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As the system voltages and fault levels
increased the Bulk Oil Breakers required huge quantities of
insulating oil and
became unwieldy in size and weight.
This added enormously to the cost of a power
system. Simultaneously improvements were made in the
technique
of ceramics.
The function of oil as insulating medium in
the Bulk Oil Breakers was transferred to the porcelain
containers.
Only a small quantity of oil was used to
perform its functions as arc quenching medium. This led to
the development
of small oil volume or low oil content breakers in the
continent of Europe.
Like the Bulk Oil Breakers these have also
since then passed through many stages of development with
varying designs
of the arcing chambers. Today the small oil volume breakers
are available for voltages up to 36 kV and the fault levels
associated therewith.
Contrary to the operation of the impulse type Circuit Breaker, such as air
blast Circuit Breaker, in which arc extinction
and dielectric recovery
are affected by means of an external quenching medium, the
process of arc extinction in the small
oil
volume
Circuit Breaker is of internal
thermo- dynamic origin.
During the tripping operation an arc strikes
in oil between the moving contact and the fixed contact's
This arc is elongated vertically in the explosion pot until
the distance traveled is sufficient to withstand the
voltage between contacts.
The increase in internal pressure due to the
Splitting up
and vaporization of oil by the arc creates a rapid movement
of the extinguishing
medium round
the arc This self-quenching effect causes a rapid cooling of
the ionized column along its whole
Length due
to partition of the explosion pot and the dielectric
recovery is sufficiently rapid.
To prevent the arc
restricting after a natural Passage Through zero. The
electric arc itself has, therefore,
Supplied the
necessary energy for its own extinction. There are now
numerous
manufacturers of
small oil volume
breakers
However, to
illustrate the principles of working, the sectional view of
working portion of 170 kV 3500 MVA.
Breakers of
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Fig (4) |
M/s Delle France have been shown in Fig. (4) the most
important part of the breaker is its extinguishing chamber.
This takes the form of an insulating cylinder containing
oil, in the axis of which moves the contact rod and within
which breaking occurs.
The arcing chamber is supported at its base
by a casing enclosing a mechanism whose function is to
move the contact rod According to the
impulses given by the control mechanism. In the on position,
the current flows from
the Upper current terminal (1) to the contact fingers, (2) Follows the
movable contact rod (7) and reaches the current terminal
(10)
across the lower contact fingers (8). At the beginning
of the stroke and before breaking, the contact rod strongly
pulled down.
Wards by the tripping springs, starts a high
speed opening motion.
Then, an arc strikes between the contact rod tips (6) and
the stationary
Arcing ring (3) protecting the upper contact
fingers.
At this moment gases escape without hindrance
towards top of the apparatus.
The contact rod rapidly reaches a very high
linear speed; it moves
the arc downwards and forces it to enter the explosion pot
(5) where it is maintained rectilinear and is elongated in a
direction opposite
to the release of gases towards fresh
oil. Since the arc is as short as possible the arc voltage
is minimized and the energy dissipated is
reduced.
Still,
since the gases can no longer develop freely,
they generate a considerable pressure in the explosion pot
(5), thus producing a violent
upward axial blast of oil
vapor, exhausting the highly ionized gaseous mass.
The optimum distance is thus obtained, the
jet of oil causes the dielectric strength to be rapidly
increased, and at the
following current zero, the arc is impeded from restricting
and the breaking is thus achieved.
The explosion pot (5) is intended to
withstand high pressures.
It is partitioned into several components by means of discs
whose function is to retain a certain quantity of fresh oil
while the first break
is proceeding; this allows a second
break to occur with complete
safety at the full short circuit current. The low oil
content Circuit Breakers require separate current Transformers of wound type. Still at
all voltages from 33 kV
and above the costs of these breakers
inclusive of current Transformers compete favorably with
that of the Bulk Oil Breakers.
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