FIELD OF THE INVENTION
The present invention relates to a support insulator device in metal enclosed gas
insulated co-axial systems using gas as main insulation to maintain isolation
between high voltage and grounded conductors.
BACKGROUND OF THE INVENTION
In order to support high tension (HT) electrode in grounded metal enclosures,
different types of spacers/insulators with variety of profiles are used (Figure 1
shows some of these spacers used for GIS application). In GIS, performance of
such insulators is affected adversely in the presence of metallic/non-metallic
particles and dielectric strength across the insulator decreases significantly. To
cope with such situations conventionally spacers are made with a plurality of fins
formed on the surface of insulator and increasing the creepage length (ref: US
4818825). However, these insulators have a problem of particle collection in fins
and enhancing local field especially at higher system voltages, damaging
insulator surface during service. Figure 1 shows an epoxy insulator referred in
this patent.
US 4145565 describes that the electrostatic field, between high voltage and low
voltage electrodes can also be controlled by embedding the insulator in the
conductor. The embedded length changes significantly with increase of system
voltage. Most importantly, height of the insulator has to be increased to control
surface stresses and beyond certain system voltages, triple junction breakdowns
become critical.

Different designs optimize electrostatic field and its distribution at operating
voltages. These stresses, in general, are additionally controlled by application of
profiled high tension (HT) Shields integrated to epoxy or attached externally.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a support insulator device in
metal enclosed gas insulated co-axial systems using gas as main insulation to
maintain isolation between high voltage and grounded conductors, which
eliminates the disadvantages of prior art.
Another object of the invention is to propose a support insulator device in metal
enclosed gas insulated co-axial systems using gas as main insulation to maintain
isolation between high voltage and grounded conductors, which constitutes
composite device with Alumina filled epoxy insulator including electrostatic
shields for high voltage (HV) and Ultra high voltage (UHV) applications.
A still another object of the invention is to propose a support insulator device in
metal enclosed gas insulated co-axial systems using gas as main insulation to
maintain isolation between high voltage and grounded conductors, in which the
insulator withstanding mechanical forces induced by normal and fault currents.
Yet another object of the invention is to propose a support insulator device in
metal enclosed gas insulated co-axial systems using gas as main insulation to
maintain isolation between high voltage and grounded conductors, in which
integrated electrostatic shields are provided to lower dielectric stresses on the
insulator, the insulator withstanding higher differential pressures.
A further object of the invention is to propose a support insulator device in metal
enclosed gas insulated co-axial systems using gas as main insulation to maintain
isolation between high voltage and grounded conductors, which is enabled to
develop optimized electrostatic field at a triple point junction.
SUMMARY OF THE INVENTION
The invention provides support insulator (cone/funnel) featuring stress control on
the surface of insulator by means of external electrostatic shield as well as an
integrated shield. As service stresses are well below breakdown stresses,
improved reliability of insulators is ensured.
According to the invention, the device is provided with) a dielectric spacer located
between two electrodes not only isolates the conductors electrically but also
provides mechanical support to the conductors and isolation between two
pressurized systems. The electrostatic field distribution across the dielectric
spacer depends on profiles of insulation and connected electrodes. In general,
electrical breakdown occurs in solid or gaseous insulation when the applied
electric field exceeds the withstand strength of the insulating media. Due to a
phenomenon called triple junction effect (the meeting point of materials of
different dielectric permittivity) the spacer breakdown voltage is much lower than
the gaseous medium alone. The intersection of gaseous medium, solid insulation
and electrical conductor is very common to a gas insulated system.
In the construction of metal clad switchgear equipment, sulphur hexa fluoride
gas is used, which has better insulation properties at increased pressure so that
higher voltages can be safely handled without increasing the size of high voltage
and low voltage electrodes significantly. At higher system voltages, breakdown
levels are further reduced due to metallic particles left over during assembly or
generated during operation.
The support insulator device of the invention having internal as well as external
HT shields improving electrostatic field distribution for normal and adverse
operating conditions in metal enclosed gas insulated high and ultra high voltage
systems.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS :
The invention is described with the help of Figures 1 to 8, where:
Figure 1 Shows a support insulator used for optimization of electrical stress
according to prior art.
Figure 2 Shows a Gas insulated metal clad switchgear system.
Figure 3 Integrated HT Shield of the support insulator device according to
invention.
Figure 4 Integrated LT insert of the Support insulator device according to
invention.
Figure 5 Support insulator device according to the invention.
Figure 6 Externally mounted Electrostatic field controlled shields according
to the invention.

Figure 7 Shows the device of Figure 5 with External Mounted shields.
DETAIL DESCRIPTION OF THE INVENTION
In the present invention, the high voltage (HV) conductor [01], made of a high
conductivity material is held inside a Grounded enclosure [02] or low voltage
(LV) conductor made of a material with good conductivity and mechanical
strength. The annular gap between HV and LV conductor (01, 02) is filled with
Sulphur Hexa Fluoride (SF6) gas. A cone insulator [03] made of epoxy is used to
locate HV conductor centrally in a grounded enclosure and provide mechanical
support to the HV conductor against mechanical forces/vibrations. This support
insulator commensurates to a metallic flange [04] of the grounded enclosure
[02]. This metal clad enclosure assembly is maintained at designed gas pressure
using the gas valve fixed to the cover plate [Q5]. Figure 2 shows the
configuration of gas insulated metal-clad switchgear system.
The cone insulator [03] has three parts. First one is High tension (HT) integrated
shield [06], Second one is low tension (LT) insert [07]. The third one is casted
epoxy body [08]. The support insulator (17) has cylindrical profile and can be
fastened to the tubular grounded enclosure. An HT integrated shield [06] has a
spherical profile [09] and its radius depend on the operating voltage for which
insulator is used. The HT integrated shield [06] is fastened electrically to the HV
conductor [01] of an external HT (high tension) shield [10]. There is a provision
for low contact resistance current transfer from the externally mounted HT shield
[10] to the HV conductor [01]. To improve mechanical strength of the interface
between the HT integrated shield [06] and an epoxy body [08], a number of
holes/slots [11] have been made on periphery of the insert (06) on either side.
Figure 3 shows the HT integrated shield (06) proposed as part of the device. An
LT insert [07] with a profile improves mechanical strength of the insulator (12)
and limits weight of the insulator. The material used for the LT insert (07) has
good electrical conductivity and mechanical strength. Figure 4 shows the LT
insert (07) proposed as part of the embodiment. The LT insert [07] has inner
grooves [12] (refer Figure 5) with a specific angle depending on its metallic ribs
[13] required for fastening [14] to the grounded enclosure [02]. The mechanical
strength at the interface between the LT insert [07] and the epoxy body [08] is
improved by means of a plurality of grooves [12] which are filled with epoxy.
The profile of the epoxy of the body (08) is based on optimization of electrostatic
stress on the HT conductor (01), integrated HT shield (06), LT insert (07),
insulator surface and tri-junction point.
As desired, electrostatic field level at tri-junction point is much less than that E-
field level on the HT conductor (01). In adverse operating conditions or due to
presence of foreign particles, the stress at tri-junctiqn may become critical for
the propagation of ionization/streamers/leaders created by the foreign
metallic/non-metallic/ insulating particles. Thus it is important to design
EHV/UHV insulators (17) by considering this aspect. In addition to this, the
profile of insulator (17) differs of two sides for example, concave and convex
side of the insulator (17). The thickness of the insulator (17) near the integrated
shield [06], the LT insert [07] and at its middle varies for mechanical
considerations. These geometries decide the electric stress level across the
insulator (17) as well as at the tri-junction point. The externally mounted
electrostatic field shields [10] shift the highest E-field point from the tri-junction
to a safer zone. Figure 6(a) shows the externally rnounted electrostatic field
controlled shield. This helps in improving voltage withstanding capabilities of the
insulator (17) and provide operate margins. These shields are also used for
supporting current transfer. The external shields have spherical profile [15] and
dimensions 10 to 20% higher than that of the of the integrated HT shield (06).
This arrangement improves performance of the support insulator (17) under
adverse service conditions as described above. For terminating ends of the high
voltage conductor (01) a plurality of spherical shields [16] are proposed. The
externally mounted HT (10) shields and the spherical shields (16) have identical
spherical profiles [15] and dimensions. Figure 6 (b) shows the externally
mounted electrostatic field controlled spherical shield (10) for the open end
support insulators (17).
Figure 7 shows proposed support insulator [17] with the features described
above.
APPLICATION :
Gas insulated support insulator is primarily used to support high voltage
conductors in a gas insulated substation equipment.
WE CLAIM :
1. In metal enclosed gas insulated co-axial systems, a support insulator
device to maintain isolation between high voltage and grounded
conductors the system, comprising:
- a high voltage (HV) conductor disposed inside a low-voltage conductor
(LV) or grounded enclosure (02), an annular gap between the HV and LV
conductors filled with sulphur hexa fluoride (SF6) gas;
- a cone insulator (03) enabling the HV Conductor to centrally locate in the
grounded enclosure (02) to provide mechanical support against
mechanical forces and vibrations;
- a cover plate (05) having a gas valve to maintain a metal-clad enclosure
assembly comprising the support insulator (03) with a metallic flange (04)
of the grounded enclosure (02) to maintain, at a pre-determined gas
pressure; the device comprising :
- a support insulator (17) having cylindrical profile (09) and fastened to the
ground enclosure (02);
- the cone insulator (03) comprises a height tension (HT) integrated shield
(06), a low tension (LT) insert (07), and a casted epoxy body (08), the HT
shield (06) electrically fastened to one of the HT conductor (01) and an
external high tension (HT) shield (10), the HT-shield (06) having a
Spherical profile (09) including a plurality of slots (11) to accommodate
the epoxy body (08);
- the LT insert (07) having a profile to improve the mechanical strength of
the insulator (17) including a plurality of inner grooves (12) configured at
specific angle in registration with corresponding number of metallic ribs
(13) for fastening to the grounded enclosure (02), the interface between
the LT insert (07) and the epoxy body (08) provided with a plurality of
grooves (12) filled with epoxy;
- a plurality of spherical shields (15, 16) having profiles identical to that of
the externally mounted HT-shields (10) for terminating the ends of the
high voltage conductor (01), and
- the insulator (17) is configured with a convex side and an opposite
concave side, the thickness of the insulator (17) adjacent the HT Shield
(06), LT-insert (07), and at the middle of the insulator (17) is different to
maintain a desired electric stress level across the insulator and at a tri-
junction point, the externally mounted electrostatic field shields (10)
enabled to shift the highest E-field point from the tri-junction point to a
safer zone.
2. A support insulator device in metal enclosed gas insulated co-axial
system to maintain isolation between high voltage and grounded
conductors as substantially herein described and illustrated with
reference to the accompanying drawings.

The invention relates to aft metal enclosed gas insulated co-axial systems, a
support insulator device to maintain isolation between high voltage and
grounded conductors the system, comprising: a high voltage (HV) conductor
disposed inside a low-voltage conductor (LV) or grounded enclosure (02), an
annular gap between the HV and LV conductors filled with sulphur hela fluoride
(SF6) gas; a cone insulator (03) enabling the HV Conductor to centrally locate in
the grounded enclosure (02) to provide mechanical support against mechanical
forces and vibrations; a cover plate (05) having a gas valve to maintain a metal-
clad enclosure assembly comprising the support insulator (03) with a metallic
flange (04) of the grounded enclosure (02) to maintain at a pre-determined gas
pressure; the device comprising : a support insulator (17) having cylindrical
profile (9) and fastened to the ground enclosure (02); the cone insulator (03)
comprises a height tension (HT) integrated shield (06), a low tension (LT) insert
(07), and a casted epoxy body (08), the HT shield (06) electrically fastened to
one of the HT conductor (01) and an external high tension (HT) shield (10), the
HT-shield (06) having a Spherical profile (09) including a plurality of slots (11) to
accommodate the epoxy body (08); the LT insert (07) having a profile to
improve the mechanical strength of the insulator (17) including a plurality of
inner grooves (12) configured at specific angle in registration with corresponding
number of metallic ribs (13) for fastening to the grounded enclosure (14), the
interface between the LT insert (07) and the epoxy body (08) provided with a
plurality of grooves (12) filled with epoxy; a plurality of spherical shields (15, 16)

having profiles identical to that of the externally mounted HT-shields (10) for
terminating the ends of the high voltage conductor (01), and the insulator (17) is
configured with a convex side and an opposite concave side, the thickness of the
insulator (17) adjacent the HT Shield (06), LT-insert (07), and at the middle of
the insulator (17) is different to maintain a desired electric stress level across the
insulator and at a tri-junction point, the externally mounted electrostatic field
shields (10) enabled to shift the highest E-field point from the tri-junction point
to a safer zone.