Multi-objective cooperative controller design for rapid state-of-charge balancing and flexible bus voltage regulation in shipboard DC microgrids.

In this paper, a multi-objective cooperative (MOC) controller based on average consensus algorithm is designed to achieve rapid State-of-Charge (SoC) balancing, proportional load current sharing, and flexible DC bus voltage regulation for parallel battery storage units (BSUs) in shipboard DC microgrids. Different from the conventional secondary controllers, the designed MOC controller can simultaneously achieve the above three control objectives with a fully distributed manner without requiring multiple controllers, thereby effectively improving the system stability and reducing the communication burden. Furthermore, an optimized convergence factor is designed to accelerate SoC balancing, and pinning control is introduced to obtain flexible and accurate DC bus voltage regulation. The process of SoC balancing and current sharing analysis, SoC convergence performance analysis, large-signal stability analysis, and global steady-state analysis verifies the rationality and stability of the MOC controller. Finally, the Matlab/Simulink simulation and StarSim HIL experimental results demonstrate the effectiveness and robustness of the designed MOC controller in a shipboard DC microgrid under various testing scenarios.

A novel control strategy for mode seamless switching of PV converter in DC microgrid based on double integral sliding mode control.

With the photovoltaic (PV) penetration increases in dc microgrid, the traditional PV converters with maximum power point tracking (MPPT) control, which are equivalent to current sources, can hardly meet the needs of coordinated operation. The PV converter should operate in both MPPT and constant voltage control (CVC) modes. When the operation mode changes, the PV converter needs to switch between current and voltage sources. Inevitably, the bus voltage instability would be caused. This paper presents a novel switching strategy for the smooth and seamless operation of the PV converter between MPPT and CVC modes. The PV converter under these two modes is always controlled as a voltage source, and its control variables are both the output voltage of PV arrays. When its operation mode changes from MPPT to CVC, a small offset signal based on DC bus voltage would be introduced to the reference voltage of inner loop of the PV converter, so as to shift the maximum power point (MPP) voltage. Meanwhile in the voltage inner loop, a double integral sliding mode controller (DISMC) is used to improve the performance and anti-interference ability of the PV converter. This method unifies MPPT and bus voltage adjustment strategy without any communication and changing any hardware structure and control parameters. When mode switching occurs, only the reference value of voltage inner loop would be slightly adjusted near the maximum photovoltaic power point, so the seamless transition for PV converter between the two modes can be ensured. Finally, the effectiveness of the mode seamless switching control strategy is demonstrated by the simulation and experimental tests.

A multi-mode operation control strategy for flexible microgrid based on sliding-mode direct voltage and hierarchical controls.

Multi-mode operation and transient stability are two problems that significantly affect flexible microgrid (MG). This paper proposes a multi-mode operation control strategy for flexible MG based on a three-layer hierarchical structure. The proposed structure is composed of autonomous, cooperative, and scheduling controllers. Autonomous controller is utilized to control the performance of the single micro-source inverter. An adaptive sliding-mode direct voltage loop and an improved droop power loop based on virtual negative impedance are presented respectively to enhance the system disturbance-rejection performance and the power sharing accuracy. Cooperative controller, which is composed of secondary voltage/frequency control and phase synchronization control, is designed to eliminate the voltage/frequency deviations produced by the autonomous controller and prepare for grid connection. Scheduling controller manages the power flow between the MG and the grid. The MG with the improved hierarchical control scheme can achieve seamless transitions from islanded to grid-connected mode and have a good transient performance. In addition the presented work can also optimize the power quality issues and improve the load power sharing accuracy between parallel VSIs. Finally, the transient performance and effectiveness of the proposed control scheme are evaluated by theoretical analysis and simulation results.