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An adaptive algorithm to prevent undesirable distance protection operations during voltage instability is proposed. The algorithm is based on mathematical logic blocks and uses the rate of change of voltage as an additional relay criteria to increase the relay security with respect to voltage instability. Studies are based on simulations using two different test systems; a 15-bus system developed by the authors and the Nordic32 system. The investigation shows that undesirable zone 3 distance relay operations can be the difference between a blackout and a recovering system during voltage instability. However, simulations have shown that the adaptive algorithm may save the system from a collapse. Although the algorithm prevents mal-trips due to voltage instability, the reach of the distance relay will not be restricted.