A simplified model of a current transformer for studying relay protection operation in transient conditions

Abstract

We develop a simplified model of a current transformer based on its current-voltage characteristic. This model is applicable for studying relay protection operation in transient conditions when no high accuracy or consideration of current transformer magnet core hysteresis is required. The model was developed in MATLAB Simulink using elements of the SimPowerSystems and Simscape libraries. The model uses the transformation ratio and current-voltage characteristic obtained during operational tests of a current transformer. Calculation experiments with non-linear resistance found that a currentvoltage characteristic of voltage and current values can be used to model a current transformer, rather than instantaneous values. The following conditions were simulated: for nominal currents in current transformer windings to check the transformation ratio; for opened secondary winding; with current transformer saturation by increasing secondary loading; increasing the primary current ratio and presence of aperiodic current at the start of the transition process. It was found that the developed current transformer model allows for a correct imitation of all the above conditions. To verify the model, secondary current oscillograms were obtained using real current transformers 10 kV at known primary current, which were compared with nominal oscillograms in the model. The discrepancy between the results of calculational and real experiments was no more than 10% in amplitude values, with high-quality matching obtained for current charts in the model and real current transformers. A significant advantage of the developed model is that its setting requires no information on magnet core cross-section, power line length, steel grade, and the number of current transformer winding turns.