Emergency control measures against long term voltage instability

System collapse may be caused by undesired protection relay operations during voltage instability. In this paper, the analysis of different emergency control measures is presented as means to prevent voltage collapse when long term voltage instability is evolving in the system. Being the first line of defence after such unexpected relay trip, the emergency control of load tap changers (LTC) of power transformers during undervoltage conditions is investigated. Alternatively, load shedding is applied to avoid undesired Zone 3 distance relay trips. Sensitivities of Zone 3 trip margin to load powers are calculated to determine the most efficient load shedding actions to prevent undesired operation of critical relays. The effectiveness of the proposed emergency control measures is demonstrated though simulation results for the Hellenic Interconnected System model.