Research on the Synchronized Carrier-Based PWM Strategy Under Low Switching Frequency for Three-Level Neutral Point Clamped Inverter

According to the different directions of carrier disposing, carrier-based PWM (CBPWM) can be classified into in-phase disposition PWM (IPDPWM) and phase opposition disposition PWM (PODPWM). To design a synchronized modulation strategy for three-level neutral point clamped (3-L NPC) inverter, which is convenient to implement and satisfies waveform symmetry, the harmonic characteristics of IPD PWM and PODPWM are analyzed by using double Fourier series in this paper firstly. On this basis, the carrier ratio that can make the phase voltage of IPDPWM and PODPWM meet three-phase symmetry (TPS) and half-wave symmetry (HWS) are deduced. By mixing IPDPWM and PODPWM, a synchronized CBPWM (SCBPWM) that makes the phase voltage waveform satisfies TPS and HWS at the carrier ratio is an integer multiple of 3 is further proposed. Finally, a full frequency range modulation strategy based on SCBPWM is designed, which is easy to implement and ensures the maximum switching frequency is not more than 450Hz. The simulation results verify the validity of the SCBPWM.