Experimental study on characteristics of superconducting fault current limiters connected in series

Superconducting fault current limiters (SCFCLs) can reduce fault current; thus SCFCLs allow the capacity of circuit breakers to be reduced and lend flexibility to power system network designs. Furthermore, SCFCLs are expected to improve power system stability, as has been confirmed by some experiments. Therefore, various kinds of SCFCLs were proposed and have been studied. For introducing SCFCLs in power systems, there should be some specifications, such as the impedance, the trigger current level, and the recovery time. Fault analyses pointed out that accuracy of the trigger current level is required. A transformer type SCFCL with an adjustable trigger current level was proposed, designed, made, and tested. Then, limiting tests on three-phase SCFCLs by use of an artificial transmission line with a small generator were carried out. The test results indicated that at phase-to-phase faults, the SCFCLs for the fault phases do not always turn into the limiting mode. From two points of view, it is important to study characteristics of the series connection for SCFCLs: one is as mentioned above, and the other is that a series connection of some SCFCLs of small impedance may be installed in place of an SCFCL of larger impedance. This paper describes the results of limiting tests on two SCFCLs connected in series. The tests were carried out under some different conditions of fault currents, fault phases, and trigger current levels of SCFCLs. Some discussion on characteristics of series connection of SCFCLs appears also.