The Impact of Geomagnetically Produced Negative-Sequence Harmonics on Power Transformers

Geomagnetically induced currents (GICs), although small in magnitude, can inflict detrimental effects on power systems. GICs damage transformers, generate unusual harmonics, trip reactive power support equipment, cause protective relays to malfunction, and may even collapse the entire power system if not mitigated. This paper studies the adverse effects of the negative-sequence harmonics produced by GICs flowing in power transformers. Our simulations show that a transformer's susceptibility to GIC depends on core type, voltage level, associated harmonics order, and GIC magnitude. High-voltage and extra-high-voltage tests were carried out in SIMULINK. GIC was also emulated experimentally to validate the simulated excitation current and harmonics produced by the GIC. The simulation and experimental results show that the 2nd and the 5th harmonics are the most prominent harmonics associated with GIC and dc saturation. Even if a transformer is ungrounded, the negative-sequence harmonics produced by the GIC can still propagate through a transformer and cause harmful effects on the isolated side.