  Future power grids will offer enhanced controllability due to the increased
availability of power flow control units (FACTS). As the installation of
control units in the grid is an expensive investment, we are interested in
using few controllers to achieve high controllability. In particular, two
questions arise: How many flow control buses are necessary to obtain globally
optimal power flows? And if fewer flow control buses are available, what can we
achieve with them? Using steady state IEEE benchmark data sets, we explore
experimentally that already a small number of controllers placed at certain
grid buses suffices to achieve globally optimal power flows. We present a
graph-theoretic explanation for this behavior. To answer the second question we
perform a set of experiments that explore the existence and costs of feasible
power flow solutions at increased loads with respect to the number of flow
control buses in the grid. We observe that adding a small number of flow
control buses reduces the flow costs and extends the existence of feasible
solutions at increased load.
