Multi-mode Operation Control Research For Ship Microgrid Based On Adaptive Sliding Mode Control

With the depletion of fossil energy and the increasing seriousness of environmental pollution, reducing the supply of fossol energy in the ship and using clean energy to supplement and gradually replace fossil fuels, have become the trend of the times. However, due to the randomness and uncontrollability, the intermittent distributed energy that connected into the ship grid would give the security and stability of the ship power system a negative impact. So as an effective solution, the research on ship microgrid has important theoretical and realistic significance. The research object of this dissertation is the ship microgrid based on multiple inverters, and its target is to realize the multi-modes operation control. Then the next three problems are focused on:the disturbance rejection of the micro-source inverter, load sharing accuracy between paralleled micro-source inverters, seamless mode switch based on multi-modes operation. The main contents of this dissertation are as follows:The closed-loop control strategy based on adaptive three-order sliding mode is proposed to eliminate the over-voltage, over-current, and unstable phenomenons, which may happen in micro-source inverters under the process of mode switch or load mutation. Then the strategy is applied to the voltage control of the supporting inverter and the power control of the grid-connected inverter, respectively. Based on the traditional proportional-differential sliding mode control, the integration element is added to eliminate the steady-state error, and the adaptive observation item is used to compensate the uncertain disturbances caused by states change. Then the stability analysis and parameter selection method are elaborated. Finally, simulation results demostrate the proposed control strategy can improve the power quality of the micro-source inverters, and has good robustness and dynamic performances.An improved droop method based on virtual negative impedance is proposed to solve the circulating-current and power sharing problems. The method considers the difference of the inverter equivalent output impedances and its influences on the circulating-current restraining and power sharing. With considering the inverter topology and voltage closed-loop, the virtual negative resistance offsets the line resistance and the virtual inductance realizes the equivalent output impedance matching. Then combined with the variable reactive-voltage droop coefficient, dynamic differential item, and virtual inertial link, the reactive circulating current caused by line impedance drift and inaccurate parameter mesurements can be inhibited, and the power sharing dynamic characteristics can be improved. Simulation results demostrare the proposed control method can effectively restrain the circulating current and improve the power sharing accuracy between the micro-sources. Meanwhile, the microgrid dynamic performance under the islanding mode can be ensured.A hierarchical structure based on autonomous, cooperative, microgrid management, and scheduling controls is proposed to realize the microgrid stable operation and seamless mode switch. The composition and functions of the hierarchical structure are expounded in the paper. Then the cooperative control, which is composed of secondary control and synchronization control, is designed. And its power quality improvement and stability are analyzed in subsequent. Meanwhile, the microgrid multi-modes operation control process and constant power control strategy are elaborated. The proposed hierarchical structure can realize the microgrid optimized management and control strategy smooth transition under the process of mode switch. Except the local control ability, this method also has the function of global adjustment. Furthmore, this method can effectively solve the low power sharing accuracy and poor power quality problems that existed in the traditional micro-source inverters.Based on the proposed control strategy, the system structure and experimental platform of the ship microgrid containing the distributed generations are built. Based on their simplified model, the transient characteristics simulation analysises under various working conditions are carried out. The following four conditions are analysed in detail: the transient stability and power quality of the microgrid under the condition of mode switch, the transient characteristics influence caused by sudden power change, the microgrid output performance under the motor load condition and nonlinear load condition. Simulation results demonstrate that the proposed control methods can maintain the microgrid bus voltage stability, and have a good power quality. The transient process is smooth and rapid. Meanwhile, the ship microgrid can realize the accurate power sharing and its output power has good dynamic and static performances.