SF6 - MV circuit breakers
fp2 = 1
2 p
C 1 + C 2
L 0 C 1 C 2
(100 - 500 kHz)
Due to the large inductive values of the
source and load inductances the
remainder of the circuit does not
become involved in second parallel
oscillations. The third phenomenon is
termed main circuit oscillation and
involves the total circuit with a
frequency of:
fm = 1
2 p
L 1 + L 2
L 1 L 2 (C 1 + C 2)
(5 - 20 kHz)
which for highly inductive load may be
simplified to
fm = 1
2 p L 1 (C 1 + C 2)
as L 1 < L 2
These phenomena occur simultaneously
after reignition and the
resultant overvoltage is therefore a
super-imposition of the three waveforms
however actual development of
the waveforms depends on circuitbreaker
characteristics, damping and
circuit values.
Single reignition
Immediately following reignition across
the circuit-breaker the first parallel
oscillation occurs at a rapid frequency
discharging Cp1. The second parallel
oscillation then predominates causing
an oscillating current which may or may
not cause a current zero and if so may
or may not be interrupted by the circuitbreaker.
If not then the oscillation
continues until it is sufficiently damped
at which time the main circuit oscillation
predominates.
Multiple reignitions
"Multiple reignitions" is a phenomena
which may occur during the opening
sequence of a circuit-breaker. For
highly inductive circuits the current
leads the voltage by almost 90°. Thus
after current interruption near the
current zero the voltage is almost at a
peak value. On the load side the
voltage begins to oscillate at frequency
fL while on the source side the osillation
occurs at fs. If the momentary
difference in the oscillating voltages
exceeds the rate of rise of dielectric
strength a reignition of the arc takes
place at time t1. The load side now
experiences first and then second
parallel oscillations.
These second parallel oscillations
cause a high frequency current to flow
through the circuit-breaker. This
current, when superimposed on the
power frequency current, may produce
several high frequency current zeros.
The circuit-breaker will attempt to
interrupt at these current zeros.
Whether or not the circuit-breaker can
interrupt the current successfully is
determined by the di
dt
of the current
as it crosses the zero point and the
ability of the breaker to interrupt this di
dt
This is determined by the dielectric
recovery rate of the circuit-breaker. For
breakers with high dielectric recovery
rates interruption may occur at the first
high frequency current zero where as
circuit-breakers with slower dielectric
recovery rates may take several
oscillations or may not be able to interrupt
the high frequency current at all.
The high frequency current is initiated
by second parallel oscillations and
therefore it flows in the internal circuit
illustrated previously in figure 10. Its
frequency is governed by the components
in this loop. Thus the current
flowing through the load inductance L2
is unaffected by the second parallel
oscillation loop apart from small
perturbations.
During the period of reignition the load
terminal oscillates about a voltage level
determined by its value at the time of
the first interruption. The average
voltage level during this period causes
the load current to increase.
During the period of interruption the
transient recovery voltage rises until it
exceeds the dielectric strength of the
electrode space causing a restrike. It
must be remembered that during the
opening sequence of the circuit breaker
the contacts are separating and
therefore the dielectric strength of the
gap and consequently the breakdown
voltage increases with each restrike.
During this time the load current will
increase and then decrease depending
on whether the instantaneous value of
the load terminal voltage exceeds the
voltage level-E/2 as shown in figure 12.
The additional magnetic energy from
the power frenquency source which
accumulates in the load inductance and
the increasing value of breakdown
voltage permit the second reignition to
occur at a higher mean voltage than the
first reignition.
The sequence of successive regnitions
and high frequency current interruptions
which makes up the multiple restrike
process can occur several times as the
increasing amount of energy in the load
inductance and the increasing contact
spacing enable each successive reignition
to occur at a higher mean voltage.
The reignition/interruption process can
terminate in either of two ways:
n the first way involves cessation of the
reignition phenomena. As the contact
spacing increase the minimum transient
recovery voltage required for breakdown
also increases. The magnitude of the
TRV is dependent upon the value of
chopped current which is given by the
value of current flowing through the load.
After a certain number of restrikes the
load current and therefore the chopped
current increases more slowly. When the
rate of rise of TRV is overtaken by the
rate of rise of dielectric strength, the
interruption is definitive.
n the second method involves cessation
of the current interruption phenomena. If
the high frequency current continues for
several cycles without interruption by the
circuit breaker it will be quickly damped
to zero and the circuit breaker will have
to wait for the next power frequency zero
before it can interrupt the current. This
occurs if the magnitude of the high
frequency current component is not
sufficient to cause the net current to
reach the current zero or if the circuit
breaker does not have a sufficient
dielectric recovery rate to enable it to
successfully interrupt the high frequency
current as it crosses the current
zero point.
The prospective restriking overvoltage is
governed by the circuit and circuitbreaker
characteristics. The slope of the
reignition wave is a function of the
frequency of the second parallel
oscillation circuit while the prospective
magnitude is determined by the available
energy, that is the energy stored in the
circuit inductance and capacitance at the
time of interruption. In practise the
magnitude is restricted by the dielectric
strength of the contact gap which is
increasing during opening. Thus the
actual restriking voltage magnitude is a
function of available energy and
dielectric recovery so a general
maximum value cannot be given. The
phenomenon of multiple restrikes
depends on the ability of the circuitbreaker
to interrupt high frequency
current. Vacuum circuit-breakers exhibit
excellent dielectric recovery rates and
are therefore able to interrupt high
frequency currents. Therefore when
vacuum circuit-breakers and contactors
interrupt circuits where sufficient energy
is available to initiate a restrike, an
interruption/restrike process (multiple
restriking) often results.
current chopping
It is defined as "an abrupt current
interruption in the circuit-breaker away
from the natural power-frequency
current zero of the circuit connected to
the circuit-breaker".
Although the current in a circuit-breaker
can chop to zero almost instantaneously
the current in the load
inductance requires time to dissipate
the stored magnetic energy and allow
the magnetic field to collapse. When a
current is chopped by the circuitbreaker
the energy stored in the load
inductance is transfered to the load side
capacitance and produces an
overvoltage.
The mathematic relation between the
chopped current Ia and the over voltage
value is well know. Thank's to the
conservation of energy:
1/2 C 2 U 2 max = 1/2 C 2 U c
2 + 1/2 L 2 I a
2
U max = U c
2 + L 2 / C 2 x I a
2
where
Uc = voltage across downstream
capacitor
C2 = value of downstream capacitor
L2 = value of downstream inductance
If the load side neutral is not earthed,
as is the case for most motors, a
voltage displacement will occur in the
neutral after interruption of the first
phase (fig. 13) and the resulting
overvoltage will be:
U max = 0.5 + (1.5 V) 2 + 1.5 L 2 / C 2 I a
2
The above equation neglects damping
and in practice the oscillations will
decay progressively depending on
circuit damping which is governed
primarily by the nature of the load.
If we take into account the actual
circuits, it is necessary to introduce the
damping factor K.
The lowest value of Umax is obtained
without current chopping:
K = 1 + X
1.5 V
(Umax) mini = 0.5 V + X
= 0.5 V + (K - 1) 1.5 V
= 1.7 V with K = 1.8
= 1.4 V with K = 1.6
With a non negligeable current
chopping, Umax is, of course, higher.
The highest values can be obtained
when interruption of the first phase
causes interruption of the remaining
phases almost simultaneously. This
phenomenon is called virtual current
chopping.
The process of virtual current chopping is
entirely dependent on specific circuit
conditions and interphase coupling. It
can therefore occur with any circuitbreaker
type. Similar to other
phenomena involving high frequency
current interruptions, virtual current
chopping is much more likeky to occur in
vacuum circuit-breakers than circuitbreakers
which use other interruption
techniques. Due to the high values of
chopped current which may occur, the
energy stored in the load circuit at the
time of interruption can be very high
leading to excessive overvoltages.
prestrikes
During the closing operation of all
switches a position is reached where the
dielectric strength between the closing
contacts falls below the voltage across
the contacts
At this point a flashover, termed a
prestrike, will occur. The source and
load side voltages will reach some
intermediate voltage very rapidly and
the voltage across the terminals of the
switch falls to a very low value. This
rapid change of voltage results in the
injection of a steep fronted voltage
wave into both load and source sides.
The magnitude of this wave can be as
high as the crest value of the system
line to neutral voltage.
The high frequency current now flows
as an arc across the closing contact
gap. Both current and voltage waves
flow down the cable to the load where
reflection takes place. The reflected
wave returns to the breaker terminals
where its effect depends on relative
surge impedance magnitudes. The
prestriking arc may then be interrupted
at or near to a current zero. Interruption
depends on the rate of change of
current as it passes through the current
zero.
If interruption does occur the dielectric
strength will recover until once again
the voltage across the contacts
overcomes the dielectric strength of the
decreasing gap. The process may
repeat several times until the contacts
touch.
In practise most motor surge impedance
are within the range of 200 -
8,000 W while most cable surge
impedances fall within the range 20 -
50 W. Thus the voltage appearing at the
motor terminals experiences a
"doubling effect" due to reflection
(usually in the order of 1.8 times the
injected voltage). After reaching a crest
value Vn, the voltage wave will decay
slowly due to travelling waves in the
cable. A discontinuity then occurs on
arc interruption and the wave decays
as a function of circuit RC values. The
voltage across the contact gap will then
increase again and the process may
repeat itself.
The process consists of prestrike
followed by high frequency current flow
and current interruption and is therefore
similar in nature to the restrike
phenomena already described.
However the prestriking process occurs
during circuit-breaker closure and the
dielectric profile of the closing contacts
is decreasing. Thus the magnitude of
prestriking transient wavefronts is
limited to a progressively decreasing
envelope.
In practise the ability of vacuum circuitbreakers
to interrupt high frequency
currents makes them much more
susceptable to mlutiple prestriking than
other types of circuit-breakers.
On prestriking of the first pole a steep
fronted wave of 1.8 p. u can be injected
at the motor terminals as explained
above. This voltage propagates through
the windings and will be seen at the
terminal of the second winding as a
"slow" oscillation of magnitude 1.8 p. u.
At this point in time (with source voltage
of phase "A" at maximum) the source
voltage of phases "B" and "C" will be
0.5 p. u. Thus in the worst case, when
reignition of the second pole occurs at
a time when the motor terminal voltage
of the second pole phase is 1.8 p. u, a
circuit breaker terminal voltage of
2.3 p. u is injected into load and source
sides as a steep fronted wave in a
similar fashion to the first pole. This
wave also undergoes reflection at the
motor terminals resulting in a steep
fronted wave of up to 4.14 p. u
(2.3 x 1.8) at the motor terminals.
In practise, the prestrike phenomena is
very complex and difficult to predict.
The resulting overvoltages depend on
many factors including circuit-breaker
characteristics, dielectric properties,
high frequency current interruption
capability and pole scatter, circuit
characteristics (surge impedances and
natural frequencies) and point of wave
of closing. The inability of the SF6
circuit-breaker to interrupt high
frequency current usually results in a
single prestriking transient.
results with Fluarc CB
A lot of test campaigns has been
performed in different laboratories for
some years, directly with MV motor or
with motor circuit subtitutes.
Recently, according to IEC draft which
is about to be adopted the following
tests have been performed in Volta
laboratory (test report AC 1239
and 1240).
test circuit (fig. 14)
Drawing of the test circuit:
100 A 7.3 kV and 280 A 7.3 kV
Circuit motor parameters
current: 280 A
power factor: < 0.2
oscillation frequency: 27 kHz
current: 100 A
power factor: < 0.2
oscillation frequency: 11.7 kHz
fig. 14
Cable characteristics:
the two extremities of the screen of the
radial field cable are earthed.
lenght: 100 m
voltage: 12/20 kV
current: 295 A
type: Pirelli X 23
insulation: polyethylene
capacitance by metre: 0.22 nF/m
characteristic impedance: 40 W
conclusions
Fluarc CB and Rollarc contactor are
suitable for MV motor switching.
When comparing the motor insulation
and the performances of these
apparatus, a great margin of security
does exist.
More precisely, the main conclusions
are:
n at 6.6 kV, the most popular motor
voltage, the BIL of the circuit is 60 kV
(peak value), the motor insulation
between line and earth is 31 kV (peak
value).
The overvoltage due to the current
chopping is compatible with this motor
insulation for many types of CB. In this
field Fluarc FG2 is a good response in
a large range of rated currents,
voltages and breaking capacities.
Sometimes, taking into account the low
insulation of old motors and/or the
aging of these motors, we recommand
the best solution for the smallest sizes
of motor (below 250 kW): the rotating
arc technique (Fluarc FG1 or contactorfuses
Rollarc).
n on the other hand, the interturn
insulation of the motor has to be saved
thank's to restrike free CB. All the types
of Fluarc CB and Rollarc contactor do
not create multiple reignitions.
n protection devices as Zno surge
arresters (for overvoltage limiting) and
capacitor resistance (for HF currentshunting)
are not necessary with Fluarc
CB and Rollarc contactor.
n a great experience, based on the
servicing time and on the quantity,