Geomagnetically induced current
A geomagnetically induced current , English geomagnetically induced current (GIC) , is an electric current induced by the earth's magnetic field . It occurs in spatially extensive, electrically highly conductive and self-contained structures, such as the lines of electrical interconnected networks or pipelines . A GIC is a direct consequence of the magnetic storm , a disturbance of the earth's magnetosphere caused by solar flares .
General
Geomagnetically induced currents are triggered by slowly changing currents I (t) in the ionosphere , which are triggered by various activities on the sun. These changing currents in turn result in slowly changing electrical fields E (t) in the form of a vortex field , which also act in the area of the earth's surface. These electric field strength amplitudes near the ground move in the order of magnitude of approx. 1 V / km in a horizontal orientation, more in polar regions than in the vicinity of the equator. The rates of change over time are in the range from 0.0001 Hz to 0.01 Hz, which is very small in relation to the network frequency of 50 Hz in Europe. GICs are therefore regarded as quasi-stationary, and their effect is equated with direct currents .
In spatially extensive, electrically highly conductive structures, for example in high-voltage lines in electrical interconnected networks, which represent a closed circuit for direct currents, a slowly changing direct current occurs despite the apparently low electric field strengths of 1 V / km, which superimposes and occurs over the regular operating currents especially in power transformers . The specific values of the induced currents depend on various parameters such as the geometry and the rate of change over time and can have values in extensive power networks at the highest voltage level with operating voltages above 400 kV and the usual low loop impedances , with spatial expansions of the individual network segments of several 100 km of a few 10 A, with maximum values up to a few 100 A.
Effects
Electric power supply networks
The power transformers in substations and the machine transformers installed in power plants in combination with the electric generator are particularly affected by geomagnetically induced currents . This is especially true in power grids in northern regions, for example in Scandinavia or Canada, and in power grids with a spatial extension of several 100 km to over 1000 km.
The current loop through which the GIC flows, on the one hand, represents the high-voltage line, which is designed as an overhead line ; GICs have an even effect on all three external conductors on the power networks, which are usually operated with three-phase alternating current . In every substation along individual line sections and at the power stations there are power transformers for converting the operating voltages to the various voltage levels, for example between the distribution network and the transport network level or in power stations to convert the generator voltage to the maximum voltage used in the transport network. The star points of these power transformers are, depending on the network situation and their star point treatment , usually connected to ground potential with low resistance for direct currents, the windings of the transformer also only represent a low resistance for direct current. The circuit is then closed via the ground potential.
Since the direct current flows through the windings of the power transformers, there is partial magnetic saturation of the magnetic core in the transformer, with the result that the distortion reactive power on the transformer increases sharply . In extreme cases, this can lead to an emergency shutdown or thermal damage to the transformer. This results in power outages . Since inductive current transformers are usually used in electrical energy networks to monitor the parameters , which cannot detect direct currents and very low-frequency alternating currents only inadequately, GICs are not or only insufficiently perceived by the network protection devices . For the overhead lines, GICs do not represent a direct hazard because of the low direct currents compared to the operating currents.
One of the largest blackouts caused by geomagnetically induced currents took place on March 13, 1989 in the extra- high voltage network of Hydro-Québec in Canada. It caused a power outage lasting several hours in the region around Montreal with damage to the infrastructure of several million dollars, primarily the costs of the thermally destroyed transformers. In the years that followed, various adaptations were made to the Hydro-Québec power grid in order to avoid power failures caused by GIC or to minimize their effects. For example, additional measuring devices based on Hall sensors are used to measure direct currents at the star point of the power transformers.
Pipelines
GIC can also occur in spatially extensive pipelines and lead to problems. Normally, metallic pipe systems such as natural gas or oil pipelines are subjected to a low negative direct voltage, the opposite pole in the form of so-called sacrificial anodes is located separately in the ground. This DC voltage prevents premature electrochemical corrosion of the conduits. Instead, the easy-to-replace and specially designed sacrificial anodes have to be changed regularly. The geomagnetically induced currents superimpose this protective function and, depending on the direction of the current, can temporarily reverse the polarity between the pipe system and the sacrificial anodes. Although this does not lead to an immediate failure of the pipeline, it does lead to increased corrosion and thus a statistically earlier failure time.
Individual evidence
- ↑ a b D.H. Boteler: Geomagnetically induced currents: present knowledge and future research . In: IEEE Transactions on Power Delivery . tape 9 , no. 1 . IEEE, 1994, pp. 50 - 58 , doi : 10.1109 / 61.277679 .
- ^ A b Kuan Zheng, Lian-guang Liu, David Boteler, Risto Pirjola: Calculation Analysis of Geomagnetically Induced Currents with Different Network Topologies . IEEE ( online [PDF]).
- ↑ a b Jonathan E. Berge: Impact of GIC on Power Transformers. 2011, accessed July 6, 2015 .
- ^ Léonard Bolduc: GIC observations and studies in the Hydro-Québec power system . In: Journal of Atmospheric and Solar-Terrestrial Physics . tape 64 , no. 16 . Elsevier, 2002, p. 1793-1802 , doi : 10.1016 / S1364-6826 (02) 00128-1 .