A. Power converter controlConsideri

A. Power converter controlConsidering a DC charging system for a DC-based propulsion system applicable for a scheduled ferry, as the system shown in figure 2 (a), the power converters involved in the charging path have different control objectives. At the onshore station, the grid is interfaced through a transformer and a rectifier. Converter C35 is responsible for the shore bus voltage or DC side voltage control and can provide AC side voltage control or reactive power control taking into account power quality issues for the grid. Usually, a two-level voltage source converter is selected for C35. In this regard, a common control system for the C35 is showed in the figure 14, which includes a synchronization block, which is typically a phase locked loop (PLL), a current controller, usually a Proportional-Integral (PI) controller, pulse width modulation (PWM) generator, which generates signals for driving switches in the power converter, and outer loop controllers for power flow or DC voltage and AC voltage or reactive power, providing the active and reactive (d- and q-axis) current references. Besides the grid interface, there is an onshore battery pack which is connected to the shore bus through a bidirectional DC-DC converter. There are two modes of operation for the converter C36: 1) charging from the grid 2) discharging to the ship. For Li-ion batteries, typically charging process is started with a constant current while the battery voltage rises until the moment that the battery voltage reaches a specified value, then the charging would continue with a constant voltage and descending current. The controller for such DC-DC converter is