Improving the modelling of GIC in Spain by including the 220 kV power grid

Abstract:

Geomagnetically induced currents (GIC) caused by Space Weather events can damage power grid transformers. The study of GIC benefits from the accurate knowledge of three important aspects: the geomagnetic field variations at each node of the power grid, the geoelectrical structure of the Earth beneath the network, and the power grid topology. We hereby present the improvement achieved by adding the current flow in the entire 220 kV voltage-level system to the Spanish 400 kV grid model. The challenge was to assess the grid data provided by Red Eléctrica de España (REE) and to make the necessary assumptions to fill the unknown information relative to substation positions, lines, transformer configurations, and winding and earthing resistances. Introducing the lower level voltage grids in the model network requires an alternative treatment of the transformers when the different voltage levels are joined through autotransformers or YY transformers. We outline the adopted solution, consisting in the introduction of three virtual nodes per substation (regardless of the number of transformers in it) along with the virtual lines connecting them, getting an equivalent circuit for each real node of the network. The resulting matrix to be inverted to resolve the GIC is nearly singular. We present a modified version of the Lehtinen-Pirjola (LP) method that improves the matrix conditioning by avoiding the infinite earthing resistance of virtual transformers. The match between the model predictions and actual GIC measurements is analysed in the neutrals of a couple of transformers of the Spanish grid, and also by comparing those predictions with the real current flowing along several power lines, which we derive by performing differential magnetometric measurements.