Discontinuous PWM Strategy for Neutral Point Clamped Three-Level Inverter to Achieve Multiple Control Objectives

In order to reduce switching loss of neutral-point clamped (NPC) three-level inverter (TLI), a switching loss minimized discontinuous PWM (SLMDPWM) strategy is proposed firstly. There are DC offset and AC ripple on the neutral-point (NP) voltage when SLMDPWM is used, which causes an appearance of low frequency harmonics (LFHs) in the output voltage. Aiming at this drawback, reduced LFHs (RLFHs) method is then proposed. While using RLFHs, the self-equilibrium ability of NP voltage is thus lost. So, active neutral-point voltage control (ANPVC) is also proposed. The performances of NPC TLI with the proposed DPWM and other DPWMs are compared by simulation and experimental results. The results show that switching loss, LFHs can be reduced significantly and the NP voltage can be controlled within an allowable range. Therefore, the multiple control objectives of reduced switching loss, no LFHs and allowable NP voltage are achieved.