HOW TO MAINTAIN SF6 CIRCUIT BREAKER

Transcription

1 HOW TO MAINTAIN SF6 CIRCUIT BREAKER 1 Snigdha Sharma, 2 Hemant bharadwaj, 1 Faculty of EE Department, SIT, Meerut, India 2 Faculty of EE Department, SIT, Meerut, India Abstract India s growing economy needs an efficient power transmission system to meet the increasing demand for reliable and affordable power. Circuit breakers play an important role in protection system of electrical power transmission networks. A circuit breaker is an automatically operated electrical switch which detects a fault condition and interrupt immediately to discontinue electrical flow. So its maintenance deserves special considerations in order to prevent the equipments and transmission system from damage. This paper deals with high voltage SF6 circuit breaker. This paper shows construction and operation of SF6 circuit breaker, properties of SF6 circuit breaker, importance of its maintenance, rating and testing and gas handling. The aim of this paper is to make a review of maintenance of SF6 circuit breaker. Keywords: Arc extinguishes, circuit breaker, compressed SF6 gas, Electric arc. I. Introduction A circuit breaker is a switching device which can open or close a circuit allowing interrupting the circulation of current. Previously oil and air circuit breakers were used commonly but SF6 circuit breaker is most widely used CB now a days for high voltage applications worldwide. Some of the outstanding properties of SF6 making it desirable to use in power applications such as high dielectric strength, unique arc-quenching ability, excellent thermal stability and good thermal conductivity. The only industrial process currently in use is the synthesis of sulfur hexafluoride by allowing fluorine obtained by electrolysis to react with sulfur according to the exothermic reaction: S + 3F2 SF kcal SF6 is a gaseous dielectric used in high voltage electrical equipment as an insulator or arc quenching medium. The separation of the CB contacts which are carrying current give rise to an arc. During arcing the circuit breaker maintains a relatively low pressure inside the chamber and there will be no danger of explosion and spilling of gas around. Any leakage from the chamber will not create a problem since SF6 can undergo considerable decomposition in which toxic products may stay inside the chamber in the form of white dust. Normally at least once a year or after every 500 operations, the circuit breaker must be maintained. Maintenance of circuit breakers at appropriate time is important as these are used to protect power network from short circuits and other problems environmental problems. However, it has its limitations while maintaining. This paper will contribute in studying that why is it necessary to maintain circuit breaker. II. Construction and Operation of SF6 circuit breaker It consists of two parts mainly: II.1 The interrupter unit: It consists of fixed contacts which comprise a set of current carrying fingers and an arcing probe. When the breaker is in close position, the fingers make contact round the circumference of the moving contact which has the arcing probe enclosed within its hollow end. The contacts are surrounded by interrupting nozzles and a blast shield which controls are displacement and the movement of hot gases. The moving contact is in the form of hollow nozzle sliding in the second set of spring loaded fingers. Side vents in the moving contact allow the high pressure gas into the main tank. As soon as the moving contact is withdrawn from the fixed finger contact an arc is drawn between the moving nozzle and the arcing probe. As the contacts move further apart, the

2 arc is extended and attenuated. It is finally extinguished by the gas flow from the high pressure to the low pressure systems. II.2 The gas system: Since the gas pressure is very high, lot of care is to be taken to prevent gas leakage at joints by providing perfect scaling. The low and high pressure systems are fitted with low pressure alarms and set of lock out switches which give a warning the moment the gas pressure drops below a certain value, because otherwise there will be reduction in the dielectric strength and arc quenching ability of the breaker is endangered. If the danger limit is reached the safety devices immobilize the breaker. The over riding safety devices see to it that a fault in the control circuit does not permit the compressor to build up excessive pressure in the high pressure reservoir or continue to pump gas into the atmosphere in the event of major leak. The gas is stored in the high pressure chamber at 16 atmospheres whereas the gas pressure on low pressure side is 3 atmospheres. The temperature is 20 C. in order to prevent liquefaction of a gas in the high pressure chamber at low temperature; a heater is fitted in the high pressure chamber. A thermostat is set to the switch on when the ambient temperature falls below 16 C. III. Importance of adequate maintenance The maintenance of circuit breaker is necessary because of their importance for routine switching and for protection of other equipments. Electric transmission system breaks up and equipment destruction can occur if a circuit breaker fails to operate due to lack of preventive maintenance. Breakers that remain idle for 6 months or more should be made to open and close several times in succession to verify proper operation and remove any accumulation of dust or foreign material on moving parts and contacts. IV. Properties of SF6 gas IV.1 Toxicity: SF6 is odorless, colorless, tasteless, and nontoxic in its pure state. It can, however, exclude oxygen and cause suffocation. If the normal oxygen content of air is reduced from 21 percent to less than 13 percent, suffocation can occur without warning. IV.2 Physical properties: SF6 is one of the heaviest known gases with a density about five times the density of air under similar conditions. SF6 shows little change in vapor pressure over a wide temperature range and is a soft gas in that it is more compressible dynamically than air. The heat transfer coefficient of SF6 is greater than air and its cooling characteristics by convection are about 1.6 times air. Its density at 20 degree centigrade is kg/m3. IV.3 Dielectric strength: SF6 has a dielectric strength about three times that of air at one atmosphere pressure for a given electrode spacing. The dielectric strength increases with increasing pressure; and at three atmospheres, the dielectric strength is roughly equivalent to transformer oil. The heaters for SF6 in circuit breakers are required to keep the gas from liquefying because, as the gas liquefies, the pressure drops, lowering the dielectric strength. IV.4 Thermal conductivity: The thermal conductivity of SF6 is below that of air but its overall heat transfer capability, in particular when convection is taken into account is excellent being similar to that of gases such as hydrogen and helium and higher than that of air. At high temperatures, the thermal conductivity curve of SF6 reveals one of the exceptional qualities of the gas, which allows it to be used for extinguishing arcs by thermal transport. The peak of the thermal conductivity corresponds to the dissociation temperature of the SF6 molecule at 2100 to 2500 K. The dissociation process absorbs a considerable amount of heat which is released when the molecules reform at the periphery of the arc, facilitating a rapid exchange of heat between the hot and cooler regions.

3 faulted SF6 gas from the circuit breaker and flush with fresh air before working on the circuit breaker. V. Rating of circuit breakers A circuit breaker has to work under different circumstances. It is rated in terms of V.1 Number of poles: the number of poles per phase of a breaker is a function of operating voltage. IV.5 Arc quenching: SF6 is approximately 100 times more effective than air in quenching spurious arcing. SF6 also has a high thermal heat capacity that can absorb the energy of the arc without much of a temperature rise. IV.6 Electrical properties: The excellent dielectric properties of SF6 are due to electronegative character of its molecules. It has a pronounced tendency to capture free electrons forming heavy ions with low mobility making the development of electron avalanches very difficult. Because of the arcquenching ability of SF6, corona and arcing in SF6 does not occur until way past the voltage level of onset of corona and arcing in air. SF6 will slowly decompose when exposed to continuous corona. IV.7 Chemical properties: SF6 fully satisfied the valence requirements of the sulfur molecule. Its molecular structure is octahedral with a fluorine molecule at each apex. The effective collision diameter of the SF6 molecule is 4.77 Å. The six bonds are covalent which accounts for the exceptional stability of this compound. SF6 can be heated without decomposition to 500 C in the absence of catalytic metals. SF6 is non-flammable and hydrogen, chlorine and oxygen have no action on it. SF6 is insoluble in water and is not attacked by acids.. IV.8 Toxicity of arc products: Toxic decomposition products are formed when SF6 gas is subjected to an electric arc. The decomposition products are metal fluorides and form a white or tan powder. Toxic gases are also formed which have the characteristic odor of rotten eggs. Do not breathe the vapors remaining in a circuit breaker where arcing or corona discharges have occurred in the gas. Evacuate the V.2 rated voltage: The voltage levels at various points in a system vary depending upon the system conditions and as a result the breaker has to operate under such variable voltage conditions. The breaker is expected to operate at a maximum voltage which normally is higher than rated nominal voltage. V.3 rated current: It is the maximum value of current in RMS amperes which it shall carry continuously without exceeding the temperature limits of various parts of breaker. V.4 rated frequency: It is the frequency for which it is designed to operate. V.5 Rated making capacities: The making current is the peak value of maximum current loop, including dc component, in any phase during the first cycle of current when the circuit breaker is closed. The capacity of breaker to make currents depends upon its ability to withstand and to close successfully against the effect of electromagnetic forces. The making capacity is specified by the product of making current it can make and carry instantaneously at the rated service voltage. V.6 Rated braking current: The braking current depends upon the instant on the current wave when the contacts begin to open. The braking capacity of a breaker is the product of breaking current and recovery voltage. V.7 operating duty: In case the circuit breaker is not used for auto reclosing, it must be capable of following short circuit operating duty. B-3-MB-3- MB where B is braking operation, 3 is the duration in minutes, B-Dt-MB M is making operation; Dt is dead time in terms of cycle.

4 VI. Testing of circuit breakers VI.1 Short circuit tests: These tests are carried out in short circuiting testing stations to prove the ratings of circuit breakers. There are two types of testing stations (1) Field type In case of field type stations the power required for testing is directly taken from a large power system. The breaker to be tested is connected to the system. Whereas this method of testing is economical for high voltage CBs, it suffers from the following drawbacks: The test cannot be repeatedly carried out for research and development as it disturbs the whole network. Test conditions like the desired recovery voltage cannot be achieved conveniently. The power available depends upon the location of testing stations. (2) Laboratory type In this case the power required for testing is provided by specially designed generators. This method has following advantage: Several indirect methods can be used. Test conditions can be controlled accurately. Test can be repeated. VI.2 Dielectric tests: In this, the test voltage is applied for a period of one minute between (1) Phases with a breaker closed. (2) Phases and earth with circuit breaker open. (3) Across the terminal with breaker open. With this the breaker must not flashover or punctures. These tests are normally made on indoor switchgear. Since the outdoor switchgear is electrically exposed, they will be subjected to over voltages caused by lightening. Therefore this class of switchgear is subjected to wet dielectric test. In this the external insulation is sprayed for two minutes while the rated service voltage is applied, the test overvoltage is then maintained for 30 seconds during which no flashover should occur. VI.3 Thermal test: These tests are made to check the thermal behavior of breakers. In this test the rated current through all three phases of switchgear is passed continuously for a period long enough to achieve steady state conditions. Temperature readings are obtained by means of thermocouples whose hot junctions are placed in appropriate positions. The temperature rise above ambient must not exceed 40 C when the rated normal current is less than 800 A and 50 C if it is above 800 A. An additional requirement is the measurement of the contact resistances between isolating contacts between moving and fixed contacts. These points are generally main points of excessive heat generation. VI.4 Mechanical tests: A circuit breaker must open or close at the correct speed and perform such operations without mechanical failure. B.S.116:1952 requires 500 such operations without failure and with no adjustment of the mechanism. VII. Test plant The essential components of a test plant are: VII.1 Generator: The power is supplied by short circuit generator driven by induction motors. The magnitude of the voltage is varied by adjusting excitation of generator or the transformer ratio. The capacity of these generators may be of the order of 2000MVAand very rigid bracing of conductors and coil ends is necessary in the view of high electromagnetic forces possible. VII.2 Resistors and reactors: These are used to control the power factor of current and the rate of decay of dc component of current. There are number of coils per phase and by combination of series and parallel connection, desired value of resistance and reactance can be obtained. VII.3 Capacitors: These are used for braking line charging currents and for controlling the rate of re striking voltage. VII.4 Transformer: The leakage reactance of the transformer is low so as to withstand repeated short circuits. Since they are in use intermittently, they do not pose any cooling problem. For voltage higher than generated voltages, usually banks of single phase transformers are employed. In short circuit station at Bhopal there is three single phase units each of 11KV/76KV. The normal rating is 30 MVA but their short circuit capacity is 475 MVA.

5 VII.5 Master circuit breaker: These breakers are provided as back up to interrupt the short circuit current if the test breaker should fail. VII.6 Make switch: The make switch is closed after other switches are closed. The closing speed is high so that the contacts are fully closed before the short circuit current reaches its peak value. VIII. Test procedure Before the test is performed all the components are adjusted to suitable values so as to obtain desired values of voltage, current, rate of rising of re striking voltage, power factor etc. The measuring circuits are connected and oscillograph loops are calibrated. During the test several operations are performed in a sequence in a short time of 0.2 sec. This is done with the help of a drum switch with several pair of contacts which is rotated with a motor. This drum when rotated closes and opens several control circuits according to certain sequence. IX. SF6 gas handling to minimize losses It is used to prevent faulted gas release into the atmosphere IX.1 Atmospheric safeguards: Do not release the gas into atmosphere except the minor amount while measuring the gas for moisture content. SF6 gas must be removed from equipment and stored in SF6 gas carts prior to internal maintenance or equipment salvaging. The removed gas is to be reconditioned and reused whenever possible. Faulted SF6 gas in power equipment must not be dumped to the atmosphere. Faulted SF6 gas may be toxic and must be passed through an external scrubber filter unit to remove impurities before pumping the gas into the cart for storage. Power equipment such as SF6 filled instrument transformers which are removed from service but are scheduled either for return to the warehouse or to be re energized may not require all the SF6 gas to be removed prior to movement or transportation. Whether the SF6 gas is removed or gas pressure reduced, depends upon the equipment manufacturer s recommendations. IX.2 SF6 gas handling procedures: Proper gas handling procedure must be followed to eliminate gas release to the atmosphere Pull a vacuum on interconnecting hoses prior to passing SF6 gas through the hoses. This eliminates contamination of the SF6 gas with air. Never purge SF6 gas through interconnecting hoses as a way of cleaning the hoses. This vents SF6 gas to the atmosphere, and also may contaminate the SF6 gas with air remaining in the hoses. When removing SF6 gas from power equipment, pull a long hard vacuum to remove the maximum amount of SF6 gas; any gas left in the equipment, will end up in the atmosphere. When filling equipment with SF6 gas, first pull a hard vacuum to remove as much air and moisture as possible. Any air left in the equipment will contaminate the SF6 gas on filling. IX.3 SF6 gas filing carts: A special SF6 gas filling cart was designed and fabricated. This cart pulls a vacuum on all hoses prior to filling the power equipment directly from the SF6 gas cylinders. The filling cart is built on a hand truck which can carry one to two SF6 gas cylinders, and has a small vacuum pump which is permanently mounted to it. The filling cart also has a SF6 gas pressure vacuum/gauge, and control valves. IX.4 Power equipment salvaging, removal & reclamation of SF6 gas: There are presently two of these carts in the system. These carts are equipped with external scrubber filter units to reclaim the SF6 gas before it enters the storage tank on the cart. IX.5 Air removal from SF6 gas: When the contaminated SF6 gas is removed from equipment, the SF6 gas is liquefied in the gas cart s storage tank, but the air is not liquefied, and eventually impedes further removal of the SF6 gas from the equipment. Liquid SF6 gas is in the bottom of the tank. The atmosphere on top of the tank is a mixture of the air contaminants and SF6 gas. The procedure is to slowly open a sampling valve on top of the tank for 1 to 3 minutes venting the gas mixture to atmosphere. The valve is then closed for

6 half an hour, allowing the SF6 gas within the tank to reliquefy. The procedure is then repeated until the pressure on top of the tank is proper for the tank s temperature. This procedure is effective however, during the process, some SF6 gas is vented to the atmosphere. One alternative is to purchase a SF6/nitrogen separation and filtration plant, but the cost of this alternative is prohibitive. X. Arc- Interruption theories for sf6 circuit breaker When a short circuit occurs on the system the relay signal s to C.B. to trip and isolate the healthy section of the system from the faulted without causing any harm to the system. There are two methods by which an Arc interruption is done: X.1 High resistance method: In this method arc resistance is increased in to such a high value that it force the current to reaches to zero without possibility of arc being restruck.the rate at which resistance is increased or the current is decrease is not abnormal so as to cause harmful induced voltage in the system. The arc resistance may be increased due to any or all of the de ionizing methods i.e. cooling, lengthen and splitting of the arc. Because of the resistive nature of the arc discharge, most of the energy in the system is received by C.B. X.2 Low resistance or current zero interruption: This method is used only in ac circuit interruption because there is natural zero of current present in such systems. In case of 50 Hz supply there are 100 zeros per second. This property of ac circuit is exploited for interruption purpose and the current is not allowed to rise again after a zero occurs. Also it is neither necessary nor desirable to cut off the current at any other point on the ac wave because this will induce high voltage in the system. This phenomenon of arc extinction is explained by two theories: Energy band theory: This theory is based on the fact that if the rate at which the heat generated between the contacts is dissipated the arc will be extinguished, otherwise it will restrike. The heat generated varies from time to time depending upon the separation of contacts. Initially when the contacts are about to open, the restriking voltage is zero and, therefore, the heat generated is zero. Again when the contacts are fully open, the resistance between the contacts is almost infinite and hence the heat generated is zero. Between these two limits the heat generation reaches a maximum. Now, if the heat so generated could be removed by cooling, splitting the arc at a rate faster then the generation the arc is extinguished Voltage race theory: In this stage ionization is at minimum and if the ions could be removed either by recombining them into neutral molecule or by sweeping them away by inserting insulation at a rate faster then rate of ionization, the arc will be interrupted. The ionization at current zero depends upon the voltage appearing between the contacts this voltage is known as restriking voltage. XI. Conclusion Harnessing the utilities of a SF6 gas in power system and power system equipments makes it more reliable and safe for use. The countless advantages of this SF6 gas will take us to new heights in the power system. REFERENCES [1] ANSI/IEEE C IEEE Application Guide for AC High-Voltage Circuit Breakers Rated on a Symmetrical Current Basis [2] SF6 Switchgear, H. M. Ryan and G. R. Jones C.L. Wadhwa, Electrical power systems. John wiley & Peregrinus Press, 1989 [3] IEEE C IEEE Standard General Requirements for Liquid-Immersed Distribution, Power and Regulating Transformers [4] U.S. Weather Service, Duluth, MN [5] Watlow Electric Manufacturing, St. Louis, MO [6] N I Supply, Hermantown, MN sons, Inc.1991