Microgrids rely on both primary and secondary frequency control techniques to maintain system stability. Secondary frequency control effectively minimizes frequency fluctuations by adjusting the active power reference in each power inverter, but requires complex and costly interequipment communication. In this paper, we propose a distributed secondary frequency control strategy for microgrids containing multiple virtual synchronous generator (VSG) units based on a new maximum power loading factor (MPLF) concept. The MPLF algorithm facilitates power sharing by dynamically identifying the maximum VSG loading factor at each time instance, and then using this value as a unified reference signal for all the VSGs in the microgrid. The active power reference for each VSG will be adjusted based on the unified reference signal, subsequently the secondary frequency control can be realized. The proposed strategy does not require high-bandwidth communication since the MPLF data are transmitted among the VSGs using low-bandwidth communication. We also develop small-signal models for the control architecture to analyze the influence of major proportional-integral control parameters and communication latency. The MPLF control strategy is implemented using custom digital signal processor controllers, and experimentally validated using hardware in loop simulations. Finally, the new control paradigm demonstrates significant tolerance for communication delay or failure, which we purposely introduced in our investigation.

Conventionally, additional control levels are often introduced in a dc microgrid (MG) to ensure equal power sharing among distributed energy resources (DERs). To achieve this, it is necessary for the supervisory controller to collect multiple operation signals of all DERs. Thus, the burden of communication increases with increased usage of DERs. Additionally, high requirements on communication links decrease the flexibility and reliability of dc MGs. In this paper, an improved secondary control strategy with maximum loading factor (MLF) for accurate power sharing in dc MGs is proposed. In this method, the MLF is used to achieve an auto-selection of the leader converter with differential delay methods. Additionally, the compensation value for power sharing improvement is calculated at the secondary control level. In contrast with the conventional approaches, the only signal transmitted in the communication link corresponded to the MLF, and thus power sharing is realized with ultra-low bandwidth communication. After receiving the MLF signal, the compensation value is calculated in the local converter. Thus, the centralized secondary controller or master controller is not required. Finally, performance of the proposed control scheme with different time latencies is verified via hardware-in-the-loop tests for different scenarios.

Poor power management of bidirectional interlinking converter (BIC) and sub-grids in a hybrid AC/DC microgrid can lead to a heavy or light load state of some distributed energy resources (DERs), and large deviations of AC bus voltage/frequency and DC bus voltage. This study proposes a distributed power management strategy based on the concept of coordination factors for hybrid microgrids to achieve accurate active power sharing among all AC/DC DERs (instead of the same within one sub-grid), reactive power sharing among AC DERs, and AC voltage/frequency and DC voltage restorations. Power management within a sub-grid is achieved by following the leader DER with the maximum coordination factor. Meanwhile, the BIC can establish a dynamic connection among all DERs by tracking the leaders of each sub-grid to realize power management between sub-grids. Compared to conventional methods, the proposed method can achieve accurate power sharing and voltage/frequency restorations simultaneously. Additionally, communication data and bandwidth can be significantly reduced. Finally, hardware-in-loop experiments are also performed to prove the feasibility of the proposed strategy, even during communication latency or failure.

Grid-Forming converters (GFMs) are faced with the threat of inrush current and sever transient instability issues during grid faults, which calls for fault ride through (FRT) capability. Several FRT control strategies are proposed for single GFM connected to grid system in existing literatures while transient interaction mechanism among paralleled GFMs still remain unrevealed and corresponding FRT control are rarely discussed. To fill this gap, coordinated FRT control based on mode-switching is proposed for paralleled GFMs in this paper. The proposed FRT control would turn GFMs into current control mode during grid fault periods and return to voltage control mode after fault clearance, which enables current control flexibility during grid fault. To guarantee stable operation throughout fault period, transient interaction models in current control mode and voltage control mode are established respectively. Then, design-oriented transient stability analysis is carried out using equal area criterion (EAC) theory. Finally, detailed simulation results based on PSCAD/EMTDC and experimental results are provided to validate the effectiveness of proposed method.

Voltage fluctuation suppression and reactive power proportional sharing in islanded microgrids (MGs) are often difficult to maintain simultaneously. Hence, a dynamical reconfigurable master–slave control architecture is proposed herein. When a load fluctuates, the leader will be dynamically selected through specific features (such as the output power, output voltage, and power capacity); subsequently, the leader sends his information to all other distributed generators (DGs) as a unified signal in the MG. Meanwhile, the other DGs adjust their local output based on the unified reference to achieve accurate power sharing in each DG. Moreover, system voltage recovery can be realized simultaneously since each DG contains closed-loop voltage control. In addition, for the automatic selection of the leader, a dynamically selected method based on a weight-voting-based differential delay method is proposed herein; the method realizes the unicity and automatic selection of the leader during communication failure. Additionally, the stability of the proposed method is proven mathematically. The results show that the proposed method is stable for any communication period and DG quantity. Furthermore, the communication delay and communication failure are derived mathematically. The results show that the proposed method demonstrates robustness against communication delay. Finally, experimental results verify the effectiveness of the proposed method under communication delays, failures, and power failures.

Solid oxide fuel cells (SOFCs) are promising distributed generation technologies for their controllability and efficiency. Traditional SOFCs are controlled as a current source to deliver predefined power to the grid. Since a microgrid can operate in both grid-connected mode and islanded mode, grid-forming control ability is required for SOFCs to achieve voltage/ frequency regulation. However, when subjected to load transients, nonlinear output characteristics of SOFCs would probably lead to the collapse of dc-link voltage and unintentional disconnection of SOFCs. To deal with this issue, grid-forming control for SOFCs is presented considering nonlinear characteristics between output current and voltage. The maximum power point (MPP) of SOFCs is tracked using proposed incremental impedance (INI) method and dc-link voltage difference is delivered to adjust output frequency of ac side. In this condition, stable operation and load sharing can be achieved simultaneously during load transients. Simulation results based on PSCAD/EMTDC are provided to validate the effectiveness of proposed grid-forming control.