Surge current protection using superconductors


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