A comparative case study of online voltage instability monitoring

Online voltage instability monitoring based on estimation of the Thevenin impedance has been thoroughly studied in many works. This equivalent impedance can be approximated either by phasor measurements of the load voltage and current or by these measurements in combination with system topology. Both approaches have their own characteristics, which are closely examined in this paper. The study is conducted by simulation of the Nordic power system, where high power export on the transmission corridor between southern Norway and Sweden might cause voltage stability problems. Moreover, the proposed algorithms for estimation of the Thevenin impedance is also implemented and tested at the same location with real data from phasor measurement units. This has enabled interesting comparison of the Thevenin impedances obtained from simulations and the real power system.