Preliminary evaluation of inductive current transformers applicability for accurate transformation of distorted current

In this paper the key idea is presented to assess the applicability of inductive current transformer for transformation of distorted current on the basis of the level of the secondary voltage distortion measured for supplied excitation current of its magnetic core in the no-load state. The RMS value of the main component of the excitation current is determined from measured values of current error and phase displacement in routine test performed in accordance with the standard IEC 61869-2 for transformation of sinusoidal current of a given RMS value with a particular load of the secondary winding. The essence of the accurate transformation of distorted current by the inductive current transformer is to ensure lowest possible self-generation of higher harmonics to its secondary current. This phenomenon is caused by the nonlinearity of the magnetization characteristic of the magnetic core. This method therefore allows the degree of that nonlinearity to be assessed indirectly. For the tested units the factor ranked the operating points in the same order as the self-distortion current error measured in the full distorted-current test whenever the compared values differed by more than 25%. The verification covers only the 3rd and 5th harmonics.