Voltage instability - the different shapes of the “Nose”

This paper was elaborated to enlarge the understanding on the dependence of the shape of the "nose" of a Power-Voltage (PV) curve in a given EHV bus, by the power system dynamics. The paper shows that the local control loops are strongly involved in the voltage instability phenomenon and how they modify the shape of the nose curve. The paper also analyzes how these local control loops, together with the local load characteristics, impact on the "speed to run" the PV curves and on the stability of their equilibrium points. A detailed analysis of the nose shapes and characteristics is preliminarily performed for each relevant control loop and different types of loads. The subsequent analyses refer to a realistic load representation and compare, in terms of nose characteristics, the results of some Italian EHV buses with a proposed equivalent dynamic model, giving evidence of the strong correlation between the two and confirming the relevant differences with the corresponding static nose curves. The focus given in the paper was mainly motivated by the new opportunities to estimate the proximity to voltage instability from fast and synchronized bus measurements, provided by a local Phasor Measurement Unit (PMU).