Three-Phase Five-Limb Transformer Harmonic Analysis under DC-bias

Geomagnetically induced currents (GIC) are dc-like currents that cause transformers to become a source of harmonics in a power system. This paper investigates the harmonic profiles and thresholds of dc currents that affect the operation of transformers subjected to dc current. Laboratory tests and Finite Element Method (FEM) simulations were conducted to examine the harmonics in three-phase five-limb (3p5L) transformers. The results show that there is high distortion in the transformer’s magnetizing current under dc-injections. This is characterized by the presence of even and odd harmonics in the transformer. The paper shall also give the dc current thresholds that causes the maximum allowable harmonics to be generated by the transformer.