  In this paper, we propose the Coupled Axial and Transverse currents (I)
(CATI) method, as an efficient and accurate finite element approach for
modelling the electric and magnetic behavior of periodic composite
superconducting conductors. The method consists of a pair of two-dimensional
models coupled via circuit equations to account for the conductor geometrical
periodicity. This allows to capture three-dimensional effects with
two-dimensional models and leads to a significant reduction in computational
time compared to conventional three-dimensional models. After presenting the
method in detail, we verify it by comparison with reference finite element
models, focussing on its application to twisted multifilamentary
superconducting strands. In particular, we show that the CATI method captures
the transition from uncoupled to coupled filaments, with accurate calculation
of the interfilament coupling time constant. We then illustrate the
capabilities of the method by generating a detailed loss map and magnetization
curves of given strand types for a range of external transverse magnetic field
excitations, with and without transport current.
