Title of Invention

AN IMPROVED GAS INSULATED SUBSTATION (GIS) DISCONNECTOR SWITCH

Abstract

An improved gas insulated substation (GIS) disconnector switch (DS) comprising of : a fixed contact shield (04) covering a fixed contact (FC) (01), a moving contact shield (06) surrounding a moving contact (MC) (05), wherein both the FC shield and MC shield are insulated from a grounded metallic enclosure (09) by means of SF6 gas with appropriate density which fills the enclosure (09), and MC shield is separated from FC shield be a distance D which is proportional to the system voltage and SF6 gas density, characterized in that the E-field around the contact system is made uniform by profiling the fixed contact and moving contact shields and the axial E-field between the MC and the FC shield is made uniform by keeping radius of curvature of moving contact main body (R1) greater than the radius of curvature (R2) at the throat of the moving contact while keeping the outer radius of curvature of the FC shield (R3) higher than the radius of curvature (R4) of the FC Shield around the central axis .

Full Text

Field of the Invention: The present invention relates to an improved Gas Insulated Substation (GIS) Dis-connector Switch. Disconnector switch (DS) is primarily used to isolate sections of high voltage installations. When a DS is closed, the capacitive charging current flows through the contacts of the disconnector in proportion to the line voltage and the line length. The disconnector during switching operation suffers re- strikes, leading to arc breakouts. The phenomena of the arc breakouts is associated to Very Fast Transient Over-Voltages (VFTOs) generated during this operation. Any development in high voltage disconnector switch is to meet the IEC procedures to establish such conditions in a test laboratory, for evaluation and certification of disconnector switch performance. Background of the Invention Very fast transient over-voltages (VFTOs) are common to disconnector switch operation and are result of rapid dielectric breakdown under severe electrostatic stresses experienced by the insulating media (SF6) for relatively slow operating speeds of the disconnector switch. Some times these breakdowns lead to secondary breakdowns such as arc breakouts. The voltage collapse across switching contacts take place in a time duration of 3-20 ns depending on line voltage, electric field non-uniformity and SF6 gas density. This resultant microwave emission propagating from switching contacts travel along the gas insulated bus. The wave is reflected back from terminations and modulates the principal wave in magnitude and time, which is expressed as VFTOs. The wave-shape of these transients to a great extent depends on the configuration of the gas-insulated circuit and associated terminations. The number of VFTOs generated during a disconnector switch operation is considerably high compared to the other fast acting switches. The slow moving contact sees many voltage peaks before the DS operation as well as the interruption is completed. The operating speeds of the moving contact and inter-electrode gap are the vital parameters for a SF6 gas insulated disconnector switch. Disconnectors are employed in a system to interrupt / make capacitive currents in principle. When the capacitive current is interrupted a spark is formed, which shunts the potentials on the two electrodes. When this spark extinguish, a few micro seconds later, results in different potentials on the source side and the load side of the disconnector. US patent No.4,413,166 discloses a disconnect switch and grounding apparatus therefor particularly adopted for use in gas insulated substations. The disconnect switch disclosed in this document has fixed and moving contacts insulated with SF6, but it does not make any mention about controlling the axial and radial E-fields during operation of the switch, nor about profiling of contacts or shields. US 5,177,664 discloses a three-phase "lumped together" gas insulated switchgear, where the object of the invention is to achieve a small-sized and simply constructed unit which includes disconnectors. This document too does not have any reference to control of the E-fields to eliminate arc breakouts, as is disclosed in the present invention. US 7,091,493 B2 discloses an SF6 filled device which contains an isolator, but this document also does not mention anything about the E-fields or their control through profiling of contacts or their shields. During evaluation of a disconnector switch (for bus charging current test) the moving contact of the disconnector switch is kept at a DC potentiai of -1.1 p.u., while the fixed contact is maintained at a AC voitage equal to 1.1 times of the system voltage. When contacts of a disconnector switch separate, several restrikes (spark/discharge) take place, causing transition of E-field distribution from axial to radial direction. This process tends to influence the axial discharges, deflecting it towards the grounded metallic enclosure. This recurrent phenomenon at times leads to secondary breakdowns (arc breakout) to the grounded enclosure. The VFTOs magnitude at / near the disconnector switch contacts during its operatbn may also lead to voltage multiplication as high as three times depending on the observation point and the substation configuration. Intense VFTOs may result to the voltage close to basic insulation levels (BIL) leading to frequent VFTO lead breakouts in very high voltage systems. To meet the above disasters Conventional disconnector switches were employed primarily constituted by a fixed contact, a moving contact and an inter-electrode gap. A metallic grounded enclosure was used to contain SF6 gas insulation at specified density. The system was designed for minimum average electrostatic fields, without considering the impact of axial and radial electrostatic fields. The switching (transition) of electrostatic fields during presence of a spark/discharge due to restrikes was also seldom considered. The conventional disconnector switch in above situation suffered the following deficiencies: 1. Highest magnitude of VFTOs around DS contacts 2. Field non-uniformity between/surrounding the contacts during discharges 3. Long discharge lengths/lnter-electrode gap. 4. Requirement of higher operating densities for SF6 gas. 5. Voluminous designs Description of the Invention The main objective of the Invention to overcome the prior art difficulties is developing a secondary break-down free gas insulated disconnector switch by controlling the radial electric field around the contact system during a discharge between the contacts. According to another objective of the invention the conventional contact system of the disconnector switch is replaced by an improved arrangement to achieve optimized electrostatic fields in the electrode gap. According to a further objective of the invention the discharge length/inter-electrode gap is optimized. According to a still further objective of the invention radial E-field between the contact system of the disconnector switch and the metallic grounded enclosure is improved. According to yet another objective of the invention specific profile of electrostatic shields for contacts of disconnector switch is developed. The proposed Invention has designed constructional features of the fixed contact (Fc) and moving contact (Mc) of the disconnector switch to meet the following requirements. 1. Axial E-field must be uniform between the contact shields, when DS is in fully open condition. 2. Axial E-field must also be uniform between moving contact and fixed contact shield during DS is in operation. 3. During contact movement, axial E-field should be highest between FC shield and the moving contact around the central axis. 4. During a discharge between the contacts, the radial E-field from the moving contact tip / discharge towards grounded metallic enclosure shall be controlled with suitable profiles of FC and MC shields. To comply with the above requirements to maintain optimal efficiency of the switch without arc breakout and VFTO lead breakouts the fixed contact covered by a fixed contact shield is maintained axial with annular space between the fixed contacts shield to control the axial field between the fixed and moving contact shields. The entire contact system is insulated from a grounded metallic enclosure maintained with SF6 gas with appropriate density and the moving contact is located in a MC housing. A gap proportional to system voltage and SF6 gas density is maintained between the moving contact shield and fixed contact shield. The maintenance of discharged length and axial E-field between the Mc and Fc shield is made uniform and keeping the highest axial E- field at the central axis and not on the surface of the tip is carried out by using well profiled FC shield and the moving contact keeping radius of curvature of the moving contact main body higher than the radius of curvature at the throat of moving contact. The outer radius of curvature of the FC shield is maintained much higher than the radius of curvature of the FC shield around the central axis. According to the invention there is provided a GIS disconnector switch (DS) comprising of a fixed contact shield covering a fixed contact (FC) and a moving contact shield surrounding a moving contact (MC), both the FC shield and MC shield for DS operation being Insulated from a grounded metallic enclosure filled with appropriate SF6 gas density wherein MC shield is separated from FC shield by a distance 'D' proportional to the system voltage and SF6 gas density, the highest discharge length and radial E-field from the discharge are controlled by profiling fixed contact shield around the central axis of the DS moving contact shield and high voltage shield around the fixed contact in which discharge length, uniform axial E- field between the MC and FC by keeping radius of curvature of moving contact main body (R1) higher than the radius of curvature (R2) at the throat of the moving contact, keeping the outer radius of curvature of the FC shield (R3) much higher than the radius of curvature (R4) of the FC shield around the central axis, the said R1, R2, R3 & R4 are being maintained with correlated relationship with diameter (D1) of the MC and diameter (D3) of the FC shield. The invention will be better understood by description with the help of the accompanying drawings which Figure 1 represents the conventional contact system for the gas insulated disconnector switch. Figure 2 represents the invented moving contact for the secondary breakdown-free disconnector switch. Figure 3 represents the invented fixed contact shield for the secondary breakdown-free disconnector switch. Figure 4 represents the Fixed contact assembly with shield for the invented gas insulated disconnector switch. Figure 5 represents the Assembly of the invented secondary breakdown-free disconnector switch, (a) DS is in fully open condition (b) DS Is under operation. As shown in Figure 5 the fixed contact [01] of the disconnector switch (DS) Is made of a high conductivity material with low erosion refractory material tip held on a support insulator [02] with suitable adapter [03]. The fixed contact is covered by a fixed contact shield [04] with designed axial and annular space to control the axial field between the fixed and moving contacts. The moving contact [05] Is surrounded by a moving contact shield [06], when DS is in fully open condition. The moving contact is located in housing called as MC housing [07]. The DS operation is achieved by using an insulator [08]. The entire contact system is insulated from the grounded metallic enclosure [09] with designed SF6 gas density. The moving contact shield is separated from the fixed contact shield by a distance 'D' proportional to the system voltage and the SF6 gas density. A secondary breakdown-free disconnector switch shall meet the following requirements: The profile of the fixed contact and the moving contact shields are designed in such a way that the E-field around the contact system is highly uniform. This criterion must be met as long as the DS is in fully open condition and is in closed condition. The axial E-field between the MC and the FC shield (seen by moving contact) is made uniform by using a well-profiled FC shield [04] and the moving contact tip. The Moving contact is designed and constructed in a manner so that the field shall be highest at the central axis rather than on the surface of the tip. Figure 2 shows the profile of moving contact. For keeping the highest axial E-field along the central axis, R1 (radius of curvature of the moving contact main body) must be much higher than R2 (radius of curvature of the throat of moving contact). With this design, the highest axial field will be close to the central axis Instead of near the outer surface of the moving contact. Further, the discharge will be bound to the central axis. Once the axial field is highest along the central axis, the discharge length may not be the optimum value. The highest discharge length is a function of the diameter (D1) of the moving contact with respect to the FC shield. By using the highest possible value of R1 for the moving contact (depending on diameter of the moving contact and the throat of moving contact (D2) for gas flow), optimum discharge length can be achieved. Since highest discharge length can not be optimized beyond certain extent, the radial E-field from the discharge is still a parameter to be controlled. The diameter of the FC shield (D3) and its profile are selected in such a way that the radial E-field around the discharge / moving contact tip can not force the discharge from the central axis towards the grounded enclosure of the disconnector switch. The diameter of the shield is required to be at least 3 to 4 times highest possible discharge length depending on the system voltage. When moving contact is approaching the fixed contact during the DS operation, the highest axial E-field is forced to be around the central axis. This is possible by profiling the fixed contact shield around the central axis, where moving contact enters and engage the fixed contact. Figure 3 shows the profile of FC shield by meeting the above features. Here, R3 is outer radius of the shield and R4 is radius of the shield around the central axis. For optimal design of the shield, R3 must be much higher than R4. Moreover, R3 is comparable to the diameter of the moving contact. Figure 4 shows the fixed contact assembly for the gas insulated disconnector switch, according to the proposed invention. The Invention is illustrated with an exemplary illustration in Table 1. Table 1 For 145 kV GIS, the dimensions as stated herein above are as follows: R1 = 12.0 mm R2 = 2.0 mm R3 = 35.0 mm R4 = 2.0 mm D1 = 45.0 mm D2 = 8.0 mm D3 = 125.0 mm D = 60.0 mm The said dimensions change with voltage class. However, the relation between these parameters will be maintained in the similar manner. The highest discharge length for 145 kV GIS is in the order of 30 mm and is in the same level as R3. This value is again in the same order as that of diameter of moving contact (40 to 45 mm). The diameter of FC shield is 3 to 4 times of the highest discharge length. As stated herein above, the profile of FC shield is function of highest discharge length. It is important to optimize the highest discharge length during the disconnnector switch operation. The greater the distance between the contacts at the moment of strike, the greater the radial E-field driving the discharge towards the grounded enclosure. Hence, while designing and constructing high voltage gas insulated disconnector switch, it is essential to consider the highest radial E-field level during Its operation (i.e. in the presence of discharge between the contacts). Figure 5 shows the secondary breakdown-free disconnector switch during its fully open condition and in operation. The invention as herein described above should not be read in a restrictive manner as various modifications, alterations and changes are possible within the scope and limit of the invention as encampused within the appended claims. WE CLAIM: 1. An improved gas insulated substation (GIS) disconnector switch (DS) comprising of: a fixed contact shield (04) covering a fixed contact (FC) (01), a moving contact shield (06) surrounding a moving contact (MC) (05), wherein both the FC shield and MC shield are insulated from a grounded metallic enclosure (09) by means of SF6 gas with appropriate density which fills the enclosure (09), and MC shield is separated from FC shield be a distance D which is proportional to the system voltage and SF6 gas density, characterized in that the E-field around the contact system is made uniform by profiling the fixed contact and moving contact shields and the axial E-field between the MC and the FC shield is made uniform by keeping radius of curvature of moving contact main body (R1) greater than the radius of curvature (R2) at the throat of the moving contact while keeping the outer radius of curvature of the FC shield (R3) higher than the radius of curvature (R4) of the FC Shield around the central axis . 2. A GIS disconnector switch as claimed in claim 1 wherein the axial E-field between the MC and FC shield is made uniform by using a well profiled FC shield (4) and the moving contact tip. 3. A GIS disconnector switch as claimed in claim 1 wherein the MC is designed and profiled in such a way that the field is always maintained highest at the central axis rather than on the surface of the tip. 4. A GIS disconnector switch as claimed in claim 1 wherein for keeping the highest axial E-field along the central axis radlus of curvature of the moving contact main body (R1) is maintained much higher than radius of curvature of the throat of MC (R2). 5. A GIS disconnector switch as claimed in the proceeding claims wherein the highest discharge length is a function of diameter (D1) of the MC with respect to FC shield and by using the highest possible value of R1 for the moving contact (depending on diameters of the moving contact and the throat of the moving contact (D2) for gas flow), optimum discharge length is achieved. 6. A GIS disconnector switch as claimed in the preceeding claims wherein the diameter of the FC shield (D3) and its profile are selected in such a way that the radial E-field around the discharge/moving contact tip cannot force the discharge from the central axis towards the grounded enclosure of the DS. 7. A GIS disconnector switch as claimed in claim 6 wherein the diameter of the shield is kept 3 to 4 times the highest possible discharge length depending on the system voltage. 8. A GIS disconnector switch as claimed in the preceeding claims wherein R3 is maintained much higher than R4 and R3 is comparable to the diameter of the moving contact. 9. A GIS disconnector switch as claimed in the preceeding claims wherein for a 145 kV GIS the dimensions of the disconnector switch are as follows R1 = 12.0 mm R2 = 2.0 mm R3 = 35.0 mm R4 = 2.0 mm D1 = 45.0 mm D2 = 8.0 mm D3 = 125.0 mm D = 60.0 mm 10. A GIS disconnector switch as herein described and illustrated.

Documents:

00883-kol-2006-abstract.pdf

00883-kol-2006-claims.pdf

00883-kol-2006-correspondence others.pdf

00883-kol-2006-description(complete).pdf

00883-kol-2006-drawings.pdf

00883-kol-2006-form-1.pdf

00883-kol-2006-form-2.pdf

00883-kol-2006-form-3.pdf

00883-kol-2006-g.p.a.pdf

883-KOL-2006-ABSTRACT.1.1.pdf

883-KOL-2006-CANCELLED PAGES.pdf

883-KOL-2006-CLAIMS.1.1.pdf

883-KOL-2006-CORRESPONDENCE-1.1.pdf

883-KOL-2006-CORRESPONDENCE.pdf

883-KOL-2006-DESCRIPTION (COMPLETE).1.1.pdf

883-KOL-2006-DRAWINGS.1.1.pdf

883-kol-2006-examination report.pdf

883-KOL-2006-FORM 1-1.2.pdf

883-KOL-2006-FORM 1.1.1.pdf

883-kol-2006-form 18.pdf

883-KOL-2006-FORM 2.1.1.pdf

883-KOL-2006-FORM 3.1.1.pdf

883-kol-2006-form 3.pdf

883-KOL-2006-FORM 5.pdf

883-KOL-2006-FORM-27.pdf

883-kol-2006-gpa.pdf

883-kol-2006-granted-abstract.pdf

883-kol-2006-granted-claims.pdf

883-kol-2006-granted-description (complete).pdf

883-kol-2006-granted-drawings.pdf

883-kol-2006-granted-form 1-1.1.pdf

883-kol-2006-granted-form 1.pdf

883-kol-2006-granted-form 2.pdf

883-kol-2006-granted-specification.pdf

883-kol-2006-reply to examination report-1.1.pdf

883-KOL-2006-REPLY TO EXAMINATION REPORT.pdf

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