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, 2006
COMPLETE SPECIFICATION
(See section 10 and rule 13)
TITLE OF THE INVENTION
Oil filled internally clamped transformer having improved short circuit withstand capability
APPLICANTS
Crompton Greaves Limited, CG House, Dr Annie Besant Road, Worli, Mumbai 400 030,
Maharashtra, India, an Indian Company
INVENTORS
Vaidya Sameer Sharadchandra, Joshi Vikrant Shashikant and Vanage Manish Madhukar, all of Crompton Greaves Limited, Transformer Division (Tl), Kanjurmarg (E), 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 an oil filled internally clamped transformer having
improved short circuit withstand capability.
BACKGROUND OF THE INVENTION
An oil filled transformer is single phase or multiphase. Each phase of an oil filled transformer comprises a conductor coil assembly mounted on one limb of a magnetic material core and located between the bottom yoke and top yoke of the core. The coil assembly comprises a low voltage winding and a high voltage winding wound concentrically in radially spaced apart relationship with each other. Each of the low voltage winding and high voltage winding comprises a plurality of turns of conductors with dielectric material layers between the turns of conductors. The core coil assembly is mounted in a transformer tank filled with transformer oil. The core coil assembly is clamped in position in the transformer tank internally or externally. An internally clamped transformer comprises a bottom clamping arrangement and a top clamping arrangement. The bottom clamping arrangement comprises a pair of bottom frame members one located at one side of the bottom yoke and the other located at the opposite side of the bottom yoke. The bottom frame members are clamped together with bottom yoke studs running across the bottom yoke. The ends of the bottom yoke
2
31 MAR 2009

studs are tightened against the bottom frame members with nuts. The bottom frame members are further clamped together with bottom end tie rods running across the bottom frame members at their ends over the bottom yoke ends. The ends of the bottom end tie rods are tightened against the bottom frame members with nuts. The coil is supported at the bottom thereof against bottom coil support members which are normally welded to the bottom frame members at one ends thereof. The bottom coil support members are located at the top ends of bottom gussets which are disposed vertically across the bottom frame members and fixed to the bottom frame members. Each of the bottom gussets comprises two gusset members spaced apart from each other. The bottom gussets are supported at the bottom ends thereof on base pads which in turn are supported on base feet located across the bottom yoke. The core coil assembly is mounted on the bottom of the transformer tank against the base feet. The top clamping arrangement comprises a pair of top frame members one located against one side of the top yoke and the other located against the opposite side of the top yoke. The top frame members are clamped together with top yoke studs running across the top yoke. The ends of the top yoke studs are tightened against the top frame members with nuts. The top frame members are further clamped together with top end tie rods running across the top frame members at the ends thereof over the top yoke ends. The ends of the top end tie rods are tightened against the top frame members with
3
3 1 MAR 2O09

nuts. The top frame members are further held together at the top by top stays disposed across the top yoke and fixed to the top frame members. The top and bottom frame members are joined together at each side of the top and bottom yokes with flitch plates which run from top to bottom along each limb of the core and are located in the top and bottom frames using flitch plate pins and are tied to each limb of the core. The top of the coil is clamped against the top frame members by clamping screws tightened against a coil clamping ring located at the top end of the coil. The coil clamping screws are held in screw clamping bosses in thread engagement therewith. The screw clamping bosses laterally extend from the top frame members and are welded to the top frame members at one ends thereof.
During operation of the transformer, short circuits can develop on the transformer terminal due to various abnormal conditions in the electrical power system. Under short circuit conditions, the coil is subjected to axial and radial forces. The axial forces are very high and the transformer must sustain them successfully. As the bottom gussets are of two piece construction and discontinuous and disconnected, the axial forces acting on the bottom coil support members are not effectively transferred to the transformer tank bottom via the bottom gussets, base pads and base feet. Further the bottom coil support members fixed to the bottom
4
31 MAR 2009

frame members only at their one ends are like cantilevers. Therefore, the axial forces acting on the bottom coil support members tend to twist and bend the bottom frame members. Also the surface area of contact between the bottom faces of the coil clamping screws and the coil clamping ring is small and limited. Therefore, the axial forces acting on the coil are not effectively transmitted to the top stays and top frame members via the screw clamping bosses. Further the screw clamping bosses fixed to the top frame members only at their one ends, are like cantilevers. Due to the axial forces acting on the coil clamping screws, the clamping screws may bend or break. The clamping screw bosses also may break off. Also, the axial forces acting on the coil tend to twist and bend the top frame members. As a result of all this, misalignment and deformation of the coil will take place and the transformer will start malfunctioning and reliability and life of the transformer will be reduced.
OBJECTS OF THE INVENTION
An object of the invention is to provide an oil filled internally clamped transformer having improved short circuit withstand capability because of which the reliability and life of the transformer is increased.
5
31 MAR 2009

Another object of the invention is to provide oil filled internally clamped transformer having improved short circuit withstand capability, which transformer is simple in construction and cost effective
DETAILED DESCRIPTION OF THE INVENTION
According to the invention there is provided an oil filled internally clamped transformer having improved short circuit withstand capability, the transformer comprising atleast one phase having a conductor coil assembly mounted on one limb of a magnetic material core and located between the bottom yoke and top yoke of the core, the core coil assembly being mounted in a transformer tank filled with transformer oil, the core coil assembly being clamped in position in the transformer tank by a bottom clamping arrangement and a top clamping arrangement, wherein the bottom clamping arrangement comprises a bottom frame comprising a pair of bottom frame members, one located against one side of the bottom yoke and the other located against the opposite side of the bottom yoke, the bottom frame members being clamped together with bottom yoke studs engaged across the bottom yoke and with bottom end tie rods engaged across the bottom frame members at their ends over the bottom yoke ends, a plurality of bottom gussets of single piece construction vertically located against the bottom frame members and fixed to the bottom frame members, a plurality of base pads
6
31 MAR 2009

each located at the bottom of pairs of bottom gussets and a plurality of base feet each located below each of the base pads and across the bottom yoke, a plurality of pairs of bottom coil support members connected together with a web portion, the web portions being located at the upper surface of the bottom frame members with their one end extending into the bottom frame and their other end extending out of the bottom frame, the web portions being welded to the bottom frame members at the inner and outer surfaces of the bottom frame members and a plurality of pairs of bottom frame bands disposed between pairs of bottom yoke studs and running across the bottom yoke over the top of the bottom yoke with their ends fixed to the bottom frame members and the top clamping arrangement comprises a top frame comprising a pair of top frame members one located against one side of the top yoke and the other located against the opposite side of the top yoke, the top frame members being clamped together with a plurality of top yoke studs running across the top frames through the top yoke and with top end tie rods running across the top frame members at their ends over the ends of the top yoke, a plurality of top coil support members disposed against the bottom surface of the top frame members with their one ends extending into the top frame and their other ends extending out of the top frame, the top coil support members being welded to the top frame members at the inner and outer surfaces of the top frame members, atleast one insulator support corresponding
7

to each of the top coil support members and disposed on a coil clamping ring located at the top of the coil and packing provided in the space between each insulator support and corresponding top coil support member and a plurality of mounting brackets each corresponding to each of the top coil support members and positioned above each of the top coil support members and mounted to the top frame members and a plurality of pairs of top frame bands each pair being provided between each pair of top yoke studs and extending over the top yoke underneath the top yoke with their ends fixed to the top frame members and a plurality of top stays disposed across the top yoke and fixed to the upper ends of the top frame members and a plurality of top gussets corresponding to each of the top coil support members and fixed to the top brackets, top frame members and top stays, the top and bottom frame members being joined together at each side of the top and bottom yokes with flitch plates which run from top to bottom along each limb of the core and are located in the top and bottom frames using flitch plate pins and are tied to the outside of each limb of the core.
The following is a detailed description of the invention with reference to the accompanying drawings, in which :
8
31 MAR 2009

Fig 1 is an isometric view of an oil filled internally clamped transformer having improved short circuit withstand capability according to an embodiment of the invention;
Fig 2 an isometric view of the bottom of the transformer of Fig 1;
Fig 3 is a crosssectional view of the transformer of Fig 1 across the bottom thereof;
Fig 4 is a plan view of a pair of bottom coil support members;
Fig 5 is an enlarged view at A in Fig 3;
Fig 6 is an isometric view of the top of the transformer of Fig 1;
Fig 7 is a crosssectional view across the top of the transformer of Fig 1;
Fig 8 is an enlarged view at B in Fig 7; and
Fig 9 is a view in the direction X in Fig 7.
9
31 MAR 2009

The transformer 1 as illustrated in Figs 1 to 9 of thee accompanying drawings is a single phase transformer comprising a coil assemply 2 mounted on a limb 3 of a magnetic material core 4 and held between the bc*tom yoke 5 and top yoke 6 of the core. The end limbs of the core are marked 7 and 8. 9 is a bottom frame comprising a pair of bottom frame members 10, 1°, one located against one side of the bottom yoke 5 and the other located against the opposite side of the bottom yoke. The bottom frame members 10, 10 are held together against the bottom yoke with bottom yoke studs 11 running across the bottom yoke 5 and tightened on the bottom frame members with nuts 12. The pottom frame members are also held together with bottom tie rods 13 running across the bottom frame members at the ends thereof over the ends of the bottom yoke and tightened against the bottom frame members with nuts 14. A pluralith of bottom gussets 15 are vertically disposed against the bottom frame menders and welded to the bottom frame members. The bottom gussets are single piece construction. Base pads 16 are located against pairs of bottom gussets at the lower ends thereof. A plurality of base feet 17 are disposed across the bottom y^ke with their ends fixed to the base pads. A plurality of pairs of bottom coil support members 18 interconnected with web portions 19 are located at the top surface of the bottom frame members with one ends 19a of the web portions extending into the bottom frame and the
10
31 MAR 2009

other ends 19b of the web portions extending out of the bottom frame member (Fig 5). The bottom coil support members are welded to the upper ends of the respective bottom gussets. The web portions of the bottom coil support members are welded to the bottom frame members at the inner and outer surfaces of the bottom frame members marked 19c and 19d, respectively (Fig 5). Pairs of bottom frame bands 20, 20 preferably, made of fibre glass are disposed across the bottom yoke over the top of the bottom yoke and between pairs of bottom yoke studs with their ends fixed to the bottom frame members. 21 is a top frame comprising a pair of top frame members 22, 22, one located against one side of the top yoke 6 and the other located against the opposite side of the top yoke. The top frame members are clamped together against the top yoke with top yoke studs 23 running across the top frame members through the top yoke and tightened on the top frame members with nuts 24. The top frame members are also clamped together at their ends by top end tie rods 25 running across the top frame members at their ends over the ends of the top yoke and tightened on the top frame members with nuts 26. A plurality of top coil support members 27 are located against the bottom surfaces of the top frame members with their one ends 27a extending into the top frame and the other ends 27b extending out of the top frame (Fig 8). The top coil supporting members are welded to the top frame members at the inner and outer surfaces of the top frame members marked 27c
11
31 MAR 2009

and 27d, respectively (Fig 8). A plurality of hydraulic jacks 28 corresponding to each of the top coil support members are mounted to a mounting bracket 29 which in turn is disposed above each of the top coil support members and mounted to the respective top frame members. Each of the hydraulic jacks comprises a piston 30 reciprocating in a hydraulic cylinder 31. The piston is extendable downwardly beyond the cylinder and the top coil support members 27 through openings 32 in the top coil support members. 33 is a top coil clamping ring located against the top end of the coil assembly 2. Insulator supports marked 35 and 36, preferably wooden supports are disposed against the top coil clamping ring one on top of the other. 37 represents packing material between the top coil supporting members and the top insulator supports 35. A plurality of top stays 38 are disposed across the top yoke with their ends fixed to the top frame members. A plurality of top gussets 39 are fixed to the mounting brackets and top frames and top stays. A plurality of pairs of top frame bands 40 are disposed across the top yoke 6 underneath the top yoke between pairs of top yoke studs 23 with their ends fixed to the top frame members. The top frame members 22,22 and bottom frame members 10,10 are joined together at each side of the top yoke 6 and bottom yoke 5 with flitch plates 42 which run from top to bottom along each limb 3, 7, 8 of the core and are located in the top and bottom frames using flitch plate pins 43 and tied to the outside of each limb 3, 7, 8 of the core.
12
31 MAR 2009

According to the invention, the coil 2 is clamped to the design pressure by operating the hydraulic jacks 28. On pressuring the hydraulic cylinders 31 with hydraulic fluid (not shown) the pistons 30 move down through openings 32 in the top coil support members 27 and tighten the top insulator supports 35, bottom insulator supports 36 and the top coil clamping ring 33 against the top of the coil assembly 2. This pressing creates gaps or spaces (not marked) between the top coil support members and the top insulator supports 35, which are packed with packing material 37 so as to maintain the design clamping pressure against the coil assembly. The jacks are used only during the manufacturing of the transformer for clamping the coil assembly and are removed once the coil clamping is complete. During operation of the transformer, axial forces developed due to short circuits are effectively transmitted to the bottom of the transformer tank via the bottom coil supporting members, bottom gussets, base pads and base feet as the bottom gussets are single piece construction and without any break or discontinuity. The bending and twisting forces developed along the bottom frame members due to the axial forces acting on the bottom coil support members are counterbalanced by the web portions of the bottom coil supporting members with their one ends extending into the bottom support frame and their other ends extending out of the bottom support frame. The one ends of
13
31 MAR 2009

the web portions of the bottom coil supporting members will bend in the direction opposite to the direction of bending of the other ends of the web portions of the bottom coil supporting members so as to counterbalance the axial forces acting on the bottom coil support members. Also the axial forces on the bottom coil support members are shared by the interconnecting web portions thereof and transmitted to the area reinforced by the respective bottom yoke studs. Further, the bending and twisting of the bottom frame members are prevented by the bottom yoke studs in conjunction with bottom frame bands. The surface area of contact between the coil clamping ring and the insulator supports and the top coil clamping members are considerably large. Therefore, the axial forces developed in the coil assembly are effectively transmitted to the top stays via the mounting brackets and top gussets and top frames. Since the top coil supporting members are supported at the bottom of the top frame members with their one ends extending into the top frame and the other ends extending out of the top frame members, the axial forces acting on the other ends of the top coil supporting members extending out of the top frame are counterbalanced by the one ends of the top coil supporting members extending into the top frame in that when the other ends of the top coil supporting members bend in one direction due to the axial forces, the one ends of the top coil supporting members bend in the opposite direction so as to counterbalance the axial forces acting on the top coil
14
31 MAR 2009

supporting members. Since the top coil supporting members are offset from the top frame members along transverse direction away from yoke , it causes bending of top frame members. This bending is counterbalanced by yoke studs in conjunction with top frame bands. This prevents bending and twisting of the top frame members. The flitch plates and flitch plate pins together also resist the axial forces produced due to short circuit. As a result of all this, the short circuit withstanding capability of the transformer is improved and reliability and life of the transformer are increased. The transformer is also simple in construction and requires reduced maintenance expense as the clamping screws and associate parts have been eliminated.
The above embodiment of the invention is by way of example and should not be construed and understood to be limitative of the scope of the invention. The jack need not be hydraulic and can be pneumatic or any other jack. The transformer can be multiphase. There can be one or more than two layers of insulator supports. Embodiments of the invention comprising such variations are obvious to those skilled in the art and are to be construed and understood to be within the scope of the invention.
15
3 1 MAR 2009

We claim:
1) An oil filled internally clamped transformer having improved short circuit withstand capability, the transformer comprising atleast one phase having a conductor coil assembly mounted on one limb of a magnetic material core and located between the bottom yoke and top yoke of the core, the core coil assembly being mounted in a transformer tank filled with transformer oil, the core coil assembly being clamped in position in the transformer tank by a bottom clamping arrangement and a top clamping arrangement, wherein the bottom clamping arrangement comprises a bottom frame comprising a pair of bottom frame members, one located against one side of the bottom yoke and the other located against the opposite side of the bottom yoke, the bottom frame members being clamped together with bottom yoke studs engaged across the bottom yoke and with bottom end tie rods engaged across the bottom frame members at their ends over the bottom yoke ends, a plurality of bottom gussets of single piece construction vertically located against the bottom frame members and fixed to the bottom frame members, a plurality of base pads each located at the bottom of pairs of bottom gussets and a plurality of base feet each located below each of the base pads and across the bottom yoke, a plurality of pairs of bottom coil support members connected together with a web portion, the web portions being located at the upper surface of the bottom frame members with their one end
16

extending into the bottom frame and their other end extending out of the bottom frame, the web portions being welded to the bottom frame members at the inner and outer surfaces of the bottom frame members and a plurality of pairs of bottom frame bands disposed between pairs of bottom yoke studs and running across the bottom yoke over the top of the bottom yoke with their ends fixed to the bottom frame members and the top clamping arrangement comprises a top frame comprising a pair of top frame members on^ located against one side of the top yoke and the other located against the opposite side of the top yoke, the top frame members being clamped together with a plurality of top yoke studs running across the top frames through the top yoke and with top end tie rods running across the top frame members at their ends ove.r the ends of the top yoke, a plurality of top coil support members disposed against the bottom surface of the top frame members with their one ends extending into the top frame and their other ends extending out of the top frame, the t0p coil support members being welded to the top frame members at the inner and outer surfaces of the top frame members, atleast one insulator support corresponding to each of the top coil support members and disposed on a coil clanging ring located at the top of the coil and packing provided in the space between each insulator support and corresponding top coil support member and a plurality of mounting brackets each corresponding to each of the top coil support members and positioned above
17
31 MAR 2009

each of the top coil support members and mounted to the top frame members and a plurality of pairs of top frame bands each pair being provided between each pair of top yoke studs and extending over the top yoke underneath the top yoke with their ends fixed to the top frame members and a plurality of top stays disposed across the top yoke and fixed to the upper ends of the top frame members and a plurality of top gussets corresponding to each of the top coil support members and fixed to the top brackets, top frame members and top stays, the top and bottom frame members being joined together at each side of the top and bottom yokes with flitch plates which run from top to bottom along each limb of the core and are located in the top and bottom frames using flitch plate pins and are tied to the outside of each limb of the core.
or 2, wherein the insulator
2) The oil filled transformer as claimed in claim 1, wherein the bottom frame bands and top frame bands are made of fibre glass.
3) The oil filled transformer as claimed in claim 1 supports are wooden supports.