Swing Curve

friends,i have a great problem of understanding the stability concept in power system,

figure shown above is the curve of power angle Vs time. figure a is stable ,but the figure b is unstable. i cant undrstnd the reason.

the critical clearing angle of figure (a) is 0.3s and for figure (b) is 0.4s.....

we know that stablility is nothing but ability of generator to attain synchronous speed,for that the delta should be zero.

according to this figure (a) only reaches the zero ,but the figure(b) is not

little bit confusing friends,please help me,i will be thankful to u.......