To: Bilow who wrote (2321 ) 5/19/2001 5:27:14 AM From: frankw1900 Read Replies (1) | Respond to From May 1998 IEEE Spectrum. article on Super Conducting Fault Current Limiter (SCFCL). [Power grid stuff]"Superconductors, because of their sharp transition from zero resistance at normal currents to finite resistance at higher current densities, are tailor-made for use in FCLs [Fault Current Limiters]. "Super conductors lose their electrical resistance below certain critical values of temperature, magnetic field, and current density. A simplified phase diagram of a superconductor defines three regions [Fig.1}. In the innermost, where the 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, resistivity rises steeply as values for the three variables go higher. Outside that area, resistivity is in essence independent of field and current density, as with ordinary conductors." "All the same, the superconductor's impedance is truly zero only for DC currents. The more common ac applications are affected by two factors. First, the finite length of the conductor produces a finite reactance, which, however, can be kept low by a special conductor architecture. Second, a superconductor is not loss free in ac operation; the magnetic ac field generated by the current produces so called ac losses -- basically just eddy current losses. .... They barely contribute to the total SCFCL impedance but dissipate energy in the superconductor thus raising cooling costs." "It was earlier assumed that while a fault current is being limited, the voltage drop is uniform throughout the conductor. But in practice superconductors tend to develop thermal instabilities, called hot spots, connected with the strong current and temperature dependence of their resistivity in the transitional state. If a part of the superconductor sees a greater voltage drop than the rest, as a result of an inhomogeneity, this part will heat up even faster, leading to an even greater voltage drop at that point and further accelerated heating. Burn-through can result."
