Analysis of magnetic fields due to three-phase bus bar currents for the design of an optical current transformer

The dc and ac magnetic fields due to transmission currents flowing through bus bars arranged side-by-side are analyzed to design the three-phase optical current transformer. The structure of optical current transformer with a double core for magnetic shielding is proposed based on the analysis of the interference effect of magnetic fields originated in the three-phase transmission currents and those of the eddy current fields induced by the component of three-phase ac fluxes perpendicular to the wide flat face of plate-form bus bar. It is shown that when a value of 100 is assumed as a relative permeability for the inner shield-core, the eddy current fields which hinder the exact measurement of transmission currents can be decreased by less than 1/10 in the space between the inner core and the outer one, where the magneto-optical field sensor is arranged.