  In the transition to achieving net zero emissions, it has been suggested that
a substantial expansion of electric power grids will be necessary to support
emerging renewable energy zones. In this paper, we propose employing
battery-based feedback control and nonlinear negative imaginary (NI) systems
theory to reduce the need for such expansion. By formulating a novel
Lur\'e-Postnikov-like Lyapunov function, stability results are presented for
the feedback interconnection of two single nonlinear NI systems, while output
feedback consensus results are established for the feedback interconnection of
two networked nonlinear NI systems based on a network topology. This
theoretical framework underpins our design of battery-based control in power
transmission systems. We demonstrate that the power grid can be gradually
transitioned into the proposed NI systems, one transmission line at a time.
