Many uncertainties exist in power systems and they will affect the stability analysis results. Voltage stability considering uncertainty in load parameters will be discussed. With the assumption that parameter variation is normal distribution, the probabilistic characteristics of eigenvalues under the uncertainties of dynamic load parameters can be obtained. Distribution of the critical eigenvalue will determine the stability probability of a power system. The stability margin can be inferred from the probabilistic critical load level, which is the maximal load level where system is 'probabilistically' stable. Case studies on three test systems illustrate that the stability margin will be reduced with load uncertainty. The proposed probabilistic results are validated using deterministic method of Monte Carlo on multi 10 000 sample studies.

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In power system studies, it has been long recognized that power system stabilizer (PSS) aims to provide appropriate phase lead in order to compensate the phase lag of generator excitation system. Therefore, lead compensation prevails in almost all stability studies and the effectiveness of PSSs with lead compensation was demonstrated by eigenvalue and swing plots, to confirm that their PSS designs can enhance the system damping under both small and large disturbances. The present paper will (a) explore whether the lead compensation of PSS has provided the correct phase compensation even with substantial improvements of system damping, and (b) highlight the risks due to lead compensation. A systematic technique based on eigenvalue sensitivity is recommended to effectively provide correct phase compensation and to suppress the risks in PSS tuning.

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