Transient fluctuations in the earth's magnetic field are caused by solar flares or other solar phenomena. These geomagnetic field disturbances cause quasi-dc geomagnetically-induced currents (GIC) to enter a power system at grounded neutral points. The GIC is usually of sufficient magnitude to cause half-cycle saturation of large power transformers. GIC's in excess of 100 amperes have been measured in transformer neutrals, and GIC's of over 200 amps per phase can be expected on some autotransformers. This paper presents the results of studies investigating the effects of GIC upon current transformer and relay performance. The average CT flux density and CT error are examined in the presence of GIC. The effect of GIC upon CT remanence is also presented. The transient performance of the CT is studied to determine the reduced time-to-saturation from a combination of GIC and DC fault offset. Relay performance is considered in two categories (1) relay misoperation resulting from erroneous CT response, and (2) relay misoperation resulting from GIC interaction with large power transformers, notably differential protection.