FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
As amended bv 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
An assembly for electrically isolating a core frame assembly from a bottom plate of an oil tank
disposed within a transformer
APPLICANT
Crompton Greaves Limited, CG House, Dr Annie Besant Road, Worli, Mumbai 400 030,
Maharashtra, India, an Indian Company
INVENTORS
Vaidya Sameer Sharadchandra, Crompton Greaves Ltd, Power transformers (Tl) Division,
Kanjur Marg, Mumbai, Maharashtra, India, both Indian Nationals
PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the invention and the manner in which it is to
be performed:

FIELD OF THE INVENTION
The present invention relates generally transformers and more particularly, to
electrical isolation of a frame assembly from oil tank bottom plate within transformers so as to prevent circulating current through core.
DESCRIPTION OF THE BACKGROUND ART
The core-frame-tank isolation is required for prevention of circulating current
around core and hence is important for the smooth functioning of magnetic circuit i.e. heart of transformer. According to conventional standards, the insulation is designed to withstand voltages up to 2 kV voltages. However, nowadays various customers are specifying this withstand voltage as 10 kV to increase safety margin of magnetic circuit of transformer. Upgrading to such high KV voltages is not easy. In power distribution devices, core-frame-tank isolation is typically achieved through the following ways. Isolation between the core frame and the yoke stud, fibre glass washers and tubes are used as aninsulating material to isolate yoke stud from core and frame. However, with the existing design being used in providing the insulation, such upgrading is not easy because higher rating may lead to low creepage path for electric charges that may be due to crack in the insulating materials. Further, due to sharp corners at the edges of the insulating materials, stress concentration is more at the edges.
For base foot to bottom tank isolation, fibre glass tube locating pin isolates
base foot from the bottom tank. However, the existing design is applicable only for 2 kV. However, this arrangement doesn't withstand 10 kV voltage due to following reasons. This will require increase of clearance between hole in the base foot and the locating pin diameter to accommodate bigger size FG tube. But this will weaken the very purpose of design of locating pin - Locating of CCA to withstand Transport accelerations. Practical difficulties observed

during further isolation of Locating Pin. This Joint becomes inaccessible after tanking of core coil assembly.
Still the need exists for high voltage core-frame-tank isolation in transformers
which having increase of clearance between hole in the base foot and the locating pin diameter to accommodate bigger size fibre glass tube to adversely affects withstand for transport accelerations and isolation of locating pin and joint access after core coil assembly.
SUMMARY OF THE INVENTION
Accordingly disclosed herein is an assembly for electrically isolating a core
frame assembly from a bottom plate of an oil tank disposed within a transformer, the core frame assembly including a low voltage side and a high voltage side, the bottom plate and the high voltage side electrically maintained at earth potential, the assembly including a base foot plate contactably positioned over the bottom plate of the oil tank, the base foot plate having a pair of slots formed therein, a base pad having a hole formed therein protruding outwardly from each of the low voltage side and the high voltage side of the frame assembly, each of the base pad supportively positioned over the base foot plate and electrically insulated therefrom through a first insulating member sandwiched therebetween, holes of the base pad matches the corresponding slots of the base foot plate when the base pad is positioned over the base foot plate, and a base foot screw having an insulating tube covering leg of the screw and insertable within the matching hole and slot of the base pad and the base foot, respectively, a head of the base foot screw separated from the base pad through a second insulting material sandwiched therebetween.

In some embodiments, the bottom plate of the tank contacts the base foot plate
and is electrically shorted so that both the bottom plate and the foot plate is at the same earth potential.
In some embodiments, thickness of the FG material is chosen in such a
manner that the FG material atleast provides a creepage distance of 30mm between the base foot plate and the bottom plate on each of the high voltage and the low voltage sides.
It is to be understood that both the foregoing general description and the
following detailed description of the present embodiments of the invention are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention and are incorporated into and constitute a part of this specification. The drawings illustrate various embodiments of the invention and together with the description serve to explain the principles and operation of the invention.
A BRIEF DESCRIPTION OF THE DRAWINGS
The above-mentioned and other features and advantages of the various
embodiments of the invention, and the manner of attaining them, will become more apparent and will be better understood by reference to the accompanying drawings, wherein:
FIG. lis a perspective view of a typical core-frame assembly according to an
embodiment of the present invention;
FIG. 2 is a side elevational view of the core-frame assembly of FIG. 1 showing
a zoomed-in view of typical yoke stud-frame isolation according to an embodiment of the present invention;

FIG. 3 is a side elevational view of the core-frame assembly of FIG. 1 showing
a zoomed-in view of typical frame to tank isolation arrangement according to an embodiment of the present invention; and
FIG. 4 is another zoomed-in view of the frame to tank isolation arrangement
of FIG. 3.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 illustrates a perspective view of a typical core frame assembly 100
according to an embodiment of the present invention. As seen in FIG. 1, a winding assembly is supportively positioned on a limb (not shown). The winding assembly 102 is surrounded by a yoke 104 namely, between a top yoke 106 and a bottom yoke 108. The winding and the yoke 104 assembly acts as magnetic and electric circuit that moves around the yoke 104 in a known manner well understood by a skilled person in the art. The entire core frame assembly 100 sits on a bottom plate 110 of a tank within the transformer (not shown). It is important to provide isolation between the core-frame-tank at various positions within the transformer so as to prevent circulation of the current around core which is important for the smooth functioning of magnetic circuit.
As seen in FIG. 1, the yoke 104 is defined by six (no. of limbs in this case - 3
x 2) sides 112 and having a central part 114 in which the winding assembly 102 is supportively positioned. To one of the pair of opposite sides 112 of the yoke 104 and near a top end thereof, a pair of top frames 116is detachably attached. Each of the top frames 116is attached on the opposite sides 112 of the yoke 104. Similarly, a pair of bottom frames 118 is also attached to the same opposite sides 112 of the yoke 104 to which the pair of top frames 116is attached (FIG. 1). All of the frames are preferably made from a conductive material known in the art. Further, the oppositely positioned top frames 116 and bottom frames 118 are fastened together by means of

yoke studsl20 and end tie rods 122 as shown in FIG. 1. Particularly, the yoke studsl20 are extremely important from the point of view of clamping the yoke 104 together. Therefore, as seen, a pair of two yoke studsl20 is fastened with the yoke 104. Few / required no. of pairs of yoke stud 120 are used to fasten the pair of top frames 116 with the yoke 104 whereas Few / required no. of pairs of yoke stud 120 are used to fasten the pair of bottom frames 118. These yoke studsl20 apply the predetermined clamping pressure to tighten the yoke: 104. However, the yoke studsl20, the frame and the yoke 104 are isolated from each other so that no additional current loop is generated as a result of creepage path of charge carriers.
Reference is now given to FIG. 2 that illustrates a zoomed-in view of an
assembly 124 for electrically isolating the yoke stud 120 from the top frame 116 /bottom frame 118 of a core frame assembly 100 within the transformer. For the purpose of the specification, reference will only be given to one of the yoke studsl20 that are inserted within one of the pair of the opposite sides 112 of the top end of the yoke 104. However, the constructional and functional features discussed below will be applicable to all of the isolations between the yoke studsl20 and their corresponding frames.
As seen in FIG. 2, between the yoke 104 and the top frame 116 a yoke
insulation member 126 is positioned that is used to isolate the frame from the yoke 104. The top frame 116 has a hole formed therein that is aligned with the hole within the yoke insulation 126and the corresponding holes formed within the yoke 104. These linearly aligned holes are aligned with a corresponding holes formed within the top frame 116 and the yoke insulation, respectively, disposed on the opposite side of the yoke 104. Thus, a clear passage is formed that extends between the pair of opposite sides 112. Further, a first insulating material 128 is inserted within the hole of the frame and extends upto the hole formed within yoke insulation. The first insulating member is defined by an elongated portion 130 extending from a circular outer end 132. and a central opening that extends between the outer end 132 and the elongated portion 130.

The elongated portion 130 is inserted within the hole formed within the frame and the yoke insulation 126with the circular outer end 132 is positioned outside an outer surface 134 of the frame in contacting relationship. Preferably, the first insulating material 128 is formed of a fibre glass bush for providing the necessary insulation properties. However, other insulating materials known in the art may also be used instead of fibre material and considered to be within the scope of the present invention.
Further, as shown in FIG. 2, a washer 136, preferably formed of mild steel
(MS), and having a hemispherical peripheral surface 138 is disposed adjacent to the first insulating material 128 in contacting relationship. The washer 136 has a central hole as well that is aligned with the central opening of the first insulating material 128. The yoke stud 120, which is formed of a conductive material, is formed of to have an elongated portion 140 that is covered within a second insulating material 142. Preferably, the second insulating material 142 is formed of fibre glass tube however other insulating materials known in the art may also be used. The conductive yoke stud 120 is insertable within the central hole of the washer 136 and the central opening of the corresponding first insulating material 128. The yoke stud 120 is inserted in such a manner that the elongated portion 140 of the yoke stud 120 protrudes from the central hole of the washer 136 contacting the oppositely disposed first insulating material 128. The first insulating material 128 and the second insulating material 142 acts as a double insulating layer between the conductive stud and the top frame 116. As seen in FIG. 2, the second insulating material 142 also provides insulation between the yoke stud 120 and the yoke 104.
During isolation testing, a potential difference is established between the yoke
stud 120 and the frame as a result of voltages (KV) applied therebetween and tested for withstanding KV between the two. Experimental details with respect to the current set up has shown that the double layered insulation between the frame and the yoke stud 120 can withstand a potential difference of 10 KV. This is achieved because the hemispherical peripheral surface

138 of the washer 136 and a portion of the circular end of the first insulating material 128 provide an enhanced creepage path or distance, upto 30mm, for the electric charge to travel between the conductive yoke 104 stud and the frame. This happens because the stress concentration on the washer 136 and the first insulating material 128 reduces. Thus, when the transformer is in operation and operating at nearly 10KV voltages, the insulation between'the two does not breaks which would result in formation of creepage path which would result in formation of an additional loop, via the frames, for the charge carriers to travel thereby resulting in local heating and which may ultimately lead to failure of the transformer.
The above noted embodiments establish the importance of sturdy isolation, of
upto 10KV, between the frame and yoke stud 120 isolation. Sturdy isolation of similar ratings between the core-frame and the base of the tank is also equally important, which is described by way of the following embodiments. FIGS. 1 and 3 shows an assembly 100' for electrically isolating the core frame assembly 100' from a bottom plate 110 of the oil tank (not shown) disposed within the transformer, according to another embodiment of the present invention. The core frame assembly 100' includes a low voltage side 146 and a high voltage side 148. The bottom plate 110 of the oil tank and the high voltage side 148are electrically maintained at earth potential. As seen in FIG. 3, a base foot plate 150 is contactably positioned over the bottom plate 110 of the oil tank. The base foot plate 151 extends from the low voltage side 146to the high voltage side 148 and has a pair of slots 152 formed therein on its opposite ends thereof, as shown in FIG. 3. It is to be noted that in available design, there is an insulation provided between the bottom plate 110 and the base foot plate 150, however in the various embodiments of the present invention, the two are joined together. This is being done so as to allow the bottom plate 110 of the tank to be electrically shortened with the base foot plate 150 so that both the bottom plate 110 and the foot plate are at the same earth potential. It is to be noted that the bottom plate 110 of the tank and the base foot plate are formed of a metallic material, and wherein contact area of both

the base foot plate and the bottom plate 110 are unpainted so as to ensure metal-to-metal contact therebetween.
Further, a base pad 154 having a hole formed therein protrudes outwardly
from each of the low voltage side 146 and the high voltage side 148of the frame assembly 100. Each of the base pads 154 is supportively positioned over the base foot plate and electrically insulated therefrom through a first insulating member 156 sandwiched therebetween. It is to be noted that the holes 158 of the base pads 154 matches the corresponding slots 152 of the base foot plate when the base pad 154 is positioned over the base foot plate. Preferably, the first insulating material is formed of a fibre glass material however alternatives are also considered to be within the scope of the present invention. Preferably, thickness and dimensions of the fibre glass material are chosen in such a manner that the fibre glass material atleast provides a creepage distance of 30mm between the base foot plate and the base pads 154 on each of the high voltage and the low voltage sides.
Furthermore, a base foot screw 160 having an insulating tube covering leg 162
of the screw is insertable within the matching hole and slot of the base pad 154 and the base foot, respectively. A head 164 of the base foot screw is separated from the base pad 154 through a second insulting material 166 sandwiched therebetween. Preferably, the second insulating material 166 is also formed of a fibre glass material and its thickness and dimensions are chosen is such a manner that the fibre glass material atleast provides a creepage distance of 30mm between the head 164 of the screw 160 and the base pad 154 on each of the high voltage and the low voltage sides.
It will be apparent to those skilled in the art that various modifications and
variations can be made to the present invention without departing from the spirit and scope of the invention. Thus it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.

We Claim:
1. An assembly for electrically isolating a core frame assembly from a bottom plate of an oil tank
disposed within a transformer, the core frame assembly including a low voltage side and a high
voltage side, the bottom plate and the high voltage side electrically maintained at earth potential,
the assembly comprising:
a base foot plate contactably positioned over the bottom plate of the oil tank, the base foot plate having a pair of slots formed therein;
a base pad having a hole formed therein protruding outwardly from each of the low voltage side and the high voltage side of the frame assembly, each of the base pad supportively positioned over the base foot plate and electrically insulated therefrom through a first insulating member sandwiched therebetween, holes of the base pad matches the corresponding slots of the base foot plate when the base pad is positioned over the base foot plate; and
a base foot screw having an insulating tube covering leg of the screw and insertable within the matching hole and slot of the base pad and the base foot, respectively, a head of the base foot screw separated from the base pad through a second insulting material sandwiched therebetween.
2. The assembly according to claim 1, wherein the bottom plate of the tank contacts the base foot plate and is electrically shorted so that both the bottom plate and the foot plate is at the same earth potential.
3. The assembly according to claim 1, wherein the first insulating material, the second insulating material, and the insulting tube are formed of fibre glass material.

4. The assembly according to claim 3, wherein thickness of the fibre glass material is chosen is such a manner that the fibre glass material atleast provides a creepage distance of 30 mm between the base foot plate and the bottom plate on each of the high voltage and the low voltage sides.
5. The assembly according to claim 3, wherein thickness of the fibre glass material is chosen is such a manner that the fibre glass material atleast provides a creepage distance of 30 mm between the head of the screw and the base pad on each of the high voltage and the low voltage sides.
6. The assembly according to claim 1, wherein the bottom plate of the tank and the base foot plate are formed of a metallic material, and wherein contact area of both the base foot plate and the bottom plate are unpainted so as to ensure metal-to-metal contact therebetween.