Abstract: The synchronous grid connection of medium-voltage mobile generator units stands as a pivotal link in live-line operations of distribution networks, and its success rate directly underpins operational safety and power supply reliability. This paper presents an in-depth analysis of a tripping incident encountered during the synchronous grid connection process of a medium-voltage mobile generator unit. First, it outlines five typical operating modes of such units and identifies four critical determinants of successful grid synchronization. Building upon this theoretical foundation, an investigation is conducted in conjunction with a real-world case study, focusing on the issue of generator unit tripping induced by undervoltage at the terminals of a long transmission line. This paper aims to provide empirical insights and references for the on-site application of mobile generator units in live-line distribution network operations.