Operation Control for Improving Energy Efficiency of Shipboard Microgrid including Bow Thrusters and Hybrid Energy Storages

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 article, a hybrid energy storage system, including batteries and ultracapacitors (UCs), 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 (DG) sets provide the required average power to the bow thruster; 2) the batteries smooth the active power fluctuations; and 3) the UCs 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 DG sets to the bow thruster being equal to the required average power. Simulation results shows that this control scheme can effectively reduce the capacity of DG 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.