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Surge current protection using superconductors |
INTROUCTION
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. According
to the accumulated intelligence of many utility experts, an ideal fault current
limit would:
(i) Have zero impedance throughout normal operation
(ii) Provide
sufficiently large impedance under fault conditions
(iii) Provide rapid detection
and initiation of limiting action within less than one cycle or 16ms. (iv)
Provide immediate (half cycle or 8ms) recovery of normal operation after clearing
of a fault.
(v) Be capable of addressing tow faults within a period of 15 seconds.
Ideal
limiters would also have to be compact, light weight inexpensive, fully automatic,
and highly reliable besides having long life. In
the past, the customary means of limiting fault current have included artificially
raising impedance in the system with air-coil rectors or with high stray impedance
of transformers and generators or splitting power-grids artificially to lower
the number of power sources that could feed a fault current. Nut such measures
are inconsistent with today's demand for higher power quality, which implies increased
voltage stiffness and strongly interconnected grids with low impedance. What
is need is a device that normally would hardly affect a power system bit during
a fault would hold surge current close to nominal value that is a fault current
limiter. Until recently most fault current limiter concepts depend on mechanical
means, on the detuning of L_C resonance circuit or use of strongly non-linear
materials other than High Temperature super conditions (HTS). None is without
drawbacks.
Super
conductors because of their sharp transition from zero resistance at normal currents
to finite resistance at higher current densities are tailor made for use in fault
current limiters. Equipped with proper power controlled electronics, a super conducting
limiter can rapidly detect a surge and taken and can also immediately recover
to normal operation after a fault is cleared. Superconductors
lose their electrical resistance below certain critical values of temperature,
magnetic field and current density. A simplified phase diagram of a super conductor
defines three regions
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