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This paper discusses methods for voltage-sag source detection, which are based either on energy, current, or impedance criteria. It is shown that methods known from the literature do not work well, particularly in cases of asymmetrical voltage sags. Therefore, generalized methods for voltage-sag source detection are proposed, using a vector-space approach. All the discussed methods, both the already known and the proposed ones, were tested by applying extensive simulations, as well as laboratory and field tests. The correctness of the proposed generalized methods was verified in this way. The obtained results show that all the proposed methods are highly effective in all cases of voltage sags, whereas the overall effectiveness of those methods known from literature is mainly unacceptable.