Sag voltage affects the quality of the grid voltage and the working conditions of the electrical equipment. In order to prevent effective this problem, the authors researched the method estimation of fast and accurate voltage and phase angle in the presence of sag voltage. The method proposed Least Error Squares (LES) were constructed on the basis of the least squares algorithm and Taylor series expansion. Compared with conventional Clark and Park methods, the LES approach has the following major advantages: faster detection time, reduced deviation of estimated voltage values at the time of failure; faster detection time, reduced deviation of estimated voltage values at the time of failure; Not affected by other fundamental components 50Hz, the DC components in the grid. The effectiveness of the method is verified by simulation model of a distribution grid in Vinh Phuc province by Matlab / SIMULINK software
