  It is challenging to assess the vulnerability of a cyber-physical power
system to data attacks from an integral perspective. In order to support
vulnerability assessment except analytic analysis, suitable platform for
security tests needs to be developed. In this paper we analyze the cyber
security of energy management system (EMS) against data attacks. First we
extend our analytic framework that characterizes data attacks as optimization
problems with the objectives specified as security metrics and constraints
corresponding to the communication network properties. Second, we build a
platform in the form of co-simulation - coupling the power system simulator
DIgSILENT PowerFactory with communication network simulator OMNeT++, and Matlab
for EMS applications (state estimation, optimal power flow). Then the framework
is used to conduct attack simulations on the co-simulation based platform for a
power grid test case. The results indicate how vulnerable of EMS to data
attacks and how co-simulation can help assess vulnerability.
