FIELD OF THE INVENTION
Present invention relates to an improved performance of insulation system under high voltage applications. This is achieved by using novel high voltage and low voltage shields along with progressive gas gap approach around Tri-junction region of insulation system of different materials in gas insulated switchgear.
BACKGROUND OF THE INVENTION
The present invention relates to three phase gas insulated switchgear, particularly to support insulators of different insulating materials being used. The support insulators in three phase gas insulated switchgear are generally made of different insulating materials depending on the application. Arcing insulator in gas circuit breaker is made of fibre reinforced plastic (FRP) material and is used for holding contact system of gas circuit breaker. The insulated arcing chamber with integrated shields keep the electrostatic field across the arcing contacts, current carrying contacts and on the surface of insulated arcing chamber all the time close to uniform irrespective of moving contact position. Support insulators are made of fibre reinforced plastic (FRP) material and is used to support movable arcing contact system of gas circuit breaker and at the same time to isolate the second terminal of gas circuit breaker from ground potential. Insulated operating rods are made of fibre reinforced plastic (FRP) material and is used to operate contact system of switchgear equipment like gas circuit breaker, disconnecter switch etc. These operating rods shall be capable to operate under electrical conditions and have good tensile and compression strength.
Further, the insulated nozzle made of filled PTFE is used in gas circuit breaker to discharge hot gas, to interrupt the arc, to insulate the contact system and to operate second movable contact of dual motion circuit breakers. Arcing takes place between arcing contacts and contained

within nozzle. Hence, nozzle material shall have excellent electrical, mechanical and thermal properties.
Moreover, the three phase gas insulated bus bar module consists a metallic chamber filled with insulating gas and the bus bars are supported by a single insulator concentrically or multiple insulators located radially / non-concentrically with respect to Metallic Chamber. Most of these support insulators are radially connected or post type with respect to metallic chamber. In some of the conventional bus bar arrangements, each bus bar is supported by multiple post insulators. These post insulators are either single post or tri post type. If it is single post type, there is requirement of minimum two to three post insulators for the support of bus bar at each location of the bus bar enclosure. If we use post insulators of one or two only for the support of bus bar, it may be susceptible to unsymmetrical electro-dynamic forces during short circuit. If three phase insulator is used to support three phase bus bars, flexibility in the extension of bus is limited. Rejection rate of three phase insulator or tri post insulator is more than an independent insulator both at production and operation and hence uneconomical. Assembly of multiple post insulators on each phase of three phase bus bar arrangement becomes complex and may result to bulky in size or complex in nature.
The life span and safety of the Gas insulated high voltage Switchgear(GIS) depends upon the electric field stress along the surface of the supporting spacer which must be uniform. The electric field stress should be as low as possible at the Triple Junction which is formed by the conductor, SF6 gas and the supporting insulator to avoid any discharge extended failures.
According to patent 6521839B1 and JPH06318415A, the insulated operating rod has cylindrical insulating rod and a connecting rod are connected to each other by a metallic shield ring and a connecting pin.

This type of operating rods cannot be used for higher voltages as the tri-junction formed around insulating rod, shield and gaseous insulation may intensify the electric field levels and initiate discharges across insulating rod at higher voltages. In some of the conventional operating rods, threaded connections between metal shields and insulating tubes may critically enhance electric field levels. Fig. 1 shows conventional insulated operating rods for switchgear equipment. Cone insulators, disc insulators or post insulators are used to support bus bars in gas insulated equipment. Gas-insulated bus bars are enclosed in a metal encapsulation that is filled with an insulating gas, e.g., SF6 or N2 or mixture of these gases or mixture with any other compatible gas. With reference to Pat. No. US 7485807 and US20030079906A1, the three bus bars are arranged in a particular orientation and are supported by post insulator. The grounded enclosure dimensions’ increase significantly with system voltage depending on post insulator configuration requirements. In conventional arrangements, post insulator height along with minimum distance between buses decide the grounded enclosure dimensions. In some of the conventional bus bar arrangements pat. No. US 7612293B2, DE 3137783 and US 4404423 each bus bar is supported by post insulator. These post insulators are either single post or tri post type. If it is single post, then there is requirement of minimum two to three post insulators for the support of bus bar at each location. Fig. 2 shows the conventional bus bar arrangements with various insulators. Application of this type of insulators is limited for lower voltages. At higher voltages, the tri-junction region formed around post insulators may initiate discharges and hamper performance of equipment. Therefore, there remains a need for mechanisms to improve performance of insulation system under high voltage applications in three-phase gas insulated switchgears.

OBJECT OF THE INVENTION
It is therefore primary objects of the invention to solve one or more of the above problem by providing progressive gas gap method for improved shielding from electric field across tri- junction of the insulators.
Yet another object of the invention is to improve tangential electric field along insulator surface by means of integrated high voltage and low voltage shields.
Still another object of the invention is to limit electric field intensification around tri-junction region of insulator of different materials and relative permittivity.
Yet another object of the invention is to provide electric field insulation system for high voltages without increasing radial distance of insulator.
Still another object of the invention is to limit electric field intensification around tri-junction region of insulator.
SUMMARY OF THE INVENTION
The present invention is therefore intended to solve one or more of the above problems by providing a method for improved electric field intensity around tri-junction region of the insulators.
Accordingly, there is provided a solid insulator which supports high voltage conductor from grounded body that in turn is immersed in gaseous environment. This configuration generates a very high electric field at the junction of metal (high voltage conductor), insulator (solid insulation) and gaseous insulation. The same is true even for LT conductors or LT shields. Conventional shields cannot be placed around insulator in such locations as the electric field around such insulation system is two to three times more than average electric field and results to premature failures. A novel HT and LT shields along with progressive gas gap approach have been proposed around such insulators and electric field intensification

could be limited. The present invention also aimed to cover broad range of insulators with different relative permittivity to limit such high electric field intensification around tri-junction region. The invention discloses a method for improved insulation from high electric field intensity.
In an embodiment, the method comprises providing a progressive gas gap around the tri-junction regions of said devices (01,07,08 and 13). In an embodiment the tri-junction is formed by a metal shield, insulating gas and a supporting insulator while increasing the gas gap from the tri-junction region to the shielding region of said devices for improved insulation; In another embodiment the progressive gas gap around the tri-junction is based on two parameters:
(i) depth of the gas gap [05A] region from the surface of the metal shield; and
(ii) the gas gap [05B] between the metal shield and the insulator.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The illustrated embodiments of the subject matter will be best understood by reference to the drawings, wherein like parts are configured by like numerals throughout. The following description is intended only by way of example, and simply illustrates certain selected embodiments of apparatus that are consistent with the subject matter as claimed herein. Figure 1 illustrates conventional insulated operating rods.
Figure 2 illustrates conventional support Insulators.
Figure 3 illustrates progressive Gas Gap configuration for Insulated Arcing Insulator of gas circuit breaker.
Figure 4 illustrates Progressive Gas gap configuration for the support Insulator of gas circuit breaker.

Figure 5 illustrates Progressive Gas gap and Auxiliary Shield configuration for insulated operating rod of switchgear modules
Figure 6 illustrates Progressive Gas gap and Auxiliary Shield configuration for Insulated nozzle of dual motion type gas circuit breaker.
Figure 7 illustrates Progressive Gas gap configuration for a Rib Insulator of a compact bus bar.
DETAIL DESCRIPTION OF THE INVENTION
The embodiments as described below are exemplary means to describe the invention and may subject to various modifications in alternate form falling within the scope of the invention.
Drawings as illustrated are for explaining details of the present disclosure in pertinent way and could be easily understood by person ordinarily skilled in the art.
Referring to fig.3, according to an embodiment, an insulated arcing chamber (01) assembly comprises of shielding electrodes such as HT shield I (02) and HT shield II (03) integrated to an insulating tube (04) on either side. The insulated tube (04) is made of Fibre Reinforced plastic (FRP) or Aramid / Kevlar or a combination of these fibres or equivalent material wound, impregnated with epoxy resin in vacuum/ pressure. The internal and external surfaces of the insulating tube (04) are resistant against arced SF6 gas. The relative permittivity of FRP tube is in the order of 2.2 to 2.4. A tri- junction is thus formed by the FRP, the gaseous insulation and the HT shields (I and II). To optimise electric field levels around tri-junction region of arcing insulator, a progressive gas gap (05) is proposed. The gas gap increases progressively from tri-junction point G11 to G14(05B). It also means that the gas gap increases from the tri-junction point to a region of shielding, G14, so that the electric field intensification is shifted towards region of shielding. The entire contact system is placed

inside the arching chamber (01) and insulated from the grounded metallic enclosure with configuration SF6 gas intensity. The progressive gas gap technique as implemented through tri-junction configuration having insulator integrated with the high voltage or the low voltage shields through gluing or crimping or locking or physical tightening process. Additionally, the arching chamber can also act as a support insulator (06) for insulating circuit breaker pole from ground potential.
Progressive gas gap are good at providing insulation from electric field intensity depending upon the depth up to which the gas gap [05A] is maintained. The depth of progressive gap [05A], which is to be maintained should be such that the tangential field along insulator and, electric field levels around high voltage (or low shield) is optimal and remain unaffected by the tri-junction configuration. The electric field intensity in the gas gap region remain below the tangential field intensity of the insulator.
Referring to figure 4, is an insulated Arcing insulator of gas circuit breaker illustrating the Progressive Gas Gap (05) increasing from G21 to G24(05B), wherein G24 being the region of the shielding.
Further, implementation of the progressive gas gap method of insulation has been illustrated through following examples:
Insulated Pull rod: Referring to figure 5, is an insulated pull rod (07) having limited dimension and the electric field intensity around HT Shield-I (02), HT Shield-II (03) remain so high that cannot be shielded by high voltage shielding of the rod. In these situations, one more high voltage or low voltage shield is required to protect the tri junction region and profile of insulated operating rods. These shields are called as auxiliary shields [02A, 03A]. These shields can be both at high voltage or at ground potential for higher voltage class equipment, it is essential to limit electrical stresses on surface of insulated operating rod [07] to improve reliability. The auxiliary shield configuration shall be such that the insulated operating

rod [07] shall have lower working stress all along the surface. The profile of auxiliary shield [02A, 03A] is such that there is no sudden increase of electrical stress at any point on the surface and the levels shall be within permissible limits. To optimise electric field levels around tri-junction region of the insulating operating rod [07], the progressive gas gap [05] is introduced. Accordingly, the gap increases from G41 to G44 [05B] i.e. from tri junction point to the region where the auxiliary shield [02A,03B 03A] profile prevails i.e., G44.
Insulating Nozzle: Referring to fig.6 is an insulated nozzle (08) having a second movable contact /pin (09) engaged to the main drive through the nozzle (08) for improving the electric field intensity across the inter – electrode gap. The nozzle having a primary moving contact assembly/socket contact assembly (11) integrated to first assembly of the nozzle. Moving contact assembly [11] consists of arcing contact [11A], movable current carrying contact (11B) and CC contact collar (11C). Alternatively, the nozzle (08) is connected to socket contact assembly in self-locked manner which helps to work against mechanical forces. Second terminal of the nozzle is coupled to a pin (09) through a mechanical arrangement and an energy storage device.
The second terminal of the nozzle [08] is at a fixed potential rather than at floating potential as in the some of the conventional systems. The second terminal of nozzle (08) is connected to a profiled terminal clamp (10). The clamping arrangement is connected to mechanical arrangement through an auxiliary movable shield (12). Profile of nozzle terminal clamp (10) may hamper the withstand voltage across nozzle (08). The electric field level across the nozzle is improved by proposing auxiliary movable shield (12). The profile of nozzle (08) is such that the push from arcing energy to the pin (09) along with the energy stored in the pin helps in achieving required arcing contact (09, 11A) separation in few milliseconds. The nozzle is connected to movable current carrying contact (CC) (11B) in self-locked

manner. Precisely the CC contact collar (11C) does not allow the nozzle to come out in normal working conditions. The CC contact (11B) and nozzle (08) along with SF6 gas form a tri-junction point that impede the performance of nozzle (08) for higher test voltages. For configuration the CC contact (11B) integration with nozzle (physical locking) for better performance, predefined gas gap has been created which generally increases as we go away from CC contact collar (11C) to its contact surface. This gas gap improves tangential field across nozzle surface and limits electrostatic field intensification around tri-junction region. To control local electric field enhancement, it is important to provide divergent gas gap or progressive gas gap starting from “GG” to “G1” and then finally to “G2” between CC contact collar (11C) and the nozzle surface. At the same time the collar (11C) shall have suitable profile to limit these electrical stresses further. In this present situation, the metallic part i.e. CC contact (11B) is not integral part, it is only locked manually. Similarly, the gas gap between nozzle terminal clamp (10) and nozzle (08) is varied from “G3” to “G4” and then finally to “G5” near the auxiliary movable shield (12). This type of progressive gas gap improves the surface stress on the nozzle and the gas gap G5 ensures the highest stress levels on nozzle surface less than the allowed limits. The relative permittivity of nozzle material is possible up to 3 depending on its filler composition.
Compact Bus Bar: Referring to figure 7 is a rib insulator with proposed progressive gas gap configuration (05). A three phase compact bus consists of three high tension (HT) buses arranged circularly or in a triangular configuration supported by compact insulator or post insulator or a rib insulator. These insulators take support from grounded metallic enclosure. The proposed configuration is such that instead of cost intensified and bulky compact insulator or post insulator with bulky grounded enclosure, a Rib Insulator (13) is used to support all three phases of compact bus independently. The Rib Insulator (13) consists of three parts: HT insert (14) with integrated shield (14A), low tension (LT)

insert (15) or metallic studs and casted epoxy body (16). Rib insulators (13) having two ribs and having a tapered profiled from centre i.e., high voltage side to tail, i.e., ground potential. Precisely, the rib insulator gets supported at two locations on a grounded enclosure (17). These supporting points are located non-concentrically to the grounded enclosure. An additional compact LT shield (18) for limiting electrical stresses along with the Rib insulator is provided which is made of high conductive material, and used as a connector between the rib insulator and the grounded metallic enclosure (17). These LT shield are flexible and keep the rib insulator in position. An epoxy body (16) always remain on a floating voltage so that its creepage across the body should be sufficient enough for limiting the surface intensity of electric field. In other words, the composite insulation formed by epoxy and gas gap (radial distance between HT conductor and LT shield (18)) shall be such that the electric field level on the rib insulator body shall not be sufficient to create surface discharges. This helps in improving voltage withstanding capabilities of the Rib insulator (13) and provide good safety margins of the Rib insulators against basic insulation levels.
Similarly, the compact LT shield (18) is profiled in such a way that the electrostatic field level at tri-junction point (junction of gas, metal and epoxy insulation) is much less than that E-field level on the insulator body. To limit stress at this junction, the gaseous gap (GG) (05B) is created between LT shield (18) and the rib insulator (13). The LT shield is of spherical type and gas gap “GG” (05B) between spherical LT shield (18) and epoxy body (16) shall be such that the electrical stress on epoxy body is much less than on the epoxy body (16) near high voltage conductor. Here, LT insert (15) is an integral part of epoxy body i.e., LT insert (15) is casted in the epoxy body (16). If we tighten directly LT shield (18) to the LT insert (15), gas gap created is not sufficient to supress the electric field intensity around tri-junction region. Hence, profile of LT shield (18) shall be made in such a way that, sufficient gas gap “GG” (05B) is created. In this

situation, progressive gas gap has to be created to improve electric field levels around insulator and withstand for higher voltages demanded by the system. The progressive gas gap increased from fraction of millimetre to few millimetres depending on voltage level. In the present design progressive gas gap (05B) increased from GG to G and then finally to GGG. This variation is within few millimetre distance depending on type of insulator material. In the present invention, the relative permittivity is considered up to 6. Most importantly, the tangential field around insulator surface remains much less than the electric field level around high voltage and low voltage shields.

WE CLAIM:
1. In three phase gas insulated switchgear comprising of an insulated
arcing chamber (01), an insulated pull rod (07), an insulating nozzle
(08), a rib insulator (13), and a method for improved insulation system
from high electric field intensity, comprising the steps of:
- providing a progressive gas gap around the tri-junction regions of said devices wherein the tri-junction is formed by a metal shield, insulating gas and an insulator (FRP tube, epoxy, filled nozzle etc.);
- increasing the gas gap from the tri-junction region to the shielding region of said devices for improved insulation; wherein,
the progressive gas gap around the tri-junction is based on two parameters:
(i) depth of the gas gap [05A] region from the surface of the metal shield; and
(ii) the gas gap [05B] between the metal shield and the supporting insulator.
2. The method as claimed in claim 1, wherein the progressive gas gap (05) around the tri-junction region of the insulated arcing insulator/chamber (01) is provided in the tri-junction region by integrating the insulator with high voltage or low voltage shield either by gluing or crimping or locking or physical tightening.
3. The method as claimed in claim 2, wherein the progressive gas gap of the insulated arching chamber increased from G11 to G14 so that the electric field intensification shifting from the tri-junction region to the shielding region for better insulation system.

4.The method as claimed in claim 1, wherein the insulated arching chamber comprises a HT shield I (02), HT shield II (03) integrated to an insulating tube (04) on either side, and wherein the insulating tube made of Fibre Reinforced plastic (FRP) or Aramid/ Kevlar or a combination of both wound and impregnated with epoxy resin in vacuum/pressure.
5. The method as claimed in claim 1,4, wherein the insulated arcing chamber (01) configured as a support insulator (06) for insulation of circuit breaker pole from ground potential providing said progressive gas gap (05) method from G21 to G24(05B).
6. The method as claimed in claim 1, wherein the insulated operating/pull
rod (07) having auxiliary shields (02 A and 03A) is configured to shift so the progressive gas gap (05) region gets from a G41 to a G44 (05B) i.e. from tri junction point to the region where the auxiliary shield [02A, 03A] profile prevails i.e. G44.
7. The method as claimed in claim 1, wherein the insulating nozzle (08) for improving electric field intensity around inter-electrode gap of gas circuit breaker by providing said progressive gas gap method starting from “GG” to “G1” and then finally to “G2” (05B) between CC contact collar (11C) and the nozzle surface.
8. The method as claimed in claim 1, wherein the progressive gas gap (05) between a nozzle terminal clamp (10) and a nozzle (08) is increased between G3 to G5 around auxiliary movable shield (12) region.

9. The method as claimed in claim 1, wherein the Rib insulator (13) having
a rib insulator body (16), High Tension (HT) insert (14), Low Tension (LT) insert (15) casted on the epoxy body, a casted epoxy body (16), a grounded metallic enclosure (17) and an additional compact LT shield (18) for limiting electric field intensity along the rib insulator (13).
10. The method as claimed in claim 8, wherein between the rib insulator (13) and the grounded metallic enclosure (17) is provided a flexible LT shield as a connector which is to be interoperable to the rib insulator (13) for providing increment in the progressive gas gap from GG(05B) to (GGG).
11. The method as claimed in claim 1, wherein the progressive gas gap varies depending on voltage level and relative permittivity.