SF6 - MV circuit breakers
rotating arc technique
The motion of the arc is caused by a
magnetic field produced by the load
current itself, and applied on to
the arc.
The process is clearly illustrated in
figure 5.
As the main contacts separate, the
current to be broken is diverted
through a solenoid down to the arcing
rings, an arc appears between the two
rings, perpendicular to its own
magnetic field B produced through the
solenoid.
The current I is flowing into a
conductive plasma. Simultaneously,
the magnetic field B perpendicular
to I.
The effect of this combination is a
force F exerted upon the arc, which is
consequently accelerated into a
circular motion along the arcing ring.
The solenoid is designed in such a
way that the resulting arc speed is
high during the arcing period. This
have several advantages:
n the cooling of the arc is effective in
the surrounding SF6.
n hot spots creating metallic vapors
and excessive wearing are avoided
through the motion of the arc roots.
This rotation of the arc will proceed no
more than half a period, until a current
zero.
The speed of rotation of the arc has
been measured. It varies with the
intensity of the current to be
interrupted and can reach the speed
of sound in the gas for the highest
currents (fig. 6).
When short-circuit currents are
interrupted, the speed slightly before
the current zero is high enough to
keep the arc in rotation. The field is
out of phase with the current and the
product of the two is still significant.
On the other hand, the speed is very
low just before the current zero when
small current are interrupted. This is
the reason for the smooth breaking
made possible by this technique and
switching of small inductive currents
Switching of motor starting currents is
the most frequent and the most severe
case:
n the switching of small inductive
currents, as unloaded transformers
switching, is easier, thanks to the
dumping factor of the transformers,
n the motor insulation is lower than the
insulation of the rest of the circuit. Due
to the large number of medium voltage
motors used in industry and their
relative importance, it is necessary to
ensure their continuous reliable
operation.
The price of these motors is much
higher than the price of the switchgear,
leading the users to be prudent and
sometimes anxious.
During their lifetime, these machines
are subject to many forms of voltage
surges with varying magnitudes and
wavefronts. Surges which originate on
the machine voltage system are likely
to be most severe due to their short
propagating distances and no intervening
transformers. The most
common sources of these surges are
restriking during interruption and
prestriking during energisation and
current chopping.
Switching overvoltages have recently
undergone much investigation due to
the discovery of motor failures and the
introduction of new switching technologies.
To determine the effects of
switching overvoltages on motor
insulation, it is necessary to investigate
the characteristics of the overvoltage
waveforms generated by switching
operations and the effect of the
waveforms on the various forms of
insulation.
Previous studies of motor overvoltages
have proved that for all types of
switching devices the overvoltages
injected at the terminals of a motor
running at speed, whether it be under
load or unloaded, are in almost all
cases, not of sufficient magnitude to
damage motor insulation.
This is because the low surge
impedance and the back EMF present
in the winding of a rotating machine are
sufficient to reduce the net switching
overvoltage seen by the motor
insulation to an insignificant level. For
motors under starting conditions
however, the rotor is stationery and
therefore no back EMF has been
generated.
motor insulation
Most stator windings of AC machines
are composed of form wound coils
which are joined together to form a
phase winding. The coils consist of
several turns in series, each of which
must be insulated from each other and
the earthed steel laminations. Thus the
insulation can be divided into "winding
to earth" and interturn insulation types.
To achieve magnetic and thermal
performance, the coils are placed close
together near the air gap in slots in the
grounded stator core laminations, with
several turns in the same slot. This
results in thin, dry-type insulation
between turns and thicker insulation to
earth to withstand the machine voltage.
The construction results in large
capacitances between turns and
between coils and the slot. This in turn
results in a slow surge velocity in motor
windings.
The machine winding to earth insulation
or main insulation, is normally
subjected to high dielectric stress which
only increases by a factor of three to
ten times during surges. The interturn
insulation however, is normally
subjected to very low stress levels
which may increase 100 to 1,000 times
under surge conditions. This makes
interturn insulation very difficult to test.
It also leads to varying opinions on
testing conditions. Consequently, there
exists no standards for normalisation
on the subject.
propagation of steep fronted waves in motor windings
As previously described, the motor
winding consists of many turns
connected in series. The exact
distribution of voltage stress placed on
interturn insulation by a steep fronted
wave is very difficult to determine, due
to the many parameters involved with
the winding construction. Many
models have been developed to
approximate the phenomena, the most
popular being a capacitor model, or a
ladder network of coils and shunt
capacitors.
Using the ladder network, the winding
can be considered to have travelling
wave properties with a given surge
impedance and transit time. Thus a
steep fronted wave will take a certain
time to travel through each turn. This
time (Tt) is usually much smaller than
the wave-front time (Tf) as shown in
figure 7.
If the wave amplitude is Vmax, then
the voltage developed across the turn
V2 = Vmax. (Tt/Tf). Thus, for a
particular motor with a fixed wave
propagation characteristic (Tt) the
voltage appearing across the first turn
is dependent on the magnitude and
front time of the surge (i.e. rate of
change of surge voltage).
Results of experimentations, have
revealed that wave front times in
excess of 3 ms result in a negligibly
small voltage build up across the first
turns however for waves with front
times less than 10 ms voltage
distribution across initial turns is
significant, in particular for very fast
front times of 0.2 to 0.5 ms a major
percentage of the wave front
magnitude can appear across the line
end coil. This is illustrated in figure 8.
It must be remembered that the
duration of such overvoltages is very
short so the energy accumulated in
this time is very small. As a result the
damage to insulation is very limited
and usually undetectable.
