Page 671 - Industrial Power Engineering and Applications Handbook

19/636 Industrial Power Engineering and Applications Handbook
Splitter or Arc plasma rising
quenching plates to the arc splitters

- Housing of non
magnetic material
(Asbestos, bakelite,
FRP or DMC etc

Lengthening of
arc
Arc moving
into arc chute

Fixed & moving
arcing contacts

Figure 19.11 Process of arc formation and quenching in an
ACB using splitter plates

axial air blast through the arc plasma, when the contacts
have just begun to separate. (See Figure 19.13(b).) The
compressed air has greater dielectric strength and thermal
properties than ordinary air at atmospheric pressure.
The conventional pressure of air blast is generally 218
to 900 lb/in2 (1.5 to 6.2 MN/m2) up to 110 kV, up to
3000 lb/in2 (20.7 MN/m2 (for voltages up to 400 kV, and
still higher pressures for higher voltages. The pressure
requirement will change from one manufacturer to another,
depending upon contact design, interrupting mechanism
and design and value of resistors. Since most quenching
is through a predetermined force of an air blast, the force
of arc plasma quenching and extinction (deionization)
remains the same for a particular size of breaker, a Top terminal e9 Compression spring
b Moulded-plastic base
f2 Operating shaft
irrespective of the amount of current the interrupting b5 Fixed contact g1 Insulating barrier
device may have to break. This factor inherits a tendency d Arc chute g2 Insulating barrier
to break small currents, even before their natural current dZ Arc runner h4 Bimetal strip
Intermediate shaft
zeros, causing current chopping (Section 19.6). Current d4 Arc splitters h, hs Current transformer
d5 Leaf spring
chopping may raise the TRV up to 2.5-3 p.u. However, el Moving contact n3 Intermediate shaft
it is possible to design these breakers to contain the value e2 Contact carrier n4 Magnet core
of a TRV to a non-striking level. e, Tension spring p Bottom terminal
The dampening of a TRV is achieved through low- e, Flexible connector
and high-resistance units provided across the contacts.
The low unit will short at higher TRVs and the higher unit Figure 19.12(a) Operating mechanism of an LTACB showing
the arc chute with splitter plates (Courtesy: Siemens)
at lower TRVs. The arc interruption is fast and generally
at the first current zero due to dampening. When the
breaker is in the closed position, the resistors are open pressure and volume at all times. This requires an elaborate
circuited. As soon as the main contacts begin to interrupt, arrangement for air compression, an air storage facility,
the contacts of resistor make first, before the main contacts a network of feed pipes, valves and safety devices besides
separate and provide the required dampening. After their regular maintenance and upkeep. All this is costly
extinction of the arc, the resistor circuit opens again particularly when only a few breakers are at a particular
automatically and restores to the original position. installation.
Until a few years ago these breakers had been quite Moreover, compressed air is released through an orifice
common for medium voltages, up to 33 kV. Since they at the exhaust point at a high velocity and causes a sound
require a powerful blast of air at high pressure and velocity like thunder. This may be frightening to people in the
into the arcing region, they require a reliable source of vicinity. Sub-stations involving a number of such breakers
air supply. Air should be clean and dry and at the correct are a nuisance to residents nearby. The larger the breaker