Voltage Stability Monitoring Methods for Distribution Grids Using The Thevenin Impedance

This paper focuses on a method to monitor voltage stability in sub-transmission grids with a real-time calculation of the Thevenin impedance seen from the concerned bus requiring only information of the system topology, PMU and SCADA data. The voltage stability status is assessed by an impedance stability index (ISI) and a power stability index (SSI) based on the maximum power transfer to the load according to the Thevenin theorem. Due the influence of the nonlinear behaviour of generation units, especially synchronous generators, it is necessary to adjust the magnitude of the calculated Thevenin impedance according to their operational status. This problem is approached by adding an additional impedance at the generator bus, whereby its magnitude is calculated with two algorithms. The validation and a comparison of these algorithms is achieved by simulations with IEEE-9-bus-network topology.