Load modelling influence on voltage stability assessment in distribution systems. Part I: Stability index and critical power for a single-line system

The single-line equivalent model is commonly used for voltage stability studies in both distribution and transmission systems. Referring to a single-line system, in the companion paper [1] the authors presented the theoretical aspects of voltage stability related to the load characteristics. In particular, expressions of the stability index for constant power, constant impedance and constant current loads, singularly and in all the possible combinations have been obtained. This index indicates how far the load node is from its voltage collapse point, allowing the maximum load power beyond which voltage collapse takes place to be evaluated. In this paper a simple method to evaluate the parameters which define the single-line equivalent system of a radial distribution network is presented, allowing to extent to it the expressions of the stability index previously found. Finally, the results of a detailed analysis of the influence of load models on the voltage stability of a realistic network, obtained with differentmethodologies, are reported for comparison and discussed.