A 3-D generalized direct PWM for 3-phase 4-wire APFs

A 3-dimensional (3D) generalized direct PWM algorithm for multilevel converters in a three-phase four-wire active power filter is proposed. It is proved to be equivalent to the newly proposed generalized 3D space vector modulation (SVM). However, the direct PWM greatly simplifies the control algorithm and is much easier to implement in digital processors. The direct PWM is applied to an active power filter, where a three-level NPC inverter is used. Important issues for implementing the DPWM in three-level inverter, such as dc voltage variation control and over-modulation, are discussed in detail. A dc voltage variation control strategy in a-b-c coordinates is also proposed. Simulation results are given to show the validity of the proposed control strategy.