Convex Relaxation of Optimal Power Flow, Part I: Formulations and   Equivalence
  This tutorial summarizes recent advances in the convex relaxation of the optimal power flow (OPF) problem, focusing on structural properties rather than algorithms. Part I presents two power flow models, formulates OPF and their relaxations in each model, and proves equivalence relations among them. Part II presents sufficient conditions under which the convex relaxations are exact. 
Steven H. Low
math.OC cs.SY