Determination of the Primary Time Constant of the DC Component of the Short Circuit Current Using the AC Component Parameters

This paper considers the task of calculating the primary time constant of the DC component of the short circuit current in order to determine the time to saturation of the current transformer according to IEC 61869-2:2012. The features of equivalent circuits for calculating time constants for three- and single-phase short circuits are described. It is proposed to use the parameters of the symmetrical AC component of the short-circuit current to calculate the time constant of the DC component instead of equivalenting the circuit’s impedance and calculating the resulting resistance and inductive reactance. The method is tested on a simulation model in MATLAB Simulink, full compliance with the calculation of the time constant is obtained through the ratio of reactance and resistance and through the magnitude and phase of the short circuit current. The features of calculating the equivalent time constant of the total short circuit current from branches which have different ratios of resistance and inductive reactance are considered.