The method to detect phaseloss and phasesequence of quadrant frequency converter was focused on. Lock the phase and obtain the amplitudes of positive and negative phasesequence separately using dual second order generalized integrator PLL (DSOGIPLL). Analyze all possible threephase voltage of grid and compute the corresponding positive and negative amplitudes, combined with calculation of DSOGIPLL in real system, predict the phasesequence and faults happened to power supply. Simulation based on MATLAB/Simulink and experiment in real system showed that the method could make accurate judgment to faults and sequence of grid within 15 ms.