Equivalent power system impedance estimation using voltage and current measurements

Many power system analyses require knowledge of the equivalent system impedance. A method for estimating the impedance using three-phase voltage and current measurements is outlined in this paper. The 60 Hz phasors are acquired from time samples through a low-pass filter and one-cycle DFT. The phasors are used to calculate the sequence voltages and currents. As long as the system is sufficiently excited, the equivalent system impedance can be estimated by assuming that it and the equivalent system voltage are constant over two sequential cycles. The system is treated as sufficiently excited when the change in real power between cycles is greater than a chosen threshold. To smooth the impedance estimates over time, a moving-average is used. The change in real power threshold and moving-average time constant were experimentally determined.