Capability curves are a plot of apparent power capability (MVA), at rated voltage, using active power (MW) and reactive power (MVAR) as the two principle axis.
Circumferences drawn with their centers at the origin represent curves of constant stator current. A capability curve (see figure below) separates the region of allowed operation, inside the curve, from the region of forbidden operation, outside the curve.
On an x-y graph where the x axis represents MW and the y axis represents MVAR, a circumference represents a constant MVA. If, as in the case of a machines capability curve, the voltage is kept constant (at rated value), a circumference also represents a constant-current trajectory.
On the same graph, any line starting at the intersection of the axis represents a particular power factor. Different parts of the capability curve are limited by different machine components. There is a part limited by field winding capability, a part limited by stator winding capability (the circular part), and a part limited by core-end heating.
As the power factor is varied from fully overexcited, through unity to fully underexcited, first the field current, then the stator current, and then the stator core-ends are limiting. Accordingly curves that define a turbine-generator’s capability have three segments that pictorially describe the effect of the capability of the three machine components.