A Novel Modulation Method to Reduce Leakage Current and Balance Neutral-Point Voltage for Neutral-Point-Clamped Inverters

Due to the low device voltage stress and the small output harmonics in high-power applications, neutral-point-clamped(NPC) three-level inverters are widely used in grid-connected photovoltaic inverters. However, the traditional modulation strategy may lead to low-frequency oscillation of the neutral point voltage, increasing the output harmonic content. Moreover, the large leakage current can reduce the efficiency and life of the system. To address these issues, this paper presents the extended discontinuous phase opposition disposition PWM (EDPOD PWM) strategy. By using the hybrid H-P modulation which is based on carrier-based PWM modulation strategy (CBPWM) and extended modulation discontinuous PWM modulation strategy (EDPWM), the neutral point potential can be balanced in full power factor and modulation ratio ranges with low switching losses. By inversing the carrier of a specific phase, the small vector switching level generating large common mode voltage is eliminated. Then the amplitude of common-mode voltage can be limited to 1/6 bus voltage. The proposed strategy is verified by simulation. Compared with the original H-P modulation without optimized common-mode voltage, the proposed strategy can reduces the leakage current by 66%. Compared with the common mode voltage suppression strategy based on CBPWM and VSVPWM hybrid modulation, the leakage current suppression effect is more obvious, and the effective value is reduced by 33%. Finally, the feasibility and superiority of the proposed method are experimentally verified.