A method and an apparatus for detecting a vector shift in an AC power system, the apparatus comprising frequency measuring means (10) adapted to determine a frequency of the AC power system, derivation means (20) adapted to determine a time derivative of the frequency, filtering means (30) adapted to filter the time derivative of the frequency, and detection means (40, 50, 60, 70) adapted to calculate time derivatives for the filtered time derivatives of the frequency over a time period having the length of the fundamental period of the AC power system, to determine a number of positive and negative time derivatives among the calculated time derivatives, and to detect a vector shift in the AC power system in response to an absolute value of a difference between the determined number of positive time derivatives and negative time derivatives exceeding a predetermined threshold value.