With the increase of the harmonic content of the metro power distribution system

the traditional harmonic control device is replaced gradually by the shunt active power filter（SAPF）. A mathematical model of output voltage and power on the AC side in PWM inverter in α-β coordinate system for shunt active power filter（SAPF）is established. For keeping the SAPF still have perfect harmonic suppression result without the need of accurate mathematical model

a kind of power flatness control strategy is proposed by way of choosing the output power of AC side of inverter as the state variable and the output of the system as well as AC output voltage as the input control variable. The design of the controller is achieved with reference of such two parts as feed forward control trajectories and the error feedback compensation. The feed forward plans to control variables’ reference trajectories according to desired system output and relationship between output and input control variables

while the error feedback compensation eliminates the error between the actual value of the output and the desired value. It is shown by the simulation result that the effectiveness and harmonic compensation result of the proposed control strategy is significant

which provides a kind of new idea for the design of SAPF power controller.