Page 678 - Industrial Power Engineering and Applications Handbook

Circuit interruDters 191643
blayt. The blast also helps the main moving contacts to hence the arcing time are lob. requiring lex\ energy
inow farther away and thus reduce the energy requirement to interrupt (Figure 19.6).
bl the moving mechanism to interrupt the breaker. This 5 As the arcing time is very low. it causes no or only :I
makes the whole process of arc extinction easy and smooth. small amount of contact erosion.
This technique. instead of equalizing the arc heat, reduces 6 It is highly suitable for hazardous locations.
the arc energy itself, facilitating a quicker and smoother
extinction of the arc. The moving tnechanim that in a 19.5.6 Vacuum circuit breakers (VCRs)
normal puffer is usually hydraulically or pneumatically
operated (Figure 19.16(a)) due to the higher energy Refer to the general arrangement ofa loow breaker xhou ti
requirement by the moving mechanism can now be in Figure 19.22 and its housing (Figure 19.23).
achieved through a simple spring mechanistn (Figure The electrical breaking capacity in vacuum has been
l9.l6(b)). long known. But it was not until 1970 that it wii\ used in
Figure 19,16(c) illustrates a comparison of energy the making and breaking of currents at high voltages. It
requirements between the various techniques in practice. has been a widely recognized and accepted breaker. lea\ ing
Future technology on which some manuf. acturera are behind all other techniques of arc breaking and extinction
already working may be in the form of a double volume in its voltage range. In vacuum. a IO mm gap at about
technique. In which an attempt is being made to make l/IOhmm vacuum of mercury is capable ofwithstanding
the I'ixed main and arcing contacts also moving. This a peak voltage up to around 240 kV (Figure 19. I ). These
odd enhance gas compression as well as the separation breakers require no maintenance and are very compact.
01' the arcing (interrupting) contacts, and further reduce Unlike other mediums, the dielectric strength of a
the arc energy requirement, making arc extinction easier. vacuum increases with a gap. but only marginally. which
smoother and quicker. The approximate improvised energy is the limiting factor in producing such breakers beyond
require-inent is shown in Figure 19.16(c). 36 kV. These breakers are therefore used only for niediun-
As SF6 breakers can be designed to provide a very voltage systems (2.4-36 kV). Some inanufacturcn have
smooth interruption of an arc, devoid of current chopping attempted to produce them up to 66 kV but they have not
or ;I restrike of the arc plasma by accurately controlling shown the desired results so far. The application of these
the supply of gas to the required level of cooling. they breakers therefore continues to be up to 36 kV onl!.
ma! also be termed soft break interrupters. SF, breakers A comparison of dielectric strength of high wciiutn
are the most extensively used and are suitable for with the other available mediums is shown in Figure
practically all applications and voltage systems up to 19.1. The very high dielectric strength 01' vactiiini makes
765 kV and more. The other advantage with SF, it possible to quench an arc with a ver) small contact
switchgear is a space saving of up to 70-90'3 over the gap and breakers with very coinpact di mensions can be
conventional type of switchgears. Since these breakers designed.
are totally enclosed and sealed from the atmosphere, Because of the low contact gap. lo\\ arc resistance and
the! are also the most recommended choice for a11 fast clearance. the arc energy dissipated in vacuum for :I
installation\ that are ha,mdous and prone to explosions. particular current is 1/10 that of oil and l/1 that of SF,.
and is illustrated in Figure 19.6.
Vacuum is finally judged to be the best medium to
Pwiiiwrtioii resistoi. quench the arc plasma and interrupt ;I circuit under the
These breakers, when used for switching long transmission most adverse conditions. Figure 19.23 gives cross-\ectional
line\ at 120 kV and above, are pro\,ided with a pre- views of one pole of a vacuum circuit breakcr and a
insertion resistor across each interrupting contact to limit typical construction of the arcing contact\ and Figure
o\ ervoltages that may occur during a closing or opening 19.25 shows its assemblq.
sequence. as a result of heavy charging currents as noted
in Table 24.2. The value of the resistor may be around Advantages
400 R for the line parameters, considered in Table 24.2
and may \ary with line parameters. The resistors are Some advantages of vacuum circuit breakers arc sutii-
connected so that during a closing sequence they short- marized below:
circuit the making contacts before closing the main
contact\ for. say. 8-1 0 milliseconds. and open immediately I They have a lungcr life span. during which they do
after the contacts are made. This also happens during an not deteriorate or lose their dielectric properties.
opening wquence. 2 They require extremely low niainteiiance.
3 At lower currents. say, up to I kA, the inaxitnuin
duration of arc even at low p.f.s. is of the order of just
I,ow gas velocity and pressure minimizes the tendency onc-half to one cycle of the natural frequency of the
towards current chopping. system. as against nearly two
A closed recycling of gas causes no noise or contatnina- The low exciting currents, at low p.f.s. are more
tion of the atmosphere. difficult to interrupt rather than large current\ at high
There is no carbonization and therefore no tracking. p.f.s. due to an extremely adverse wltage-current
(conduction of the insulating medium). phasor disposition. For more clarity rcfer to Section
Because of the extremely good dielectric properties 17.62. The current now is nearly 90" lagging the
of SF, yas. the arc gap and the contact travel and applied voltage and the TRV approaches ;I full applied