Voltage sag detection algorithm for voltage sag corrector is proposed in this paper. To quantify the standard of voltage unbalance under the faulted conditions, the 3-phase unbalanced voltages are decomposed into two balanced 3-phase symmetrical components of the positive and negative sequence voltages, which is defined by the magnitude factor (MF) and unbalance factor (UF). It is analyzed that MWF and UF values are given as the dc constant values even though under the voltage unbalance condition. This paper also proposes the control scheme of the instantaneous voltage sag corrector based on this detection algorithm. The validity of the proposed algorithm is verified through the EMTDC simulation and experiments.