MgB₂-based superconductors for fault current limiters

Abstract

Abstract. A promising solution of the fault current problem in power systems is the application of fast-operating nonlinear superconducting fault current limiters (SFCLs) with the capability of rapidly increasing their impedance, and thus limiting high fault currents. We report the results of experiments with models of inductive (transformer type) SFCLs based on the ring-shaped bulk MgB 2 prepared under high quasihydrostatic pressure (2 GPa) and by hot pressing technique (30 MPa). It was shown that the SFCLs meet the main requirements to fault current limiters: they possess low impedance in the nominal regime of the protected circuit and can fast increase their impedance limiting both the transient and the steady-state fault currents. The study of quenching currents of MgB 2 rings (SFCL activation current) and AC losses in the rings shows that the quenching current density and critical current density determined from AC losses can be 10-20 times less than the critical current determined from the magnetization experiments. IntroductionOne of the promising solutions of the fault current problem in power systems is the application of fast-operating nonlinear current limiters (FCLs) with the capability of rapidly increasing their impedance, and thus limiting high fault currents. Superconducting fault current limiters (SFCLs) based on two properties of superconducting materials: an ideal conductivity and a quick phase transition to the normal conducting state under the influence of the current, magnetic field and temperature. An SFCL is one of the most attractive applications of superconductors in power systems, which has no classical equivalent.The utility requirements to an FCL have been discussed in many publications [1-4, and see references therein]. The main of them following are: -limitation of the first peak of a fault current (transient current); -limitation of the steady-state fault current; -low impedance under nominal operation conditions of the protected circuit: the voltage drop across an FCL should be usually less than 5% of the rated circuit voltage; -low power loss under the nominal conditions: the total losses in an FCL must be much less than the transformer loss: in the range of 0.01%-0.1% of the nominal power of the protected circuit. In a

Bulk MgB2 Superconducting Materials: Technology, Properties, and Applications

“…The superconducting characteristics of MgB 2 materials depend on many factors and their combination. Very deep and comprehensive studies of the synthesis process of MgB 2 -based materials, the correlation between material structure and superconducting characteristics, and the manufacturing technology have been performed by the authors of [ 7 , 8 , 9 , 10 , 11 , 16 , 19 , 73 , 76 , 82 , 84 , 85 , 98 , 99 , 101 , 102 , 103 , 104 , 105 , 106 , 107 , 108 , 109 , 110 , 111 , 112 , 113 , 114 , 115 , 116 , 117 , 118 , 119 , 120 , 121 , 122 , 123 , 124 , 125 , 126 , 127 , 128 , 129 , 130 ]. These correlations were comprehensively studied for materials prepared using initial powders of MgB 2 and stoichiometric Mg:2B mixtures (typical characteristics are given in Table 1 ) at manufacturing temperatures in the range of 600–1100 °C under different pressure conditions using the methods noted above.…”

“…Figure 6 presents one of the important characteristics of superconductors, which determines the field of their application, the upper critical magnetic field, B c2 . Figure 6 shows the temperature dependences of the highest upper critical magnetic fields for the HP, SPS, and HotP materials [ 120 , 132 ].…”

“…Two properties of superconducting materials are the bases of SFCLs: an ideal conductivity in the superconducting state and a fast phase transition from this state into the normal conducting state with an increase in the current, magnetic field, or temperature above their critical values. SFCLs are one of the most attractive applications of superconductors in power systems, and there have been no classical equivalents up to now [ 120 , 136 , 145 , 146 ]. These devices meet all the power system requirements; this has been confirmed experimentally by testing models, prototypes, and experimental power devices of various types of SFCLs, based on different superconductors.…”

“…These deviations are associated with the transition of the ring from the superconducting to the resistivity state, and with the quenching (critical) current of the ring. A set of FCL models with MgB 2 rings prepared using various techniques and initial materials and additions has been built and successfully tested [ 91 , 120 ].…”

“…The sizes and synthesis conditions of the rings that have been tested as elements of an inductive SFCL are presented in Table 9 . Note, that the experimental set-up for SFCL model testing ( Figure 27 a) can be used for measuring a “transport” critical current, AC losses, and voltage–current characteristics [ 120 , 151 ]. The “transport” critical current of the various rings was estimated as a quenching current, causing the transition.…”