Displacement polarization is a process in which positive and negative charge centers are pulled apart by electric field. You should know that alternating current contains alternating current component and direct current component. The DC component makes it produce displacement polarization, but the AC component changes too fast to reverse the orientation polarization, so the displacement polarization plays a major role at this time. Not all microscopic processes are instantaneous, and the orientation polarization of electric dipole moment has a relaxation time. When the alternating current period is much less than the relaxation time, the oriented polarization can be ignored.