FIELD OF THE INVENTION:
The present invention generally relates to design and fabrication of epoxy-cast
bushings capable of serving multi purpose functions in high voltage gas insulated
switchgear (GIS). More particularly, the present invention relates to a
multipurpose epoxy-cast bushing for gas insulated switchgears.
BACKGROUND OF THE INVENTION:
Electrical bushings are usually used to conduct high voltage electrical power from
a power line into another electrical device such, as transformers through
switchgears. The metal housing of such electrical equipment is an electrical
ground that is insulated from the high voltage power, being conducted into the
electrical equipment, generally through an opening in the housing. Electrical
bushings therefore comprise a conductor for high voltage power, an insulation
material and means for mounting the bushing on the electrical equipment.
There are usually known about two types of insulation for high voltage
switchgears, one is Air Insulated Switchgear (AIS), and the other is called Gas
Insulated Switchgear (GIS). GIS contains an insulating gas such as SF6. The
manufacturing process as well as the design of GIS bushing varies from case to
case depending on the application criteria and other associated requirements.
European Patent Number EP0075471 Bl dated Mar 26,1986 describes electrical
bushings that comprise an electrical conductor, a first layer of insulation
surrounding the conductor, a ground plane, and a stress-grading material
surrounding the insulation, a flange for mounting the bushing to the electrical
equipment or apparatus with which it is to be used, and an outer insulating

layer. The stress-grading material can extend a predetermined distance from the
ground plane. The insulation of the first layer is, for example, a cured epoxy
resin, or any similar material. A typical bushing of this type is disclosed in U.S.
Patent No, 3 646 251 to Friedrich. It is taught by this prior art that the insulation
layer, the interface between the insulation and stress grading material, and the
interface between the insulation and the conductor becomes void-free. When an
epoxy resin system is used that generally requires the epoxy resin to be
degassed and then to be casted in a vacuum and further to be cured under
pressure to prevent any void formation. U.S. Patent No. 3 312 776 discloses
bushings in which resinous insulation material is cast around a stress shield and
a conductor, the conductor being encompassed by a metallic braid.
European Patent Number EP0993005 Al dated Apr 12, 2000 describes a bushing
device, which is hermetically mounted in a hole of an electrical equipment,
insulated with oil fluid or gas, and is provided with a metallic fixing flange having
a diameter larger than the diameter of this hole. The bushing device is adapted
thereof to interconnect an electrical distribution cable external to the equipment
with a device internal to the equipment. The invention also describes addition of
"anti-migratory additive" to the elastomeric material for improving its
permeability in order to make it compatible with the insulating medium in the
epoxy-cast bushing.
European Patent Number EP1889265 Al dated Feb 20, 2008 describes a resin-
impregnated graded bushing that has a conductor and a core surrounding the
conductor in which the core comprises a sheet-like spacer, wherein the spacer
contains filler particles. As per the invention, the spacer may comprise paper, in
particular creped paper and the filler particles are preferably electrically
insulating or semi conductive particles that allow an increased thermal
conductivity of the core.

European Patent Number EP2415133 Al dated Feb 8, 2012 describes a bushing
for connecting both GIS with AIS in which the bushing comprises a busbar and a
shield. The busbar and the shield are embedded in an electrically insulating
body. The GIS-side of electrically insulating body is conical and the AlS-side of
electrically insulating body is hollow. The electrically insulating body comprises
an assembling flange between the conical side and the hollow side. On the
hollow side, a number of sheds are evenly distributed oh the exterior. The angle
formed by the axis from the centre of the assembling flange to the end of conical
side and the axis from the centre of the assembling flange to the end of the
hollow side is less than 180 degree.
European Patent Number EP1202417 A2 dated May 02, 2002 describes a gas-
insulated switchgear provided with a resin mold bushing for supporting and
insulating a main circuit conductor from a ground tank. A container portion is
formed by opening an end inside of the tank of a resin mold bushing for
supporting and insulating an internal conductor from a ground tank where a
breaker and a disconnector are accommodated. A blade-supporting a conductor
of the disconnector for joining the internal conductor and the disconnector is
installed in the container portion.
EP1160945A1 discloses a gas-insulated switchgear which comprises a cable
bushing. This kind of cable bushing is widely used as a connection device in the
switchgears.
In this context, the present invention describes the design and fabrication of a
so- called, "multipurpose Epoxy Bushings" primarily used for insulation for high
voltage applications. Such bushings have not yet been used as structural
member, or as a medium of current transfer. The unique design in the invention

makes the bushing to serve multiple usages either in a single or several multiple
combinations depending on the area and or exact conditions of the application/s
in this area.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a multipurpose epoxy-cast
bushing for gas insulated switchgears.
Another object of the invention is to propose a multipurpose epoxy-cast bushing
for gas insulated switchgears, which overcomes possible dielectric breakdown in
high voltage stress locations of the switchgear.
Another object of the invention is to propose a multipurpose epoxy-cast bushing
for gas insulated switchgears, which is provided with sealing arrangement/s
between high voltage chambers to prevent the arc, in case of any fault traveling
from one chamber to the other.
A still another object of the invention is to propose a multipurpose epoxy-cast
bushing for gas insulated switchgears, which includes rigid fixed contact to
prevent mechanical bending upon engagement of the moving contacts.
A further objects of the invention is to propose a multipurpose epoxy-cast
bushing for gas insulated switchgears, which is enabled to provide structural
support to the interrupters.

Yet another object of the invention is to propose a multipurpose epoxy-cast
bushing for gas insulated switchgears, which prevents leakage of medium gas
(SF6) to the atmosphere.
A further object of the invention is to propose a multipurpose epoxy-cast
bushing for gas insulated switchgears, which is flexible to act as a gas
communicator.
SUMMARY OF THE INVENTION
According to the invention, there is provided a multipurpose epoxy-cast bushing
for gas insulated switchgears. The gas insulated switchgear has at least three
high voltage chambers, the bushing connects any of two chambers. The three
chambers Busbar Chamber, Breaker Chamber and Cable Termination Chamber.
The Busbar Chamber comprising bus bars, interpanel barriers, droppers, and on
load isolators, and the Breaker chamber comprising a plurality of Vacuum
interrupters and a driving mechanism. An insulating body is embedded in the
bushing with a conducting link to accommodate a conducting copper terminal to
allow current transfer, on the hollow side of which at least one vacuum
interrupter is provided. The current transfer from the bushing to the conductor
takes place via a multi lam contact mounted on a sliding conductor which can be
slided into the embedded copper terminal. The bushing comprises a plurality of
slots at 120 degrees on the insulated body to enable the bushing to
communicate gas between two adjacent chambers. The bushing when not
provided with slots, enables the bushing to seal the chamber it is connected to,
and allow generation of a pressure difference between the two adjacent
chambers.

BRIEF DESCERIPTION OF THE ACCOMPANYING DRAWINGS
The above and other objects and features of the invention will become more
apparent and the invention itself will be best understood by referring to the
following description of an embodiment taken in conjunction with the
accompanying drawings, in which:
Figure 1A describes a side view of a multi-purpose bushing with a slot (optional).
Figure IB describes a side view of the multi-purpose bushing without the slot for
gas communication.
Figure 2A describes a sectional side view of the bushing.
Figure 2B describes the bushing of Figure 2A without the slots for gas
communication. This feature enables to have pressure difference between the
adjacent chambers.
Figure 3 describes adaptation of the multi-purpose bushing as a medium for
current transfer, the current transfer taking place through the gas communicator
embedded in the epoxy casting.
Figure 4 describes a clamping arrangement of the multi purpose bushing.

DETAIL DESCRIPTION OF THE INVENTION
The present invention describes a multipurpose epoxy-cast bushing for gas
insulated switchgears. As shown in figure 1A, the bushing is optionally provided
with a slot (1). The slot as shown in figure 1A serves the purpose for the bushing
to be used as a gas communicator between the adjacent chambers, in addition
to its basic aim of providing insulation. The alternative embodiment of the
bushing is shown in figure 2A. The bushing shown in figure 2A contains
circumferential slots on both sides in the form of O-ring slot. The O-ring slots are
used to provide a sealed interface between the chambers and atmosphere.
Figure 3 describes as to how the inventive bushing acts as a medium for current
transfer. In this case, the current transfer takes place through the gas
communicator (3) embedded in the epoxy casting. The clamping arrangement is
shown in figure 4. The bushing of figure 4 has metal fasteners (4) embedded in
the epoxy casting. The tightening between the bushing and any surface takes
through the metal fasteners (4), hence minimizing the compressive stresses on
the epoxy body.
The construction of the bushing takes into consideration:
- the dielectric strength required at the specified locations;
- mechanical stability of the bushing so as to use it as a structural support;
and
- close tolerance and sealing prequisites of the bushing so as to provide a
leak proof chamber for gas.

Uses:
a) The basic purpose of the multipurpose Epoxy Bushing is to ensure sealing
between two high voltage chambers of Gas insulated Switchgear (GIS). GIS
consists of three high voltage chambers: for example Busbar chamber, Breaker
chamber, and Cable Termination chamber.
- Busbar chamber includes busbars, Interpanel barriers, droppers, and on load
isolators.
- Breaker chamber includes the Vacuum Interrupters and the drive mechanism
of the chamber.
- Cable Termination chamber includes the CT Bushing, and current transfer
contacts.

b) The multipurpose Epoxy bushing can also be used for sealing between
Breaker chamber and Cable Chamber in order to prevent movement of arc in
case of any phase-to-earth fault in Breaker Chamber or Cable Chamber.
c) Gas communication slots can also be incorporated if same gas pressure is to
be maintained between the two adjacent chambers.
d) Current transfer takes place from vacuum interrupter to casted Copper
through specially designed circular copper component (refer figure 3). The
current transfer from Bushing to Cable Chamber is achieved through multilam
contact mounted on Copper conductor.
Clearances:
Phase to earth clearance is very critical and in case of higher rated GIS, bell
shape is provided (refer figure 1). The geometry and shape is made to ensure
thatnall necessary clearances are achieved between phase to earth. The finish

and parallelism are also very critical to reduce contact resistance and to ensure
smooth current transfer.
Mechanical Strength:
Mechanical strength of the bushing is designed in such a manner that all
electrical dynamic forces are absorbed by the bushing. It is also ensured that
while tightening of fasteners on the Bushing, stresses are not transferred on the
bushing. Brass inserts with M12 hardware are used for fastening. (Refer figure
4). Inserts are used in top and bottom so that in case of non-communicable
design, gas in the breaker chamber does not leak while removing the Bottom
Cable Termination Chamber.
Prevention of leakage:
Leakage of SF6 is prevented by grooves present on the circumference (refer
figure 2). The grooves are designed to ensure gas leakage of less than 0.1% by
volume per annum.

WE CLAIM:
1. A multipurpose epoxy-cast bushing for gas insulated switchgears, the
gas insulated switchgear having at least one each busbar chamber,
breaker chamber, and cable transmission chamber all being high
voltage chambers, the breaker chamber comprising a plurality of
vacuum interrupters and a driving means, the bushing comprising :-
- an insulated body embedded in the epoxy casting with at least one
conducting link provided at a first end to accommodate a copper terminal
to allow current transfer, the insulated body having a sliding conductor
accommodating a multi lam contact,
- at least one of said plurality of vacuum interrupters resting on said second
side of the insulating body to allow current transfer via said multi lam
contact from the bushing to the copper terminal;
wherein the bushing when provided with a plurality of slots configured at
120° on the insulated body enables communication of gas between the
two chambers connected by the bushing, and wherein when the bushing
is absent of slots allows sealing of the chambers which the bushing
connects to allow generation of a pressure differential between the two
connected chambers.

2. The bushing as claimed in claim 1, wherein the busbar chamber of the
gas insulating switchgear comprising bus bars, interrupted barriers,
and on-load isolaters.

ABSTRACT

The invention relates to a multipurpose epoxy-cast bushing for gas insulated
switchgears, the gas insulated switchgears having at least one each busbar
chamber, breaker chamber, and cable termination chamber, all being high
voltage chambers, the breaker chamber comprising a plurality of vacuum
interrupters and a driving means, the bushing comprising :- an insulated body
embedded in the epoxy casting with at least one conducting link provided at a
first end to accommodate a copper terminal to allow current transfer, the
insulated body having a sliding conductor accommodating a multi lam contact, at
least one of the said plurality of vacuum interrupters resting on said second side
of the insulating body to allow current transfer via said multi lam contact through
the bushing to the copper terminal; wherein the bushing when provided with a
plurality of slots configured at 120° on the insulated body enables
communication of gas between the two chambers connected by the bushing, and
wherein when the bushing is absent of slots allows sealing of the chambers
which the bushing connects to allow generation of a pressure differential
between the two connected chambers.