Emergency tap-blocking to prevent voltage collapse

This paper proposes a technique for the selection of tap changers to be blocked as an emergency measure, in the case of long-term voltage instability. The instability is identified using a long-term sensitivity index and the tap changers to be blocked are selected based on the column sum of a voltage sensitivity matrix. This method is applied to a severe contingency in the heavily loaded Hellenic Interconnected Power System and it is shown to be able to mitigate the voltage degradation in the event of a long-term instability, so that an imminent voltage collapse is prevented