  False Data Injection (FDI) attacks are one of the challenges that the modern
power system, as a cyber-physical system, is encountering. Designing AC FDI
attacks that accurately address the physics of the power systems could
jeopardize the security of power systems as they can easily bypass the
traditional Bad Data Detection (BDD) algorithm. Knowing the essence of the AC
FDI attack and how they can be designed gives insight about detecting the
system again these attacks. Moreover, recognition of the nature of these
attacks, especially when they are designed optimally, is essential for
benchmarking various defensive approaches to increase the resilience of power
systems. This paper presents a unified approach to demonstrate the process of
designing optimal AC FDI attack. In this connection, we first define the
process of designing an AC-based FDI attack that satisfies AC power flow
equations. We then formulate an optimization problem to design an optimal AC
FDI attack that both satisfies AC power flow equations and overloads a specific
line in the system. The objective function is defined to optimize the magnitude
of the attack vector in such a way that it can evade residue-based BDD
approaches. The proposed approach for designing AC FDI attacks is applied to
the IEEE 118-bus test case system. Various comparisons are conducted to
elaborate on the impact of optimally designing AC FDI attacks on the residual
for the AC state estimation algorithm. Comparing the results of optimal and
non-optimal AC FDI attacks demonstrates the impact on the difficulty of
detecting FDI attacks and the importance of optimally designing these attacks.
