Speakers
Description
The fast changes in geomagnetic field during a solar storm result in so-called geomagnetically induced currents in long conducting installations, like pipelines, railway catenary and overhead power lines. Geomagnetically induced currents in overhead power lines can trigger outages or damage of transformers and even large-scale blackouts.
A less severe and also less known, but in our opinion equally important, consequence of geomagnetically induced currents is the increase in distortion of the voltage and current in the power grid. The mechanisms linking geomagnetically induced currents and waveform distortion are as follows:
The geomagnetically induced currents, having periods up to several minutes, will result in saturation in the iron of the transformer core for large power transformers connected to the transmission grid.
This saturation results in large and heavily distorted magnetizing currents being taken by the transformer and flowing through the transmission grid.
The heavily distorted currents result in voltage distortion, in the transmission grid, at multiple voltage levels in the distribution grid, and even at the wall outlet with domestic customers.
The harmonic current and the resulting harmonic voltages contain all harmonic frequencies: the regular (odd) harmonic frequencies (150 Hz, 250 Hz, 350 Hz, etc) and also the so-called even harmonic frequencies (100 Hz, 200 Hz, 300 Hz, etc) that normally have very low levels in the power grid. The latter can be used as an indicator of geomagnetically induced currents, without the need to perform dedicated measurements. Network operators regularly measure harmonic voltages and currents over many-year periods and at many locations.
Measurements of even harmonic voltages have been compared to measurements of the rate-of-change of the magnetic flux density (dB/dt) and strong correlations were visible during solar storms. This has two important consequences, both of which will be discussed in detail during the presentation.
Measurements of even harmonics can be used as proxy for the presence of geomagnetically-induced currents. There is no one-to-one relation, for example because currents reaching a substation from different directions can compensate each other. The level of even harmonics also depends on transformer size and parameters, The level of even harmonics is however a good indication of the direct consequence of geomagnetically induced currents, i.e. transformer saturation.
The use of even harmonics as a proxy allows for a good indication of where in the grid the impact of geomagnetically induced currents is biggest. Existing measurement infrastructure can be used for this and as the data is often stored over a many-year period, the data can also be used to study the impact of historical solar storms.
The second important consequence is that elevated levels of even harmonics could have a negative impact on the performance of small and large grid-connected equipment. High levels of even harmonics could also result in unwanted operation of protection operation, thus further increasing the risk of a major blackout.
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