Development of a fiber optic current sensor for low DC measurements in the power grid

Preserving the stable operation and proper functionality of the electric power grid is of utmost importance. Integral grid components such as power transformers are negatively affected by occurring disturbances. In particular, the time course and distribution of the different phases of direct currents (dcs) within the transmission grid, among them geomagnetically induced currents (GICs), is of great interest. In this article, we report on the development of a fiber optic current sensor (FOCS) exploiting the Faraday effect intended to observe these dcs on all three phases of a high-voltage (HV) power grid. The proposed low-cost sensor is constructed with an adjustable phase retarder and a single-mode (SM) sensing fiber. A linear current relationship for dcs down to 1 A was observed. From the sensor signal, the presence of a dc bias superimposed on an alternating current can be identified. Simulations based on a Jones formalism model provided application limits of the SM sensing fiber. Furthermore, we developed a calibration procedure to counteract temperature-induced variations of the sensor scale factor within the fiber-optic coil and achieve a maximum current error of 0.5% down to a rated current of 5 A in a range from 15 °C to 65 °C.