FIELD OF THE INVENTION

This invention relates to an electrical bushing for high voltage applications, in
particular to a bushing comprising sheet like spacers having multiple holes for
filling matrix material. The invention further relates to a spacer having net
shaped/ meshed/ synthetic fabric/fibers like glass fiber or basalt fiber. The
invention further relates to a method of producing such an insulator in less
time.
BACKGROUND OF THE INVENTION
A high voltage bushing is a device used to carry current at high potential though
a grounded barrier like transformer tank, generator or high voltage applications
like in gas insulated substation or as a test bushing. A simplest bushing type, a
stud or bulk type bushing has no condenser/capacitive grading and typically
uses porcelain or cast resin as its main insulation material. Stud bushings are
available for lower system voltages, where no fine field grading is needed. For
higher voltages condenser bushings, also called fine-graded or capacitance
graded bushings, are used. The aim of the condenser bushing is to reduce the
maximal field stress and optimize the field distribution in both the axial as well as
in the radial direction. In the traditional technologies of condenser bushings, the
main electrical insulation consists of a band of paper, coiled up around the
conductor or supporting tube, which is subsequently impregnated by oil or epoxy
resin. During winding, sheets of electrically conductive material are inserted
between the

layers of wound paper band to form field grading layers of the condenser core
of the bushing, concentrically arranged around the conductor. Insulators are
among the most important components used in different parts of a power
system including substations, transmission and distribution lines. So their
performance has a significant effect on power system characteristics. The
intensity of voltage and electric field, in addition to creating corona effect on
insulator surface, makes partial discharges on existing cavities on the insulator
surface. This changes the electrical and mechanical characteristics of insulator
and makes it completely damaged. To avoid corona and partial discharges in
power transformer bushings, the electric field on bushing and its insulator, using
the change in the structure of floating condenser plates, should be controlled. In
order to hold the high voltage conductor firmly inside the electrically grounded
metal enclosure, in a position sufficiently far away from the grounded enclosure
to avoid dielectric breakdown, an insulator is provided inside the enclosure. A
material, such as epoxy, is usually selected which has coefficient of expansion
similar to the metal conductor so as to minimize the possibility of voids being
formed at the critical interface where the insulator meets the conductor. This is
because such voids are subjected to high electrical fields at the critical insulator
conductor interface region, which can lead to ionization within the voids,
flashover and a reduced life expectancy for the insulator. This high electrical field
at this critical region approaches a value equal to the product of the field at the
inner conductor in the gas and the dielectric constant of the insulator.

Conventional high voltage bushing uses condenser core wound from Kraft paper
or crepe paper as spacer and metallic insert like aluminum foil or non metallic or
semi conductive ink or graphite powder as stress grading layer.
US Patent US 20100206604, teaches a high voltage outdoor bushing which
includes a conductor extended along an axis, a condenser cored and an
electrically insulating polymeric weather protection housing molded on the
condenser core. The condenser core can contain an electrically insulating tape
which is wound in spiral form around the conductor. Capacitance grading
insertions can be arranged between successive windings of the tape. A cured
polymeric insulating matrix embeds the wound tape and the capacitive grading
insertions. A moisture diffusion barrier can be incorporated inside the condenser
core prior to molding the weather protection housing.
EP 1798740 Al relates to the field of high-voltage technology, which discloses a
bushing and a method for the production of a bushing and an electrically
conductive layer for a bushing. Such bushings find application e.g. in high
voltage apparatuses like generators or transformers or in high voltage installation
like gas-insulated switchgears or as test bushings.
EP 2053616A1 describes a high-voltage outdoor bushing comprising a conductor
extended along an axis, a condenser core and an electrically insulating polymeric
weather protection housing molded on the condenser core. The condenser core
contains an electrically insulating tape which is wound in spiral form around the

conductor, capacitance grading insertions arranged between successive windings
of the tape and a cured polymeric insulating matrix embedding the wound tape
and the capacitive grading insertions. Such a bushing is used in high voltage
technology. In particular in switchgear installations or in high-voltage machines,
like generators or transformers, for voltages upto several hundred kV, typically
for voltages between 24 and 800 kV.
The need exists for an electrical bushing for high voltage application, to replace
the crepe paper which absorbs the humidity or moisture present in air and the
need also exists for the less expensive insulator with spacer materials which are
having multiple holes which will allow the matrix material for easy filling of the
core of the bushing.
The need also exists for less expensive resin systems, manufacturing process
and reduced final production time for the manufacture of high voltage insulating
products such as electrical bushing and other high voltage insulating products,
which are void free and which meet the depicted high voltage insulation
requirements.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a paper (crepe/Kraft paper)
less electrical bushing for high voltage electrical application using basalt fibre or
glass fibre.

Another object of the invention is to propose a paper (crepe/Kraft paper) less
electrical bushing for high voltage electrical application using basalt fibre or glass
fibre, which includes an epoxy resin system consisting of a bisphenol nA" epoxy,
a carboxylic acid anhydride based liquid hardener, the tertiary amine accelerator
and the suitable filler suitable for application as an electrically insulating material.
A still another object of the invention is to propose a paper (crepe/Kraft paper)
less electrical bushing for high voltage electrical application using basalt fibre or
glass fibre, which adapt an impregnating process under vacuum and heating for
application as an electrically insulating material.
A further object of the invention is to propose a paper (crepe/Kraft paper) less
electrical bushing for high voltage electrical application using basalt fibre or glass
fibre, which has dielectric properties of enhanced capacitance value, breakdown
strength, partial discharge and non hydroscopic nature.
BRIEF DESCRIPTION OF THE ACCOMPANYNG DRAWINGS
Figure 1 shows a Cross sectional view of the electrical bushing of the invention.
DETAILED DESCRIPTION OF THE INVENTION
The present invention refers to a paperless bushing for high voltage applications
using basalt fibre or glass fibre as a spacer material instead of crepe or Kraft
paper which would absorb the water or moisture in atmospheric air, which has

better dielectric property of enhanced capacitance, tan delta value, partial
discharge and which would serve as an electrical insulation in high voltage
applications.
The present invention further describes a process of producing a paperless
bushing for high voltage applications using pre-filled epoxy resin matrix (the neat
epoxy resin filled with micro alumina/micro silica/ combination of micro and
nano filler) for impregnation purpose which will enhance the electrical properties
because of better wetting properties due to net/ mesh shaped spacer material.
In a preferred embodiment of the present invention, the basalt fabric/glass fabric
of 150-200 grams per square meter density and a thickness of 0.10 to 0.20
millimetres is chosen in this invention.
According to the invention, the bushing has a conductor, the conductor typically
is a rod or a tube and a core surrounding the conductor, wherein the core
comprises a sheet like spacer, which spacer is impregnated with an electrically
insulating matrix material. Typically, the core is substantially rotationally
symmetric and concentric with the conductor. The spacer is wound in spiral form
around an axis, the axis being defined through the shape of the conductor. Thus

a multitude of neighboring layers is formed. The core further comprises
equalization elements, which are arranged in appropriate radial distances to the
axis. The equalization elements are inserted into the core after certain numbers
of windings, so that the equalization elements are arranged in a well-defined,
prescribable radial distance to the axis.
As per the invention, commercially available unfilled low viscosity liquid epoxy
resin system of bisphenol A epoxy resin or neat epoxy system with filler materials
is mixed with the carboxylic acid anhydride based liquid hardener and the tertiary
amine accelerator in pre-determined proportions using an anchor shaped
laboratory mixer with de-gassing attachment for a sufficient period of time,
maintaining a vacuum level of 3 - 5 mbar.
The mixed epoxy resin system is admitted into the cavity of the die containing
the said embodiment of the invention and de-gassed under heat of set
temperature of 80 - 90 degree Celsius preferably at 80 Celsius for a period of
1-2 hours. The temperature is then increased 140 - 150 Celsius preferably at
140 Celsius for a period of 4 - 6 hours and then is cooled down to ambient
temperature.

The die is then opened to obtain the paperless electrical bushing.
The paperless electrical bushings are then tested for the dielectric properties of
enhanced capacitance value, breakdown strength, and partial discharge.