Ricerca sul Sistema Energetico - RSE S.p.A. has been involved in a six-year long project, funded in the framework of the Italian research fund Ricerca di Sistema (RdS), for the design, development, installation and in-field testing of a 9kV/3.4MVA resistive-type Superconducting Fault Current Limiter (SFCL). The result was a three-phase BSCCO-based prototype installed in a MV substation belonging to the Italian utility A2A Reti Elettriche S.p.A. inside the Milan distribution grid. For the first time in Italy (and one of the first worldwide) a SFCL device was in-field tested for more than two years (March 2012 - June 2014) and its limitation capability was proved during a real three-phase short-circuit.
The main outcomes from the experimentation will be summarized in the first part of this paper; conversely, the second part will be dedicated to the results of the activities undertaken with the aim of developing the upgraded 9kV/15.6MVA SFCL device to be installed in the Milan city distribution grid in late 2015. Initially, two single-phase prototypes were developed: YBCO-based (2G) and BSCCO-based. Both prototypes were DC and AC laboratory tested at the temperatures of 77K and 65K and then short-circuit tested under severe fault conditions. Further to this preliminary activity, a single-phase 2G-based SFCL prototype, representing the first phase of the final three-phase prototype, was developed. The layout consists of two parallel-connected coaxially-arranged 2G windings, each made of two layers anti-inductively wound on a fiberglass-cylinder. The amount of 2G tape used for each winding is 96m. The SFCL device was tested in January 2015 at the nominal current of 1kA and was able to properly limit a reduced short-circuit current (6kA_rms) and then a full-current of 11kA_rms corresponding to a peak short-circuit current as high as 26 kA_P. The details of the prototype architecture, along with the tests findings, will be described and discussed, as well as the next research steps.

A superconducting power cable has features of a high power transmission capacity, reduced transmission losses, a compactness, etc., and, therefore, it is expected as an alternative technique for a conventional power cable, especially for power grid applications. We have been studying the feasibility of applying superconducting power cables to DC electric railway feeding systems. Our previous studies indicated that superconducting cables could improve the regeneration rate and system energy saving, and also could reduce the substation capacity. On the other hand, it has been concerned that the introduction of superconducting cables would lead to an increase in short circuit currents. A superconducting fault current limiter (SFCL) would be expected for a stable and reliable operation of the DC electric railway systems with superconducting cables.
Requirements for circuit breakers are strict to protect the superconducting cables and the system. However, as the possibility of breaker operation failure cannot be neglected, it is necessary to develop a method to protect the superconducting power cables when the breaker operation fails.
In this study, electric circuit analysis models of the system were created and the system behaviors were investigated by simulation using MATLAB-Simulink. Currents, voltages, temperatures, etc. of the substations, trains and SFCLs were evaluated under various conditions of the location of SFCLs, the operation parameters of circuit breakers, etc. Some fundamental experiments of an SFCL were also carried out. From these results, the effects of introducing SFCLs to the DC electric railway feeding systems with superconducting power cables are discussed.

Saturated Iron Core Superconducting Fault Current Limiter (SICSFCL) is one of a few types of superconducting fault current limiters that have been installed for real power grid operation. SICSFCLs become more and more attractive due to its good performance in both normal power transmission and fault current limiting as well as the intrinsic advantage in recovery time and cryogenic load. Since SICSFCL is generally new to most engineers and technicians of utilities and equipment manufacturers, it may be worthwhile to summarize the critical issues and newly developed technologies in designing an SICSFCL. This paper is focus on how to design an SICSFCL properly and efficiently. Key factors in designing each major sub system of an SICSFCL, including the superconductor coil, the reactor, the dc magnetization circuit, the control and protection, and the cryogenics, are introduce and discussed.

A resistive superconducting fault current limiter (SFCL) has been developed using round magnesium diboride (MgB₂) wire. The SFCL coil was wound using an interleaved coil arrangement to minimize the total coil inductance. The SFCL coil demonstrated reliable and repeatable current limiting properties during testing. However, the wire temperature of the SFCL coil increases quickly during quench tests and several minutes are required for temperature recovery after the fault is cleared. The SFCL coil therefore was fully integrated with a vacuum interrupter to quickly remove the SFCL coil from the circuit once a fault occurred. This allowed the SFCL coil to recover quickly whilst a bypass resistor acted as the current limiting resistance. A fast-acting actuator and its control circuit were designed and built to provide automatic control for the operation of the vacuum interrupter. The SFCL with the integrated prototype vacuum interrupter was successfully tested. The energy dissipated in the SFCL coil was significantly reduced by integrating the vacuum interrupter. The fault tests with different potential fault currents also proved that the operation of the vacuum interrupter is independent of the fault current level. This prototype demonstrated the potential of a cost-effective and compact integrated SFCL and vacuum interrupter for power system applications.