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A method to determine the origin of voltage sag disturbance is proposed in this paper. Its principle is to estimate the equivalent impedance of the nondisturbance side by utilizing the voltage and current changes caused by the disturbance. The sign of the real part of the estimated impedance can reveal if the disturbance is from upstream or downstream. The proposed method can be easily implemented into power quality meters for troubleshooting applications or into digital revenue meters to document the responsibility for disturbances. Extensive field and laboratory test results have proven the effectiveness and robustness of the proposed method.