Abstract: Active power filter（APF）has become an important measure for harmonic control because it can

track and compensate reactive power and suppress harmonics adaptively，dynamicly，and in real time. APF was

taken as the research object，and on the premise of verifying the differential flatness of the system，a differential

flatness based control（DFBC）of APF was proposed under the condition of unbalanced power grid. Firstly，the

differential flatness of APF system was verified theoretically based on the voltage and current model of APF. Then

the corresponding positive and negative sequence control system were designed. Finally，the Matlab simulation

experiment was carried out under the condition of unbalanced power grid，and the simulation waveform of

traditional PI control strategy was compared to verify the effectiveness and superiority of the control system

designed.

      Key words: active power filter（APF）；differential flatness based control（DFBC）；the condition of unbalanced power grid；PI control