As can be seen, the VQ curve does not depart very much from a straight line
around this point. Curve 2 refers to more loaded situation. The operating point without
compensation is O´, where the curvature of the curve is more pronounced. The Q1 and Q2
values shown in figure 2.10 are reactive power margins with respect to the loss of an
operating point. These corresponds to the minimum amount of reactive load increase for
which there is no operating point any more [3]. Finally, curve 3 corresponds to a situation
where the system cannot operate without reactive power injection. It might result from a
severe disturbance that increases X. the shown margin Q3 is negative and provides a
measure of the Mvar distance to system operability