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.

中文翻译：

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无变压器光伏并网逆变器概述

无变压器并网逆变器（TLI）由于既不使用工频变压器也不使用高频变压器，具有高效、低成本、体积小、重量轻的特点。因此，TLI 在学术界得到了广泛的研究，并在过去十年中广泛安装在分布式光伏并网系统中。本文分析并总结了 TLI 技术的最新进展，从一开始的通用 CMV 分析模型得出了在开关频率下保持 TLI 的恒定共模电压 (CMV) 的三个规则。其次，对泄漏电流 (LC) 和直流电流注入 (DCCI) 的抑制方法，即第一代 TLI 的两个关键挑战，分别进行了分类和详细讨论。最后，