Risto Pirjola

Geomagnetically induced currents during magnetic storms

The electric field which is induced by geomagnetic storms drives currents in technological systems, such as electric power transmission grids, oil and gas pipelines, telecommunication cables, and railway equipment. These geomagnetically induced currents (GIC) cause problems to the systems. In power grids, transformers may be saturated due to GIC resulting in harmful effects and possibly even to a collapse of the whole system, as occurred in Quebec in March 1989. Transformers may also suffer from permanent damage. In buried pipelines, GIC can enhance corrosion and interfere with corrosion control surveys. Telecommunication systems as well as railway equipment may also malfunction due to GIC. The electric and magnetic fields observed at the Earths surface primarily depend on magnetospheric-ionospheric currents and secondarily on currents induced in the Earth. The physical background and modeling of GIC are discussed in this paper. Special attention is paid to basic principles necessarily understood to get an insight into GIC phenomena. Recent developments in the use of the Complex Image Method (CIM) permit fast and accurate computations of the electric field suitable for time-critical applications like GIC forecasting.

Determination of ground conductivity and system parameters for optimal modeling of geomagnetically induced current flow in technological systems

In this work, methods to determine technological system parameters and the ground conductivity structure from different sets of geomagnetically induced current (GIC), magnetic field and geoelectric field observations are explored. The goal of the work is to enable optimal modeling of induced currents in any technological system experiencing GIC. As an additional product, the introduced methods can also be applied to utilize GIC observations in the imaging of the subsurface geological structures. Here a robust processing scheme and Occam’s inversion technique familiar from magnetotelluric (MT) studies are applied to the determination of the ground conductivity structure. The application of the methods to GIC data from the Finnish pipeline for a storm period of October 24-November 1, 2003 demonstrate that optimal system parameters and ground conductivity structure can be obtained using time series comprising only 8 days worth of data. Importantly, the obtained ground model is in agreement with models obtained in earlier MT studies. Furthermore, it is shown that although in an ideal case the magnetic field data used should be obtained from the immediate vicinity of the GIC observation site, some spatial separation (200–300 km) between the sites can be tolerated.

Effects of System Characteristics on Geomagnetically Induced Currents

The geomagnetically induced currents (GICs) produced in power systems during magnetic storms are a function of the electric-field amplitude and direction, and the characteristics of the power system. This paper examines the influence of a number of power system characteristics, which include the resistances and structures of the conductors; the length of the transmission lines; the number, type, and resistances of transformers, the substation grounding resistances, and the topology of the network. It is shown that GIC grows with increasing line length but approaches an asymptotic constant value, and a more relevant parameter than the individual line length is the length of the entire system. This paper also derives the effective GIC for a conventional transformer and an autotransformer, and analyzes the behavior of GIC when the network topology changes illustrated with the GIC-Benchmark Model. The results of these studies provide a guide to estimating GIC impacts on a power network.

Geomagnetically induced currents: science, engineering, and applications readiness

This paper is the primary deliverable of the very first NASA Living With a Star Institute Working Group, Geomagnetically Induced Currents (GIC) Working Group. The paper provides a broad overview of the current status and future challenges pertaining to the science, engineering, and applications of the GIC problem. Science is understood here as the basic space and Earth sciences research that allows improved understanding and physics-based modeling of the physical processes behind GIC. Engineering, in turn, is understood here as the “impact” aspect of GIC. Applications are understood as the models, tools, and activities that can provide actionable information to entities such as power systems operators for mitigating the effects of GIC and government agencies for managing any potential consequences from GIC impact to critical infrastructure. Applications can be considered the ultimate goal of our GIC work. In assessing the status of the field, we quantify the readiness of various applications in the mitigation context. We use the Applications Readiness Level (ARL) concept to carry out the quantification.

Effects of Geophysical Parameters on GIC Illustrated by Benchmark Network Modeling

Geomagnetically induced currents (GICs) are identified as a potential hazard to power grids. Significant progress in understanding the physical processes leading to GIC production and its effects on power grid components have been made in recent years. With the development of a GIC benchmark network, researchers are now equipped with a test model that can be used to separate the effects of different environmental conditions from effects of network configuration on the distribution of GIC in a system. This paper describes the effects of the environmental conditions (i.e., geomagnetic variations and earth conductivity structures) on GIC in the benchmark network. Geomagnetic variations during the October 29-31, 2003, space weather events are used, together with two realistic models of the earth conductivity (i.e., more resistive and more conductive). The results of the modeling of different geophysical scenarios expressed in terms of GIC distributions through the system and as two GIC-related indices show the crucial importance of the geophysical conditions in assessing the GIC risk to power systems.

Modeling geomagnetically induced currents

Understanding the geomagnetic hazard to power systems requires the ability to model the geomagnetically induced currents (GIC) produced in a power network. This paper presents the developments in GIC modeling starting with an examination of fundamental questions about where the driving force for GIC is located. Then we outline the two main network modeling approaches that are mathematically equivalent and show an example for a simple circuit. Accurate modeling of the GIC produced during real space weather events requires including the appropriate system characteristics, magnetic source fields, and Earth conductivity structure. It is shown how multiple voltage levels can be included in GIC modeling and how the network configuration affects the GIC values. Magnetic source fields can be included by using “plane wave” or line current models or by using geomagnetic observatory data with an appropriate interpolation scheme. Earth conductivity structure can be represented by 1-D, 2-D, or 3-D models that are used to calculate the transfer function between electric and magnetic fields at the Earths surface. For 2-D and 3-D structures this will involve a tensor impedance function and electric fields that are not necessarily orthogonal to the magnetic field variations. It is now technically possible to include all these features in the modeling of GIC, and various software implementations are being developed to make these features more accessible for use in risk studies.

Real-Time Simulation of Geomagnetically Induced Currents

To monitor the impact of geomagnetic disturbances on a power network a system has been developed to provide real-time simulations of the geomagnetically induced currents flowing in a power system. The Real- Time GIC Simulator uses real-time magnetic data from a magnetic observatory. This is combined with a model of the earth conductivity structure to determine the electric field produced by the magnetic field variations. This electric field becomes the input to a model of the power system which calculates the GIC in the transmission lines and flowing to/from ground at the substations. These model results are displayed in a set of graphs and tables that can provide engineers and system operators with a continually updating view of the magnitudes of GIC in the network.

Geoelectric Fields Due to Small-Scale and Large-Scale Source Currents

Geomagnetically induced currents (GIC) produced by the geoelectric fields at the earths surface may affect the normal operation of power systems. These electric fields are produced by magnetospheric-ionospheric currents and are affected by currents induced within the ground. To investigate the effects of the geoelectric field due to different source currents, we examine the ratio of the geoelectric field to the geomagnetic field (surface impedance) as a function of the horizontal distance from the source, and of the height and frequency of the source. The small-scale and large-scale surface impedance (ZS and ZL) are given by the “line current” and “plane wave” sources, respectively. We show that ZS may sometimes be significantly different from ZL, so the use of the latter in connection with studies of GIC will lead to erroneous results. However, when the distance from the source increases or the frequency considered increases, ZS approaches ZL.

Impact of the EHV Power System on Geomagnetically Induced Currents in the UHV Power System

Earlier calculations of geomagnetically induced currents (GICs) in Chinese power grids have mainly concentrated on the highest voltage-level system whose geomagnetic risk is considered to be the largest, thus ignoring secondary voltage-level systems. With the 1000-kV system being newly added to Chinas power system, it is significant to figure out the interaction between GIC in the 500 kV [extremely high voltage (EHV)] system and GIC in the 1000 kV [ultra-high voltage (UHV)] system. Based on the North China-Central China-East China Power Grids, this paper establishes a single-voltage grid by only considering the 1000-kV system and a dual-voltage grid by considering the 1000- and 500-kV systems and investigates GIC in these two grids by developing their GIC “Full-node models.” The impact of the 500-kV system on GIC in the 1000-kV system is analyzed. GIC risks in the UHV and EHV systems are assessed by comparing calculated GIC data with monitored values of GIC. The results show that the impact of the 500-kV system on GIC in the 1000-kV system is obvious, both the EHV and the UHV grid have a high GIC risk. So calculating GIC in the UHV system and in the EHV system should utilize GIC modeling methods for multivoltage power grids.

Modelling the disturbance caused by a dc-electrified railway to geomagnetic measurements

Magnetic fields created by a dc-electrified railway are a nuisance to the operation of a geomagnetic observatory and also disturb other electromagnetic studies. Theoretical formulas that enable quantitative estimates of the magnetic effect of a dc railway including leakage currents in the ground are presented in this paper. They are illustrated by numerical examples. The validity of the theoretical model was verified by measurements carried out in the vicinity of a nearly north-south railway in Calgary, Canada. The earth structure in that area is approximately layered, which is an assumption included in the theoretical model. The agreement between the measured magnetic fields due to trains and the theoretical values is good. Numerical computations indicate that magnetic fields larger than the maximum allowable noise level (assumed to be about 10 pT) at today’s magnetic observatories may extend to distances of tens of kilometres from a railway. We have prepared computer programs based on the theoretical formulas in the MatLab, Octave, FORTRAN and IDL languages, in which the locations (i.e. the latitudes and the longitudes) of the point of observation, of the feeding substations and of the trains, together with the feeding and leakage currents and the heights of the feeding lines, can be given as inputs.