Geomagnetically Induced Currents (GIC)
Cause of the low frequency currents
The cause of the low frequency currents (LFC) has already been identified as geomagnetic disturbances (GMD), which are caused by the interaction of the Sun with the Earth's magnetic field (Figure 1). These geomagnetically induced currents are also referred as GIC, which can be calculated by an in house programmed software-tool.
Figure 1: Principle of geomagnetic induction in the Earth
Other causes and sources of the low-frequency currents are the subject of current research.
The low-frequency currents in the high-voltage network cause problems, especially in transformers. The currents cause increased noise, distortion of the voltage and heating of the transformer. This also negatively affects the stability and the transmission grid. In order to investigate the effects of the low-frequency currents in more detail and to better understand them, experiments with transformers are being carried out in the institutes own laboratory as part of the “Nf-Sternpunkt 2” project. For this purpose, two distribution grid transformers, such as those found on power poles in rural areas, were converted. The conversion allows the transformers to be connected in any vector group on the low-voltage side and makes the high-voltage neutral point accessible from the outside.
Figure 2: Laboratory at IEAN with modified power transformers
In the experiments, the transformers are connected to each other, e.g. on the high-voltage side, and a direct current is fed in via the neutral point. In this way, an arrangement comparable to that in the high-voltage grid can be tested in the laboratory.
In the simulation program (Figure 3), GICs and their effects on transformers and transmission systems can be calculated. Magnetic field data, earth conductivities and the use of a plane wave method lead an electric field. With transmission network data, including the coordinates of the substations, voltages and subsequently the currents in transformers, lines and earth electrodes are calculated. Furthermore, constant electric fields can be specified for worst-case analyses.
Figure 3: GIC calculation tool
From these calculations, recommendations for the safe operation of transmission systems can be derived.
Currently, the neutral point currents are measured at 7 transformers in APG's Austrian high-voltage grid (Figure 4).
Figure 4: Austrian transmission grid with measurement locations
The measurement data in 1-second intervals allow the validation of the calculations from the software and the laboratory tests. The measurement system was developed specifically for these measurements at the institute (Figure 5, Figure 6). It allows automated measurement and remote access to the measurement data. All signals with a frequency greater than 0.5 Hz are filtered out of the measurement signal. Thus, only the low-frequency currents are recorded
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Albert, D.; Maletic, D., Renner, H.
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Low Frequency Neutral Point Currents on Transformer in the Austrian power Transmission Network
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Messung von Gleichströmen am Sternpunkt von Drehstromtransformatoren im Höchstspannungsnetz
Master Thesis, Graz University of Technology, Graz, Austria, 2014