A Hybrid Transmission Grid Architecture Enabling Efficient Optimal Power Flow

TL;DR

This paper proposes a hybrid AC-HVDC transmission grid architecture that allows for efficient, globally optimal solutions to the OPF problem, increasing capacity and reducing costs.

Contribution

It introduces a systematic upgrade to HVDC on selected lines creating a hybrid grid that enables convex relaxation of the OPF problem for the first time.

Findings

Hybrid architecture increases transmission capacity.

It reduces generation costs significantly.

Enables polynomial-time globally optimal solutions.

Abstract

The recent rise of electricity generation based on renewable energy sources increases the demand for transmission capacity. Capacity expansion via the upgrade of transmission line capacity, e.g., by conversion to a high-voltage direct current (HVDC) line, is an attractive option. In this paper, it is shown that if the upgrade to HVDC is applied systematically to selected transmission lines across the grid, a hybrid architecture is obtained that enables an efficient and globally optimal solution of the optimal power flow (OPF) problem. More precisely, for conventional meshed AC transmission grids the OPF problem is nonconvex and in general NP-hard, rendering it hard to solve. We prove that after the upgrade to the proposed hybrid architecture, the same mesh topology facilitates an exact convex relaxation of the OPF problem, enabling its globally optimal solution with efficient polynomial time algorithms. This OPF method is then employed in simulations, which demonstrate that the hybrid architecture can increase the effective transmission capacity and substantially reduce the generation costs, even compared to the AC grid with optimal transmission switching.