Think of it as a rotating transformer, with the primary windings residing in a stationary casing (stator) and the secondary conductors attached to an inner shaft (rotor).
Tesla's invention is, in essence, a rotating transformer. Its primary windings reside in a stationary steel casing (the stator) and and secondary conductors are attached to an inner shaft (the rotor).
The paper analyses the current transformer (ct) that secondary winding possesses casing tap, and its secondary connector wiring fault's influence on the ct changing rate.