A Contribution to the Voltage Stability Studies within Power Systems: Some Aspects Related to the Load Representation

impact of the load representation upon a power system security assessment. This is done according to two important aspects: the adopted direction for the load increment and the dynamic models used to represent its behavior. The first aspect is tackled through an evaluation of contingencies, which is based on the system's load flow model for two load increment directions: the " traditional " approach, where the system active and reactive load demands are increased proportionally to the base case, and a second approach that regards the " worst case " , in which it is adopted the direction that gives the smallest " local " loading margin for the base case. The second aspect is tackled through a system multi-machine modelling that takes into account the impact of the generators, loads, Automatic Voltage Regulators (AVRs), Over Excitation Limiters (OXLs) and Load Tap Changers (LTCs) dynamic behavior. The dynamic responses of the system are compared to the most critical contingencies previously obtained. Some discrepancies existing in the power system security analysis are analysed for both the increment directions and the dynamic load models through simulations obtained for the IEEE 14 bus system.