  We study moving-target defense (MTD) that actively perturbs transmission line
reactances to thwart stealthy false data injection (FDI) attacks against state
estimation in a power grid. Prior work on this topic has proposed MTD based on
randomly selected reactance perturbations, but these perturbations cannot
guarantee effective attack detection. To address the issue, we present formal
design criteria to select MTD reactance perturbations that are truly effective.
However, based on a key optimal power flow (OPF) formulation, we find that the
effective MTD may incur a non-trivial operational cost that has not hitherto
received attention. Accordingly, we characterize important tradeoffs between
the MTD's detection capability and its associated required cost. Extensive
simulations, using the MATPOWER simulator and benchmark IEEE bus systems,
verify and illustrate the proposed design approach that for the first time
addresses both key aspects of cost and effectiveness of the MTD.
