Semidefinite Relaxation of Optimal Power Flow for AC-DC Grids

The proliferation of technologies operating on dc power has motivated the system planners toward integration of dc and ac grids. The optimal power flow (OPF) analysis is widely used to determine the economically efficient operating points of the power grids. The OPF problem in ac-dc grids is a non-convex optimization problem due to the nonlinear power flow equations and the operating constraints imposed by the ac-dc converters. In this paper, we study the OPF problem in ac-dc grids to address the non-convexity of the problem. The objective of the ac-dc OPF problem is to jointly minimize the generation cost and the losses on the lines and converters. The optimization problem is subject to the ac and dc power flow constraints, the limits of the voltages and line flows, and the operating limits of the converters. We use convex relaxation techniques and transform the problem to a semidefinite program. We derive a sufficient condition for zero relaxation gap to obtain the global optimal solution. Simulations are performed on an IEEE 118-bus test system connected to sample dc grids. We show that the zero relaxation gap condition holds for the case study and the global optimal solution can be obtained.