As the steady-state equilibrium is disturbed by a fault, the effective reactance between the two
machines changes, and it changes again when the fault is cleared. As the reactance changes, and
the rotor angles cannot move instantaneously to compensate for this change, the rotor is subjected
to an accelerating torque, which depends upon the angle δ. The resulting equation of motion can
be solved by the familiar equal-area criterion2 in the case of a two-machine system.
Consider the simple two-machine power system shown in Figure 10.5. Before the occurrence of
the fault F the machine is operating at an initial rotor angle δ0 and the accelerating power is zero
as shown in the following equation: