Unwanted DC currents in our 50 Hz AC network, which for instance are induced by changes in the earth's magnetic field in the overheadlines of the grid, cause problems in substations in our transmission grid. Particularly in the case of transformers, there is a reduction in efficiency, increased operating noise and, in the worst case, destruction of the transformer. If several transformers fail at the same time, there is a risk of a large-scale blackout, which would shut down public life and cause enormous economic damage. In order to mitigate these direct currents or to compensate their negative effects, it is essential to know the precise magnitude and time course of these DC currents.
The project FORESEEN aims at the application of an optical measurement using the Faraday effect to measure these DC currents in high-voltage lines for the first time during normal operation. For this purpose, a measuring system is being developed which can measure DC currents in the range of a few milliamperes on individual phases at operating voltages of the grid from 220 kV to 380 kV, despite superimposition with the 50 Hz AC current. This system will be based on known approaches from the literature, like a Sagnac interferometer for instance, which will be optimized and further developed to meet the special requirements.