  The dynamic performance of the generators is a critical factor for the safe
operation of the power grid. To this extent, the stability of the frequency of
generators is the target of cyber attacks since its instability may lead to
sizable cascade failures in the whole network. In this paper, we perform the
vulnerability analysis in a developed power grid Hardware-In-The-Loop (HITL)
testbed with a Wago 750-881 PLC sending control commands to the generators and
a 750 Feeder Management Relay connected to a local load. A process-aware
coordinated attack is demonstrated by spoofing control commands sent by the PLC
and the relay to the simulated power system which is modeled as a
single-machine infinite-bus (SMIB). Based on the reachability analysis, the
attacker can find the optimal attack signal to drive the system state out of
their safe set of values. Thereafter, it is experimentally demonstrated that
the attacker does not need to send attack signal continuously if he implements
a carefully designed coordinated attack on the PLC and the relay. The presented
assessments provide information about the best time to launch an attack in
order to destabilize the power system.
