A Modified Variable Carrier-Based Discontinuous PWM Strategy

Due to switching loss limitations, low carrier ratio operating conditions are common for medium- or high-voltage, high-power converters. In grid-connected applications, commonly used modulation techniques such as Sinusoidal Pulse Width Modulation (SPWM) and Space Vector Pulse Width Modulation (SVPWM) often fail to meet the grid requirements for current harmonic distortion under low carrier ratios. The adoption of multilevel techniques and Discontinuous Pulse Width Modulation (DPWM) can significantly reduce switching losses and improve the carrier ratio. However, conventional modulation strategies based on fixed carriers tend to generate substantial low-frequency harmonics at low carrier ratios, complicating the design of grid-connected filters. Selected Harmonic Elimination PWM (SHEPWM) can eliminate low-frequency harmonics, which can facilitate higher-order filters to meet grid harmonic requirements. However, its limited dynamic response hinders its widespread application. This paper presents a modified DPWM strategy based on variable triangular carriers (VC-DPWMA), implemented on a 3-level inverter. The proposed method achieves harmonic performance comparable to SHEPWM at low carrier ratios while overcoming the drawbacks of slow dynamic response. A low-power hardware platform is used to realize the proposed approach. Both simulation and experimental results validate the theoretical analysis and demonstrate the feasibility of the proposed method.