An Effective DPWM Realization Method to Suppress the CM Resonance for MLCL-Filtered Grid-Connected PV Inverters

The modified LCL (MLCL) filter is widely used in the transformerless PV string inverter to suppress the common mode (CM) harmonic current flowing into the power grid. Whereas, the CM current resonance may arise in the inverter side current due to the MLCL filter. The CM harmonic in the inverter side current is related not only to the control strategy but also to the modulation method. In this paper, we compare the CM harmonic suppression ability of two discontinuous pulse width modulation (DPWM) realization methods, the negative carrier flipped (NCF) method and the negative carrier shifted (NCS) method. Since the NCF method has a smaller CM voltage compared with NCS method in the zero-clamped region, we conclude that the CM harmonics with NCF method should be much smaller than those with NCS. Finally, the theoretical analysis is verified by the experiment.