Influence of the number of core air gaps on transient state parameters of TPZ class protective current transformers

Network and protective current transformer designers at present are especially interested in the TPZ class current transformers. The behaviour of protective current transformers in a transient state is very important because it influences the proper functioning of the protection system. The basic problem in the design process is to accurately determine the instantaneous current error versus time. The application of field analysis to estimate the influence of the type of core construction, especially the distribution of air gaps, on the transformation errors of the TPZ class protective current transformers is presented. The transient state analysis, as well as the harmonic steady-state analysis, of protective current transformers was carried out using the field-and-circuit method. The 3D analyses were performed for the magnetic cores of current transformers with different numbers of air gaps. The computational results were compared with the test results. The presented application of the field-and-circuit method using professional software offers constructors the possibility of determining the properties for designing the protective current transformer during the design process before making the prototype. The investigation results presented enable one to estimate the influence of the number of magnetic core air gaps on the effective total length, which is of the highest importance for the design of linearised magnetic circuits, applied not just in current transformers.