Operation Mode Transition Technique of Flexible Modulation Scheme for Single-phase Transformerless PV Inverters

The advanced modulation scheme with reactive power injection is preferred in single-phase transformerless PV inverters, (e.g., H5, H6, and HERIC), which can achieve high efficiency as well as flexible power control capability. Considering the turn-on and turn-off periods of real semiconductor devices, the deadtime is indispensable between the switching of bridge switches to avoid the short-through phenomenon. The deadtime should be inserted into not only the switching state transition (in one switching cycle) but also the operation mode transition (in one grid cycle). Among, the deadtime insertion during the operation mode transition always causes current distortions at the AC voltage zero-cross point, or even fails the mode transition. Thus, this paper proposes an improved switching mode transition strategy to ensure the exact mode transition and the high power quality of PV inverters. Simulation and experiments have been carried out on a grid-connected transformerless PV inverter prototype, verifying the excellent performance on the power quality of the proposed technique.