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HOW TO MAINTAIN SF6 CIRCUIT BREAKER

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HOW TO MAINTAIN SF6 CIRCUIT BREAKER Powered By Docstoc
					    International Journal of Scientific Research Engineering &Technology (IJSRET)
    Volume 1 Issue1 pp 030-035 March 2012             www. ijsret.org ISSN 2278 - 0882


              HOW TO MAINTAIN SF6 CIRCUIT BREAKER
                                 1
                                  Snigdha Sharma, 2Hemant bharadwaj,
                              1
                               Faculty of EE Department, SIT, Meerut, India
                              2
                                Faculty of EE Department, SIT, Meerut, India



Abstract                                                     SF6 is a gaseous dielectric used in high voltage electrical
India’s growing economy needs an efficient power             equipment as an insulator or arc quenching medium.
transmission system to meet the increasing demand for        The separation of the CB contacts which are carrying
reliable and affordable power. Circuit breakers play an      current give rise to an arc. During arcing the circuit
important role in protection system of electrical power      breaker maintains a relatively low pressure inside the
transmission networks. A circuit breaker is an               chamber and there will be no danger of explosion and
automatically operated electrical switch which detects a     spilling of gas around. Any leakage from the chamber
fault condition and interrupt immediately to                 will not create a problem since SF6 can undergo
discontinue electrical flow. So its maintenance deserves     considerable decomposition in which toxic products may
special considerations in order to prevent the               stay inside the chamber in the form of white dust.
equipments and transmission system from damage.              Normally at least once a year or after every 500
This paper deals with high voltage SF6 circuit breaker.      operations, the circuit breaker must be maintained.
This paper shows construction and operation of SF6           Maintenance of circuit breakers at appropriate time is
circuit breaker, properties of SF6 circuit breaker,          important as these are used to protect power network
importance of its maintenance, rating and testing and        from short circuits and other problems environmental
gas handling. The aim of this paper is to make a review      problems. However, it has its limitations while
of maintenance of SF6 circuit breaker.                       maintaining. This paper will contribute in studying that
                                                             why is it necessary to maintain circuit breaker.
Keywords: Arc extinguishes,            circuit   breaker,
compressed SF6 gas, Electric arc.                            II. Construction and Operation of SF6
                                                              circuit breaker
         I.     Introduction
                                                             It consists of two parts mainly:
A circuit breaker is a switching device which can open or
close a circuit allowing interrupting the circulation of     II.1 The interrupter unit:
current. Previously oil and air circuit breakers were used
commonly but SF6 circuit breaker is most widely used         It consists of fixed contacts which comprise a set of
CB now a days for high voltage applications worldwide.       current carrying fingers and an arcing probe. When the
Some of the outstanding properties of SF6 making it          breaker is in close position, the fingers make contact
desirable to use in power applications such as high          round the circumference of the moving contact which
dielectric strength, unique arc-quenching ability,           has the arcing probe enclosed within its hollow end. The
excellent thermal stability and good thermal                 contacts are surrounded by interrupting nozzles and a
conductivity. The only industrial process currently in use   blast shield which controls are displacement and the
is the synthesis of sulfur hexafluoride by allowing          movement of hot gases. The moving contact is in the
fluorine obtained by electrolysis to react with sulfur       form of hollow nozzle sliding in the second set of spring
according to the exothermic reaction:                        loaded fingers. Side vents in the moving contact allow
          S + 3F2 →SF6 + 262 kcal                            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

                                                     IJSRET @ 2012
    International Journal of Scientific Research Engineering &Technology (IJSRET)
    Volume 1 Issue1 pp 030-035 March 2012             www. ijsret.org ISSN 2278 - 0882

arc is extended and attenuated. It is finally extinguished
by the gas flow from the high pressure to the low            IV.2 Physical properties:
pressure systems.                                            SF6 is one of the heaviest known gases with a density
                                                             about five times the density of air under similar
II.2 The gas system:                                         conditions. SF6 shows little change in vapor pressure
                                                             over a wide temperature range and is a soft gas in that it
Since the gas pressure is very high, lot of care is to be    is more compressible dynamically than air. The heat
taken to prevent gas leakage at joints by providing          transfer coefficient of SF6 is greater than air and its
perfect scaling. The low and high pressure systems are       cooling characteristics by convection are about 1.6 times
fitted with low pressure alarms and set of lock out          air. Its density at 20 degree centigrade is 6.139 kg/m3.
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         IV.3 Dielectric strength:
chamber. A thermostat is set to the switch on when the       SF6 has a dielectric strength about three times that of air
ambient temperature falls below 16◦C.                        at one atmosphere pressure for a given electrode
                                                             spacing. The dielectric strength increases with increasing
III. Importance of adequate maintenance                      pressure; and at three atmospheres, the dielectric
                                                             strength is roughly equivalent to transformer oil. The
The maintenance of circuit breaker is necessary because heaters for SF6 in circuit breakers are required to keep
of their importance for routine switching and for the gas from liquefying because, as the gas liquefies, the
protection of other equipments. Electric transmission pressure drops, lowering the dielectric strength.
system breaks up and equipment destruction can occur if
a circuit breaker fails to operate due to lack of preventive IV.4 Thermal conductivity:
maintenance. Breakers that remain idle for 6 months or The thermal conductivity of SF6 is below that of air
more should be made to open and close several times in but its overall heat transfer capability, in particular
succession to verify proper operation and remove any when convection is taken into account is excellent
accumulation of dust or foreign material on moving parts being similar to that of gases such as hydrogen and
and contacts.                                                helium and higher than that of air. At high
                                                             temperatures, the thermal conductivity curve of SF6
IV. Properties of SF6 gas                                    reveals one of the exceptional qualities of the gas,
                                                             which allows it to be used for extinguishing arcs by
IV.1 Toxicity:                                               thermal transport. The peak of the thermal
SF6 is odorless, colorless, tasteless, and nontoxic in its conductivity corresponds to the dissociation
pure state. It can, however, exclude oxygen and cause temperature of the SF6 molecule at 2100 to 2500 K.
suffocation. If the normal oxygen content of air is re- The dissociation process absorbs a considerable
duced from 21 percent to less than 13 percent, suffocation amount of heat which is released when the
can occur without warning.                                   molecules reform at the periphery of the arc,
                                                             facilitating a rapid exchange of heat between the hot
                                                             and cooler regions.

                                                      IJSRET @ 2012
    International Journal of Scientific Research Engineering &Technology (IJSRET)
    Volume 1 Issue1 pp 030-035 March 2012             www. ijsret.org ISSN 2278 - 0882

                                                             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.

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


                                                     IJSRET @ 2012
    International Journal of Scientific Research Engineering &Technology (IJSRET)
    Volume 1 Issue1 pp 030-035 March 2012             www. ijsret.org ISSN 2278 - 0882

                                                               continuously for a period long enough to achieve steady
VI. Testing of circuit breakers                                state conditions. Temperature readings are obtained by
VI.1 Short circuit tests: These tests are carried out in       means of thermocouples whose hot junctions are placed
    short circuiting testing stations to prove the             in appropriate positions. The temperature rise above
    ratings of circuit breakers. There are two types           ambient must not exceed 40◦C when the rated normal
    of testing stations                                        current is less than 800 A and 50◦C if it is above 800 A.
                                                               An additional requirement is the measurement of the
     (1) Field type                                            contact resistances between isolating contacts between
     In case of field type stations the power                  moving and fixed contacts. These points are generally
     required for testing is directly taken from a             main points of excessive heat generation.
     large power system. The breaker to be tested is
     connected to the system. Whereas this method              VI.4 Mechanical tests: A circuit breaker must open or
     of testing is economical for high voltage CBs, it         close at the correct speed and perform such operations
     suffers from the following drawbacks:                     without mechanical failure. B.S.116:1952 requires 500
                 The test cannot be repeatedly                such operations without failure and with no adjustment
         carried out for research and development              of the mechanism.
         as it disturbs the whole network.
                 Test conditions like the desired             VII. Test plant
         recovery voltage cannot be achieved
         conveniently.                                         The essential components of a test plant are:
                 The power available depends upon
         the location of testing stations.                     VII.1 Generator: The power is supplied by short
                                                               circuit generator driven by induction motors. The
     (2) Laboratory type                                       magnitude of the voltage is varied by adjusting
     In this case the power required for testing is            excitation of generator or the transformer ratio. The
     provided by specially designed generators.                capacity of these generators may be of the order of
     This method has following advantage:                      2000MVAand very rigid bracing of conductors and
      Several indirect methods can be used.                   coil ends is necessary in the view of high
      Test conditions can be controlled accurately.           electromagnetic forces possible.
      Test can be repeated.
                                                               VII.2 Resistors and reactors: These are used to
VI.2 Dielectric tests: In this, the test voltage is applied    control the power factor of current and the rate of
    for a period of one minute between                         decay of dc component of current. There are
      (1) Phases with a breaker closed.                        number of coils per phase and by combination of
 (2)      Phases and earth with circuit breaker open.          series and parallel connection, desired value of
 (3)      Across the terminal with breaker open.               resistance and reactance can be obtained.

With this the breaker must not flashover or punctures.         VII.3 Capacitors: These are used for braking line
These tests are normally made on indoor switchgear.            charging currents and for controlling the rate of re
Since the outdoor switchgear is electrically exposed, they     striking voltage.
will be subjected to over voltages caused by lightening.
Therefore this class of switchgear is subjected to wet         VII.4 Transformer: The leakage reactance of the
dielectric test. In this the external insulation is sprayed    transformer is low so as to withstand repeated short
for two minutes while the rated service voltage is             circuits. Since they are in use intermittently, they do
applied, the test overvoltage is then maintained for 30        not pose any cooling problem. For voltage higher
seconds during which no flashover should occur.                than generated voltages, usually banks of single
                                                               phase transformers are employed. In short circuit
VI.3 Thermal test: These tests are made to check the           station at Bhopal there is three single phase units
thermal behavior of breakers. In this test the rated           each of 11KV/76KV. The normal rating is 30 MVA
current through all three phases of switchgear is passed       but their short circuit capacity is 475 MVA.


                                                           IJSRET @ 2012
    International Journal of Scientific Research Engineering &Technology (IJSRET)
    Volume 1 Issue1 pp 030-035 March 2012             www. ijsret.org ISSN 2278 - 0882


VII.5 Master circuit breaker: These breakers are                IX.2 SF6 gas handling procedures:
provided as back up to interrupt the short circuit              Proper gas handling procedure must be followed to
current if the test breaker should fail.                        eliminate gas release to the atmosphere

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

                                                            IJSRET @ 2012
    International Journal of Scientific Research Engineering &Technology (IJSRET)
    Volume 1 Issue1 pp 030-035 March 2012             www. ijsret.org ISSN 2278 - 0882

half an hour, allowing the SF6 gas within the tank to             contacts is almost infinite and hence the heat
reliquefy. The procedure is then repeated until the               generated is zero. Between these two limits the heat
pressure on top of the tank is proper for the tank’s              generation reaches a maximum. Now, if the heat so
temperature. This procedure is effective however,                 generated could be removed by cooling, splitting the
during the process, some SF6 gas is vented to the                 arc at a rate faster then the generation the arc is
atmosphere. One alternative is to purchase a                      extinguished
SF6/nitrogen separation and filtration plant, but
the cost of this alternative is prohibitive.                      •      Voltage race theory: In this stage ionization is
                                                                  at minimum and if the ions could be removed either
X. Arc- Interruption theories for sf6                             by recombining them into neutral molecule or by
  circuit breaker                                                 sweeping them away by inserting insulation at a rate
                                                                  faster then rate of ionization, the arc will be
When a short circuit occurs on the system the relay               interrupted. The ionization at current zero depends
signal’s to C.B. to trip and isolate the healthy section          upon the voltage appearing between the contacts this
of the system from the faulted without causing any                voltage is known as restriking voltage.
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.                       XI. Conclusion
                                                               Harnessing the utilities of a SF6 gas in power system and
X.2 Low resistance or current zero interruption: This          power system equipments makes it more reliable and
method is used only in ac circuit interruption because         safe for use. The countless advantages of this SF6 gas will
there is natural zero of current present in such systems.      take us to new heights in the power system.
In case of 50 Hz supply there are 100 zeros per second.
This property of ac circuit is exploited for interruption      REFERENCES
purpose and the current is not allowed to rise again after
a zero occurs. Also it is neither necessary nor desirable to   [1] ANSI/IEEE C37.010-1979 - IEEE Application
cut off the current at any other point on the ac wave          Guide for AC High-Voltage Circuit Breakers Rated on
because this will induce high voltage in the system. This      a Symmetrical Current Basis
phenomenon of arc extinction is explained by two               [2] SF6 Switchgear, H. M. Ryan and G. R. Jones
theories:                                                      C.L. Wadhwa, Electrical power systems. John wiley &
    •       Energy band theory: This theory is based on        Peregrinus Press, 1989
                                                               [3] IEEE C57.12.00-1993 - IEEE Standard General
    the fact that if the rate at which the heat generated
                                                               Requirements for Liquid-Immersed Distribution,
    between the contacts is dissipated the arc will be
                                                               Power and Regulating Transformers
    extinguished, otherwise it will restrike. The heat
                                                               [4] U.S. Weather Service, Duluth, MN
    generated varies from time to time depending upon
                                                               [5] Watlow Electric Manufacturing, St. Louis, MO
    the separation of contacts. Initially when the contacts
                                                               [6] N I Supply, Hermantown, MN sons, Inc.1991
    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


                                                           IJSRET @ 2012

				
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