In order to identify unbalanced three-phase voltage sags characteristics, as well as to meet the real time voltage detection requirements of dynamic voltage restorer, a new method based on traditional dq transformation to detect voltage sags is presented. At first, the fuzzy control is applied to mathematical morphology filter to pretreat harmonics and random/impulse noise in power grid. Then, a derivative method is used for each single-phase voltage to obtain a three-phase virtual voltage. Finally, voltage sags can be detected instantaneously with duration, magnitude and phase-angle jump when three-phase virtual voltage is treated by dq transformation and new filter replaces Butterworth low-pass filter. Compared with traditional dq detection algorithm, this method has advantages of good anti-jamming capability, high speed in execution as well as simple implementation.