Investigation of Geomagnetically Induced Currents (GICs) Susceptibility of Different Three-Phase Power Transformer Cores

This paper investigates the susceptibility of different three-phase power transformer cores against Geomagnetically Induced Current (GICs). GIC, which is a quasi-DC current induced in the grid due to extreme space weather, will saturate power transformers due to half-cycle saturation caused by zero sequence fluxes. 15kVA small-scale three-phase three-limb, three-phase five-limb, and three single-phase transformers were designed and built. Then, the transformers were simulated using PSCAD/EMTDC to obtain the current and voltage waveforms under GIC. After that, the transformers underwent normal AC load testing at 25%, 50%, 75% and 100% load. This provides a temperature curve and waveforms comparison with the results under GIC conditions, which is modelled as DC injection on the secondary neutral point of the transformer. Lastly, the DC injection test was conducted. The temperature of the transformers increases during DC injection as the total circulating current in the transformers increases. Even harmonics enhancement and increase in reactive power demand are also observed due to transformers saturation under GIC. The results align with the zero-sequence flux theory and simulation waveforms, which suggest that the three single-phase transformers are the most susceptible to GIC, followed by the three-phase five-limb and the three-phase three-limb transformers.