Long-Term Reliability of Fiber-Optic Current Sensors

We present studies on the long-term reliability of interferometric fiber-optic current sensors (FOCS) for use in electric power transmission systems. Accelerated ageing tests are performed on crucial optical sensor components and a three-phase sensor system is subjected to an extended field trial. The sensor components under test include the sensor's superluminescent light emitting diode light source, the integrated-optic phase modulator and various passive components such as fiber couplers, fiber polarizers, polarization-maintaining fiber connectors, and fiber coatings. The components are exposed to accelerated ageing conditions for extended periods of time, i.e., temperature cycling (between -25 °C and 65 °C for up to 15000 cycles), constant high temperature at dry conditions (up to 115 °C for up to 20000 h), and damp heat (85 % relative humidity at 85 °C for up to 7900 h). Crucial component parameters such as the source wavelength and polarization extinction ratios are repeatedly measured as a function of temperature at defined intervals during the ageing periods and examined for potential drift of component failures. The field trial is carried out for a three-phase FOCS system integrated into 420 kV double-chamber circuit breakers over a period of more than three years. The sensor signals are compared to the signals of conventional current transformers. In addition, the evolution of various operational parameters such as the light source power is continuously recorded. The results prove a high degree of reliability of modern FOCS systems.