  Robust operation of power transmission grids is essential for most of today's
technical infrastructure and our daily life. Adding renewable generation to
power grids requires grid extensions and sophisticated control actions on
different time scales to cope with short-term fluctuations and long-term power
imbalance. Braess' paradox constitutes a counterintuitive collective phenomenon
that occurs if adding new transmission line capacity to a network increases
loads on other lines, effectively reducing the system's performance and
potentially even entirely removing its operating state. Combining simple
analytical considerations with numerical investigations on a small sample
network, we here study dynamical consequences of secondary control in AC power
grid models. We demonstrate that sufficiently strong control not only implies
dynamical stability of the system but may also cure Braess' paradox. Our
results highlight the importance of demand control in conjunction with grid
topology for stable operation and reveal a new functional benefit of secondary
control.
