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Pole-mounted transformers are especially vulnerable to lightning damage. The progress of the information society imposes increasingly stringent requirements for the reliability of electric power supply, and this in turn necessitates a reduction in lightning damage to pole-mounted transformers. Lightning protective devices (surge arresters) are now being installed around the primary bushing of the transformers, which has decreased the number of disconnections around the primary bushing due to lightning. But surge arresters installed on the primary side of the transformer cannot protect it against backflow lightning entering the secondary side of the transformer. The characteristic of transformer damage by backflow lightning is that the electromagnetic force produced by the current flowing into the secondary side deforms the transformer windings. This paper elucidates the mechanism of transformer damage by lightning flowing into the secondary side by comparing actual lightning damage cases with the results of verification tests using a short-circuit generator. Effective countermeasures against transformer damage by backflow lightning are examined by EMTP calculations, which indicate that neutral grounding on the low-voltage distribution line is the most effective way of decreasing the current flowing into the transformer. The lower the grounding resistance, the less current flows into the transformer. In addition, decreasing the voltage on the secondary side is important in order to protect the secondary-side bushing. The calculation results indicate that surge arresters installed around the secondary side of the transformer are effective in decreasing the voltage on the secondary side. © 2010 Wiley Periodicals, Inc. Electr Eng Jpn, 171(2): 1–11, 2010; Published online in Wiley InterScience (www.interscience. wiley.com). DOI 10.1002/eej.20921

Superconducting fault current limiter (SCFCL) is expected to be a first application of high-TC superconductors to electric power systems. We have been developing a magnetic shielding type of SCFCL. In this paper, we propose a design method for the magnetic shield SCFCL. The method is based on a newly developed computer program which can evaluate the dynamic thermal and electromagnetic behaviors of high-TC superconducting cylinder in fault current limiter operation. As an example, the trial design of a model system, which is 6.6 kV (rms) in rated voltage, 400 A(rms) in rated current, and 12.5 kA(rms) in maximum prospective short-circuit current, is shown. © 2003 Wiley Periodicals, Inc. Electr Eng Jpn, 145(2): 50–57, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.10197

A novel three-phase conducting fault current limiting reactor (SCFCLR) is fabricated. The SCFCLR has three superconducting windings with the same number of turns wound on an iron core. The rating of SCFCLR is 200 V, 10 A. Two SCFCLRs are inserted in the sending and the receiving ends of the model power-transmission line. The line fault test of a model power system with two SCFCLRs is undertaken. The fundamental behavior of this reactor is confirmed. In the case of single-line-to-ground fault, the fault current is limited to a very small value by the large zero-phase sequence reactance of the SCFCLR without quench. In the case of a three-phase short circuit, the SCFCLR quenches, and the short-circuit current is limited by the normal conducting resitance of the winding. It is confirmed that the transient stability of the system during line faults is greatly improved by the insertion of SCFCLRs.