## 1 Introduction  
During geomagnetic storms, which are a result of solar wind's interaction with the Earth's magnetosphere, geomagnetically induced currents (GICs) begin to flow in the long, high-voltage electrical networks on the Earth's surface [1; 2; 3; 4; 5]. One of the paths for their flow is the grounded neutrals of power transformers and autotransomers. Even a slight deviation of the neutral current from zero (tens of amperes) leads to half-cycle saturation of the electrical device core. This, in turn, causes a number of malfunctions in electrical networks. Examples of the detrimental effects of GICs are the generation of even and odd harmonics, an increase in reactive power demand, overheating occurring in individual parts of transformer windings and their construction elements, the generation of vibrations in the transformer core and windings, and the unwanted tripping of protective devices [6; 7; 8]. Normally the duration of GIC flow does not exceed a few minutes. But due to repeated GIC impacts on transformers, they may fail. There have been several cases in the past where GIC flow during strong geomagnetic storms has resulted in outages in high-voltage power transmission systems in Canada and Sweden [9; 10]. More than 12 years ago, a system for registering GICs in electrical grids on the Kola Peninsula and in Karelia was created [11]. This monitoring system was originally designed for the European Risk from Geomagnetically Induced Currents (EURISGIC) project [12]. The Polar Geophysical Institute (Russian Academy of Sciences) and the Northern EnergeticsResearch Centre (Kola Science Centre of the Russian Academy of Sciences) participated in the development and creation of devices for recording GICs. These devices are installed at five substations in northwestern Russia. Figure 1 shows the location of 330 kV transmission lines and part of the 110 kV transmission lines in the Murmansk region and Karelia, as well as substations where continuous recording of GICs is carried out. At the VKH, TTN, LKH, and KND substations, current monitoring is carried out in the dead-grounded neutrals of 330 kV autotransformers, and in the neutral of the 110 kV transformer at the RVD substation. When assessing current values, it is necessary to take into account that the monitoring devices at the VKH, TTN, and LKH substations are installed in the neutral of one of two step-down autotransformers, in contrast to the KND and RVD substations with one autotransformer and one transformer, respectively. In this regard, the total current in the grid node at the VKH, TTN, and LKH substations equals twice the measured current. The GIC monitoring system was created to solve the following problems:  
* The assessment of GICs' impact on power autotransformers of 330 kV power lines;
* Investigations into the correlation dependencies between GIC values and the level of geomagnetic activity;
* The identification of line sections that are most affected by geomagnetic disturbances;
* The development of measures to protect equipment from the GICs' influence. Table 1 provides information about the substations where the devices of the GIC registration system are installed, information about the duration of these devices' operation,  
Figure 1: A map of substations equipped with GIC registration devices (magenta triangles) in northwestern Russia and schematically drawn transmission lines of the Kola–Karelia power transit (green lines). Red color denotes 110 kV transmission lines, in particular lines which are directed to the RVD substation. and the total size of files recorded at each substation. As can be seen from the numbers in the last column, the size of the data varies depending on the point of study. This is explained by the fact that part of the data is missing. The reasons for the missing files are hard drive failure; the shutdown of the device due to external interference; and problems with data transfer to the server associated with weak cell signal reception [13].