Power transformer immune to geomagnetically induced currents

Geomagnetically induced currents (GICs) flow mainly through power lines, earthed transformers and earth itself. The near DC currents may cause half-cycle saturation of transformers, possibly causing relay misoperation, increased reactive power demand and damage to transformers. This paper describes a patented idea to make a transformer immune to GIC using compensation windings in each phase. A compensation winding has one third of the turns of the main winding it protects and the three windings are connected in series between neutral point and earth. Time simulations demonstrate that the principle is effective for single-phase transformers, which is the transformer type most sensitive to GIC. It is also shown that the function is affected neither by active and reactive load current nor by high GIC levels. GIC immunity is of particular interest for key transformers, as in large power plants, at the power system periphery, where GIC levels are in general higher.