 100 which comprises a main voltage winding section 10 and a tap winding section 20. Both the main voltage winding section 10 and tap winding section 20 are positioned around a magnetic core center 30. The main voltage winding section 10 comprises a plurality of discs 10(1), 10(2)...10(n), where n is the total number of discs in the main voltage winding section 10. The tap winding section 20 comprises a plurality of
discs 20(1), 20(2) 20(y), where y is the total number of discs in the tap winding
section 20. Each of the discs of the main winding section 10 and tap winding section 20 comprises a plurality of conductor turns. For example, each disc of the main winding section 10 and the tap winding section 20 comprises total three conductor turns.
It may be noted that the tap winding section 20 provide a lower voltage rating than the main voltage winding section 10 and therefore anytime, the total number of conductor turns in the tap winding section 20 are less than the total number of conductor turns in the main voltage winding section 10.
In the main winding section 10. a tap terminal 1 is located at top of the main winding section 10, connected to first turn of the first disc 10(1). A first tap terminal 3 is connected to an inner turn, preferably innermost turn of the last disc 10(n) and a second tap terminal 4 is connected to an outer turn, preferably outermost turn of the last disc 10(n).

In the tap winding section 20, each outermost conductor turn is connected to a tap terminal, such that the tap winding section 20 comprises a plurality of tap terminals 6, 7,
8 'x' extending out from respective outermost conductor turn. It may be noted that the
number of conductor turns selected between any two adjacent tap terminals of the tap winding section 20 is twice the number of turns selected between tap terminals 3 and 4 of the main winding section 10. For example, the number of conductor turns that are selected between the tap terminals 3 and 4 is three, whereas the number of conductor turns that are selected between the tap terminals 6 and 7 is six.
The power transformer 100 may comprise a first tap changing mechanism for selecting one of the first and second terminals 3 and 4 for connecting to the tap terminal 5. The tap terminal 5 is connected to a first tap terminal 6 of the tap winding section 20.
The power transformer 100 may comprise a second tap changing mechanism for connecting a tap terminal 2 to any one of the plurality of tap terminals 6, 7, 8...x, such that between terminals 1 and 2, a desired voltage rating is obtained.
The tap terminals 2.3,4,5...x may be interconnected in various possible ways so as to add none, part or all of the voltage of the last disc 10(n) and tap winding section 20 to voltage of (n-1) discs of the main winding section 10.
The total voltage rating of the transformer 100 is dependent upon the connection arrangement of the tap terminals 2, 3. 4. 5....X.
Fig.2 illustrates an exemplary tapping arrangement in the power transformer 100 in accordance with an embodiment of the present invention. The power transformer 100

may be a low voltage class power transformer of 3.6 to 33kv class having capacity more than 5 MVA.
The main voltage winding section 10 comprises total 213 conductor turns, with 3 conductor turns in each disc. The initial 210 turns are not connected to any tap terminal, whereas three turns of the last disc are selected between the tap terminals 3 and 4 respectively. The tap terminals 3 and 4 provide one tapping step of voltage rating 1.25%.
The tap winding section 20 comprises total 48 conductor turns, and a tap terminal is provided at every sixth turn, thus providing total 8 tap terminals (6, 7... 14) in the tap winding section. It can be said that total 8 tap steps each of voltage rating 2.5% are provided in the winding section 20.
When the tap terminal 5 is connected to the tap terminal 3, total 210 turns are selected in the main winding section 10, whereas when the tap terminal 5 is connected to the tap terminal 4, total 213 turns are selected in the main winding section 10; When the tap terminal 5 is connected to either of tap terminals 3 and 4, then by connecting the tap terminal 2 to any of the tap terminals 6, 7, ....and 14, a desired amount of turns are selected between tap terminal 1 and 2, and thus a desired voltage rating of transformer is obtained.
The number of conductor turns that can be added to the 210 turns of the main winding section 10 using tap terminals 2, 3, 4... 14 are in multiples of 3, even though minimum number of turns selected in the tap winding section 20 is six. Thus, by providing two tap terminals between last three turns of the main winding section 10, and

providing tap terminals between every six turns of the tap winding section 20, one is able to obtain a minimum voltage variation corresponding to three turns. Thus, there is no need to provide tap terminals between every three turns of the tap winding section 20.
FIG. 3 is a table 300 representing some possible tap connection arrangements of the transformer 100 and corresponding voltage rating. While measuring voltage ratings due to various tap arrangements, it has been assumed that the main voltage winding section 10 has a voltage rating of 90%, whereas the last disc 10(n) of the main voltage winding section 10 has a voltage rating of 1.25%. Further, every tap step of the tap winding section 20 have a voltage rating of 2.5%. Other voltage ratings are also within the contemplation of this invention.
As illustrated in the table 300, when the tap terminal 5 is connected to the tap terminal 4 and the tap terminal 2 is connected to the tap terminal J4, the voltage rating of the transformer 100 is maximum, i.e. 110% of the rated voltage. When the tap terminal 5 is connected to the tap terminal 3, the tap terminal 2 is connected to the tap terminal 14,
the last disc 10(n) is bypassed and a voltage rating of 108.75% is obtained. The voltage
i
rating decreases with the connection of the tap terminal 2 to upper tap terminals of the tap winding section 20. When the tap terminal 2 is connected to terminal 6, the tap winding section 20 is bypassed and a voltage rating of 90% is obtained. It is apparent from the table that the voltage ratings are obtained with a minimum increment of 1.25% voltage, although the minimum voltage variation provided by the tap winding section 20 is 2.5%. Thus, there is no requirement of providing a tap step of 1.25% in the tap winding section 20, when a tap step of 1.25% is provided in the main winding section 10.

In prior art, in order to obtain a tap step of 1.25%, total 16 tap terminals were provided in the tap winding section 20, whereas, in the present invention, in order to obtain a tap step of 2.5%, total 10 tap terminals are provided. It will be appreciated by a person skilled in the art that the present invention provides A tapped winding arrangement for a low voltage transformer which is of disc winding type and gives the same tap range and tapping increment with a smaller number of tap terminals. Further, the tap terminals of the tap winding section 20 are not connected to innermost turns of discs of the disc winding, thus reducing the complexity in the system and easing manufacturing of the transformer.
Although the invention has been described with rederence to a specific embodiment, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiment, as well as alternate embodiments of the invention, will become apparent to persons skilled in the art upon reference to the description of the invention. It is therefore contemplated that such modifications can be made without departing from the scope of the invention as defined in the appended claims.

We claim:
1. A power transformer comprising:
a main winding section comprising a plurality of interconnected discs, a plurality of conductor turns in each disc, a first tap terminal being connected to an inner turn of a last disc and a second tap terminal being connected to an outer turn of the last disc; and
a tap winding section comprising a plurality of interconnected discs, a plurality of conductor turns in each disc, and a plurality of tap terminals connected to outermost conductor turns thereof, such that two adjacent tap terminals select twice the number of conductor turns selected between the first and second tap terminals.
2. The power transformer as claimed in claim 1, which comprises a first tap changing mechanism for selecting one of the first and second tap terminals of the main winding section for connecting to a first tap terminal of the tap winding section.
3. The power transformer as claimed in claim 1, which comprises a second tap changing mechanism for selecting a tap terminal from the plurality ofjtap terminals.
4. The power transformer as claimed in claim 1, wherein the number of conductor turns selected between the first and second tap terminals of the main winding

section provide a voltage rating equal to 1.25 percent of overall voltage rating of the power transformer. 5. The power transformer as claimed in claim 1, wherein the number of conductor turns between two adjacent tap terminals of the tap winding section provide a voltage rating equal to 2.5 percent of overall voltage rating of the power transformer.