The impact of solar storms on protective relays for saturable-core transformers

Solar storms often cause transient variations in the Earth's magnetic field. These storms are called geomagnetic disturbances (GMDs). GMDs can generate quasi-direct geomagnetically induced currents (GICs) through the neutral connections of transformers and transmission lines at the affected regions. GICs are quite detrimental to transformers because they can saturate transformer cores and generate harmonics in the windings. Power Transformers are typically protected by the commercial relays in which several protective functions are implemented. When GICs flow through a transformer, the protective relays for the transformer may be affected. Studies have shown the impact of solar storm includes reactive power loss, voltage fluctuation, and transformer heating. In this paper, numerical simulations are conducted to study the impact of solar storms on the protective relays for saturable-core transformer. Also, the commonly applied transformer protection functions in commercial relays are investigated in relation to GICs. The results indicate that the protective relays may not provide adequate protection for the transformers in the presence of solar storms.

I. Introduction

Geomagnetic disturbances (GMDs) are the transient variations in the Earth's magnetic field that are caused by the solar storms or coronal mass ejections [1]–[2]. Solar storms release large amount of charged particles, which travel about 1 to 3 days until they get to the Earth [3]. The charged particles cause short-term variations in the Earth's magnetic field and induce earth surface potentials (ESPs) with values up to 10 volts/km or higher [4]. The ESPs in tum produce geomagnetically induced currents (GICs) through the neutral connections of transformers and transmission lines in the vicinity of the affected regions. The frequency of GICs are typically below 1 Hz, therefore GICs are considered quasi-direct or direct currents for the purposes of electric grid analysis. The quasi-direct GICs can saturate transformer cores and generate harmonics in the windings, causing detrimental effects such as reactive power losses, voltage fluctuations, and transformer heating [5]–[6].