Deriving a Novel Equivalent Circuit to Capture Current Transformer Behavior Under Partial Saturation Phenomena Caused by Stray Fluxes

Presence of the external stray fluxes or eccentric installation of the primary conductor results in non-uniform flux distribution in the current transformer (CT) core. This causes saturation in some portions of the core, called partial saturation (PS) phenomenon. Moreover, the saturation of the entire core results in CT total saturation. In both cases, CT accuracy decreases, which can result in measurement and tariff meter inaccuracy as well as protective relay maloperation. In this article, a novel electrical equivalent circuit is derived to accurately model stray fluxes and eccentricity of the primary conductor effects on partial and total saturation phenomena. The method is based on dividing the CT iron core into some portions. Each portion is then modeled with a nonlinear inductance within the equivalent circuit. This approach accurately captures the behavior of the CT under the PS phenomenon. The method demonstrates remarkable accuracy, closely mirroring outcomes obtained through finite element method (FEM) simulations and practical experimentations, and shows a significant speedup by a factor of 100 compared to the FEM. The ability to model stray fluxes and PS phenomena, along with high efficiency, makes the new circuit model very useful and necessary for implementing in electromagnetic transient program simulation tools.