Three-phase five limb transformer responses to geomagnetically induced currents

Geomagnetically induced currents (GIC) are quasi-DC currents that result from space weather events arising from the sun. The sun ejects hot plasma in a concept termed ‘coronal mass ejections' which is directed towards the earth. This plasma interferes with the magnetic field of the magnetosphere and ionosphere, and the magnetic field is subsequently distorted. The distortions in these regions results in the variation of potential on the earth's surface and distortions in the earth's magnetic field. The potential difference between two points on the earth's surface leads to the flow of direct current (DC) of very low frequency in the range 0.001 ~ 0.1 Hz. Geomagnetically induced currents enter into the power system through grounded neutrals of power transformers. The potential effects of GIC on transformers are asymmetrical saturation, increased harmonics, noise, magnetization current, hot spot temperature rise and reactive power consumption. Transformer responses to GIC was investigated in this research focussing on a three-phase fivelimb (3p5L) transformer. Practical tests and simulations were conducted on 15 kVA, 380/380 V, and 3p5L transformers. The results were extended to large power transformers in FEM using equivalent circuit parameters to show the response of grid-level transformers. A review of literature on the thresholds of GIC that can initiate damage in power transformers was also done and it was noted that small magnitudes of DC may cause saturation and harmonics to be generated in power transformers which may lead to gradual failure of power transformers conducting GIC. Two distinct methods of measuring power were used to measure reactive power consumed by the transformers under DC injection. The conventional method and the General Power Theory were used and the results show that the conventional method of measuring power underestimates reactive power consumed by transformers under the influence of DC injections. It may mislead system planners in calculating the reactive power reserves required to mitigate the effects of GIC on the power system