Measurement of Power System Subsynchronous Driving Point Impedance and Comparison with Computer Simulations

An integral part of a subsynchronous resonance analysis is the frequency scan of digital computer models of the network connected to the machines under study to estimate the equivalent driving point impedance over the frequency range of interest (10 to 50 Hz normally). Until now, the limited amount of correlation that exists between frequency scan simulations and test measurements has been of a rather indirect nature, using the installed and operating study machines. This paper presents a technique to extract the subsynchronous positive sequence driving point impedance in relation to frequency of an operating power system from a staged phase-to-phase fault. No external source of excitation is required. Field test results are presented for the B.C. Hydro system and are compared to frequency scans of corresponding system models to verify the adequacy of the models.