FIELD OF THE INVENTION
The present invention relates to an electrical bushing for high voltage electrical
applications, in particular to a bushing having sheet like spacers and/or fibrous
material having multiple holes impregnated with an impregnating medium
containing higher thermal conductivity filler particles than the impregnating
medium. The invention further relates to a process for producing an electrical
bushing for high voltage electrical application.
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/wounded 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
and /or bushings 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 metallic conductor such as aluminum/copper 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 cellulose paper or crepe paper as spacer and metallic insert and/or metal foils
like aluminum foil or semi conductive ink or graphite powder or high conductive
carbon fiber as stress grading layer impregnated with un-filled impregnation
medium. High electrical stresses may also occur around the tips of metal foils
inside the electrical bushings and high service temperatures may give rise to
thermal expansion problems in and around the electrical bushing. Such harsh
conditions in combination with difficult wetting properties and mismatch of
thermal expansion coefficients between epoxy resin and other materials can also
lead to delamination of the electric insulation material, which in turn can result in
destructive partial discharges.
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 A1 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 up to 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 and allow the matrix material for easy filling of the core of
the bushing.
The need also exists for less expensive resin systems with linear thermal
expansion near or equal to the conductive layer and/or copper conductor
adjacent to or embedded in the material during manufacturing process and
thereby avoid delamination of electrical insulation material in 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 an electrical bushing for high
voltage electrical application using basalt fiber and/or glass fiber which have
multitude holes.
Another object of the invention is to propose an electrical bushing for high
voltage electrical application using basalt fiber and/or glass fiber which have
multitude holes, which includes an epoxy resin system consisting of a bisphenol
“A” epoxy, a carboxylic acid anhydride based liquid hardener, the tertiary amine
accelerator and the high thermal conductivity filler material suitable for
application as an electrically insulating material.
A still another object of the invention is to propose an electrical bushing for high
voltage electrical application using basalt fiber and/or glass fiber which have
multitude holes, 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 an electrical bushing for high
voltage electrical application using basalt fiber and/or glass fiber which have
multitude holes, which have enhanced dielectric properties such as capacitance
value, breakdown strength, and partial discharge in addition to non-hygroscopic
nature.

A still further object of the invention is to propose a process for producing an
electrical bushing for high voltage electrical applications.
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 an electrical bushing for high voltage applications
using basalt fiber and/or glass fiber as a spacer material instead of crepe paper
or Kraft paper which would absorb the water or moisture in atmospheric air,
which has enhanced dielectric properties such as 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 an electrical
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
millimeters is chosen in this invention.
According to the invention, the bushing has a conductor, the conductor is
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/impregnating 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, the method comprises the step of impregnating the sheet
like spacers and/or fibrous material having multiple holes with impregnation
medium under vacuum and/or pressure so as to provide a void free structure not
subject to layer separation and to render the fibrous matrix impervious to
harmful conductive particle intrusion by water for example.
As per the invention, commercially available unfilled low viscosity liquid epoxy
resin system of bisphenol A epoxy resin and/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 mold containing
the said embodiment of the invention and de-gassed under heat of set
temperature of 80 – 900C preferably at 800C for a period of 1-2 hours. The
temperature is then increased 140 – 1500C preferably at 1400C for a period of
4 – 6 hours and then is cooled down to ambient temperature.
The mold is then opened to obtain the paperless electrical bushing.
The electrical bushings are then tested for the dielectric properties of enhanced
capacitance value, breakdown strength, and partial discharge.
The invention would be more understood in terms of taking various examples,
which are explained in the following:
EXAMPLE 1:
As per this example, the conductor is having certain diameter is wounded with
spacer/fibrous material with basalt fabric/glass fabric of 150-200 grams per

square meter density and a thickness of 0.10 to 0.20 millimeters to a thickness
of around 20-25 mm which form as the core material instead of Kraft/Crepe
paper which doesn’t allow resin to penetrate radially. This core material is also
wounded with equalization elements like semi-conductive tape/paint or aluminum
foils/high conductive carbon fibrous material at pre-determined radial position
from the central conductor. In this example, the number of layers of semi-
conductive tape/paint or aluminum foils/high conductive carbon fibrous material
inserted at pre-determined positions is about 4-6 layers to grade the electrical
stress from conductor to the grounded layer and the size of the filler particle is
around 50-100µm with less than 50% weight addition. The wounded core
material along with the equalization elements are impregnated with a
commercially available un-filled 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 mold containing the said embodiment of the invention and de-
gassed under heat of set temperature of 80-900C preferably at 800C for a period
of 1-2 hours. The temperature is then increased 140-1500C preferably at 1400C
for a period of 4-6 hours and then is cooled down to ambient temperature. The
cavity mold is than opened to get the bushing as per the above said
embodiments and it is subjected to certain electrical tests like capacitance, tan
delta tests, partial discharge test and electrical breakdown test. Test results
confirm that the invented bushing as in Fig.1 complies with the international
standards.


EXAMPLE 2:
In this example, the procedure and all the experimental conditions were followed
exactly the same that is described in the example 1, except that the number of
equalisation elements placed in the core material, i.e., the number of
equalisation elements were 6-8 layers instead of 4-6 Layers and the size of the
filler particles is increased from 100µm to 200µm with less than 50% weight
addition as in the example 1.
The derived bushing showed a better test results as compared to example 1 and
other commercially available electrical bushing and the test results are given
below in Table 2.


EXAMPLE 3:
In this example, the procedure and all the experimental conditions were followed
exactly the same that is described in the example 1, except that the number of
equalisation elements placed in the core material, i.e., the number of
equalisation elements were 6-8 layers instead of 4-6 layers in the example 1 and
the combination of micro and Nano filler particle sizes were added like 50-200µm
size micro filler and 10-60nm Nano fillers with less than 50% and less than 5%
volume addition of micro and Nano-fillers respectively.
The derived bushing showed a better test results as compared to example 1 and
other commercially available electrical bushing and the test results are given
below in Table 3.

WE CLAIM:
1. A Non-hygroscopic electrical bushing for high voltage electrical
application, comprising:
a bushing having at least one electrical copper conductor surrounded by
a core material;
a plurality of equalization elements disposed around the core in desired
radial positions from the center of the conductor;
wherein the core with sheet like spacer and equalization elements are
impregnated with a matrix material consisting of an un-filled low viscosity
liquid epoxy resin system of bisphenol “A” epoxy resin and/or neat epoxy
system, with filler materials being mixed with the carboxylic acid
anhydride based liquid hardener and tertiary amine accelerator in pre-
determined proportions, wherein a sheet-like spacer wound in spiral form
around an axis of the conductor instead of Kraft/crepe paper and forming
a multitude of neighboring layers, the axis being defined through the
shape of the conductor, and wherein the equalization elements comprise
electrically conductive or semi-conductive layers/Aluminum foils/semi
conductive paints/high conductive carbon fibrous material which are
capable of being applied to the core separately from the spacer.

2. Electrical bushing as claimed in claim 1, wherein the sheet like spacer
is basalt/glass fibre having multitude holes/openings
3.. Electrical bushing as claimed in claim 1, wherein the sheet like spacer i.e.
basalt/glass fibre having 150-200 grams per square meter density and a
thickness of 0.10 to 0.20 milli meters which form as the core the core
material.
4. Electrical bushing as claimed in claim 1, wherein the equalisation elements
are comprised of conductive and/or semi-conductive layers/ aluminium
foils/semi-conductive paints/high conductive carbon fibrous material.
5. Electrical bushing as claimed in claim 1, wherein the core is wound around
the conductor and equalisation elements are wound around the conductor
at pre-determined position in the core material.
6. Electrical bushing as claimed in claim 1, wherein the filler materials can be
electrically insulating and/or impregnating material.
7. Electrical bushing as claimed in claim 1, wherein the thermal conductivity
of the filler particles is higher than the thermal conductivity of the matrix
material and/or that the coefficient of thermal expansion of the polymer.
8. Electrical bushing as claimed in claim 1, wherein the thermal conductivity
of the filler particles is higher than the thermal conductivity of the matrix
material and/or that the coefficient of thermal expansion of the polymer.

9. Electrical bushing as claimed in claim 1, wherein the core with sheet like
spacer material and equalisation elements, both having multitude of
holes/opening has allowed the matrix material to penetrate to the copper
conductor both axially as well as radially.
10. Electrical bushing as claimed in claim 1, wherein the size of the micro-filler
is about 50-200µm and the size of the Nano-fillers is about 1-5nm.
11. Electrical bushing as claimed in claim 1, wherein the filler content in the
said embodiment is about 10-50% weight for the micro-filler and 1-5%
volume in the case of the Nano-fillers.
12. A process for producing a non-hydroscopic electrical bushing as claimed in
claim 1, characterized by the steps of :
- un-filled low viscosity liquid epoxy resin system of bisphenol “A’’ based
epoxy resin, carboxylic acid anhydride based liquid hardener and the
tertiary amine accelerator duly mixed with a combination of micro and
Nano alumina/silica filler in pre-determined proportions, wherein said
mixture is poured into and alloy Steel mould corresponding to the
dimension/shape of the bushing, the air bubble being removed from
the composite body through de-gassing during the impregnation
process,

- heat-treating the impregnated body in an air circulated oven in a
temperature range of 80-90°C for a period of 1-2 hours, to produce a
pre-cured electrical bushing; and
- further heat-treating the pre-cured electrical bushing in an air-
circulated oven at a temperature range of 140° - 150°C preferably at
140°C for a period of 4 - 6 hours, which produces a fully-cured
electrical bushing.