Circuit Breaker arc phenomenon.pdf engineering

Different types of circuit breaker and ther applications

1. Circuit Breaker

2. Circuit Breaker
•A circuit breaker is an equipment which is
designed to protect an electric circuits from
damage caused by short circuit or overload.
–
Make or break a circuit either manually or by
–
Make or break a circuit either manually or by remote control under normal conditions.
–Break a circuit automatically under fault
conditions
–Make a circuit either manually or by remote
control under fault conditions.

3. Operating Principles
•A circuit breaker essentially consists of fixed and
movingcontacts,calledelectrodes.
•Under normal operating conditions, these contacts
remain closed and will not open automatically until
and
unless
the
system
becomes
faulty
.
and
unless
the
system
becomes
faulty
.
•The contacts can be opened manually or by remote
controlwheneverdesired.
•Whenfaultoccursonany partof thesystem,the trip
coils of the circuit breaker get energized and the
movingcontactsarepulledapartbysomemechanism,
thusopeningthecircuit.

4. Operating Principles cont….
•When the contacts of a circuit breaker are separate d
under fault conditions, an arc is struck between th em.
The current is thus able to continue until the disc harge
ceases.
•
The production of arc not only delays the current
•
The production of arc not only delays the current interruption process but it also generates enormous
heat which may cause damage to the system or to the
circuit breaker itself.
•The main problem in a circuit breaker is to extingu ish
the arc within the shortest possible time so that h eat
generated by it may not reach a dangerous value.

5. Circuit Breaker
•In addition to these making and breaking
capabilities, circuit breaker are required to do
so under the following typical conditions:
–
Short
-
circuit interruption
–
Short
-
circuit interruption
–Interruption of small inductive currents
–Capacitor switching
–Interruption of short-line fault

6. Circuit breaker
•According to the operating mechanism of
circuit breaker they can be divided as:
•Solenoid………… up to 132 kV
•
Spring Charged …….. up to 765 kV
•
Spring Charged …….. up to 765 kV
•Hydraulic ….. up to 245 kV
•Pneumatic ….. used up to 400 kV

7. Circuit breaker
•According to the voltage level of installation
types of circuit breaker are referred as:
–High voltage circuit breaker (> 72 kV) –
Medium voltage circuit breaker (1
-
72 kV)
–
Medium voltage circuit breaker (1
-
72 kV)
–Low voltage circuit breaker (< 1 kV)

8. Short summary for breakers
•Plain-break air breakers -used in low voltage and
medium voltage up to 15 kV
•For low and medium voltages fuses can be also used,
but the main disadvantage is that they must be
replaced after fault clearing
•
In medium voltage systems
-
minimum oil, SF6 and
•
In medium voltage systems
-
minimum oil, SF6 and
vacuum breakers used
•For high voltages -minimum oil, SF6 and air blast
breakers used
•The maximum voltage per interrupter is 100 kV for a ir-
blast and SF6 breakers, 170 kV for minimum oil
breakers

9. Circuit Breaker
•According to their arc quenching (rapid
cooling) media the circuit breaker can be
divided as:
–
Air circuit breaker
–
Air circuit breaker
–Oil circuit breaker
–Vacuum circuit breaker
–SF6 circuit breaker

10. Types of circuit breaker
•Air circuit breakers (ACB) : The circuit breaker
which operates in air at atmospheric pressure.
•Air-blast break: A blast of compressed air is
directed into the arc path to cool the ionized gas
and remove it from the gap between the and remove it from the gap between the contacts.
•Oil circuit breakers (OCB):
•Vacuum circuit breakers (VCB):
•Sulfur-hexafluoride (SF6) circuit breakers: Sulfur-
hexafluoride (SF6) is an excellent gaseous
dielectric for high voltage power applications.

11. Air circuit breakers (ACB)

12. Oil circuit breakers (OCB)

13. Vacuum Circuit Breaker

14. Sulfur-hexafluoride (SF6) circuit breakers

15. Circuit breaker
•According to their services the circuit breaker
can be divided as:
–Outdoor circuit breaker
–
Indoor circuit breaker
–
Indoor circuit breaker

17. Arc Phenomena
•When a short-circuit occurs, a heavy current flows through the
contacts of the circuit breaker before they are ope ned by the
protective system.
•At the instant when the contacts begin to separate, the contact area
decreases rapidly and large fault current causes in creased current
density and hence rise in temperature.
•The heat produced in the medium between contacts is sufficient to
ionize the air or vaporize and ionize the oil. ionize the air or vaporize and ionize the oil.
•The ionized air or vapour acts as conductor and an arc is struck
between the contacts. The p.d. between the contacts is quite small
and is just sufficient to maintain the arc.
•The arc provides a low resistance path and conseque ntly the
current in the circuit remains uninterrupted so lon g as the arc
persists.
•During the arcing period, the current flowing betwe en the contacts
depends upon the arc resistance. The greater the ar c resistance, the
smaller the current that flows between the contacts .

18. The arc resistance depends upon
•Degree of ionization—the arc resistance increases
with the decrease in the number of ionized particle s
between the contacts.
•
Length of the arc
—
the arc resistance increases with
Length of the arc
—
the arc resistance increases with
the length of the arc i.e., separation of contacts.
•Cross-section of arc—the arc resistance increases
with the decrease in area of X-section of the arc.

19. Factors responsible for maintaining ARC
•P.D. between the contacts
•Ionized particles between contacts
–P.D. between the contacts:
•
When the contacts have a small separation, the
p.d
.
•
When the contacts have a small separation, the
p.d
.
between them is sufficient to maintain the arc.
•One way to extinguish the arc is to separate the co ntacts to
such a distance that p.d. becomes inadequate to mai ntain
the arc.
•However, this method is impracticable in high volta ge system
where a separation of many meters may be required.

20. Factors responsible for maintaining ARC
–Ionized particles between contacts:
•The ionized particles between the contacts tend to
maintain the arc.
•If the arc path is deionized, the arc extinction will be
facilitated. facilitated.
•This may be achieved by cooling the arc or by bodily
removing the ionized particles from the space between
the contacts.

21. Methods of arc Extinction
•High resistance method.
•Low resistance method or current zero
method.

22. High resistance method
•Inthis method, arc resistance is made to
increasewithtimesothatcurrentisreduced
toavalueinsufficienttomaintainthearc. •
Consequently,
the
current
is
interrupted
or
•
Consequently,
the
current
is
interrupted
or
thearcisextinguished.
•Theprincipledisadvantageofthismethodis
thatenormousenergyisdissipatedinthearc.
•Itisemployedonlyind.c.circuitbreakersand
low-capacitya.c.circuitbreakers.

23. Methods of increasing arc resistance
•Lengthening of arc.
•Cooling of arc.
•Reducing cross section area of arc.
•
Splitting the arc.
•
Splitting the arc.

24. Methods of increasing arc resistance cont…
•Lengthening the arc
–The resistance of the arc is directly proportional to its
length.
–The length of the arc can be increased by increasing the
gap between contacts. gap between contacts.
•Cooling the arc
–Cooling helps in the deionization of the medium between
the contacts.
–This increases the arc resistance.
–Efficient cooling may be obtained by a gas blast directed
along the arc.

25. Methods of increasing arc resistance
•ReducingX-sectionofthearc
–IftheareaofX-sectionofthearcisreduced,thevoltage
necessarytomaintainthearcisincreased.
–The cross-section of the arc can be reduced by letting
the arc pass through a narrowopening or by having
smaller
area
of
contacts
.
smaller
area
of
contacts
.
•Splittingthearc
–The resistance of the arc can be increased by splitting
thearcintoanumberofsmallerarcsinseries.
–Each one of these arcs experiences the effect of
lengtheningandcooling.
–The arc may be split by introducing some conducting
platesbetweenthecontacts.

26. Low Resistance or Current Zero Method
•Low resistance method is applicable only for ac circuit and it is
possible there because of presence of natural zero of current.
•In an a.c. system, current drops to zero after every half-cy cle. At
every current zero, the arc extinguishes for a brief moment.
•The medium between the contacts contains ions and
electrons
so that it has small dielectric strength and can be
electrons
so that it has small dielectric strength and can be
easily broken down by the rising contact voltageknown as
restriking voltage.
•If such a breakdown does occur, the arc will persist for another
half cycle.If immediately after current zero, the dielectric
strength of the medium between contacts isbuilt up more
rapidly than the voltage across the contacts, the arc fails to
restrike and the current willbe interrupted.

27. Theories of arc extinction phenomenon
Energy Balance Theory
•When the contact of circuit breaker are about to op en,
restriking voltage is zero, hence generated heat wo uld be
zero and when the contacts are fully opened there i s
infinite resistance this again make no production o f heat.
•
This means the maximum generated heat is lying
•
This means the maximum generated heat is lying between these two cases.
•This theory is based on the fact that the rate of
generation of heat between the contacts of circuit
breaker is lower than the rate at which heat betwee n the
contact is dissipated.
•Thus if it is possible to remove the generated heat by
cooling, lengthening and splitting the arc at a hig h rate
than the generation, arc can be extinguished.

28. Voltage Race Theory
•The arc is due to the ionization of the gap between the
contact of the circuit breaker.
•Thus the resistance is very small at the initial st age i.e.
when the contacts are closed
•
The resistance starts increasing as the contacts ge t
Theories of arc extinction phenomenon
•
The resistance starts increasing as the contacts ge t separated.
•If we remove ions at the initial stage either by
recombining them into neutral molecules or insertin g
insulation at a rate faster than the rate of ioniza tion, the
arc can be interrupted.
•The ionization at zero current depends on the volta ges
known as restriking voltage.

29. Low Resistance or Current Zero Method
•The rapid increase of dielectric strength
of the medium near current zero can be
achieved by :
–
Causing the ionized particles in the space between
–
Causing the ionized particles in the space between contacts to recombine into neutral molecules.
–Sweeping the ionized particles away and replacing
them by un-ionized particles

30. Deionization
The Deionization of the medium can be
achieved by following methods:
–Lengthening of the gap.
–
High pressure.
–
High pressure.
–Cooling.
–Blast effect.

31. Cont….
•Lengtheningofthegap
–The dielectric strength of the mediumis proportional to
thelengthofthegapbetweencontacts.
–Therefore, by opening the contacts rapidly, higher
dielectricstrengthofthemediumcanbeachieved.
•
High
pressure
•
High
pressure
–If the pressure in the vicinity of the arc is increased, the
density of the particles constituting the discharge also
increases.
–Theincreaseddensityofparticlescauseshigherrateofde-
ionization and consequently the dielectric strength of the
mediumbetweencontactsisincreased.

32. Cont…
•Cooling
–Natural combination of ionized particles takes place
morerapidlyiftheyareallowedtocool.
–Therefore,dielectricstrengthofthemediumbetween
the
contacts
can
be
increased
by
cooling
the
arc
.
the
contacts
can
be
increased
by
cooling
the
arc
.
•Blasteffect
–If the ionized particles between the contacts are
swept away and replaced by unionized particles, the
dielectric strength of the mediumcan be increased
considerably.
–Thismaybeachievedbyagasblastdirectedalongthe
dischargeorbyforcingoilintothecontactspace.

33. Arc Voltage, Restriking Voltage and Recovery Voltag e
•The basic principle for breaker operation is
toextinguish arcwhich take place during opening
of circuit breaker. But it does not mean that
arcing do not take place when the breaker is
closed, rather it does. closed, rather it does.
•The time duration for which arcing take place
when breaker is closed is known as Pre-arcing
Time which is typically around 2 ms and the
duration for which arcing persists when we open
the breaker is known as Arcing Period whose
value is around 6ms.

34. Arc Voltage
•As soon as the Breaker contacts open, an arc is for med
between the contacts of the Circuit Breaker.
•The voltage which appears across the contacts of th e
Breaker during this arcing period is called the Arc
Voltage.
Its value is low but when the value of arc
Voltage.
Its value is low but when the value of arc
current reaches to zero, arc voltage will shoot up to its
peak value which in turn will try to maintain the a rc
across the contacts.
•So here we come to a voltage which shoots up to pea k
when the current crosses to its zero. Actually this is the
origination of Restriking Voltage.

35. Restriking voltage
•As the arcing current crosses zero, a high
frequency transient voltage appears across the
contacts of the Circuit Breaker. This Transient
voltage is known as Restriking Voltage.
•
The power system has appreciable amount of
•
The power system has appreciable amount of inductance, thus the fault current must lag
behind the system voltage by 90°.
•Therefore, when the arcingcurrent crosses zero,
the voltage across the contacts of Circuit Breaker
shoots up to its peak value.

36. Restriking voltage
•As the voltage reaches its peak, it restrike the ar c and try to
maintain the arc. Due to this the arcing current wi ll increase from
its zero and correspondingly the voltage must also decrease. The
combined effect of increasing current and decreasin g voltage across
the contact will bring the voltage back to its norm al value within
few miliseconds.
•Restriking Voltage has a very important role in the arc extinction
process. If the Restriking Voltage rises more rapid ly than the process. If the Restriking Voltage rises more rapid ly than the dielectric strength of the medium between the conta cts of the
Circuit Breaker, the arc will persists for next hal f cycle and after next
half cycle, arcing current will again reach to its zero and we will
again get a chance. If this time the rate of rise o f dielectric strength
of medium between the contacts is more than rate of rise of
Restriking Voltage then arc will extinguish.
•Therefore, for arc extinction the rate of Rise of R estriking Voltage
should be less than the rate of Rise of Dielectric Strength of
Medium

37. Restriking voltage

38. •Let us consider a simple circuit, having a circuit breaker CB, as illustrated in
Fig. and that a short circuit occurs on the feeder close to the bus-bars. Let
L be the inductance per phase of the system up to t he fault point, R be the
resistance per phase of the system up to the fault point and C be the
capacitance per phase to earth of the system.
•Consider the opening of a circuit breaker under fau lt conditions. Before
current interruption, the capacitance C is short ci rcuited by the fault and
the short-circuit current through the breaker is li mited by resistance R and
inductance L of the system. If R is negligible comp ared to L, the short
-
Restriking voltage
inductance L of the system. If R is negligible comp ared to L, the short
-
circuit current iwill lag behind the system voltage v by 90°.
•With the contacts opened and the arc broken, curren t iis diverted through
capacitance C so that the voltage v, which has so f ar been effective only
across the inductance L, is suddenly applied to the inductance L and
capacitance C in series which form an oscillatory c ircuit, having a natural
frequency.
fn = 1 / (2ᴨ √(LC))
•The initial charging current surge tends to carry t he voltage across the
capacitor, and therefore across the circuit breaker contacts to double its
equilibrium value i.e., 2 V
max
; this is the re-striking voltage transient which
tends to re-establish the arc in the circuit breake r.

39. Recovery Voltage
•It is the normal frequency (50 Hz) r.m.s. voltage t hat appears across
the contacts of the circuit breaker after final arc extinction.
•When contacts of circuit breaker are opened, curren t drops to zero
after every half cycle.
•At some current zero, the contacts are separated su fficiently apart
and dielectric strength of the medium between the c ontacts attains
a high value due to the removal of ionized particle s. a high value due to the removal of ionized particle s.
•At such an instant, the medium between the contacts is strong
enough to prevent the breakdown by the restriking v oltage.
•Consequently, the final arc extinction takes place and circuit current
is interrupted. Immediately after final current int erruption, the
voltage that appears across the contacts has a tran sient part.
•The voltage across the contacts is of normal freque ncy is known as
recovery voltage.

40. Some basic formulae
•Rate of rise of restriking voltage = RRRV
•Recovery voltage V = I
sc* X
–I
sc= Short circuit current; X = Reactance
•Peak value of recovery voltage V
max = √2 * V
•Max value of restriking voltage = 2V
max
Time to reach the first peak restriking voltage,
t =
ᴨ
√(LC)
•
Time to reach the first peak restriking voltage,
t =
ᴨ
√(LC)
•Time to attain max RRRV, t = ᴨ√(LC) /2
•Natural frequency of the circuit, fn = 1 / (2ᴨ√(LC))
•Average rate of rise of restriking voltage,
•
RRRV
av= 2V
max /(ᴨ√(LC))
•Maximum value of RRRV
RRRV
max= V
max /√(LC)

41. Examples
•A 50 Hz, 11 kV generator is connected to a power sy stem. The system
inductance and capacitance per phase are 10 mHand 0 .02 µF respectively.
Calculate;
–a. max voltage across the contacts of the CB at an ins tant when it passes
through zero.
–b. frequency of transient oscillation
–c. avg rate of rise of voltage up to the first peak of oscillation, neglect
resistance.
•Solution:
a) Active recovery voltage V
max
= √2 * V
ph
=√2 * 11/ √3 = 8.98 kV
Max Restriking voltage = 2 * V
max
= 2*8.98 = 17.96 kV
b) fn = 1 / (2ᴨ√LC) = 1 / (2ᴨ√10x10 -3
x 0.02x10
-6
) = 11.254 kHz
c) Av RRRV = 2V
max
/(ᴨ√LC)
= 2*8.98/(ᴨ√10x10
-3
x 0.02x10
-6
)
= 0.404 kV/µs

42. Examples
•In a short circuit test on a circuit breaker, the f ollowing
data was obtained on a frequency transient.
(i) Time to reach the peak restriking voltage 55 µs .
(ii) The peak restriking voltage 100 kV.
Determine;
a)
Natural frequency of the circuit
a)
Natural frequency of the circuit
b) Avgrate of rise of restriking voltage
•Solution:
a)t = ᴨ√(LC) = 55 µs.
fn = 1 / (2ᴨ√(LC)) = 1 / (2*55*10
-6
) = 9091 Hz
b)Av RRRV = 2V
max /(ᴨ√(LC)) = 100/55 = 1.82 kV/ µs

43. Examples
•In a system of 132 kV, the circuit phase to ground
capacitance is 0.02 µF and the circuit inductance i s 5 H.
The circuit breaker interrupts a magnetizing curren t of
5 A (peak). Find;
–
a)
The voltage across the CB contacts after the circuit interruption
–
a)
The voltage across the CB contacts after the circuit interruption
–b) The value of resistance to be used across the
contacts to suppress restriking voltage
•Solution:
a) v = i√ (L/C) = 5* √ (5/0.02x10
-6
) = 79 kV
b) R = 0.5 √ (L/C) = 0.5 * √ (5/0.02x10
-6
) = 7.9 kΩ