OWPF Solutions Using Polyhedral and Conic Relaxations

Water distribution systems (WDS) and power grids are critical infrastructure systems ensuring everyday human activity. A significant amount of research effort has been dedicated to finding optimal operation policies for each of those systems. WDS power consumption creates a dependency between the operation of those two systems, and several recent studies have dealt with the optimization of their conjunctive operation, also known as optimal water and power flow problem (OWPF). The combination of the WDS optimal operation problem and the optimal power flow (OPF) problem results in a non-convex MINLP problem, which poses significant mathematical and computational challenges. Previous studies have used different types of approximation methods to convexify the problem and obtain feasible solutions. These methods often lead to local optima and do not provide theoretical guarantees of global optimality. This study presents a tailored solution method for optimizing the conjunctive operation of WDS and power grids. The method relies on implementing polyhedral relaxations of the non-convex hydraulic constraints, together with conic relaxations to overcome non-linearities in the OPF problem. The approach allows for fast convergence and reduces running time significantly, which allows the solution of large-scale OWPF problems. Furthermore, the use of convex relaxations provides optimality gaps for the computed solutions. The method is tested on an example application, and its performance is compared with that of an off-the-shelf non-linear solver.