The stability margin of the system pertains to the regions of acceleration and deceleration in the power-angle curve. Consider the power angle curves shown in Fig. given below.
The curve of Fig. first, is for an uncompensated system, while that of Fig. second, for the compensated system. Both these curves are drawn assuming that the base power is V2/X . Let us assume that the uncompensated system is operating on steady state delivering an electrical power equal to Pm with a load angle of δ0 when a three-phase fault occurs that forces the real power to zero. To obtain the critical clearing angle for the uncompensated system is δcr , we equate the accelerating area A1 with the decelerating area A2