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In this article, the soft-switching technique is an important way to achieve high frequency and high conversion efficiency for transformerless grid-connected inverters (TLIs) with distributed photovoltaic generation. Meanwhile, new technical standards require TLIs to have a wider power factor operation range to support grid voltage, which means that there are positive power regions and negative power regions in a grid period. In order to fulfill soft-switching for high-frequency working switches during all power regions, a pair of zero-voltage-transition (ZVT) resonant cells with different capacitor connections are first derived in this article, and two equivalent two-quadrant resonance networks are originated by using the pair of ZVT resonant cells alternatively. Furthermore, defined connection points of the obtained two-quadrant resonance networks in TLIs are explored, and a guideline to invent new ZVT-TLIs is summarized. As a result, two families of ZVT-TLIs with full power factor range (ZVT-TLIs-FPF) are obtained. In order further to verify the correctness of the proposed guideline, the obtained ZVT-HERIC-FPF is taken as an example to analyze its operation principle. Finally, experimental results from a 1 kW prototype at 100 kHz are provided to verify the effectiveness of the proposed structuring method. [ABSTRACT FROM AUTHOR]

Transformerless grid-connected inverters (TLI) feature high efficiency, low cost, low volume, and weight due to using neither line-frequency transformers nor high-frequency transformers. Therefore, TLIs have been extensively investigated in the academic community and popularly installed in distributed photovoltaic grid-connected systems during the past decade. This article analyzes and summarizes the state of the art of TLI techniques, three rules of maintaining constant common-mode voltage (CMV) of TLIs at switching frequency have been concluded from a generic CMV analysis model at the beginning. Second, suppression methods of leakage current (LC) and dc current injection (DCCI), so-called two key challenges for first-generation TLIs, have been classified and discussed in detail, respectively. Finally, future trends and some challenges of TLIs based on wide bandgap devices named second-generation TLIs have been presented in this article. [ABSTRACT FROM AUTHOR]

Unipolar sinusoidal pulsewidth modulation (SPWM) full-bridge inverter brings high-frequency common-mode voltage, which restricts its application in transformerless photovoltaic grid-connected inverters. In order to solve this problem, an optimized full-bridge structure with two additional switches and a capacitor divider is proposed in this paper, which guarantees that a freewheeling path is clamped to half input voltage in the freewheeling period. Sequentially, the high-frequency common-mode voltage has been avoided in the unipolar SPWM full-bridge inverter, and the output current flows through only three switches in the power processing period. In addition, a clamping branch makes the voltage stress of the added switches be equal to half input voltage. The operation and clamping modes are analyzed, and the total losses of power device of several existing topologies and proposed topology are fairly calculated. Finally, the common-mode performance of these topologies is compared by a universal prototype inverter rated at 1 kW. [ABSTRACT FROM AUTHOR]