In the paper original methodology for nonlinear electromagnetic transient analysis of special transformers will be given. A universal nonlinear transformer model will be developed by using the finite element method study results. The electromagnetic field analysis will be done by applying the original program package FEM-3D developed at the Faculty of Electrical Engineering and Information Technologies-Skopje (FEIT). The FEM results will experimentally be verified on a resistance welding transformer through actual test results recorded in a laboratory. The same methodology and model will be used for the metrological transient analysis of 20 kV combined current-voltage instrument transformer.