A circuit breaker is
equipment that can open or close a circuit under all conditions viz. no-load, full load, and fault conditions. It
is so designed that it can be operated manually (or by remote control) under
normal conditions and automatically under fault conditions. For the latter
operation, a relay circuit is used with a circuit breaker.
A circuit breaker is a piece of equipment which can
(i) make
or break a circuit either manually or by remote control under normal conditions
(ii) break a circuit automatically under fault conditions
(iii) make a circuit
either manually or by remote control under fault conditions
Thus a circuit
breaker incorporates manual (or remote control) as well as automatic control
for switching functions. The latter
control employs relays and operates only under fault conditions. Under normal
operating conditions, the contacts remain closed and the circuit breaker
carries the full-load current continuously.
In this condition, the e.m.f. in the secondary winding of the current
transformer (C.T.) is insufficient to operate the trip coil of the breaker but
the contacts can be opened (and hence the circuit can be opened) by manual or
remote control. When a fault occurs, the
resulting overcurrent in the C.T. primary winding increases the secondary
e.m.f. This energizes the trip coil of
the breaker and moving contacts are pulled down, thus opening the contacts and
hence the circuit. The arc produced during the opening operation is quenched by
the oil. It is interesting to note that
relay performs the function of detecting a fault whereas the circuit breaker
does the actual circuit interruption.
Operating principle.
A circuit breaker essentially consists of
fixed and moving contacts, called electrodes.
Under normal operating conditions, these contacts remain closed and will
not open automatically until and unless the system becomes faulty. Of course, the contacts can be opened
manually or by remote control whenever desired. When a fault occurs on any part
of the system, the trip coils of the circuit breaker get energized and the
moving contacts are pulled apart by some mechanism, thus opening the circuit.
When the contacts of a circuit breaker are separated under fault conditions, an
arc is struck between them. The current
is thus able to continue until the discharge ceases. 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. Therefore, the main problem in a circuit
breaker is to extinguish the arc within the shortest possible time so that heat
generated by it may not reach a dangerous value.
When a
short-circuit occurs, a heavy current flows through the contacts of the
*circuit breaker before they are opened by the protective system. At the instant when the contacts begin to
separate, the contact area decreases rapidly and large fault current causes
increased current density and hence rise in temperature. The heat produced in the medium between
contacts (usually the medium is oil or air) is sufficient to ionize the air or
vaporize and ionize the oil. The
ionized air or vapor acts as a 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
consequently, the current in the circuit remains uninterrupted so long as the
arc persists. During the arcing period, the current flowing between the
contacts depends upon the arc resistance.
The greater the arc resistance, the smaller the current that flows
between the contacts.
The arc resistance
depends upon the following factors :
(i) Degree of ionization— the arc
resistance increases with the decrease in the number of ionized particles
between the contacts.
(ii) Length of the arc— the arc resistance increases with the length of the
arc i.e., separation of contacts.
(iii) Cross-section of arc— the arc
resistance increases with the decrease in the area of X-section of the arc.
Principles of arc Extinction
Before discussing the methods of arc extinction, it is
necessary to examine the factors responsible for the maintenance of arc between
the contacts.
These are :
(i) p.d.
between the contacts
(ii) ionized particles between contacts
Taking these in
turn,
(i) 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 contacts to such a distance that p.d.
becomes inadequate to maintain the arc.
However, this method is impracticable in a high voltage system where the separation of many meters may be required.
(ii) The ionized particles between
the contacts tend to maintain the arc.
If the arc path is deionized, the arc extinction will be
facilitated. This may be achieved by cooling
the arc or by bodily removing the ionized particles from the space between the
contacts.
Methods of Arc Extinction
There are two methods of extinguishing the arc
in circuit breakers viz.
1. High resistance method. 2. Low resistance or current zero method
1. High
resistance method. In this method, arc
resistance is made to increase with time so that the current is reduced to a value
insufficient to maintain the arc.
Consequently, the current is interrupted or the arc is
extinguished. The principal disadvantage
of this method is that enormous energy is dissipated in the arc. Therefore, it is employed only in d.c. circuit
breakers and low-capacity a.c. circuit breakers.
The resistance of the arc may
be increased by :
(i) 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.
(ii) 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.
(iii) Reducing X-section of the arc. If the area
of X-section of the arc is reduced, the voltage necessary to maintain the arc
is increased. In other words, the
resistance of the arc path is increased.
The cross-section of the arc can be reduced by letting the arc pass
through a narrow opening or by having a smaller area of contacts.
(iv) Splitting
the arc. The resistance of the arc can
be increased by splitting the arc into a number of smaller arcs in series. Each one of these arcs experiences the effect
of lengthening and cooling. The arc may
be split by introducing some conducting plates between the contacts.
2. Low
resistance or Current zero method. This
method is employed for arc extinction in a.c. circuits only. In this method, arc resistance is kept low
until the current is zero where the arc extinguishes naturally and is prevented
from restriking in spite of the rising voltage across the contacts. All modern high power a.c. circuit breakers
employ this method for arc extinction. In an a.c. the system, current drops to zero
after every half-cycle. At every current
zero, the arc extinguishes for a brief moment.
Now the medium between the contacts contains ions and electrons so that
it has small dielectric strength and can be easily broken down by the rising
contact voltage known as restriking voltage.
If such a breakdown does occur, the arc will persist for another
half-cycle. If immediately after the current
zero, the dielectric strength of the medium between contacts is built up more
rapidly than the voltage across the contacts, the arc fails to restrike and the
current will be interrupted.
The rapid
increase of dielectric strength of the medium near current zero can be achieved
by :
(a) causing the ionized particles in the space between contacts to
recombine into neutral molecules.
(b) sweeping the ionized particles away and
replacing them by un-ionized particles Therefore, the real problem in a.c. arc
interruption is to rapidly deionize the medium between contacts as soon as the current becomes zero so that the rising contact voltage or restriking voltage
cannot breakdown the space between contacts.
The de-ionization of the medium can be achieved by:
(i) lengthening
of the gap. The dielectric strength of
the medium is proportional to the length of the gap between contacts. Therefore, by opening the contacts rapidly, the higher dielectric strength of the medium can be achieved.
(ii) high
pressure. If the pressure in the
vicinity of the arc is increased, the density of the particles constituting the
discharge also increases. The increased
density of particles causes a higher rate of de-ionization and consequently, the
dielectric strength of the medium between contacts is increased.
(iii)
cooling. The natural combination of ionized
particles takes place more rapidly if they are allowed to cool. Therefore, the dielectric strength of the medium
between the contacts can be increased by cooling the arc.
(iv) blast
effect. If the ionized particles between
the contacts are swept away and replaced by unionized particles, the dielectric
strength of the medium can be increased considerably. This may be achieved by a gas blast directed
along with the discharge or by forcing oil into the contact space.
Important Terms
The following are the important terms much used in the
circuit breaker analysis :
(i) Arc Voltage.
It is the voltage that appears across the contacts of the circuit
breaker during the arcing period. As soon as the contacts of the circuit
breaker separate, an arc is formed. The
voltage that appears across the contacts during the arcing period is called the arc
voltage. Its value is low except for the
*period the fault current is at or near zero current points. At current zero, the arc voltage rises
rapidly to a peak value and this peak voltage tends to maintain the current flow
in the form of an arc.
(ii) Restriking voltage.
It is the transient voltage that appears across the contacts at or near
current zero during the arcing period. At current zero, a high-frequency transient
voltage appears across the contacts and is caused by the rapid distribution of
energy between the magnetic and electric fields associated with the plant and
transmission lines of the system. This
transient voltage is known as the restriking voltage. The current
interruption in the circuit depends upon this voltage. If the restriking voltage rises more rapidly
than the dielectric strength of the medium between the contacts, the arc will
persist for another half-cycle. On the other hand, if the dielectric strength
of the medium builds up more rapidly than the restriking voltage, the arc fails
to restrike and the current will be interrupted.
(iii) Recovery voltage. It is the normal frequency (50 Hz) r.m.s. the voltage that appears across the contacts of the circuit breaker after final arc
extinction. It is approximately equal to
the system voltage. When contacts of the circuit breaker are opened, current drops
to zero after every half cycle. At some
current zero, the contacts are separated sufficiently apart and dielectric
strength of the medium between the contacts attains a high value due to the
removal of ionized particles. At such an
instant, the medium between the contacts is strong enough to prevent the
breakdown by the restriking voltage. Consequently, the final arc extinction
takes place and circuit current is interrupted.
Immediately after the final current interruption, the voltage that appears
across the contacts has a transient part. However, these transient oscillations subside
rapidly due to the damping effect of system resistance and normal circuit
voltage appears across the contacts. The
voltage across the contacts is of normal frequency and is known as recovery
voltage.
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