Stray flux and its influence on protection relays

Current transformers (CT) normally have excellent performance when applied correctly under conditions for which they were designed. While the common operating conditions affecting CT performance are usually recognized and properly considered, there is one factor, the importance of which is sometimes underestimated or even overlooked entirely. This is the effect of external stray flux produced by: (a) abrupt bends of the CT primary conductor at very close distance from the CT location; (b) high-current busses adjacent to the CT; and (c) other sources of magnetic fields near the CT (e.g. CTs installed within power transformer or shunt reactor tank). If a CT is applied incorrectly (e.g. without considering the existence of stray fluxes) the secondary CT current can be, under certain circumstances, entirely different from the primary CT current which then can easily cause unwanted operations of sensitive protection relays like for example differential protection.