“…Notice that although variations of θ i,j depend on voltage magnitudes based on the physical laws, the results of this section are independent of these effects. In fact, the variations of the relative angles will cause variations in ∆ c 1,2 and ∆ s 1,2 , which are both bounded and take a value in the set [−1, +1], in Ψ(θ(t)) (16). Thus, without making any specific assumption on the dynamics of θ i,j , we can mathematically model the variations of θ i,j as a time varying variable which takes a value in [−1, +1].…”