Optical current and voltage sensors have become attractive alternatives to conventional instrument transformers in high voltage electric power transmission systems. The optical sensors offer important benefits such as small size and weight, enhanced performance, and constitute an important part of the transition to digital substations. The sensors must comply with stringent accuracy and reliability requirements. Commonly, substation applications demand accuracy to within ±0.2% over outdoor temperature ranges. Other aspects are insensitivity to shock and vibration and stray fields as well as life times in excess of 30 years. We review the technology of the sensors and present particular measures that were necessary to achieve the required performance. This includes the exploration of different sensing fiber types, inherent temperature compensation, accelerated life tests, and, in case of voltage sensors, adequate high voltage proof insulation and packaging. We discuss the integration of a current sensor into a circuit breaker and show results from a corresponding field test.
We report the first fabrication of polarization rocking filters in a highly birefringent elliptical microfiber. The rocking filters are made by periodically heating/twisting a microfiber with an ellipticity of ~0.7 and a diameter of ~2.8 μm along its major axis. Strong input polarization suppression of ~20 dB is achieved at a resonant wavelength of ~1556.4 nm with a device length of ~3.12 mm. High-order polarization rocking filter was used to measure the refractive index with sensitivity of 32036 nm/RIU.
A highly accurate reflective interferometric fiber-optic current sensor for alternating and direct currents up to 500 kA is
investigated. The magnetic field of the current introduces a differential phase shift between right and left essentially
circularly polarized light waves in a fiber coil wound around the conductor. Technology adopted from fiber gyroscopes
is used to measure the current-induced phase shift. The sensor achieves accuracy to within ±0.1% over at least two
orders of magnitude of current and for temperatures from -40 to 80°C with inherent temperature compensation by means
of a non-90°-retarder. The paper analyzes the influence of key parameters on the sensor accuracy as well as linearity as a
function of magneto-optic phase shift. Particularly, we consider residual birefringence in the sensing fiber and its effect
on the high-current performance of the sensor as well as optimum parameters for the temperature compensation scheme.
Applications of the sensor are in high-voltage substations and in the electrolytic production of metals such as aluminum.
The nonlinearities in the response of an interferometric fiber-optic current sensor associated with deviations of the light
waves from perfect circular polarization are theoretically and experimentally investigated. The consequences for inherent
temperature compensation of the Faraday effect by means of a non-90°-retarder are investigated for currents up to
several 100 kA and temperatures between -40°C and 80°C. The results are of particular interest to sensors for high direct
currents in the electro-winning industry where measurement accuracy to within ±0.1% is required up to 500 kA.
The stability of a polarimetric Fabry-Perot fiber laser sensor for fluid pressure up to 100 MPa is investigated. The fluid acts on one of two elliptical-core fiber sections in the laser cavity producing a shift in the differential phase of the two orthogonal polarization modes and thus a variation in the beat frequencies of the corresponding longitudinal laser modes. The second fiber section, with a 90-degree offset in the core orientation, compensates for temperature-induced phase shifts. Dispersion in the birefringent fiber Bragg grating reflectors is employed to enhance the resolution of the sensor to a few parts in 106 of the free spectral range. Investigations on sensor stability address the effect of the fluid on the integrity of the fiber, creep caused by various types fiber coatings, as well as the intrinsic stability of erbium-doped and undoped sensing fibers under pressure and temperature changes. It is found that moisture-free silicone oil does not cause any fiber deterioration even in case of a bare fiber. Silicon nitride and gold coatings (1 mm thick) cause significant signal drift, whereas no drift is observed for carbon coatings. Highly doped elliptical-core sensing fiber exhibited drift in the differential optical phase while un-doped fiber did not.