The Alternate Arm Converter (AAC) is promising multilevel Voltage Source Converter (VSC) suitable for High Voltage Direct Current (HVDC) transmission systems. This converter exhibits interesting features such as a DC Fault Ride Through capability thanks to the use of Full-Bridge Sub-Modules (SM) and a smaller footprint than an equivalent Modular Multilevel Converter (MMC). After an analysis of the converter operating modes called Non-overlap and Overlap mode, a sequential representation of the AAC operation is proposed. The main originality of this paper is the use of the Petri Net to describe all the phases and to highlight their sequencing. According to the phases identified thanks to the sequential approach, models and control structures for the grid currents, the internal energy and the Zero Current Switching (ZCS) are detailed. Furthermore, the step-by-step approach proposed in this paper allows a clear and rigorous modelling of this complex converter.