Hybrid Improved Carrier-Based PWM Strategy for Three-Level Neutral-Point-Clamped Inverter With Wide Frequency Range

The three-level neutral-point-clamped inverter used in the variable frequency system has the characteristics of a wide frequency range, and the pulsewidth modulation (PWM) strategy needs to be designed for the low-frequency region with low modulation index and the high-frequency region with low carrier ratio, respectively. The traditional carrier-based PWM (CBPWM) is simple to implement, but in the low-frequency region, CBPWM will cause narrow pulse and output voltage distortion; while in the high-frequency region, CBPWM can only achieve the synchronized modulation with half-wave symmetrical (HWS) and three-phase symmetrical (TPS) waveforms when the carrier ratio is an odd integer multiple of three, and it cannot realize the square-wave modulation. To solve the defects of CBPWM, this article proposes improved CBPWM (ICBPWM) I, which can avoid narrow pulse and generate line voltage accurately at any low modulation index, ICBPWM II, which can ensure the voltage to meet TPS and HWS when the carrier ratio is an arbitrary integer multiple of three, and ICBPWM III, which can achieve square-wave modulation, separately. On this basis, by using ICBPWM I, ICBPWM II, and ICBPWM III in combination, a hybrid ICBPWM strategy with wide frequency range is further proposed. Finally, the proposed strategy is verified by simulation and experiment.