Based on the mathematic model of DFIG， the paper analyses the composition of instantaneous active and reactive power from the stator under unbalanced grid-voltage conditions. By adding a second frequency notch filter， the constant switching frequency direct power control（CSF-DPC） can be used for the unbalanced fault ride through operation. In this case， each improved CSF-DPC method presents its own unique characteristic， i.e. the rotor-flux based DPC（RF-DPC） can stabilize the flux and shorten the DFIG dynamic process; the rotor-current based DPC（RC-DPC） can suppress the oscillations of stator and rotor current; and the electromagnetic torque based DPC（EMT-DPC） has the ability of reducing the torque oscillation. On the basis of analysis on the errors caused by the DFIG parameter mismatch， the control robustness of the improved CSF-DPC schemes is further improved. In contrast to the conventional control strategy that needs negative sequence components and feedback loop， the proposed new CSF-DPC has a simpler structure and better dynamic performance.