VACUUM CIRCUIT BREAKE AND SF6 CIRCUIT BREAKER

1. Shroff S.R. Rotary Institute of Chemical Technology
Principal Supporter & Sponsor- UPL LTD./ Shroff family
Managed By Ankleshwar Rotary Education Society
Approved by AICTE, New Delhi, Govt. of Gujarat & GTU Affiliated
Department of Electrical Engineering
Active Learning Assignment (ALA)
NAME OF TOPIC :- VACUUM CIRCUIT-BREAKER & SULPHUR HEXAFLUORIDE
CIRCIT-BREAKER
NAME OF COURSE :- SWITCHGEAR AND PROTECTION (2170908)
STUDENT NAME ENROLLMENT NO.
CHAUHAN SACHIN V. 170993109001
PRAJAPATI DHAVAL R. 160993109014
VANSIYA DIVYARAJSINH K. 160993109023
Guided By: Mr. Sourav Choubey

2. CONTENT
 VACUUM CIRCUIT-BREAKER
 SULPHUR HEXAFLUORIDE(SF6) CIRCUIT-BREAKER

3. INTRODUCTION
 A circuit breaker is a device that, interrupts an electric circuit to prevent
unwarranted current, caused by a short circuit, typically resulting from an
overload. Its basic functionality is to interrupt current flow after a fault is
detected. A vacuum circuit breaker is a kind of circuit breaker where the arc
quenching takes place in vacuum medium. The operation of switching on and
closing of current carrying contacts and interrelated arc interruption takes place in
a vacuum chamber in the breaker which is called vacuum interrupter.

4.  The Vacuum interrupter technology was first introduced in the year of 1960. But
still, it is a developing technology. As time goes on, the size of the vacuum
interrupter has reduced from its early 1960’s size due to different technical
developments in this field of engineering.

5. Construction of Vacuum Circuit Breaker
 The vacuum circuit breaker comprises a steel arc chamber in the center-
symmetrically arranged ceramic insulators. The pressure inside the
vacuum interrupter is maintained below 10^-4 torr.
 The material used for current carrying contacts plays an important role in the
performance of the vacuum circuit breaker. The alloys like, Copper-bismuth or
copper-chrome are the ideal material to make VCB contacts.

7.  From the figure shown above, the Vacuum circuit breaker consists of a fixed
contact, a moving contact and a vacuum interrupter. The moving contact is
connected to the control mechanism by stainless steel bellow. The arc shields
are supported o the insulating housing such that they cover on these shields and
is prevented from condensing on the insulating enclosure. The possibility of a
leak is eliminated due to permanent sealing of vacuum chamber for that a glass
vessel or ceramic vessel is used as the outer insulating body.

8. Working of Vacuum Circuit Breaker
 The sectional view of vacuum circuit breaker is shown in the figure below
when the contacts are separated due to some abnormal conditions, an arc is
struck between the contacts, the arc is produced due to ionization of metal ions
and depends very much on the material of contacts.
 The arc interruption in vacuum interrupters is different from other types
of circuit breakers. The separation of contacts causes the release of vapor
which is filled in the contact space. It consists positive ions liberated from
contact material. The vapor density depends on the current in the arc. When
the current decreases, the rate of vapor release decreases and after current zero,
the medium regain its dielectric strength if the vapor density is reduced.
 When current to be interrupted is very small in a vacuum, the arc has several
parallel paths. The total current is divided into many parallel arcs which repel
each other and spread over the contact surface. This is called diffused arc
which can be interrupted easily.

10.  At high values of current, the arc gets concentrated in a small region. It causes
rapid vaporization of the contact surface. The interruption of the arc is possible
if arc remains in diffused state. If it is quickly removed from the contact
surface, the arc will be re-strike.
 Arc extinction in vacuum breakers is greatly influenced by material and shape
of the contacts and the technique of considering metal vapor. The path of the
arc is kept moving so that temperature at any one point will not be high.
 After the final arc interruption, there is rapidly building up of dielectric strength
which is peculiar of the vacuum breaker. They are suitable for capacitor
switching as it will give a re-strick free performance. The small current is
interrupted before natural current zero, which may cause chopping whose level
depends on the material of contact.

11. Advantages of VCB
Vacuum offers the utmost insulating strength. So it has extreme superior arc
quenching properties than any other medium.
 The vacuum circuit breaker has a long life.
 Unlike Oil Circuit Breaker (OCB) or air blast Circuit Breaker (ABCB), the
explosion of VCB is avoided. This enhances the safety of the operating
personnel.
 No fire hazard
 The vacuum CB is fast in operation so ideal for fault clearing. VCB is suitable
for repeated operation.
 Vacuum circuit breakers are almost maintenance free.
 No exhaust of gas to the atmosphere and Noiseless operation.

12. Disadvantages of VCB
 The main disadvantage of VCB is that it is uneconomical at voltages exceeding
38 kVolts.
 The cost of the breaker becomes excessive at higher voltages. This is due to the
fact that at high voltages (above 38 kV) more than two numbers of the circuit
breaker are required to be connected in series.
 Moreover, the VCBs production is uneconomical if produced in small quantities.

13. Applications of Vacuum Circuit Breaker
The vacuum circuit breaker is today recognized as most reliable current interruption
technology for medium voltage switchgear. It requires minimum maintenance
compared to other circuit breaker technologies.
The technology is mainly suitable for mainly medium voltage application. For
higher voltage vacuum technology has been developed, but it is not
commercially feasible. Vacuum circuit breakers are used in metal clad
Switchgear and also in porcelain housed circuit breakers.

14. Sulphur Hexafluoride (SF6) Circuit
Breaker
A circuit breaker in which SF6 under pressure gas is used to extinguish the arc is
called SF6 circuit breaker. SF6 (sulphur hexafluoride) gas has excellent dielectric,
arc quenching, chemical and other physical properties which have proved its
superiority over other arc quenching mediums such as oil or air. The SF6 circuit
breaker is mainly divided into three types
 Non-puffer piston circuit breaker
 Single- puffer piston circuit breaker.
 Double-puffer piston circuit breaker.
The circuit breaker which used air and oil as an insulating medium, their arc
extinguishing force builds up was relatively slow after the movement of
contact separation.

15. In the case of high voltage circuit breakers quick arc extinction properties are used
which require less time for quick recovery, voltage builds up. SF6 circuit breakers
have good properties in this regards compared to oil or air circuit breakers. So in
high voltage up to 760 kV, SF6 circuit breakers is used.
Properties of Sulphur hexafluoride Circuit Breaker :-
Sulphur hexafluoride possesses very good insulating and arc quenching
properties. These properties are
 It is colourless, odourless, non-toxic, and non-inflammable gas.
 SF6 gas is extremely stable and inert, and its density is five times that of air.
 It has high thermal conductivity better than that of air and assists in better
cooling current carrying parts.
 SF6 gas is strongly electronegative, which means the free electrons are easily
removed from discharge by the formation of negative ions.

16.  It has a unique property of fast recombination after the source energizing spark
is removed. It is 100 times more effective as compared to arc quenching
medium.
 Its dielectric strength is 2.5 times than that of air and 30% less than that of the
dielectric oil. At high pressure the dielectric strength of the gas increases.
 Moisture is very harmful to SF6 circuit breaker. Due to a combination of
humidity and SF6 gas, hydrogen fluoride is formed (when the arc is interrupted)
which can attack the parts of the circuit breakers.

17. Construction of SF6 Circuit Breakers
SF6 circuit breakers mainly consist of two parts, namely (a) the interrupter unit
and (b) the gas system.
Interrupter Unit – This unit consists of moving and fixed contacts comprising a
set of current-carrying parts and an arcing probe. It is connected to the SF6 gas
reservoir. This unit consists slide vents in the moving contacts which permit the
high-pressure gas into the main tank.
Gas System – The closed circuit gas system is employed in SF6 circuit breakers.
The SF6 gas is costly, so it is reclaimed after each operation. This unit consists low
and high-pressure chambers with a low-pressure alarm along with warning
switches. When the pressure of the gas is very low due to which the dielectric
strength of gases decrease and an arc quenching ability of the breakers is
endangered, then this system gives the warning alarm.

19. Working Principle of SF6 Circuit Breaker
 In the normal operating conditions, the contacts of the breaker are closed. When
the fault occurs in the system, the contacts are pulled apart, and an arc is struck
between them. The displacement of the moving contacts is synchronized with
the valve which enters the high-pressure SF6 gas in the arc interrupting chamber
at a pressure of about 16kg/cm^2.
 The SF6 gas absorbs the free electrons in the arc path and forms ions which do
not act as a charge carrier. These ions increase the dielectric strength of the gas
and hence the arc is extinguished. This process reduces the pressure of the
SF6 gas up to 3kg/cm^2 thus; it is stored in the low-pressure reservoir. This low-
pressure gas is pulled back to the high-pressure reservoir for re-use.
 Now a day puffer piston pressure is used for generating arc quenching pressure
during an opening operation by mean of a piston attached to the moving
contacts.

20. Advantage of SF6 circuit breaker
SF6 circuit breakers have the following advantages over conventional breaker,
 SF6 gas has excellent insulating, arc extinguishing and many other properties
which are the greatest advantages of SF6 circuit breakers.
 The gas is non-inflammable and chemically stable. Their decomposition
products are non-explosive and hence there is no risk of fire or explosion.
 Electric clearance is very much reduced because of the high dielectric strength
of SF6.
 Its performance is not affected due to variations in atmospheric condition.
 It gives noiseless operation, and there is no over voltage problem because the
arc is extinguished at natural current zero.
 There is no reduction in dielectric strength because no carbon particles are
formed during arcing.
 It requires less maintenance and no costly compressed air system is required.
 SF6 performs various duties like clearing short-line faults, switching, opening
unloaded transmission lines, and transformer reactor, etc. without any problem.

21. Disadvantages of SF6 circuit breakers
 SF6 gas is suffocating to some extent. In the case of leakage in the breaker tank,
the SF6 gas being heavier than air and hence SF6 are settled in the surroundings
and lead to the suffocation of the operating personnel.
 The entrance of moisture in the SF6 breaker tank is very harmful to the breaker,
and it causes several failures.
 The internal parts need cleaning during periodic maintenance under clean and
dry environment.
 The special facility requires for transportation and maintenance of quality of
gas.