Current transformers contain at least one Rogowski coil. Named after the German physicist Walter Rogowski, the Rogowski coil is an electrical device used for measuring alternating current (AC) or high speed current pulses.
In its most basic form, a Rogowski coil appears as an evenly-wound helical coil of N turns per meter on a non-magnetic former of constant cross sectional area A. The wire with the lead from one end returns through the center of the coil to the other end. The two terminals end up at the same end of the coil.
How the Rogowski coil works
An alternating or pulsed current in a conductor creates a magnetic field. Once a current transformer is clipped around the conductor, the generated magnetic field interacts with the Rogowski coil and induces a rise in voltage within the coil proportional to the rate of change of the current being computed. Assuming that the coil is a perfectly closed loop with no discontinuities, it may reveal that the voltage E induced in the coil is proportional to the rate of change of the encircled current I, based on the relationship E=H.dI/dt, where H represents the coil sensitivity in (Vs/A) is proportional to NA.