Tests of optical fiber current transformer for direct currents

To measure direct currents at railway DC substations, current transformers to utilize the Hall effect is used so far. The output a current transformer of this type is influenced, however, by the magnetic field due to other circuit currents. Therefore, we developed an optical fiber current transformer (optical CT) to utilize the Faraday effect to measure direct currents. The output of this prototype shows good linearity in spite of the presence of the magnetic field due to other circuit currents. Moreover, the optical CT is more lightweight than conventional current transformers.