EXECUTIVE SUMMARY

1. INTRODUCTION

1.1 A significant advancement is observed in the SF6 technology in designingof switchgear and other substation equipment This technology has beensuccessfully applied to: (a) AC circuit breakers (b) DC circuit breakers(c) Gas insulated substations and(d) Gas insulated transmission systems.

1.2 Indian manufacturers are marketing SF6 circuit breakers of values 3.6 kVto420 kV. At present there are seven manufacturers in India, for SF6 circuitbreakers, in the voltage range of 3.6 kV to36 kV and, seven manufacturers inthe voltage range of 72.5 kV to 420 kV.

1.3 International manufacturers are marketing SF6 circuit breakers in thevoltage range of 3.6 kV to 800 kV.

1.4 All the manufacturers, making SF6 circuit breakers in India, have enteredinto collaboration agreements with foreign manufacturers. The leadinginternational manufacturers such as: Brown Boveri, AEG, Siemens,Alsthom, Merlin Gerin, Hitachi, and Mitsubishi are among thecollaborators.

2. STATE OF ART

2.1 Three types of interrupters are used by international manufacturers in SF6circuit breakers : (a) Double Pressure Type (b) Single Pressure Puffer Type(c) Single Pressure Rotating Arc Type. Double Pressure Type has nowbecome obsolete.

2.2 Three types of operating mechanisms of SF6 circuit breakers are in currentuse by international manufacturers: (a) Spring Operating Mechanism(b) Pneumatic Operating Mechanism (c) Hydraulic Operating Mechanism.Operating mechanism is also being marketed, which is a combination ofbasic type of machnisms.

2.3 Closing resistors to reduce over-voltages, during, line energisation andgrading capacitors basically for, equally, distributing the voltage across theinterrupters, are used in SF6 circuit breaker of 420 kV and above.

2.4 The gas chamber in a SF6 circuit breaker is hermetically sealed. The den-sity of SF6 gas is monitored during service.

1

3. LATEST DEVELOPMENTS IN INTERNATIONAL SCENE

3.1 Research and development by international manufacturers is headingtowards a single SF6 interrupter per phase for 420 kV breakers.

3.2 At present two to three cycle interrupters are available. Development of onecycle interrupter, by a foreign manufacturer, has been reported.

3.3 Development of double nozzle interrupters, which helps in reducing thedelay in current zeros associated with generator breakers, has been reportedby a foreign manufacturer. One of the Indian manufacturers has, also,adopted the double nozzle interrupter design and others have incorporatedthe basic principle.

3.4 A 500 kV high voltage direct current (HVDC) circuit breaker proto-type hasbeen delivered by Westinghouse Corporation to Celilo terminal of Bon-neville Power Administration. USA.

3.5 Gas insulated substations are, being, increasingly used in Japan. USA.Middle East and Europe, mainly to overcome space problem, where landcosts are prohibitive and where pollution is heavy.

3.6 Gas mixtures like N2-He, N2-SF6, CO2-SF6 etc. are, being, investigated, as asubstitute for SF6 gas, particularly, for gas insulated switchgear because ofthe large quantities of gas required in these equipment

4. FUTURE SCOPE FOR SF6 CIRCUIT BREAKERS

4.1 In voltage range from 3.6 kV to 36 kV, there is a race between vacuum andSF6 technologies. Which type of these two will capture a bigger market inthe future, cannot be ascertained, at present A comparison of minimum oil,SF6 and vacuum circuit breakers is given in Table 5.1 of Chapter-5.

4.2 In the voltage range, from 72.5 kV to 145 kV, minimum oil and SF6 typecircuit breakers are presently, sharing the market SF6 circuit breakers willbe used on a large scale, in future, in this voltage range, because of thesuperior characteristics of SF6 circuit breakers.

4.3 In the voltage range, 245 kV to 800 kV, SF6 circuit breakers will capturecomplete market in the near future, because of their competitive edge inrespect of cost

4.4 Interrupters, based on rotating arc principle, will be more widely used inSF6 circuit breakers rated3.6 kV to36 kV. whereas, puffer type principle hasbeen adopted by all Indian manufacturers forSF6 breakers rated 72.5 kVto420 kV.

5. TECHNOLOGY ABSORPTION/INOIGENISATION

5.1 Indian manufacturers are manufacturing the circuit breakers rated from3.6 kV to 420 kV which are also being, manufactured by their collaboratorsabroad and as such, there is no technology gap in this range.

5.2 Castings required for SF6 circuit breakers such as aluminium alloy, S.G.iron and copper chromium castings and components required for theoperating mechanism, such as : springs, pumps, compressors and pressureswitches, have been indigenised by circuit breaker manufacturers.

5.3 Efforts should be made, to evolve a national standard, for high voltageinsulators, operating rods and for SF6 gas density monitors used in SF6 cir-cuit breakers.

5.4 Unreliable quality and deliveries of indigenised components is a constraintin rapid indigenisation of components.

5.5 A technology gap exists for meeting future needs of India Circuit breakersof 800 kV, circuit breakers above 40 kA short circuit rating and circuitbreakers for HVDC & GIS, are not included, in present collaborationagreements. The collaboration agreements of Indian manufacturers needto be extended to include 800 kV circuit breakers and circuit breakers hav-ing interrupting current of 63 kA, where necessary.

5.6 Unreliable quality and deliveries by indigenous suppliers of substituteditems, are major constraints in respect of indigenisation.

5.7 Circuit breakers, with an interrupting time of 2 cycles (0.04 seconds), are,currently, available in India. Breakers with one cycle (0.02 seconds)interrupting'time are reported by foreign manufacturers. Use of one cyclebreaker has advantages on the system. System studies conducted by anorganisation for Trombay 500 MW, Unit-5, shows that the power system isstable with a two-cycle circuit breaker, even during a sudden outage of 500MW turbine-generator. Therefore, one cycle breaker may not be a necessityfor Indian system, as long as, the unit ratings are limited to 500 MWand below.

6. RECOMMENDATIONS

6.1 Indian manufacturers of SF6 circuit breakers shall evolve a common stan-dard for high voltage insulators, operating rods, and gas monitors, so as, toaccelerate the pace and quality of indigenisation. Since, different manufac-turers have adopted different designs/materials, based on their collaboratorsdesign for these items, a national consensus is necessary, in this regard.

6.2 Research and development efforts should be directed towards developing asingle interrupter per pole for 420 kV circuit breakers.

6.3 Indian manufacturers and research organisation should, vigorously, pursuefor the development of circuit breakers of voltage ratings of 800 kV &interrupting ratings of 63 kV, suitable for HVDC & GIS applications.

6.4 Reliability of mechanical systems should be given high importance, since ithas been observed that 70.3% of major failures in circuit breakers areattributable to mechanical causes. There is, also, a need for collection offailure data of indigenously manufactured SF6 circuit breakers, now,operating in India, which is not, readily, available at present.

6.5 Commissioning of the proposed short circuit test facility at CPRI,Bangalore, having synthetic mode of testing, shall be given highpriority.

6.6 There is a need for establishing facilities for manufacture of SF6 gas inthe country.

6.7 R&D efforts in India by manufacturers and national institutions are,presently, on a limited scale. There is an, urgent, need to establish a strongR&D base in the country for development of SF6 technology. A special cellunder the auspices of a national research organisation, such as CPRI,Bangalore, exclusively, for R&D efforts associated with SF6 technologymay be formed. This calls for a group of specialist engineers in the design ofcircuit breakers and prototype testing equipment for evaluating the new cir-cuit breaker designs. The following items may be taken up for study.

(a) Development of 63 kA interrupter.(b) Development of 800 kV SF6 circuit breakers.(c) Development of SF6 equipment for Gas Insulated Sub-stations.(d) Development of SF6 circuit breakers for HVDC.(e) Development of 420 kV interrupters.(f) Development of selfblast principle of interruption.(g) Development of know-how for large scale manufacture and testing of

SF6 gas.