Distance protection and voltage stability

In this paper the influence of zone 3 distance relays in the case of voltage instability is examined. Different types of relay characteristics are analysed. An adaptive algorithm to prevent undesirable zone 3 operations during voltage instability is proposed. Studies are based on simulations in a fifteen bus system developed by the authors. Data on the mixture of electromechanical, solid-state and numerical distance relays in the Swedish transmission system are presented together with statistics of zone 3 operations from the start of year 1985 until the end of 1998. The investigation shows that undesirable zone 3 distance relay operations during voltage instability can be the difference between a total blackout and a recovering system. Simulations have shown that the adaptive algorithm may save the system from a collapse