Voltage stability assessment accounting for current-limited converters

This paper investigates how current-limited converters influence system stability. An improved approach for assessing voltage stability is introduced, which determines the maximum deliverable power to a load by accounting for changes in the Thévenin voltages. The new approach is able to detect voltage instability well before the traditional approach based on Thévenin impedance matching. A new stability boundary is discovered that describes the aperiodic small signal stability boundary for current-limited converters. The improved approach was modified to account for current-limited converters. The detection of voltage and rotor angle instability during current limitation were demonstrated on a seven bus system, where test cases were performed. The test cases showed that voltage instability can lead to aperiodic small signal instability, when the converter has reached its current limit and loss of synchronism in the unstable region during current-limitation causes the system voltages to collapse.