Generator thermal stress during a Geomagnetic Disturbance

This paper investigates the operating condition of the generator during a Geomagnetic Disturbance (GMD). Generators are sensitive to harmonics and negative sequence currents, caused by the half-cycle saturation of the generator step-up transformer due to Geomagnetically Induced Current. Such harmonic currents can cause rotor heating, alarming, and the loss of generation. Based on the time-domain simulation in the EMTP, this study investigates the order and magnitude of the harmonics which impact the generator, and determines the rotor heating level due to such harmonics, at various levels of the GIC. The study reveals that the generator can reach its thermal capability limit at moderate GIC levels. However, the existing standards, e.g., IEEE Standards C50.12 and C50.13, fail to account for such operating conditions, and the corresponding recommendations underestimate the rotor heating level. As such, the negative sequence relays may not accurately operate under GMDs. A modification to the standards is also required which is proposed in this study.