Fuzzy set approach to transformer differential relay

In the process of decision making by a transformer differential relay often occurs a measure of uncertainty. It is so, because the differential current may be caused by other reasons than internal faults. Because of that, to improve reliability the relays are comparatively slow, because the correct decision requires at least one full cycle of current, and sometimes the delay is much longer. To improve it one may use several criteria and apply the so called `a fuzzy set approach´. Fuzzy sets have become a very powerful mathematical tool to describe quantitatively uncertain values and relations between them. They are perfect to solve the decision making problems in cases when symptoms are uncertain and the situation unclear. If applied to the protective relays they make the best compromise between the speed and selectivity of the protections