Convex Relaxation of AC Optimal Power Flow with Flexible Transmission Line Impedances

Yue Song, David J. Hill, Tao Liu, Tianlun Chen

Published 2020-07-07Version 1

Flexible transmission line impedances on one hand are a promising control resource for facilitating grid flexibility, but on the other hand add much complexity to the concerned optimization problems. This paper develops a convexification method for the AC optimal power flow with flexible line impedances. First, we discover that a line with flexible impedance is equivalent to a constant-impedance line with a correlated pair of tap-adjustable ideal transformers placed at its two terminals. Then, with this circuit equivalent, we reformulate the original optimization problem as a semi-definite program under the existing convex relaxation framework, which improves the solution tractability and optimality. The proposed method is verified by numerical tests on the IEEE 118-bus system.