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Optical ZCT (Zero-phase Current Transformer) which combines a optical magnetic field sensor with a magnetic ring core of silicon steel plates has been developed. Bi-doped YIG (Yttrium Iron Garnet, (BiYbGd)3Fe5O12) having a large verdet constant is used as a Faraday element. Optical ZCT detects the conductor current of 0 to 2,000 A at high resolution and has been applied to the fault section detection system for 66 kV underground branch transmission lines which monitors a multi-terminal line having Y-branch joint box in on-line state. Optical ZCTs and a section detector/ indicator are connected with optical fibers which are capable of non-repeatedly transmitting signal over 6 km, allow to monitor a long-distance underground power transmission lines. Developed system is now operating on JR Kasai line (Shinkawa pump line) of Tokyo Electric Power Company.

The optical ZCT (Zero‐phase Current Transformer) which combines an optical magnetic field sensor with a magnetic ring core of silicon steel plates has been developed. Bi‐doped YIG [Yttrium Iron Garnet, (BiYbGd)3Fe5o12] having a large Verdet constant is used as a Faraday element. The optical ZCT detects the conductor current of 0 to 2000 A at high resolution and has been applied to the fault section detection system for 66‐kV underground branch transmission lines which monitors a multiterminal line having Y‐branch joint box in on‐line state. Optical ZCTs and a section detector/indicator are connected with optical fibers which are capable of nonrepeatedly transmitting signals over 6 km, and allow monitoring long‐distance underground power transmission lines. A developed system is now operating on the JR Kasai line (Shinkawa pump line) of Tokyo Electric Power Company.