DC and DC/AC hybrid distribution and energy storage for shipboard power systems (SPS) are becoming a major trend due to efficiency improvement, space saving, and maneuverability enhancement. This paper has taken a real hybrid-electric-ferry as a case-study to integrate battery units (BUs) to a DC bus for supplying the propulsion motors. Further, two diesel generators (DGs) are connected to the AC bus to supply the hotel loads and a bidirectional DC/AC converter with an LCL filter is responsible for the power flow between AC and DC buses. This power topology is flexible for this ferry operation in pure electric, range extended and shore power modes. DC bus voltage is stabilized and its voltage ripple is limited by BUs' interleaved three-phase bidirectional DC/DC converter with its controller considering the operation states of propulsion motors. A coordinated power flow control between DGs and BUs is presented that the system frequency is fixed for the optimal operational efficiency of the diesel engines and a Q-V droop control plus a virtual impedance loop is used to make different AC bus voltage. Synchronization with shore power and DC/AC converter is facilitated by P-f droop control. Simulation results are presented to validate the proposed control approach in different missions.
The frequent start/stop operation of a bow thruster presents a high pulsating power demand in shipboard microgrids while reducing the fuel efficiency of the diesel engines, which may induce potential instabilities. In this paper, a hybrid energy storage system, including batteries and ultra-capacitors, is connected to the DC bus of the thruster driver, and a hierarchical controller is proposed for the bidirectional DC/DC converters that interface storages thus providing the following functions: (1) the diesel-generator sets provide the required average power to the bow thruster; (2) the batteries smooth the active power fluctuations; and (3) the ultra-capacitors provide the pulsating active power due to the slow charging response of batteries. A V-I droop control and high/low-pass filters are used inside the primary controller to proper power sharing between storages in different time-scales. The secondary controller is developed to ensure the power from the diesel-generator sets to the bow thruster being equal to the required average power. Simulation results shows this control scheme can effectively reduce the capacity of diesel generator sets and batteries, and can improve the fuel consumption efficiency. When the thruster machine operates as a generator during fast breaking, the hybrid energy storage can absorb the braking energy.
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