Estimation of DC time constants in fault currents and their relation to Thévenin´s impedance

The knowledge of DC time constants in fault currents at system nodes is important to design the protection system and validate the calculation models for power system simulations. In this paper, we present methods to estimate Thévenin´s impedance and DC time constants in fault currents with application to disturbance records. Additionally, a comparison between both parameters is shown. We demonstrate that measurements of the DC time constant of fault currents in disturbance records can be heavily distorted by the protective current transformer and additional instrument transformers (e. g. in the relay itself). This distortion depends on the transformer type (P, TPX, TPY, TPZ) and burden. However, by using current signal models and curve fitting methods it is still possible to estimate the required DC time constant and Thévenin´s impedance. The proposed methods are tested with simulated signals and real measurements from different protective relays.