Page 683 - Industrial Power Engineering and Applications Handbook

19/648 Industrial Power Engineering and Applications Handbook
subject to frequent restrikes, and are vulnerable to damage,
while the actual extinction of an arc will take place only
at a natural current zero, To cope with such situations
the interrupters must be fast operating. Figure 19.27
illustrates the restriking phenomenon of the parting
contacts during current chopping that is assumed to occur
at point ‘a,’, on the current wave. The actual current and
voltage waveforms may differ from assumptions,
depending upon the speed of the breaker, rate of
deionization, current being interrupted, its p.f. and the
instant at which the interruption initiates. In addition,
the surge impedance of the circuit being interrupted. We
assume that the TRV may rise to 2.5 p.u. and is interrupted
by the immediate first current zero (i.e. at point ‘a’ in
Figure 19.27) in about 0.02 part of one half of a cycle of
a 50 Hz wave. During this period, if we consider the
surge frequency of the interrupting circuit to be of the
order of 13 kHz* the arc may restrike for nearly 2.6
cycles or 5 times before a final interruption as determined
below.
Time for 0.02 part of one-half of a cycle of a normal
frequency wave of SO Hz, during which current chopping qsb; T&T -
occurs:
Motor
I,
windings I---- ....... *.-------!
~
1
1
x
= - x - 0.02 second
2 50 = Load current
ir, iv, ib = Charging or restriking currents
:. Number of completed cycles of the TRV at the surge C,, C,, C, = Interphase dielectric leakage lumped capacitances
frequency of 13 kHz
Figure 19.28 One pole opening.
1 1
x
= 13 x lo3 x - x - 0.02 cycles Restriking phenomenon in phase ‘R’ causing charging currents
2 so
in phases ‘Y and ‘13’ which are still closed
= 2.6 cycles (approximately S restrikes)
By the immediate first current zero it is assumed that first and faces a restrike of the arc, leading to surge
the contacts have travelled sufficiently apart to achieve frequency currents, similar (balancing) currents will be
thc rcquircd deionization and have built up adequate induced in the other two phases that are still closed, in
dielectric strength to withstand at least 0.95 V,. If the addition to the normal current, I,, that these poles will
circuit does not interrupt at the immediate current zero still be carrying. The result will be that when these two
at ‘a’, which is so near to the point of chopping ‘a,’, the poles also open the charging currents at a surge frequency
intcrruption will take place only by the next current zero may virtually force a faster or premature current zero in
at point ‘b’ and result in another 260 strikes by then. To phase B, as illustrated in Figure 19.29. This is termed
study more accurate behaviour of an interrupter, with virtual current chopping and may cause an additional TRV.
the number of restrikes and the formation of the actual The amplitude of this TRV, however, may not be large
transient voltage waveforms on current chopping, due to a generally low surge impedance of the interrupting
oscillograms similar to those during a short-circuit test circuit during an interruption. It may achieve a level of
may be obtained (Section 14.3.6). only 0.6-0.7 p.u. (see Telander et al. 1986), which may
sometimes prove fatal for the insulation of the terminal
equipment due to its steepness. Charging currents would
19.7 Virtual current chopping develop in phase Y also (at point ‘y’, but not shown to
avoid overlapping of curves). But current chopping is not
This may occur during the interrupting process of a possible in this phase because pointy’ will fall further away
switching device when not all the three poles will interrupt from a current zero, on the one hand, and the Y-phase would
simultaneously. It is corollary to a closing phenomenon carry a near-maximum current at this instant, on the other.
(Section 17.7.2(ii)) when not all the three poles make at Current chopping is a phenomenon of small currents.
the same instant. This will also endanger the insulation
of the other two phases. The interphase dielectric leakage
capacitances, as illustrated in Figure 19.28, are the cause. 19.8 Containing the severity of
When one of the poles, say of phase R, starts opening switching surges
*It is seen that the surge frequency during current chopping may 19.8.1 Theory of energy balancing
rarely exceed 20 kHz and which, in the context of switching surges,
may be considered as low-frequency oscillations, easy to handle From the above we can deduce that at the instant of
and interrupt. current chopping the arc extinguishes for a moment and