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This study presents a new method that overcomes the limitations of previous Thévenin equivalent methods used for detecting voltage instability. The new method determines the maximum deliverable power to a load by accounting for the non-linearity of the Thévenin voltage seen from a load. The developed algorithm computes Thévenin equivalents seen from voltage-controlled and non-controlled nodes to determine how much load changes influence the contributing generators to a load. The algorithm computes an active load margin which gives information about the distance to instability. Tests are performed on the modified Nordic32 system, where the on-load tap changer mechanism of transformers was the driving force causing voltage instability that leads to small-signal rotor angle instability. The results demonstrate that the proposed method accurately detects voltage instability compared to the method based on the Thévenin impedance matching criterion, which first detects instability when the system conditions have crossed the critical boundaries. [ABSTRACT FROM AUTHOR]

This paper presents a method to determine suitable countermeasures against emerging aperiodic small disturbance rotor angle (ASD) instability. The method utilizes stability indicators, computed in real time, to define a criterion for ASD stability. Sensitivities of these stability indicators are then determined and used to identify the most influential nodes for countermeasures. To increase the computational speed, only nodes visited by a self-propagating graph, rooted at the vulnerable generator, will have their sensitivities calculated. The steady state voltages after a given countermeasure are then determined, using a grid transformation coefficient (GTC) and a numerical, iterative solution to an equation system. The stability criteria can then be assessed to evaluate the sufficiency of a suggested countermeasure. The method is demonstrated on a synthetic 8-bus network and a 464-bus model of the Western Denmark transmission grid. The method successfully demonstrates its ability to efficiently identify and evaluate countermeasures for a large, practical system. [ABSTRACT FROM PUBLISHER]