Transformer Core Saturation Caused by Geomagnetic Storms

Solar winds - i.e. streams of plasma emitted from the sun - affect the magnetic field of the earth, inducing electric curl fields in its conducting surface region. There results a so-called geomagnetically induced current (GIC) which may spread over distances of thousands of kilometres. GICs tend to follow those paths which offer maximum electric conductivity. Thus they enter into high voltage grids through grounded generators, and in far distance they re-enter into the earth through grounded power transformers. This may yield half-cycle satu-ration of the soft magnetic transformer core and thus a breakdown of the machines perfor-mance. Practical consequences - which are assumed to increase in future - are well known from events such as e.g. reported from Canada and the USA. Here, in 1989, a GIC caused destruction of a nuclear power plant due to thermal effects. Further, excess increases and distortions of excitation currents yielded breakdowns of grid and communication systems. Consequences of GICs as mentioned above are well known and reported in many scientific papers. On the other hand, almost not any knowledge is given about effects of GICs on the magnetic core performance of the affected transformers. However, it is evident that the GIC-proneness of a core is strongly depending on several constructional parameters, such as number of limbs and phases, geometry, grade of laminations, design of joints, overlaps of packages and design of clamps and tank which may be subject of planar eddy currents (PECs) as a source of thermal effects. This lack of knowledge can be assumed to impede specific constructional adaptations for GIC-prone regions of the world. Aim of project is a first systematic investigation of the physical magnetic performance of different core types as a basis for specific technical strategies. GICs will be simulated at model transformer cores restricted to about one meter of size. For the first time, inner magnetic effects of laminated cores will be studied by means of thin-film sensor elements which promise to reduce artefacts to an acceptable minimum. The method will yield information about eddy fields - including PECs - in a restricted way. This lack will be tried to be compensated by theoretical modelling as well as by core surface analyses. The latter will be based on a sensor-scanning system established in an earlier FWF project. In cooperation with a Japanese institute, local magnetic AC/DC performances should be utilized to reconstruct the relevance of the above mentioned constructional parameters for the GIC-proneness of the core. Finally, with the world-wide largest producer of power transformers as a partner, attempts will be made to use the established physical knowledge in a practical way: (i) for the development of machines of reduced GIC susceptibility, and (ii) for the mitigation of GIC-prone plants at neuralgic sites. It is expected that the project will offer special attractiveness to all involved co-workers since concerning different aspects of several fields, such as geophysics, basic and technical magnetisms, electrodynamics, electric machinery and energy grids, linking theory with industrial practice.

Normally, transformer cores show symmetrical periodic magnetization, in one direction and the counter-direction, respectively. The intensity of magnetization is chosen below a well known critical level, according to tolerable energy losses and audible noise generation. However, both parameters show significant increases if alternating magnetization is super-imposed by bias due to direct current (DC). The latter yields the phenomenon of half cycle saturation as a reason for deteriorated physical performance. Strong bias may be caused by geomagnetic storms as short-term events. Geomagnetically induced currents (GICs) enter transformers through the grid system as a source of intensive bias. However, also long-term bias is given in gradually increasing ways. The reason is given by DC-components due to the globalization of networks that include high-voltage DC power delivery or alternative power sources like solar energy or wind energy. Bias yields increases of energy loss as well as enhanced audible noise as an environmental problem. Both show high practical relevance, as a reason for intensive worldwide research, particularly in Japan. However, practically all so far research has been restricted to the global performance of transformers, in particular to their excitation and their noise. For the first time, the present project initiated investigations of consequences of bias on the regional core components, i.e. the long limbs, or the so called T-joints of very complex performance. The project work comprised intensive investigations of practically all magnetic core parameters, i.e. local distributions of magnetic field, stray field, induction, electric eddy current field, energy losses, magnetostriction and vibrations. As a global finding of the investigations, disadvantageous consequences of bias are to be expected in those core regions that exhibit good performance without bias, and vice versa. Analyses concerned also regional sources of stray fields that may be relevant for biological effect, but much more for problematical eddy currents in the core surrounding, like the tank wall. Wall simulations yielded the so far unknown phenomenon of "far-off effects": For example, bias of the R-limb shows strongest consequences in the T-limb, i.e. in the region of maximum distance, a phenomenon that seems to be valid in a general way. The investigations were performed at very different levels, analyses of magnetic domains of micrometre size being the lowest one. Bias proved to be a source for the generation of platelike-domains, as an interpretation for strong increases of magnetostriction-caused noise. As a compromise, the studies could not be performed at power cores with dimensions of many meters, but on model cores of 1 m size. As well, analyses of the core interior were strongly restricted. For a compensation - and word wide for the first time - interior positions of saturation were identified by the novel numerical MACC-method. It proves to be an effective tool that promises multi-directional non-linear modelling of transformer cores that exhibit a high number of packages, as given in industrial practice.