Development of Optical ZCT and its Application to Fault Section Detection for 66 kV Underground Branch Transmission Lines

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 66kV 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 6km, 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.