This book focuses on a safety issue in terms of leakage current, builds a common-mode voltage analysis model for TLIs at switching frequency scale and develops a new modulation theory referred as "Constant Common-Mode Voltage Modulation" to eliminate the leakage current of TLIs. Transformerless Grid-Connected Inverter (TLI) is a circuit interface between photovoltaic arrays and the utility, which features high conversion efficiency, low cost, low volume and weight. The detailed theoretical analysis with design examples and experimental validations are presented from full-bridge type, half-bridge type and combined topologies. This book is essential and valuable reference for graduate students and academics majored in power electronics; engineers engaged in developing distributed grid-connected inverters; senior undergraduate students majored in electrical engineering and automation engineering.