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SUMMARYThis paper deals with a new detection method based on three-phase vector operation for voltage sag. The detection method is applicable to a protection system using a high-speed interruption system which protects distributed generators and critical loads in a customer's facility. The algorithm and merits of this method are explained in detail. The detection characteristics are then analyzed using the ATP (Alternative Transients Program) and are confirmed using a real-time digital simulator, and it is shown that the method is useful for detecting voltage sag.