The all pass filter generates a virtual q-axis voltage component with $90^{circ}$ phase delay but the virtual q-axis voltage cannot detect the voltage depending on the phase of the grid voltage. To overcome the problem, q-axis voltage component is generated from difference between the current and previous value of d-axis voltage component in the stationary reference frame. However, the difference voltage between the current and previous value around the zero crossing voltage is not enough to detect the voltage sag. Therefore, the new detection scheme which can detect the sag around the zero crossing voltage is proposed.