Page 684 - Industrial Power Engineering and Applications Handbook

Circuit interrupters 19/649
Virtual current chopping in phase 'B' re-establishes as soon as the TRV reappears. At the instant
of arc extinction, it may be considered that the arc energy
is transferred to the dielectric medium of the parting
contacts. This energy reappears in the form of a TRV
across them, and tends to cause a restrike of the arc
plasma once again. We can generally consider the arc
energy as the magnetic energy caused by the magnetizing
current of the interrupting circuit and the energy received
by the dielectric medium as the capacitive energy.
If Ine is the inductive current in amperes (no-load current
of the motor or the transformer, Figure 1.15) and L the
inductance of the circuit being interrupted in henry, then
the electromagnetic energy, J, initially contained by the
arc plasma
ot - J = 4 L . Zzl Joules
.
Note
In fact, this energy should be less by the hysteresis loss (Section
1.6.2A.(iv)) which has been ignored in the present analysis. If V, is
the prospective peak surge voltage (TRV) in volts and C the dielectric
Location, r - Interruption commences capacitance of the contact gap in farad at the instant of restrike,
r' - Restrike of arc occurs in phase 'R' causing a charging then the capacitive energy J, received across the contact gap is
current 'i:
y- Charging current 'i; develops in phase 'Y (Not shown J = 3. C V: Joules
to avoid overlapping)
b - Charging current '6' develops in phase '6' For successful interruption of the arc plasma it is
b' - Charging current ib causes a virtual forced current zero essential that the energy emitted by the arc plasma is at
or virtual current chopping at this point
least equal to the capacitive energy received by the
dielectric medium. At the instant of arc extinction, this
phenomenon is termed energy balancing, when
Figure 19.29 An approximate illustration of a virtual current
chopping in Phase 'B as a consequence of restrike of arc in
Phase 'R

Arc during
T interruption -----,
or a restrike {' +$y "i
Interruptor
Interrupting + I Contact gap having a
\
device dielectric capacitance
'C which is subject to
a TRV of V, during a
restrike
No-load magnetizing circuit of an
Transformer say,
11 kVB.6 kV
cable
Interruptor
C

@Motor 6.6 kW
Figure 19.30(a) Figure 19.30(b) An energy-balancing phenomenon while
interrupting a highly inductive circuit as shown in (a)