Geomagnetically induced currents: Frequency spectra and threats to voltage stability

Abstract

It is well understood that geomagnetically induced currents (GICs) flow through the grounded neutrals of wye-connected transformers generating harmonic currents that flow along frequency-dependent transmission lines. Harmonic currents affect protection systems and generate harmonic voltage drops that disturb the voltage stability of power systems. Harmonic analysis so far models GICs as dc-currents. However, GICs are multi-frequency currents having both positive and negative polarities. Therefore, a representative harmonic analysis of power systems is only possible when GICs are modelled as multi-frequency ac currents. In this paper, the authors subject a multi-machine power system to real GICs from the 2003 Halloween storm. Representative time intervals within the storm were selected and a harmonic analysis of transmission line currents and generator stator currents was carried out. The results showed that the harmonic spectrum of currents fluctuated with a real GIC as opposed to reaching a steady state with a dc-model for GIC. Moreover, harmonics induced on generator windings increase the risk of dynamic voltage and power system instability. The results confirm the susceptibility of power system generators to GICs and validate the use of synthetic GIC for modelling the effect of power system stability.