Figure 1.1(a) shows the simplified model of a power transmission system. Two power grids are connected by a transmission line which is assumed lossless and represented by the reactance XL. V ∠δ 11 and V ∠δ 22 represent the voltage phasors of the two power grid buses with angle δ= δ1- δ2 between the two. The corresponding phasor diagram is shown in Figure 1.1(b).