RESEARCH ARTICLE A new AC OPF tool for sub‐transmission networks considering distribution switching actions and load‐ transferring capability Mohammad Ali Tavakkoli | Nima Amjady Department of Electrical Engineering, Semnan University, Semnan, Iran Correspondence Mohammad Ali Tavakkoli and Nima Amjady, Department of Electrical Engineering, Semnan University, Semnan, Iran. Email: mo‐tavakoli@semnan.ac.ir; amjady@semnan.ac.ir Abstract In practical power systems, in order to study the results of optimal power flow (OPF) for a sub‐transmission network (STN), the downstream primary distri- bution network is usually modeled as constant load. However, in practical dis- tribution grids, the operators are constantly transferring load from one primary distribution feeder to another one because of, eg, congestion problems or main- tenance plans. Considering distribution switching actions can improve the per- formance of sub‐transmission OPF tool that typically aims at reducing the operation cost of STN. This paper presents a new sub‐transmission Alternating Current (AC) OPF (ST‐AC‐OPF) considering load transferring (LT) between primary distribution feeders implemented through distribution maneuvers. The proposed OPF tool is tested on an illustrative practical example as well as on a real‐world Tehran STN. The numerical results show that the proposed model can obtain a more optimal operating point than conventional ST‐AC‐ OPF in terms of the operation cost and load shedding. KEYWORDS load transferring (LT), primary distribution feeder, sub‐transmission AC OPF (ST‐AC‐OPF), sub‐ transmission network (STN), switching action 1 | INTRODUCTION 1.1 | Motivation and background Optimal power flow (OPF) aims at optimizing the operating state of power system, eg, by minimizing the total produc- tion cost, subject to the technical and economic constraints of power system. For practical power systems, OPF is usu- ally formulated as a large‐scale complex mathematical optimization problem. For instance, AC OPF is nonlinear and nonconvex because of including AC power flow equations. Both mathematical and heuristic methods have been used to solve OPF. A review of earlier mathematical OPF solution methods, such as sequential quadratic programming, New- ton's method, linear programming, and interior point methods, can be found in previous studies. 1,2 To enhance the per- formance of these methods, various decomposition and transformation techniques, such as Benders decomposition with special ordered sets 3 and Signomial convex transformation, 4 have been presented in the newer mathematical OPF solu- tion methods. In addition, heuristic optimization methods have been also proposed to solve OPF problems and a review of these methods can be found in other studies. 5,6 Received: 23 October 2018 Revised: 18 January 2019 Accepted: 25 February 2019 DOI: 10.1002/2050-7038.12029 Int Trans Electr Energ Syst. 2019;e12029. https://doi.org/10.1002/2050-7038.12029 © 2019 John Wiley & Sons, Ltd. wileyonlinelibrary.com/journal/etep 1 of 17