Low Frequency Neutral Point Currents 2

Influence of low frequency currents on power transformers and the transmission grid

Transformer neutral point currents with frequencies close to 0 Hz, i. e. quasi DC, are caused e. g. by earth magnetic field changes (geomagnetically induced currents, GIC) or by DC systems. These low frequency currents (LFC) are undesirable in the high-voltage transmission grids. These currents can cause transformer and power grid outages. The aims of this research projects are:

  • Investigation of low frequency current bias on power transformers
  • Mitigation techniques of low frequency currents in the power grid
  • Low frequency current calculation in the power grid
  • Measurement of low frequency currents in the power grid

Figure 1: Project organization and fields of research

Power Transformers
The low frequency currents in the high-voltage grid cause problems, especially in transformers. The currents cause increased transformer noise, distortion of the voltage and heating of the transformer. This also negatively affects the stability and the power grid. In order to investigate the effects of the low frequency currents in more detail and to improve the understanding of the physical background, experiments with transformers are being carried out in the IEAN laboratory as part of the Nf-Sternpunkt 2 project. For this purpose, two distribution grid transformers, such as those found in rural areas, were modified. The modification 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

Power Grid Calculations
The developed simulation tool (Figure 3) calculates the GIC’s in the power grid with an electric field and the power grid configuration. The electric field can either be calculated from magnetic field measurement or can be entered directly. For the electric field calculation from magnetic field measurements, the plane-wave method with a 1D earth conductivity model is used.

Figure 3: LFC Simulation-Tool

The simulation tool is used to study the effect of different switching conditions, ground conductivities and magnetic field directions on the current distribution in the grid. The calculated currents in the transmission lines and power transformers can are plotted in a geographically map and as time series.

Transformer Neutral Point Current Measurement System
The transformer neutral point current measurement system is a stand-alone system (Figure 5), which can be accessed from remote. As current sensor a zero-flux closed loop current transducer is used. The neutral point current is sampled with 1 Hz and filtered with an active low-pass filter with a cut-off frequency of 0.5 Hz. The data is recorded with single board computer. With the current electronic configuration( Figure 4) transformer neutral point current with an amplitude up to 25 A and both polarities can be measured. In addition, three further voltage measurement channels are available, which are also recorded with a sampling rate of 1 Hz.

Figure 4: Measurement electronic

Figure 5: Neutral point current measurement with heating System

Figure 6: Installed measurement system at 220kV transformer neutral point

In the substation, the transformer neutral point switch is bypassed with a mobile earthing equipment. The cable is than feed through the current transducer, as depicted in Figure 6.

Time Lap Video IEAN-Transformer Modification
Transformer Sound with and without superimposed DC

Albert, D.; Maletic, D.; Renner, H.
Measurement Based Transformer Modelling Approach
VDE ETG Fachtagung 2021, Wuppertal, Germany, 18.05.2021


Albert, D.; Achleitner G.
Sternpunktsgleichströme im Hoch- und Höchstspannungsnetz - aktueller Stand der Forschung
In: Sternpunktbehandlung in Netzen bis 110 kV (D-A-CH), Online-Veranstaltung, 22.09.2020, Präsentation

Albert, D.; Schachinger, P.; Renner, H.; Hamberger, P.; Klammer, F.; Achleitner, G.
Field experience of small quasi DC bias on power transformers A first classification of low-frequency current pattern and identification of sources

In: Cigre 2020 Session, 2020, Accepted Conference Paper

Schachinger, P.; Albert, D.; Renner, H; Bailey, R. L.; Achleitner, G.; Leber, G.
Niederfrequente Sternpunktströme im Ubertragungsnetz - Ein Überblick über aktuelle und zukünftige Forschung in Oesterreich
In 16. Symposium Energieinnovation (EnInnov), Graz, Austria, 2020, Conference Paper & Presentation

Bailey, R. L.; Möstl, C.; Amerstorfer, U. V.; Amerstorfer, T.; Weiss, A. J.; Hinterreiter, J.; Reiss, M. A.; Albert, D.
PREDSTORM and SOLARWIND2GIC: Forecasting of Space Weather Effects and GIC with Python

Machine Learning in Heliophysics, Amsterdam, Netherlands, 2019, Conference Poster

Albert, D.; Halbedl, T.; Renner, H. Bailey R. L.; Achleitner, G.
Geomagnetically induced currents and space weather - A review of current and future research in Austria
In: 54th International Universities Power (UPEC), Bucharest, Romania, 2019, Conference Paper

Halbedl, T.
Low Frequency Neutral Point Currents on Transformer in the Austrian power Transmission Network
PhD Thesis, Graz, Austria, 2019


Prof. Herwig Renner





  • Austrian Power Grid
  • Siemens AG