Three-level NPC rectifier

compared to the traditional two-level rectifier

has such characteris-tics as adjustable power factor

small current harmonics at the network side

two-way energy flow and smallvoltage stress withstand by the elements etc. When PWM rectifier is used in the charging of electric vehicleat constant power

the traditional direct current control strategy will lead to secondary fluctuation of power.In this paper

the direct power control strategy of three level type NPC PWM rectifiers which is suitable forconstant power load is studied. As for the actual issue the switching frequency is not constant of elementdue to traditional three level direct power controller

the quasi fixed direct power control strategy for a newtype of switching frequency is proposed in this paper

and the fluctuation of switching frequency of elementis reduced by the introduction of vector action time duty ratio. It is shown by the simulation result that thequasi-constant switching frequency direct power control strategy

compared to traditional direct power con-trol strategy

can optimize the switching frequency of element and THD characteristic of output voltage.