FIELD OF THE INVENTION
The present invention generally relates to manufacturing process adopted for high
voltage terminal bushing and particularly relates to the process of manufacturing
nanoclay filled epoxy moulded bushings for high voltage electrical appliances.
BACKGROUND OF THE INVENTION
Conventional terminal bushings (Ref. Fig. 1) use a central high tension (HT) conductor
integrated with a radial built-up insulation of epoxy. The length of the bushing on
either side of the system is commensurate with the medium like air. The bushing
assembly is housed in a terminal box with suitable fixing arrangement.
Since conventional terminal bushings are moulded as a single piece i.e. integrated in
nature, it is difficult to replace or to maintain if there is a minor damage to the
conductor or to the insulator. The conventional bushing cannot withstand higher fault
currents as the HT conductor is integrated with epoxy insulation. This is mainly due to
difference in thermal expansion of epoxy and HT conductor. The differences in
thermal expansion of these materials may become a critical problem at higher fault
currents due to increase in dimensions of the conductor. Alternatively, performance of
the terminal bushing at higher fault currents can be improved by isolating HT
conductor from the epoxy insulation. However, the arrangement may not be able to
withstand for medium voltage systems due to discharges in service. This is due to an
air gap between conductor and epoxy body.
Traditionally, terminal bushings are moulded using epoxy resin with micro sized fillers
like silica, alumina and such ceramic materials, 45-65% of filler contents are generally
added to the epoxy resin making the bushings heavier. As the fillers are micro sized,
uniform distribution of the filler without any intra-molecular voids is difficult to
achieve. Nano sized fillers like nanoclay can easily fill up these intra-molecular voids
thereby improving the mechanical, thermal and electrical properties. Due to the
availability of more surface area for these nano sized fillers, suitable filler composition
may improve the properties of the bushing. The epoxy based terminal bushing
consists of Bisphenol 'A' epoxy resin, carboxylic acid anhydride and nanoclay filler. The
moulding of the terminal bushing is carried out through a process similar to the one
as described in the Patent No. 212079 dated 23.11.2007.
SUMMARY OF THE INVENTION
Accordingly, there is provided a nanoclay filled epoxy moulded terminal bushing
device comprising of a nanoclay filled epoxy moulded body, a high tension (HT)
conductor and locking system. The nanoclay filled epoxy moulded body comprises
brass/steel sleeve and a grounded brass/steel terminal. The grounded brass/steel
terminal being configured on a surface of the nanoclay filled epoxy moulded body the
HT conductor being inserted through the nanoclay filled epoxy moulded body to serve
as an electrical connection between the high voltage terminal and an electrical bus
transmission system. The HT conductor is configured as a separate replaceable
component being assembled in the brass/steel sleeve by means of suitable locking
arrangement.
It is object of the present invention to propose a high voltage terminal bushing
assembly using nanoclay as filler material instead of the conventional micro-sized filler
materials like silica, thereby overcoming the disadvantages of intra-molecular voids as
seen in the prior art.
A further object of the invention is to propose a high voltage terminal bushing
assembly using nanoclay as filler material instead of the conventional micro-sized filler
materials like silica, thereby reducing the weight of the bushing.
A further object of the present invention is to propose a high voltage terminal bushing
assembly using nanoclay as filler material instead of the conventional micro-sized filler
materials like silica, thereby improving partial discharge (PD) extinction voltage level
and breakdown voltage level.
A further object of the present invention is to propose a high voltage terminal bushing
assembly which can offer better mechanical stability.
Still another object of the present invention is to propose a high voltage terminal
bushing assembly which is simple in design and takes less time for manufacture and
assembly.
Yet another object of the present invention is to propose a high voltage terminal
bushing assembly by an inexpensive terminal bushing assembly in which the HT-
conductor is removable / replaceable.
BRIEF DESCRIPTION OF THE ACCOMPAYING DRAWINGS
The present invention will now be described with the help of the accompanying
drawings, where:
Figure 1. Assembly of the conventional epoxy moulded terminal bushing device
Figure 2. HT sleeve of the nanoclay filled epoxy moulded terminal bushing device
Figure 3. HT conductor of the nanoclay filled epoxy moulded terminal bushing device
Figure 4. Grounded terminal of the nanoclay filled epoxy moulded terminal bushing
device
Figure 5. Epoxy body of the nanoclay filled epoxy moulded terminal bushing device
Figure 6. Assembly of the proposed nanoclay filled epoxy moulded terminal bushing
device.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The high tension (HT) sleeve (01) made of brass or stainless steel is arranged at the
center of the mould and form an integral part of the bushing. The periphery of the
sleeve is provided with number of step cut surfaces and knurled surface (02) to
enhance the mechanical strength of the bushing at the interface of epoxy and sleeve.
The HT conductor (05) is designed in such a way that, it has provision for tightening
of lock nuts (06) at either end.
The grounded terminal (03) which is circular in shape proposed as a part of the epoxy
body.
The surface of the grounded terminal is provided with knurling (04) to enhance the
mechanical strength of the bushing at the interface of epoxy and grounding terminal.
To improve the mechanical strength of the insulator further, small dia holes are made
on the grounded terminal. These holes will be helpful for holding of epoxy and the
grounded terminal. The dimensions of the grounded terminal are based on the proof
voltage as well as test voltage required for system to be Partial Discharge (PD) free.
The grounded terminal is in circular profile for fastening bushing to the terminal box.
Suitable studs (07) have been welded to the grounded terminal and then whole
arrangement will be part of an epoxy body. The projected studs will be commensurate
with the terminal box for necessary fixing arrangements. The grounded terminal
provides enough mechanical strength and arrests the cracks at collar portions.
The HT conductor (05), which is not an integral part of the epoxy body (08) of the
proposed terminal bushing (Figure 6), is inserted through the epoxy body to serve as
an electrical connection between the electrical equipment and the incoming / outgoing
of the power supply. After inserting the HT conductor (05) through the bushing, the
lock nuts (06) are tightened at either end and securely fitted. This unique
arrangement arrests any movement of the copper conductor during service. Since the
HT conductor (05) is not an integral part of the bushing, this arrangement facilitates
easy assembly, maintenance and replacement.
The epoxy based terminal bushing of prior art consists of Bisphenol 'A' epoxy resin,
carboxylic acid anhydride and silica powder.
The epoxy based terminal bushing of present invention consists of Bisphenol 'A' epoxy
resin carboxylic acid anhydride and nanoclay filler.
The moulding of the terminal bushing is carried out through a process similar to the
one as described in the Patent No. 212079 dt. 23.11.2007.
Composition of epoxy system of prior arts-
Si. No. Material Parts by Weight
1. Solvent free bisphenol A epoxy resin 100
2. Carboxylic acid anhydride 100
3. Silica powder (filler) 375
The composition of the resin system of present invention consist of solvent less
Bisphenol-A epoxy resin 100 parts, carboxylic acid anhydride 100 parts and nanoclay
filler 12-25 parts.
SI. No. Material Parts by Weight
1. Solvent free bisphenol A epoxy resin 100
2. Carboxylic acid anhydride 100
3. Functionalized nanoclay (filler) 12 to 25
Nanoclay filler is functionalized with a silane coupling agent. Bisphenol A epoxy resin
and the functionalized nanoclay are thoroughly mixed for 6-10 hours in a chamber
maintained under vacuum of 4-5 Torr and at a temperature of about 60-65° C.
Hardener; carbolic acid anhydride is kept in another chamber maintained at the same
conditions. The operation of maintaining the vacuum and temperature in both the
chambers for 6-10 hours is necessary to remove any dissolved gasses present in the
materials.
After this operation, both the filler mixed resin and the hardener are blended together
by using a mechanical stirrer to get homogenous compound. For moulding this
homogenous compound into a bushing, an alloy steel die is fabricated to the required
dimension of the bushing. The brass tube described above is arranged at the centre
of the alloy steel die. Knurling has been made on the outer surface of the brass tube
to enhance the mechanical bonding between the brass tube and epoxy compound.
Brass sheet is profiled into the circular shaped and is also placed at the centre of the
mould. The brass sheet with diamond knurling and perforations is provided as a collar
support to the bushing for better strength. The homogenous resin mix which will be in
semi-solid state is pumped into the mould under a pressure of 2-3 atmospheres. The
temperature of the mould is maintained at about 140-145 C. Under these conditions,
the epoxy mix is kept in the mould for 3-5 hours for curing. The cured bushing is then
removed from the mould and kept in an air- circulating oven for 8-12 hours at 130-
140 C for post curing. The post curing helps in cohesive bonding of the resin and the
hardener system, thereby resulting in improved mechanical strength. A brass / copper
rod is inserted through the brass tube to serve as an electrical connection. Necessary
locking nuts and fasteners are provided to the brass/copper rod for connecting
bushing to other electric equipment.
The filler percentage in total weight of bushing is found to be effective between 5 to
11%. The weight of the terminal bushings could be reduced by about 10% by using
functionalized nanoclay. There is a significant improvement in consistency of
capacitance with respect to applied voltage. It is also found that the partial discharge
extinction level and break down voltage level could be improved by using
functionalized nanoclay.
WE CLAIM
1. A method of manufacturing nanoclay filled epoxy based terminal bushing
comprising steps of:-
• functionalizing nanoclay fillers by mixing the nanoclay filler 12-25
parts by weight with silane coupling agent, Bisphenol-A epoxy resin
100 parts by weight and hardener carboxylic acid anhydride 100 part
by weight in a vacuum chamber at about 60°-65° C and 4 to 5 torr
pressure for a period of 6-10 hours.
2. The method as claimed in claim 1, wherein solvent free Bisphenol-A epoxy
resin, hardener carboxylic acid anhydride and functionalized nanoclay fillers are
blended together by a mechanical stirrer to get a homogenous compound.
3. A method of manufacturing of the terminal bushing comprising steps of:-
- providing an alloy steel die fabricated to the required dimension of the
bushing,
- providing a brass/stainless steel tube at the centre of the mould to act
as HT sleeve (01),
- providing step cut and knurled surfaces (02) at the periphery
brass/stainless tube,
- providing a brass sheet profiled in to circular shape and placed at the
centre of the mould to act as grounded terminal (03),
- providing the said sheet with diamond knurling (04) and perforations,
- providing studs (07) welded to the grounded terminal (03), so that
whole arrangement becomes a part of the epoxy body,
- pumping homogeneous resin mix in semi-solid state into the mould
under pressure of 2-3 atmosphere,
- maintaining the temperature of mould at 140°-150° C,
- keeping the mix in such a state for 3-5 hours for curing,
- removing the cured bushing from the mould and keeping the said
bushing in an air circulating oven for 8-12 hours at 130°-140° C for post
curing.
4. A terminal bushing device for high voltage application comprising of:-
- a terminal bushing as claimed in claim 1,
- a HT sleeve (01),
- a grounding terminal (03),
- studs (07) welded to the grounded terminal,
- HT conductor (05),
- Locking arrangement (06).
5. The device as claimed in claim 4, wherein a HT conductor (05) inserted
through the nanoclay filled epoxy moulded bushing, is configured as a
separate replaceable component being assembled in the sleeve (01) and
locked by means of a suitable locking arrangement (06) on either end of the
bushing.
6. The device as claimed in claim 4, wherein means to enhance mechanical
strength of the bushing is provided with knurling (04) and perforation at the
surface of the grounded terminal at the interface of the epoxy bushing and
grounded terminal (03) along with silicone coupling agent.
7. The device as claimed in claim 4, wherein the means to free the device from
partial discharge is achieved by fixing dimensions of grounded terminals (03)
based on proof as well as test voltage.
8. The device as claimed in claim 4, wherein means to arrest the cracks at the
collar portion is achieved by welding the studs (07) to the grounding
terminal (03) and then making the whole arrangement a part of the epoxy
body.
9. The device as claimed in claim 4, wherein means to improve consistency of
capacitance value with respect to the applied voltage and reduction in partial
discharge extinction level is provided by using functionalized nanoclay fillers.
10. The device as claimed in claim 4, wherein means to improve dissipation
factor values is provided by using functionalized nanoclay fillers.
11. The device as claimed in claim 4, wherein means to improve BDV (Break
down voltage) is provided by using functionalized nanoclay fillers.
12. The device as claimed in claim 4, wherein means to improve mechanical
strength and robustness against short-time faults is provided by perforated
sleeve (01) with knurled surfaces integrated in the body of the bushing.
13. The device as claimed in claim 4, wherein means for reduction in weight of
bushing by 10% is provided by using functionalized nanoclay fillers.

The method consists of functionalizing nanoclay fillers 12-25 parts by weight with
silane coupling agent, Bisphenol-A epoxy resin 100 parts by weight and hardener
carboxylic acid anhydride 100 parts by weight in a vacuum chamber at about 60°-65°
C and 4 to 5 torr pressure for 6-10 hours for preparing a homogenous resin mix. The
method also consists of moulding a bushing with the homogenous resin mix prepared
according to the previous method by providing alloy steel die fabricated to the
required dimension of the bushing and providing a brass/ stainless tube at the centre
of the bushing to act as HT sleeve (01), and providing brass sheets profiled into a
circular shape and placed at the centre of the mould to act as grounded terminal and
securing the same to the epoxy body by providing studs (07) welds to the grounding
terminal (03). The bushing is provided with a HT sleeve (07), grounding terminal (03),
studs (07), welded with grounding terminal a separate replaceable HT conductor (05)
with locking arrangement (06) on either side of the stud.