maximum value δm. The principle which governs this motion of the rotor is known as the ‘equalarea
criterion’, and refers to the fact that at the maximum angle reached by the rotor the area in
the P − δ space while the rotor is accelerating is equal to the area when the rotor is decelerating.
It should be clear from Figure 10.5(b) that the maximum available decelerating area corresponds
to a maximum angle δm = π − δ0. If the total accelerating area is smaller than the maximum
available decelerating area, the resulting rotor oscillation is stable as shown in Figure 10.5(c) by
the solid line. Otherwise the oscillation is unstable, and rotor angle increases indefinitely, and the
machine is said to have lost synchronization. An unstable rotor motion is shown by the dotted line
in Figure 10.5(c). The protection problem associated with transient instability will be considered
in Chapter 11.