A research team at TU Graz has developed a fibre-optic measurement system that detects unwanted direct currents in the high-voltage grid in real time. This helps to prevent damage to transformers at an early stage.
The increasingly complex network of electricity generators and consumers poses major challenges for Austria’s energy transmission infrastructure. For example, the use of increasingly powerful inverters and rectifiers in the transmission grid results in unwanted currents with a very low frequency; these are known as quasi direct currents and cause problems in our power grid. The origins of these currents can include not only the connection of renewable energy sources but also wall boxes for electric vehicles or geomagnetic storms. The fault currents caused by this can significantly reduce the service life of transformers, for example in substations; in the worst-case scenario, this can lead to grid instability or even a blackout. When monitoring these currents, commercial inductive current transformers reach their limits in terms of insulation, bandwidth and saturation.
As a possible solution, a team from the Institute of Electrical Measurement and Sensor Systems at Graz University of Technology (TU Graz) (project leader Reinhard Klambauer and doctoral students Johannes Mandl, Philipp Trampitsch and Lukas Liedl), in collaboration with the company ARTEMES GmbH, has developed a fibre-optic system. This enables the direct current component to be measured precisely at the high-voltage level of 220 to 380 kilovolts. Inductive transformers used to date merely measure the total current at the earthed neutral point, where the current phases converge. The new system, on the other hand, detects unwanted direct current simultaneously with the normal operating current at various voltage levels, offers a very high frequency bandwidth and also makes high-frequency interference visible. Thanks to its compact design and insulation, it can be easily retrofitted and installed in various locations within existing systems.
Transformers will not be available for years
“The problem with direct current in an alternating current network is that it causes the iron cores of the transformers to reach magnetic saturation,” says Werner Schöffer, CEO of ARTEMES GmbH. “This can go so far as to shorten the service life of transformers or cause them to fail. Although it is possible to compensate for the failure of a transformer at a substation, it takes several years to obtain a replacement at present. Furthermore, in the case of geomagnetic storms, the phenomenon does not occur on a regional basis. That is why it is important to be able to monitor such disruptions in real time and take targeted action.”
To obtain the desired measurement results, the project team utilises the Faraday effect. This describes the behaviour of polarised light in a transparent medium while it is exposed to a magnetic field, causing the plane of polarisation of the light to rotate. As the magnetic fields generated by power lines depend directly on the current flowing through them, it is possible to measure this current, including any disturbances. To do this, the researchers use a 3D-printed coil former to wind a glass fibre around the conductor and pass light through it. The optical system and the signal processing electronics required for this were developed in-house by the team. In a Field-Programmable Gate Array (FPGA), the recorded optical signals are processed in real time using proprietary algorithms. The current value is derived from this. Security-critical communication and data export are handled by a miniature computer (DataHub) supplied by the project partner ARTEMES GmbH.
Geomagnetic storm successfully detected
In field trials supported by Austrian Power Grid AG (APG), the research team has already successfully put the system through long-term testing. For example, by measuring direct current, it was possible to detect a geomagnetic storm in real time on 1 January 2025. In further tests, two variants of the measurement method were tried out. The AC version is optimised for a wide frequency bandwidth so that it is capable of detecting even transient current peaks or drops in the microsecond range – such as those caused by inverters or switchgear. The DC variant focuses on the precise measurement of low-frequency currents, in particular geomagnetically induced direct currents. The next steps will involve further optimising both processes with a view to bringing them to market.
“Once the system is fully operational, it will provide a secure backup for the electricity supply,” says Reinhard Klambauer. “The fibre-optic system enables high-frequency, isolated and saturation-free measurements to be taken. It therefore offers added value in terms of safety for energy-related installations, both during normal operation and in specialist applications.”