The detection of voltage dips is one of the fundamental technologies to achieve renewable energy system low voltage ride-through capability.A flexible voltage sag depth detection method is presented and the influence of frequency drift and harmonics are considered in detail.In a static coordinate system，the voltage dips is calculated by use of two sample values of n sampling interval.The detection speed and precision is also analyzed when n is varying，and under abnormal frequency and waveform conditions.By changing the delay factor n，the method can flexibly compromise between the detection accuracy and detection speed.Simulation results verify the proposed detection method.