FIELD OF THE INVENTION:
The invention relates to a product and system for protection of substation equipments, i.e; switch gear, transformer etc. against lightning over voltages, switching over voltages and temporary over voltages, which is primarily installed at the line entrances and as a complementary protection at strategic points of the substation, often at the transformer,
BACKGROUND AND PRIOR ART:
Generation, transmission and distribution substations are generally subjected to lightning, switching and temporary over voltages during service. These substations are required to be protected from their destructive effects. Surge arresters, which are being used for this purpose, are primarily installed at the line entrances and as a complementary protection at strategic points of the substation, often at the transformers. Conventional porcelain/composite insulator clad surge arresters are used for yard substation application. For compact substations or highly integrated substations (HIS), if surge arresters are installed outside the GIS i.e., near the transformer, protection against over voltages is limited by the electrical distance between the GIS and the arrester. In order to maximize the benefits of the compact substations like gas insulated substations

(GIS), it is desirable to use metal-clad gas insulated surge arresters as an integral part of these substations. The main advantages of the metal-clad surge arresters over conventional porcelain/composite insulator clad surge arresters are given below:
1. Improved insulation co-ordination.
2. Quick response for steep discharge current.
3. Immunity from external environment like pollution, snow, rain etc.
4. Reduced voltage stress to the connected equipment.
5. The flexibility of placement the arrester at an optimum place in the GIS.
6. Significant reduction in size and weight.
7. Switching over voltages seen by the connected transformer/equipment is
reduced significantly. The present invention relates to single phase UHV gas insulated switchgear. Initially, Gas-insulated surge arresters arrester columns 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 US. Pat. No. 5912611, the metal-oxide blocks are placed in insulating tube for mechanical stability. In conventional stack assembly of surge arresters, the metal-oxide blocks are physically stacked and connected electrically in series through the metallic surface. The proposed design may not be useful to solid metal-oxide

blocks (different from metal-oxide blocks with central hole). Fig. 1A shows the conventional non-linear elements stack assembly. Following are some of the drawbacks with this arrangement:
1. In the absence of inter-stage element, contact resistance between blocks
is poor.
2. Due to lack of effective heat transfer medium between blocks, heat dissipation from the blocks is confined mostly through the glazed surface of SA elements only.
3. Electrostatic field enhancement at the edges of the blocks due to their sharpness/imperfections resulting to ionization of insulation medium.
4. Discharge capabilities of blocks are limited and utilized inefficiently due to poor voltage and temperature distribution.
In some of the grounding tank type arrester, the columns are made as current folding columns and current pass columns. The current folding column includes stack sets of an element unit of zinc oxide elements and an insulating spacer. (refer US 4814936 A). Fig. IB is a diagram illustrating the construction of columns applied.
OBJECTS OF THE INVENTION:
The object of the invention is to develop a compact metal clad gas insulated ultra high voltage surge arrester with improved design, as an integral part of the sub-

stations provided by, an insulating tube housing to hold metal oxide blocks stack
in position.
Another object of the invention is to design a novel surge arrester with effective
dissipation of heat and uniform electrostatic field through optimum gas
circulation.
Further object of the invention is to arrange multicolumn arresters for high
energy discharge applications.
Still further object of the invention is to design a surge arrester for improved insulation coordination, quick response for steep discharge current, immunity from external pollution, snow, rain etc.
SUMMARY OF THE INVENTION:
The main objective of the present invention is to improve the performance of high voltage gas insulated surge arrester both in terms of capacity and compactness using a novel insulating tube assembly supported on metallic support plate for all parallel and series metal oxide stack assemblies as per energy requirements, integrated with electrostatic field controlled grading HT shields, integrator and insulated from grounded enclosure through novel LT terminal assembly.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
The invention is described with the help of Figures 1 to 8, where:
Figure 1: Shows the conventional surge arresters.
Figure 2: Shows Stack of MO blocks with inter-stage element.
Figure 3: Shows the stack assembly of gas insulated surge arrester with
insulating housing. Figure 4: Shows the stack assembly of gas insulated surge arrester with high
voltage grading shield. Figure 5: Shows high voltage grading shield. Figure 6: Shows the stack assembly of gas insulated surge arrester with
Integrator. Figure 7: Shows the plug in LT Terminal assembly for three phase gas
insulated surge arrester. Figure 8: Shows the patented high voltage surge arrester assembly.
DETAILED DESCRIPTION OF THE OF THE PREFERRED EMBODIMENT:
In the proposed invention, the metal oxide elements [01] are stacked in series and connected by means of a thermally/electrically conducting inter-stage element [02] as shown in Fig. 2. The inter-stage element [2] is made from a high conductive electrical guide Aluminum or copper metal. It helps in

maintaining uniform temperature and uniform electrostatic field across MO elements [01] and overcomes thermal runaway problems in the surge arrester. During conduction of blocks, thermal energy generated within the MO elements [01] is dissipated through this inter-stage element [02] efficiently to the gas medium. The surface of contact plate/cooling element/inter-stage element [02] has provision for gas circulation through suitable flow channels [03] as shown in Rg. 2. These flow channels [03] help to circulate gas from one MO block [01] to another. The metalized surface of metal-oxide block is allowed to be exposed to the medium through these flow channels [03]. An inner passage [04] helps in take away heat from metal-oxide block [01]/inter-stage element [02] to the insulating medium. This is further possible through the peripheral surface of the inter-stage element [02]/glazed surface of the MO blocks [01]. The element maintains a lower temperature at the metalized surface of the block as it is in contact with the insulating medium during normal operation. Part of the heat, over a period of time, distributes to the adjacent blocks and temperature distribution becomes highly uniform. The arrangement helps in achieving uniform thermal distribution through gas insulation medium. The inter-stage element [02] also improves the surge current carrying capabilities of the surge arrester as it helps in creation of parallel paths across the elements. The inter-stage element [02] also ensures low contact resistance between two metallic surfaces of the

metal-oxide block [01] during their service. The profile [05] of inter-stage element [02] is designed in such a way that resultant electrostatic field across the blocks is uniform and over all electrical stress levels are reduced to overcome the problem of ionization of the insulating medium at edges of metal-oxide blocks [01]. The design of the inter-stage element [02] is decided by the requirements of heat dissipation capabilities, discharge current and the voltage rating/distribution across elements. The improved design, obtained through above arrangement maintains uniform temperature distribution across metal-oxide blocks [01], uniform thermal distribution through insulating medium and enhances surge arrester performance and efficiency.
Improvement in electro-mechanical-thermal performance of surge arrester is further achieved by keeping the stack assembly [06] in an insulating housing [07] of high mechanical and dielectric strength. The insulating housing/insulating tube [07] is terminated in field smoothen profiled metallic flanges [08] as shown in Rg. 3. For proper support of stack, the insulating housing [07] is fixed to cun-ent transfer plate [09] and current transfer cum spring holder plate [10] with openings for gas circulation as shown in Fig. 3. The current transfer cum spring holder plate [10] has provision for gas circulation through suitable flow channels. The current transfer plate [09] helps to provide connection with HT terminal. The current transfer plate [09] provides plug and socket type arrangement to give

flexibility of 2-3% to adapt the dimensional changes if occurs in the stack assembly due to thermo-mechanical effect. Necessary gas flow openings have been made to insulating housing [07] and current transfer plates [09], [10] for better gas flow and in turn effective heat dissipation from the blocks from insulating tube assembly [07,09,10]. The current transfer plate [09] also has arrangement to integrate to the invented grading HT shield [11] as shown in Fig. 4. The voltage distribution across the SA elements is further refined with the help of the invented HT shield [11]. In GIS surge arrester, a non-linear voltage distribution across the blocks is mainly due to significant capacitive coupling between the stack assemblies [06] and the grounded enclosure [12]. The leakage current through stray capacitances will be compensated partly by HT shield [11] and improve voltage distribution across surge arrester blocks. Design of HT shield [11] is based on consideration of the electrical characteristics of blocks like stray capacitance from the SA blocks [01] to grounded enclosure [12], height of the stack, capacitance of the SA blocks [01], capacitance between HT shield [11] and SA blocks to achieve optimum voltage distribution across the SA elements [01]. Fig. 5 shows the invented HT shield. HT shield consists of various parts i.e. full HT shield [13] and HT ring [14] is integrated with each other by partial window creater HT rods [15]. The developed stack assembly [06] is modular and can be connected in series or parallel depending on energy

requirements. The resultant voltage variation across the SA blocks [01] is within acceptable range. The proposed design is of using multi-stack arrangement i.e. at least two column for meeting energy requirement of the system. These stacks are placed parallel to each other. As these stacks have same voltage levels, they can be placed as close as possible, hence optimize space. For higher voltage ratings, more number of SA blocks [01] is required. High mechanical and dielectric strength is achieved by keeping the stack assemblies [06] in two different insulating housings [07], which are connected in series. The two insulating tube assemblies [7][9][10] are connected with each other by a novel integrator [16] as shown in Rg. 6. This integrator [16] will provide support for all insulating tube assemblies [7][9][10] in series and in parallel. This integrator also provided with shielding plates [17]. These shielding plates [17] will have profile is such a way that electric field is uniform between the consecutive stack assemblies [06] and to the ground. For series connected stack assemblies [06], the integrator will allow the gas to flow from one stack assembly [06] to another and improves gas circulation and heat dissipation. Integrator [16] has advantage that it ensures perfect contact between the stack assemblies [06], under transient current conditions in long run of its application. As shown in Fig. 6, metallic support plate [18] has provision for mounting of the whole stack assembly [06]. The profile of plate is made such that electric field

levels are at acceptable range. This metallic support plate [18] also integrates the stack arrangement [06] to grounded SA enclosure assembly [12] through novel insulating plate [19] arrangement. This insulating plate [19] isolates the high voltage stack assembly [06] from the grounded enclosure [12]. This isolation helps to transfer the surge current to the grounding grid without flowing through grounded enclosure [12] and helps to measure leakage current and surge count. For monitoring of leakage current through surge arrester, a special provision of LT terminal assembly [20] has been made in the present development on the low voltage side of surge arrester, as shown in Fig, 7. The LT terminal [20] is brought out using suitable LT support insulator [21] providing insulated LT terminal [20], suitable for measuring leakage currents. Inside the enclosure the LT terminal [20] is provided as plug-socket type arrangement [22]. This arrangement consists of plug-in contact and removable contact system from surge arrester assembly. This provision helps in assembly of surge arrester in short time and keep the assembly compact. The LT terminal assembly [20] shall have provision for connecting to surge counter and leakage current measurement (LCM) system.
Fig. 8 shows the invented high voltage Surge Arrester (SA) assembly. The grounded enclosure [12] consists of three parts, first one is a reducer assembly [23] connected to modules of gas insulated substation like disconnector switch,

earthing switch etc. The second one is straight chamber [24], which had provision for supporting insulating tube assemblies [06] with Integrator [16] and LT assembly [15]. Third one is dish cover assembly [25]. This provides bottom side fixing to insulating plate [19] on which the parallel stack assemblies [06] are mounted with the help of metallic support plate [18]. At high voltage side, the MO stack assembly [06] with integrated electric field controlled HT shields [11] is fixed/supported with terminal plate [26]. The terminal plate [26] is facilitated with a conductor for connection with other modules of the GIS through support insulator [27].

WE CLAIM
1) A compact high voltage gas insulated surge arrester (28), comprising parallel and series metal oxide stack assembly (06), of metal oxide elements (01), in an insulating housing (07), fixed on metallic support plate (18).
2) The surge arrester as claimed in claim 1, wherein a high conductive electrical grade aluminium or copper make inter-stage element (02), connecting the stacks, dissipating the thermal energy generated within the metal oxide elements (01).
3) The surge arrester as claimed in claim 1 wherein, flow channels (03), inner passage (4) for circulation of gas through MO blocks being exposed to the gas medium, taking away heat to the insulating medium.
4) The surge arrester as claimed in claim 1, wherein, the profile (05) of inter¬stage element (02), effecting uniform resultant electrostatic field across the M.O blocks.
5) The surge arrester as claimed in claim 1, wherein, profiled metallic flanges (08), to hold insulating tubing (07), supporting the stack assembly(06) & the insulating housing (07).

6) The surge arrester as claimed in claim 1, wherein, current transfer cum spring holder plate (10), current transfer plate (09), providing plug & socket arrangement, characterized by flexibility of 2-3 % to adapt the dimensional changes occurring in the stack assembly.
7) The surge arrester as claimed in claiml, wherein, HT shield (11) comprising full HT shield(13), HT ring (14), integrated by HT rods (15), effecting optimum voltage distribution across the stack assembly.
8) The surge arrester as claimed in claim 1, wherein, integrator (16) providing support to integrate all insulating tubes and assemblies (7), (9) & (10) in series and parallel.
9) The surge arrester as claimed in claim 1, wherein, shielding plates (17) as
part of integrator and profiled to provide uniform electric field between the
consecutive assemblies.
10)The surge arrester as claimed in claim 1, wherein, metallic support plate (18), integrating the stack assembly (06) to grounded plate of SA enclosure assembly (12).

11)The surge arrester as claimed in claim 1, wherein, the insulating plate (19), isolating the high voltage stack assembly from the grounded enclosure.
12)The surge arrester as claimed in claim 1, wherein, LT terminal assembly (20) having plug in contact and removable contact providing connection to surge counter measuring leakage current.
13)The surge arrester as claimed in claim 1, wherein, a reducer assembly (23) connecting to modules of gas insulated substation, straight chamber (24) providing support to the insulating tube assemblies(06) with integrator (16), LT assembly (15), dish cover assembly (25).
14)The surge arrester as claimed in claim 1, wherein, the terminal plate (26), facilitating connection through conductor with other modules through support insulator (27).