The recent
growth of power circuit capacities has caused fault currents to increase. Since
the protection of power systems from the fault currents is very important, it
is needed to develop a fault current limiter. A fault current limiter is
required to assure (1) rapid reaction to fault currents, (2) how impedance in
normal operation and (3) large impedance during fault conditions. A super
conducting fault current limiter (SCFCL) can meet these requirements
superconductors, because of their sharp transition from zero resistance at
normal current to finite resistance at higher current densities, are
tailor-made for use in FCLs.
Super conductors
are of two types-high temperature superconductors (HITS) and low temperature
superconductor (LTS). The HTS are substances that lose all resistance below
temperature main tamable by liquid nitrogen. LTS are substances that lose all
receptivity close to 4k, a temperature attainable only using by using liquid
helium. Cost of cooling LTS (which are mostly metals, alloys and
intermettalics) makes their use in many applications commercially impractical.
HTS material available are all made of bismuth (BSCCO) or yttrium-cup rate
(YBCO). So far, various types of SCFLS have been developed (resistance, shield
core type, hybrid etc.). The SCFCL offers efficient advantages to power system
and opens up a major application for superconducting materials.
Damage from a
short circuit is a
constant threat to any electric power system. Insulation damaged by aging an
accident or lightning strike can unloose immense fault currents practically the
only limit on their size being the impedance of the system between their
location and power sources. At their worst, faults can exceed the largest
current expected under normal load – the nominal current by a factor of 100
producing mechanical and thermal stresses in proportion to the square of the
current’s value.
All power system
components must be designed to withstand short circuit stresses for certain
period determined by time needed for circuit breakers to activate (20-300 ms).
The higher the fault currents anticipated the higher will be the equipment and
also the maintenance cost. So there obviously is a big demand for devices that
under normal operating conditions have negligible influence on power system but
in case of fault will limit the prospective fault current. A device of this
kind is called fault current limiter.
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