Self‐rehealing characteristics of a sodium current limiter and its overcurrent coordination for low‐voltage distribution system

Abstract

AbstractThe sodium current limiter developed and applied for low voltage use is not a current limiting fuse but a new type of reusable current limiting device that utilizes metallic sodium as a fusible element. Excellent current limiting performance and quick self‐rehealing properties of the sodium limiter allowed highly reliable electric power systems to be obtained economically and compactly.Here we examine experimentally the rehealing properties of the sodium limiter after current limiting operation and discuss the overcurrent coordination of the sodium limiter for a low‐voltage distribution system. The results are summarized here: (1) The resistance of the sodium limiter, being between about one thousand and several thousand times its normal resistance r0 at room temperature during a current limiting operation of short circuit fault currents, changes abruptly to 30 to 40 times its ro value just after the disappearance of the fault current and falls steadily to ro. (2) The sodium limiter can recover current‐carrying capability for an overload current even immediately after a current limiting operation and maintain its capability for the following normal load current. The upper limit of the recovery of the sodium limiter is given by its overcurrent against the time characteristics under the normal condition. (3) The self‐rehealing characteristics of the sodium limiter presented here give the ability to design a low‐voltage motor control center using the sodium limiter which maintains maximum service continuity up to high fault currents.