  Intelligently designed false data injection (FDI) attacks have been shown to
be able to bypass the $\chi^2$-test based bad data detector (BDD), resulting in
physical consequences (such as line overloads) in the power system. In this
paper, it is shown that if an attack is suddenly injected into the system, a
predictive filter with sufficient accuracy is able to detect it. However, an
attacker can gradually increase the magnitude of the attack to avoid detection,
and still cause damage to the system.
