In this paper, a fast voltage sag detector based on peak detection is proposed. In the proposed method, the (−90)-degree phase shifter is used to generate an orthogonal AC voltage (cosine wave, V cos ) from the existing AC supply voltage (sine wave, V sin ). The differentiator is designed to function as this phase shifter. The AC supply voltage can be accurately tracked by the synthetic cosine wave. The both of sine wave and cosine wave are used to calculate the DC voltage (V av ) which is reflected to the amplitude of the AC supply voltage. Lastly, the voltage sag can be observed by comparing this DC voltage to the reference DC voltage. The performance of this proposed voltage sag detector is verified by the computer simulations. It can be seen that the proposed method can detect the voltage sag in a very short time.