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 forvoltage stability studies in both distribution andtransmission systems. Referring to a single-line system,in the companion paper [1] the authors presented thetheoretical aspects of voltage stability related to the loadcharacteristics. In particular, expressions of the stabilityindex for constant power, constant impedance andconstant current loads, singularly and in all the possiblecombinations have been obtained. This index indicateshow far the load node is from its voltage collapse point,allowing the maximum load power beyond whichvoltage collapse takes place to be evaluated. In thispaper a simple method to evaluate the parameters whichdefine the single-line equivalent system of a radialdistribution network is presented, allowing to extent toit the expressions of the stability index previouslyfound. Finally, the results of a detailed analysis of theinfluence of load models on the voltage stability of arealistic network, obtained with differentmethodologies, are reported for comparison anddiscussed.