Page 665 - Industrial Power Engineering and Applications Handbook

19/630 Industrial Power Englneenng and Applications Handbook
SF6in SF6 circuit breakers This ionizes into sulphur Pre-inserting a resistor in the interrupter unit to
and fluorine. cause a voltage (TRV) drop across it and to also
In a VCB This is not the vacuum but the metal of the improve the p.f. of the interrupting circuit and
parting contacts that becomes vapourized. making arc extinction easy. The mechanism is made
such that a resistance commensurate with the system
The main problem of circuit breaking arises out of the parameters and switching conditions (which the user
formation of the arc and its prolonged extinction, which has to stipulate for the manufacturer) is inserted
may delay the circuit interruption and lead to a restrike into the switching circuit. Insertion is made through
of the arc plasma after a current zero. The basic concept the interrupting mechanism immediately, say, by
of a circuit breaking thus leads to the quickest extinction half a cycle before the contacts make or open and
of the arc plasma. It has caused many engineers and is shortened or disconnected immediately on closing
scientists to undertake extensive research and development or opening of the contacts.
on the subject over the past 50 years or so to find more
suitable mediums and to evolve better techniques to These techniques have been successfully implemented
extinguish the arc plasma. The present-day high techno- in interrupting devices as noted in Section 19.1.2, being
logy, adopted by the various manufacturers in the field commercially produced by various manufacturers for
of arc quenching, is the result of these long years of different voltage systems and applications.
consistent and continuous research and development work. The dielectric properties of different mediums at
To achieve a quicker extinction of the arc it is imperative different contact gaps are illustrated in Figure 19.1. It
to create one or more of the following conditions: may be observed, that except the medium of vacuum,
which has a near constant or very little rise in dielectric
1 To quench the arc plasma caused during the inter- strength from about a gap of 10 mm and pressure about
ruption, quickly and continuously, to ensure that by Torr or less, all other mediums, even air, have a
the next current zero, the arc path is devoid of any near-linear rise in their dielectric strength with the contact
traces of arcing. In other words, the contact gap must gap.
restore its dielectric strength before the next current The dielectric strength can also be enhanced with the
zero. rise in pressure of the medium, except oil, which cannot
2 To lengthen the arc as shown in Figures 19.1 1 and be compressed, and can be considered as having a near-
19.12. This is an effort to render the restriking voltage constant dielectric properties. The characteristic of air at
(TRV) insufficient to re-establish an arc across the very low pressures is illustrated in Figure 19.2. The
parting contacts after a current zero. The process behaviour of air at very low pressures (below 1 O4 Torr)
increases the resistance of the arc plasma that helps is extensively utilized in vacuum interrupters.
to absorb a part of the TRV by causing a voltage drop
across the resistance so created, besides improving
the p.f. of the interrupting circuit and thus dampening
the restriking voltage (TRV) to far below its peak
value by the next current zero. Dampening of TRV at 250
improved p.f. may be observed from curves LI and h
of Figure 17.1 1.
3 Splitting the arc into a number of series arcs (Figure
19.1 1 ) so that the input power to the arc becomes less
than the heat dissipated during the process of deioniza-
tion. The more efficient the process of cooling, the
better will be the chances of avoiding a restrike and t 2oo
achieving a quicker extinction of the arc. 5'150
4 A forced interruption before a current zero, as may 5
s
occur in an ABCB or VCB, may cause current chopping F
(Section 19.6, Figure 19.27) giving rise to high TRVs, 2
is not desirable. It is therefore important that the design 2 loa
c
of the interrupting device be such that a live circuit 2
interrupts only at a natural current zero, as far as 2
possible. to avoid generation of voltage surges. The
following techniques have been developed to achieve 5c
this:
Use of high pressure at the arc plasma to drive
away the same.
Adopting forced cooling to quench the arc plasma. Confact gap (mm) -
Use of such constructions that can elongate arc 0 5 10 15 20
length and reduce the concentration of ions in the
arc plasma and hence enhance the dielectric strength
between the parting contacts. Figure 19.1 Dielectric strength of different mediums as a
function of contact gap