  Power systems are increasingly operated in corrective rather than preventive
security mode, which means that appropriate control actions must be taken
immediately after a contingency has occurred. This paper proposes an online
algorithm for automatically alleviating contingencies such as voltage limit
violations and line overloads. Unlike previously proposed approaches, the
network itself serves as a natural solver of the power flow equations. This
makes it possible to start the implementation immediately and avoids problems
caused by modeling errors. Every time the controller receives measurements from
the grid, it evaluates the presence of contingencies and computes the optimal
corrective actions that can be implemented before the next sampling period,
subject to ramping constraints of the generators. These corrective actions are
implemented through the standard Automatic Generation Control. Finding the
optimal incremental corrective actions is fast because this problem is
linearized. The effectiveness of this algorithm at correcting both line
overloads and voltage violations is demonstrated using the IEEE-118 Bus test
system.
