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Prototypes Based on High Temperature Superconductors Luciano Martini*, Marco Bocchi*, Rossella Dalessandro*, Valerio Rossi*, Israel Arcos* CIGRE’ 2006 Session 41 Technical Exhibition Exposition Technique Parigi, 27 Agosto 1 Settembre 2006 * CESI RICERCA In transmission and distribution electric networks there is the need to provide effective and reliable protection to cables, transformers, switches, and other equipment against over current faults. The ideal protection device presents zero impedance at nominal current and high impedance to limit the current in the system, during the fault. This device has been already conceived and it is known as Superconducting Fault Current Limiter (SFCL). The SFCL is a new type of system component that will efficiently address the problems related to the increasing level of short-circuit currents in MV and HV networks. In particular, the use of SFCL devices could allow utilities to avoid or delay the upgrading of existing circuit breakers to handle increasingly higher electrical surges with a consequent cost reduction. Moreover, reduction of fault currents will also limit voltage variations thus improving the power quality. Among all possible SFCL design, the resistive-type SFCL is in principle the preferred choice because of its simplicity and since it is inserted directly in series with the power circuit to be protected. Utilities will benefit tremendously from this application since it provides increased safety, reliability, and flexibility of transmission and distribution systems. Within CIGRE, a number of working groups have identify the benefits that equipment like FCL can bring to the operation of T&D networks and at present the WG A3.16 activity focuses on the impact and interaction of FCL technologies on existing and new protection schemes in power systems. This paper reports on the development work performed in the frame of the LIMSAT project, which was aimed at studying, designing, manufacturing and testing a 200kVA-class 3-phase SFCL demonstrator using commercially available High Temperature Superconductors (HTS). In particular, long-length silver-sheathed Bi-based (BSCCO-2223/Ag with critical temperature Tc=110K), Ni-alloy sheathed MgB2 (with Tc=40K) tapes and short YBCO coated conductors (with Tc=92K) have been purchased and fully characterized. BSCCO-2223 and MgB2 tape conductors have been then utilized in an intermediate step of the R&D project, to manufacture single-phase resistive-type SFCL that have been tested in liquid nitrogen (64K<T<77K) and liquid neon (T=27K) respectively, to select the appropriate design and HTS material for the final 3-phase demonstrators. The electrical and thermal properties of composite HTS tapes as function of temperature and current values have been measured experimentally and then implemented in a self-developed mathematical model together with the physical properties of cryogens in a wide pressure and temperature range. This model is thus able to describe accurately the evolution of temperature and resistance of HTS windings and hence the limiting performance of SFCL prototypes in nominal conditions (I<Ic), and also in the limitation phase for at high current (I>>Ic). The comparison between the simulation work and short-circuit testing on single-phase SFCL units is reported in this paper. Actual short-circuit testing performed up to 2.2kV on single-phase units and on a 1.2MVA 3-phase SFCL prototypes confirm the simulation results and SFCL prototypes ability to effectively limit, repeatedly and in a few milliseconds, fault current values to much lower values with no degradation. In fact, in the presence of the SFCL, prospective peak short circuit currents of 15kA have been effectively reduced to 1000-3000A depending of applied voltage. Critical aspects associated to the fault event such as the HTS tape temperature rise, recovery time, and possible degradation of HTS material have been evaluated. Moreover, the time evolution of limited current and voltage across SFCL prototypes are reported, deeply analyzed and compared with simulation results. KEYWORDS Superconductivity – High Temperature Superconductor – Short-Circuit – Fault Current Limiter – Simulation – Testing
31 Dicembre 2006
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