FIELD OF INVENTION
The present invention relates to an epoxy resin moulded bushing device for high
voltage hydrogen cooled electrical generators.
BACKGROUND OF THE INVENTION
A high voltage bushing device is conventionally used to transmit electrical current
and voltage from an electrical generator to an electrical bus transmission system.
Existing high voltage bushings for electrical generators use inorganic based
ceramic material or glass epoxy/epoxy resins as the main insulation.
The epoxy resin moulded terminal bushing device is provided with a central
metal conductor integrated with a radial built-up insulation of epoxy resin. The
length of the bushing on either side of the system is selected based on the
medium (air or gas). In the conventional bushings, the conductor is directly
inserted in a pre-fabricated insulation tube assembly, which generally is not
capable to withstand higher system voltages. Similarly, the grounded terminal is
fixed on outer surface of the insulating tube instead of being disposed within the
insulation. This arrangement gives a high electrical surface stress on the
insulating tube, which is not acceptable for higher system voltages. The bushing
has a flange assembly for mounting the same to the generator body (refer Fig.
1). The reliability of the high voltage bushing devices are governed by following
parameters:

• Quality of the metal conductor and connectors during service.
• Void free preparation of the epoxy insulation.
• Heat conduction and thermal expansion based incompatibilities.
• Prevention of gas leakage
• Prevention of leakage current
Due to transmission of high current through the conductor for hydrogen cooled
generators, the heat generated over the conductor is cooled with pressurized
hydrogen gas in a system which requires appropriate sealing arrangements to
prevent leakage of the hydrogen gas from the generator.
As the bushing device is mounted on to the generator wall by adapting a flange
assembly, the possible effects of field stress need to be carefully addressed to
avoid leakage of the current and voltage to the generator ground. Some of the
electrical stress arrangements are described in US Patent No. 20050199418 and
US Patent No. 4780577. Optimum arrangements for electrical stress control with
adequate design features can improve the manufacturability of the bushing and
can reduce the cost of the bushing. Indian Patent Nos. 212079 and 226854 23 -
11 - 2007, and Indian Patent Application Nos. 440/KOL/2005, 417/KOL/2008,
and 17/KOL/2008 address some of the issues of sealing arrangements, electrical
stress control and process for moulding of bushings by epoxy resin.
As the bushing device is mounted on the flange assembly, the bending load is so
so selected that the bushing is capable of withstanding this mechanical stress.
While addressing the issue of the bending load, the issue of the electrical stress
needs to be simultaneously addressed as described hereinabove. An improved
configuration of the flange assembly can further improve the electrical stress
control. Process modifications to address the issue of interface bonding can
improve the bending load bearing capabilities of the bushing.
OBJECTS OF THE INVENTION
It is therefore an object of the present invention to propose a high voltage
generator bushing device, which eliminates the disadvantages of the prior art.
Another object of the present invention is to propose a high voltage generator
bushing device, which is enabled to withstand higher fault currents at higher
system voltages.
A still another object of the present invention is to propose a high voltage
generator bushing device which is adaptable to extend HT connection from one
media to another.
Yet another object of the present invention is to propose a high voltage
generator bushing device which is effective for dynamic forces/mechanical
loading encountered during a fault or in service conditions.
A further object of the present invention is to propose a high voltage generator
bushing device which is capable to maintain an effective heat dissipation rate.
A still further object of the present invention is to propose a high voltage
generator bushing device which provides effective sealing arrangement to
prevent the gas leakage.
Yet further object of the present invention is to propose a high voltage generator
bushing device which is enabled to optimize surface stresses including insulation
creepage.
Another object of the present invention is to propose a high voltage generator
bushing device, which possesses an enhanced bending load bearing capacity.
Another object of the present invention is to propose a high voltage generator
bushing device which is simple and easy to manufacture and assemblage.
SUMMARY OF THE INVENTION
Accordingly, there is provided a high voltage generator bushing device for
hydrogen cooled electrical generators comprising an epoxy body, a high tension
(HT) conductor, and a locking means. The epoxy body comprises a brass/steel
sleeve and a grounded aluminum/steel terminal. The grounded terminal being
configured on an outer surface of the epoxy body, the HT-conductor being
inserted through the epoxy body to serve as an electrical connection between
the generator and an electrical bus transmission system. The HT-conductor is
configured as a separate replaceable component being assembled in the
brass/steel sleeve by the locking means. The brass/steel sleeve and the
grounded terminal are integrated with the epoxy body by adapting a coupling
agent. The copper conductor (HT conductor) has a provision for a sealing means
to extend this connection from one medium to another. The grounded terminal is
further provided with a corresponding sealing means.
The inventive bushing device is compatible for hydrogen environment, capable of
withstanding high fault currents with optimized insulation creepage including
heat dissipation and higher electromechanically load bearing capacity.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - Shows schematically a conventional high voltage generator bushing
device.
Figure 2 - HT sleeve of the high voltage generator bushing device according to
the present invention.
Figure 3 - Shows grounded terminal of the high voltage bushing device
according to the present invention.
Figure 4 - Shows HT conductor of the high voltage generator bushing device
according to the present invention.
Figure 5 - Shows an epoxy body of the high voltage generator bushing device
according to the present invention.
Figure 6 - Shows a locking means of the high voltage generator bushing device
according to the present invention.
Figure 7 - Shows a high voltage generator bushing device in assembled
condition according to the present invention.
Figure 8 - Shows a system to carry-out the bending load test of the high voltage
generator bushing device of Figure - 7.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
As shown in figure-2, a high tension (HT) sleeve [01] made of brass/stainless
steel is arranged at the center of a mould. The periphery of the sleeve (01) is
provided with a plurality of profile grooves [02] to enhance the mechanical
strength of the bushing device at the interface of an epoxy body and the sleeve
(01). The periphery of the sleeve (01) is functionalized by means of a known
silane coupling agent to further enhance the mechanical strength of the bushing
at the interface of the epoxy body and the sleeve. The thickness of the HT sleeve
[01] is not the same throughout its length. The grooves [03] on the HT sleeve
[01] are located with respect to the position of a grounded terminal [04]. The HT
sleeve [01] is provided with at least two grooves [03] for inserting 'O' rings on
one side matching with a Stopper [12] of a HT conductor [11] to act as a sealing
means for hydrogen gas. At the other end of the brass sleeve (01), a provision
for the hydrogen gas inlet into an annular space between the HT sleeve [01] and
the HT conductor [11] is made for effective cooling of the HT conductor [11].
The HT conductor [11] is configured with a plurality of holes over its radial
surface to achieve an effective cooling by the hydrogen gas. The configuration of
the HT conductor [11] enables one of its ends to enter inside the epoxy body. A
locking means [13], as shown in Fig. (6), is provided for securing the HT
conductor [11] to the HT sleeve [01] of the epoxy body [10]. The locking means
[13] comprises inner threaded surface which secures the HT conductor [11]
tightly to the HT sleeve [01] at the other end, wherein the Stopper [12] sitting
over the 'O' rings ensures a complete sealing to eliminate any possibility of
hydrogen gas leakage. The radial grooves on the locking means [13] allows a
smooth flow of Hydrogen gas into the HT conductor [11] through the provision
made for the inlet of hydrogen gas on the HT sleeve [01] to the HT conductor
[11].
The grounded terminal [04] as shown in Fig. (3), with a grounded ring [08] is
inserted in the volume of the epoxy body [10] as illustrated in Fig (4). The
dimensions of Dl, D2 and D3 are optimally configured to obtain the desired
electrical field stress control so that there is no leakage of the current and
voltage to the body of the generator, which further enhance the electro-
mechanical strength of the bushing at the interface of epoxy and the grounded
terminal. The inner surface of the grounded terminal [04] is provided with a
plurality of profiled holes [05] and communicating holes [06] from inner to outer
surface to enhance the electro-mechanical strength of the bushing at the
interface of the epoxy body [10] and the grounded terminal [04]. The inner
surface of the grounded terminal [04] is functionalized by means of a known
silane coupling agent to further enhance the electro-mechanical strength of the
bushing device. The grounded terminal [04] has a provision for fastening to the
generator mounting flanges. A sealing means [07] has been provided to this
grounded terminal [04] to arrest the gas leakage. The grounded ring [08], which
is a sealing means, facilitates further improvement to arrest the gas leakage. An
arrangement to further strengthen the sealing means is provided in the
grounded ring [09] as per Fig. (3). The profile of the grounded rings [08, 09]
enhances the dielectric properties of the bushing device especially the power
frequency withstandable voltage and Lightning impulse withstandable voltage.
The HT conductor [11] as per Fig. (5) which is not an integral part of the epoxy
body (10) is inserted through the epoxy body [10] to serve as an electrical
connection between the electrical generator and the electrical bus transmission
system. After inserting the HT conductor [11] through the epoxy body [10], the
lock nut [13] as per Fig. (6) is placed and securely tightened. The unique
arrangement arrests any movement of the HT conductor [11] during service.
This entire assembly, with the grounded ring [08] as per Fig. (3) can be used as
a terminal bushing [14] for hydrogen cooled generators. To facilitate further
improvement in arresting the gas leakage the grounded ring [09] as per Fig. (3)
can also be used in the assembly of the terminal bushing [14] for hydrogen
cooled generators. The unique arrangement arrests any movement of the copper
conductor during service. Since the HT conductor [11] is not an integral part of
the bushing, this arrangement facilitates easy assembly, maintenance and
replacement.
The moulding of the bushing can be carried out in a known system and process
for example, a solventless Bisphenol 'A' epoxy resin is thoroughly mixed with
fillers, like silica powder for about 8 hours in a chamber which is maintained
under vacuum of 5 torr and at a temperature of about 65°C. The hardener used
is carboxylic acid anhydride. The hardener is also kept in another chamber
maintained under the same conditions. This operation of maintaining the vacuum
and temperature in both the chambers for about 8 hours is necessary to remove
any dissolved gases present in the materials. After this operation, the filler mixed
resin and the hardener are blended to get a homogenous material. For 100 parts
by weight of solventless Bisphenol 'A' epoxy resin, 375 parts of silica powder filler
is added and 100 parts by weight of carboxylic acid anhydride is used. For
moulding of this material into bushing of the present invention, the mould is
fabricated to the require dimensions corresponding to a high fault current proof
bushing [14]. The grounded terminal [04] and the HT sleeve [01] are
functionalized by means of a silane coupling agent, prior to starts of the
moulding process. The functionalized grounded terminal [04] and the HT sleeve
[01] are placed inside the mould. The homogenous resin mix, which is in a semi-
solid state, is pumped into the mould under pressure of 2 to 3 atmosphere. The
temperature of the mould is constantly maintained at about 130°C to 150°C and
pressure of 2 to 6 atmospheres. Under these conditions of pressure and
temperature, the resin mix is kept in the mould for 3 to 5 hours for curing. The
cured epoxy body [10] is then removed form the mould and kept in an air-
circulating oven at a temperature of 130°C to 150°C for 07 to 09 hours.
The terminal bushing [14] is tested for dielectric properties such as proof voltage
as 59 kV (rms) for one minute, impulse voltage at 120 kVp and short time
current rating at 100 kA for one second. The terminal bushing [14] is subjected
to bending load test to ascertain the electro-mechanical properties of the
bushing device as given in an arrangement shown in Fig. (8). The terminal
bushing [14] with the following properties, as given in Table 1, is obtained with
the formulations incorporating the above technical features:

WE CLAIM
1. A high voltage generator bushing device for hydrogen cooled generator
comprising :-
- an epoxy body [10] and a high tension (HT) conductor [11], an HT sleeve
[01] and a grounded aluminium/steel terminal [04];
- at least one locking means (13);
- a plurality of grounded rings [08, 09];
- characterized in that the grounded terminal [04] is located in the volume
of the epoxy body [10] and the HT-conductor [11] is plugged through the
epoxy body [10], to act as an electrical connection between the electric
generator and the electrical bus transmission system, and in that the HT
conductor [11] is configured as a separate replaceable component
assemblable inside the HT sleeve [01] of epoxy body [10] by means of the
locking means [13].
2. The bushing device as claimed in claim 1, wherein the HT conductor [11]
is made of high conductive material with a stopper (12) to arrest gas
leakage from bushing device, and wherein the stopper is rigidly fixed
against the flange of the HT sleeve [01].
3. The bushing device as claimed in claim 1, wherein the HT sleeve [01] is
made of brass or stainless steel material, and comprises a plurality of
profiled grooves [02] configured on the periphery relative to the
disposition of the grounded terminal [04], and wherein the thickness of
the HT sleeve [01] is not uniform throughout its length.
4. The bushing device as claimed in claim 1, wherein the HT sleeve [01]
comprises at least two communication holes [06] from inner to outer
surface to enhance the electro-mechanical strength of the bushing at the
interface of the epoxy body [10] and the grounded terminal [04].
5. The bushing device as claimed in claim 1, wherein the grounded terminal
is [04] made of aluminium or steel, and comprises a plurality of profiled
holes [05] configured on the inner surface, and wherein a sealing means
[07] is provided on its collar.
6. The bushing device as claimed in claim 1, wherein the HT-sleeve [01] is
provided with at least two grooves [06] for inserting 'O' rings on one side
matching with the Stopper [12] of the HT conductor [11] to act as the
sealing means for hydrogen gas.
7. The bushing device as claimed in claim 3 to 7, wherein the HT-conductor
[11] is tightened against HT sleeve [01] and the locking means (13).
8. The bushing device as claimed in claim 1, wherein the HT-sleeve [01] is
provided with a hydrogen gas inlet into an annular space between the HT
sleeve [01] and the HT conductor [11] for effective cooling of the HT
conductor [11].
9. The bushing device as claimed in claim 1, wherein at the interface of the
epoxy body [10] and the HT-sleeve [01], an interface bonding with
functionalized silane coupling agent is provided to enhance the
electromechanical strength of the housing.
10. The bushing device as claimed in claim 1, wherein at the interface of
epoxy body [10] and grounded aluminium/steel terminal [04], an interface
bonding with functionalized silane coupling agent is provided to enhance
the electromechanical strength of the bushing.
11. The bushing device as claimed in claim 1, wherein the electric field
controlled, grounded terminal (4) including the grounded ring (08) is
enabled to optimize/control the creepage / arcing distance of the bushing.
12.The bushing device as claimed in claim 1, wherein the electric field
controlled, grounded terminal (04) including the grounded ring (08) is
enabled to optimize/control the distance of the bushing.
13. The bushing device as claimed in claim 1, wherein the electric field
controlled grounded terminal [04] along with the grounded ring [08]
combined with a second grounded ring [09] is enabled to optimize/control
the creepage / arching distance of the bushing.
14. The bushing device as claimed in claim 1, wherein the profile
configuration of the grounded ring [08, 09] is enabled to enhance the
dielectric properties of the bushing device, in particular the power
frequency withstandable voltage and Lightning impulse withstandable
voltage.
15. The bushing device as claimed in claim 1, wherein the device is enabled to
enhance the bending load of the bushing by more than 130% of the
required load of 10 kN for one minute.
16. A high voltage generator bushing device for hydrogen cooled generator as
substantially described and illustrated herein with reference to the
accompanying drawings.

The invention relates to a high voltage generator bushing device for hydrogen
cooled generator comprising an epoxy body [10] and a high tension (HT)
conductor [11], an HT sleeve [01] and a grounded aluminium/steel terminal
[04]; at least one locking means (13); a plurality of grounded rings [08, 09]; the
grounded terminal [04] is located in the volume of the epoxy body [10] and the
HT-conductor [11] is plugged through the epoxy body [10], with functionalized
silane coupling agent as an interface bonding and the electrical bus transmission
system, and in that the HT conductor [11] is configured as a separate
replaceable component assemblage inside the HT sleeve [01] of epoxy body
[10] by means of a locking means [13].