Preventing blackouts through reactive rescheduling under dynamical and protection system constraints

The 28 September 2003 blackout in Italy raised doubts about the ability of distance relays in distinguishing voltage collapse and remote short-circuits. The combined presence of reduced voltages and high power flows may trigger distance relays, with the consequent tripping of healthy lines, reducing the system survivability. The paper addresses an analysis of the behavior of distance protections during the sequence of events that led to the separation of the Italian network from the UCTE. An approach based on reactive power redispatching, aimed at mitigating the effects of cascading events, is presented. The proposed methodology allows to reduce the need for remedial control actions such as load/generation shedding which have a major economical impact. The proposed approach evaluates control actions aimed at ensuring stability and keeping system trajectories off the tripping zones of distance relays. Test results are obtained by implementing this methodology on a test case based on the Italian power system configuration.