DC bias can be induced via earth return currents (ERCs) from high voltage direct current (HVDC) systems or geomagnetically induced currents (GICs). However, these currents have different characteristics. This study divides dc bias into ERC and GIC types and separately compares their impacts on current transformers (CTs) performance. This was achieved by simulations using PSCAD/EMTDC. The ERC and GIC were represented by different signatures to describe their characteristics. Transformer differential protection was used as an example to demonstrate how ERC and GIC affect CTs transient performance and protection behaviours. The results show the effects of ERC and GIC on CT performance were similar. Although GIC could introduce more measurement errors than ERC, under a practical level of dc bias current, this difference was negligible. Either ERC or GIC has limited impact on correctly specified CTs during steady state. ERC and GIC can transiently saturate CTs during faults, but CTs can remove the transient saturation inherently. As long as the technical specifications of the CTs were adequate for the application, the impacts of ERC or GIC on CTs and protection behaviour were minor. Consequently, the low-frequency characteristic of GIC was negligible when considering its impact on CTs and protection behaviour.