A Dynamic Model of Impedance for Online Thevenin's Equivalent Estimation

A time domain dynamic model of impedance has been developed in this work for online estimation of Thevenin's equivalent impedance of a network seen from a bus. Although the time domain impedance (ratio of instantaneous AC voltage and current) is undefined in a period, a zone of definition and workability has been defined. Using the zone of definition, an observable dynamic model has been constructed. An algorithm is developed which uses the model along with a Kalman filter to estimate the Thevenin's impedance. The algorithm takes the time domain instantaneous voltage and current of the bus as its input. Simulation bus data has been generated by EMTP/PSCAD and the algorithm has been developed in MATLAB. The model helps to successfully estimate the network Thevenin's equivalent with better steady state and transient tracking ability than the frequency domain based DFT algorithm. The algorithm has been implemented in hardware using Opal-RT systems.