We consider the problem of identifying the risk of mid-term voltage instabilities subsequently to an important disturbance. In this context the main physical phenomena which influence the system behavior in the post-contingency state are the action of machine overexcitation limiters and on-load tap-changers (OLTCs) which act so as to restore HV and MV voltages to their nominal value (thereby tending to restore the pre-contingency load level) and possibly secondary voltage control as well as automatic shunt-compensation.
For a mid-term dynamic system evolution to be considered as acceptable, it is generally required to reach a satisfactory equilibrium state e.g. in terms of EHV and HV voltages and/or sensitivities. In addition, to cope with possible load increase, it is often required in practice that at equilibrium the load-power margin be larger than a threshold.