SUMMARY Optical current transformers (CTs) have the advantages that they can be made much smaller and lighter than conventional electromagnetic induction transformers by their simple structure, and contribute to improvement of equipment reliability because of their excellent surge resistance performance. The authors consider optical CTs to be next‐generation transformers, and are conducting research and development of optical CTs aiming to apply them to measuring and protection in electric power systems. Specifically, we developed an independent optical CT utilizing basic data on optical CTs accumulated for large‐current characteristics, temperature characteristics, vibration resistance characteristics, and so on. In performance verification, type tests complying with IEC standards, such as short‐time current tests, insulation tests, accuracy tests, and so on, showed good results. This report describes the basic principles and configuration of optical CTs. After that, basic characteristics of optical CTs including the conditions and results of verification tests for dielectric breakdown characteristics of sensor fibers, large‐current characteristics, temperature characteristics, and vibration resistance characteristics are described. Finally, a development outline of the independent optical CT intended for application to all‐digital substations and its type tests results are described. © 2013 Wiley Periodicals, Inc. Electron Comm Jpn, 97(1): 28–36, 2014; Published online in Wiley Online Library (http://wileyonlinelibrary.com). DOI 10.1002/ecj.11487

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