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:
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.
Before examining super conducting fault current limiters some characteristics f non-linear material deserve a closer clock.
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.
In the innermost, where values for temperature, field, and current density are low enough, the material is in its true superconducting state and has zero resistance. In a region surrounding that area, resistively rises steeply as values for three variables so higher. Outside that area, receptivity is in essence independent of field and current density as with ordinary conductors.
Until the discovery of high temperature superconductors in 1986, the only material known to super conduct had to be cooled to below 23K (-2500C). The cost of cooling such low temperature superconductor which is mostly metals, alloys and inter-metallic, makes their use in many possible applications commercially impractical. The high temperature superconductors have a critical temperature in the comparatively balmily vicinity of 100 K and can be maintained at that temperature by means of liquid nitrogen (as opposed to helium) cooling. The relative immaturity of HTS materials processing and their complex ceramic structures render it difficult to draw them out into long and flexible conductors.
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