Page 86 - Offshore Electrical Engineering Manual

Types of Interrupter 73

into the arc-chute. The arc-chute is a series of parallel insulators designed to
extinguish the arc by splitting and cooling it. Because of the need for blowout
coils, performance will be partially related to the fault current magnitude. This
effect will appear as a ‘critical current’ below which the arc is not drawn into the
chute and arc contact wear will be accelerated. Thus the risk that fault clearance
will not be achieved will be increased. This type of switchgear also tends to be
bulky, mechanically complex and hence more costly compared with other forms
now available. However, this equipment is well proven and its use well estab-
lished offshore, since it has the ability to interrupt the high magnitudes of fault
current found in offshore systems.

BULK OIL CIRCUIT BREAKERS
The bulk oil circuit breaker is a device in which the moving contacts are totally
immersed in a container of mineral oil. As oil is a better insulant and has a higher
specific heat than air, contact gaps may be reduced and the better heat dissipation
means that the overall cubicle size may be reduced compared with those for an
ACB of the same rating. Arc extinguishing is assisted by convection currents in the
oil, produced by the heating effects of the arc. No weight saving is likely, however,
because of the weight of oil required. Arcing within the oil causes hydrogen gassing,
which must be vented from the container, and although with modern designs the risk
is minimal, open flames or even explosions may occur under severe fault clearing
duty or normal fault clearing with badly contaminated oil. The insulating oil can
deteriorate over a number of operations or after a long period of time, and requires
regular sampling and testing. Sufficient replacement oil must be stocked offshore or
at least made available offshore prior to testing.
Although this form of circuit breaker is still in common use onshore and has been
installed for main switchboards offshore, it is now generally considered undesirable
in an offshore environment and its use may be questioned by a number of certifying
authorities and underwriters.

LIMITED OIL VOLUME CIRCUIT BREAKERS
If oil is injected at high velocity between the contacts as they open, an efficient
means of arc extinguishing may be obtained. The amount of oil required is much less
than with a bulk oil circuit breaker, and therefore the fire risk is reduced. However,
as with the air break and bulk oil, maintenance requirements are usually heavy com-
pared with vacuum and SF6.

VACUUM CIRCUIT BREAKERS AND CONTACTORS
At the heart of the vacuum circuit breaker is a device called the vacuum interrupter,
one of which is required for each pole. The interrupter (see Fig. 2.5.5) consists of a
ceramic tube with metal seals at both ends. The fixed contact is mounted on one metal