SF6 - MV circuit breakers
historics
Many years ago, oil circuit-breakers
(OCB) and air magnetic circuitbreakers
(AMB) were the main
breaking techniques for
MV applications.
OCB, mainly in Europe, i.e. bulk oil CB
and minimum oil CB, were the single
technology in the catalogue of the
majority of world-wide CB
manufacturers.
But the qualities of AMC were very
appreciated by the users in the field of
the industrial distribution for 6.6 and
11 kV in Asia, Europe and 5 or 15 kV
in America.
With AMB, from low voltage
applications to MV ones, the users did
not worry about voltage surges and,
mostly were unawared of the existence
of such phenomena.
Thanks to the magnetic blowing, the
cooling of the arc depends on the value
of the breaking current, leading to a
very smooth breaking.
Since 1970 or 1975 (depending on the
different countries) SF6 and vacuum
breaking techniques appeared on the
MV market, for the primary substations
firstly.
The success of these new breaking
techniques is well known. The
advantages are:
n a longer life time, in comparison
with OCB,
n a better behaviour for rapid reclosing,
capacitor switching etc... in comparison
with OCB and AMB,
n a smaller volume in comparison
with AMB.
To take the place of AMB for industrial
application, and particulary for starting
motor switching, SF6 CB is the good
response, giving:
n the advantage of a smooth breaking,
n and, in comparison with AMB, the
compactness.
For these reasons, Merlin Gerin has led
AMB technology, after 35 years of
manufacturing AMB type Solenarc, in
favour of SF6 technology, which is
developped and manufactured since
1970 in the MV field.
auto compression technique
Merlin Gerin range of SF6 MV circuitbreakers
is called Fluarc. All the Fluarc
are self extinguishing CB. That means
that the same mass of SF6 gaz is
working during the whole life - more than
20 or 30 years - of the CB, in a sealed
for life enclosure.
During the arcing period the arc is
cooled by convection in that a certain
quantity of hot gas is replaced by cold
gas. This is not a surface phenomenon;
the cold gas is brought in perpendiclarly
to the direction of flow to promote the
mixing of hot gas and cold gas. Heat
exchange by radial conduction is very
low compared with this. We could also
expect heat exchange by radiation, on
account of the high temperature of the
arc. In fact, the exchanges are small
because radiation is from the peripheral
layers only. Finally the heat exchanges
during arcing take place chiefly by
convection. The energy supplied by the
system in time dt to a mass dm of gas is:
dw = VI dt = h dm,
V being the arcing voltage and
h the enthalpy per unit of mass.
We again have VI dt = hr sdx,
r being the density, s the cross section
of the arc and
dx the path taken by the mass dm in the
time dt.
Hence VI = hr su, u being the velocity of
the gases.
The power transmitted is directly
dependent on this velocity. The laws of
gas flow teach us that this velocity
cannot be indefinitely increased to
increase the mass flow of the hot gas. It
is advantageous to stay in the vicinity of
the speed of sound in the gas. This
speed can only be obtained by suitable
structural arrangements and sufficient
switching energy. The hollow tubular
contacts facilitate the rapid flow of hot
gases and causes instability of the arc
root, preventing wear on the arcing
contacts. The puffer technique is
remarkably effective, as it is sufficient to
inject only a small quantity of gas
between the contacts.
With the Fluarc FB and FG, the quantity
of compressed gas injected at the throat
of the nozzle is 5 grammes during
breaking; to limit the temperature of
the arc to 10 or 15,000 °K, it must be
possible to evacuate the heat
produced by the arc which is
approximately 30,000 joules when
interrupting a current of 25 kA.
The gas enthalpy curve shows that
one gramme of gas is sufficient to
carry this energy (fig. 1).
During arcing, the space occupied by
the arc at the throat of the nozzle
depends on the instantaneous current
value. The cross section of the arc is
proportional to this and consequently
is subject to the same sinusoidal
variation. At high current values, the
arc may occupy the whole of the
available space, blocking the flow of
gas.
Indeed, the mass flow in the arc is
very low compared with the flow of
cold gas around the arc, as the gas
density is low at the temperature of
the arc. This is what is known as the
"clogging" effect (fig. 2).
The total mass flow at the nozzle
throat is thus low when instantaneous
current values are high, but it rises
very quickly as soon as the current
falls, and shortly before current zero it
is greater than that which would occur
on opening without current (fig. 3,
fig. 4).
The clogging effect is beneficial for
two reasons:
n when the circuit-breaker opens on a
high current it keeps a larger amount
of gas in reserve than when it
interrupts lower currents.
It does not act "blindly" whatever the
current;
it is better prepared for the passage
through current zero in heavy
currents, and it avoids brusquely
interrupting low currents, as the mass
flow on non load is modest.
n interruption of a heavy current
causes braking in the opening
movement.
This braking limits the distance
between contacts, that is, the length
of the arc and thus the energy
dissipated in the arc.
It is therefore important to stress that
the arc is relatively short, in the order
of 15 mm for a 12 kV, 24 kV, or 36 kV
equipment, that is, smaller than the
contact diameter.
