Grid-forming (GFM) inverters are recognized as a viable solution to increase the penetration of renewable energy in bulk power systems. However, they are physically different from synchronous generators in terms of overcurrent capability. To protect the power semiconductor devices and support the power grid under severe symmetrical disturbances, the GFM control systems should be able to achieve the following requirements: current magnitude limitation, fault current contribution, and fault recovery capability. Various current-limiting control methods are reported in the literature to fulfill these goals, including current limiters, virtual impedance, and voltage limiters. This paper presents an overview of those methods. Emerging challenges that need to be addressed, including temporary overcurrent, unspecified output current vector angle, undesired current saturation, and transient overvoltage, are pointed out. Comparative simulations are conducted to demonstrate the performance of different methods under grid voltage drops and phase jumps. Finally, open issues of current-limiting control methods for GFM inverters, including transient stability assessment, voltage source behavior under overcurrent conditions, and windup of voltage controllers, are shared.

This letter develops an equivalent circuit model of a grid-forming (GFM) converter with a circular current limiter and analyzes its transient stability. It is revealed that the inner control loop can be simplified as a voltage source behind an equivalent resistor. Based on the developed model, theoretical analysis and experimental tests demonstrate that the transient stability of a $P-f$ droop-controlled GFM converter with the circular current limiter can be assured whenever there exist stable equilibrium points.

In DC microgrids, load power sharing and bus voltage regulation are two common control objectives. In this paper, a consensus-based algorithm is presented to achieve proportional power sharing and regulation of weighted geometric mean of bus voltages in DC microgrids with ZIP (constant impedance, constant current, and constant power) loads simultaneously. By using the virtue of the Laplacian matrices of undirected connected graphs, a lemma is derived to assist the stability analysis of the DC microgrids. Thus, a sufficient condition that stabilizes the system with ZIP loads is established. In addition, with the help of a distributed voltage regulation error estimator, the tuning of the bus voltages can be realized without the requirement on initial voltage conditions. Through Lyapunov synthesis, the large-signal stability of the closed-loop system is theoretically ensured for a wide range of load conditions. Finally, simulation studies are performed to validate the merits of the proposed consensus-based algorithm.

Grid-forming (GFM) inverters are required to operate robustly against grid faults. However, due to the limited over-current capability of inverters, current-limiting controls are usually applied to protect these semiconductor devices, which may prevent GFM inverters from a successful fault recovery. To understand this phenomenon, this study analyzes the fault recovery process of a GFM inverter with a priority-based current limiter. According to whether the GFM inverter can ensure transient stability and exit the current-limiting mode after fault clearance, three post-fault scenarios are identified, including normal operation, current limitation, and oscillations . Further, the impacts of the short-circuit ratio and control parameters on the post-fault behavior of GFM inverters are demonstrated. To illustrate the implications of these theoretical results, typical numerical examples are presented. Finally, the theoretical findings are validated through experimental tests.

Direct current (dc) microgrids have been widely used in many critical applications. Such systems avoid unnecessary ac/dc conversions and can simplify control design. To achieve high-performance control of such system, advanced control algorithm needs to be designed. This paper presents a novel decentralized output constrained control algorithm for single-bus dc microgrids. The control objectives are to realize high-performance control of dc bus voltage, user-defined load sharing, and circulating current minimization. Unlike conventional control algorithms, the control algorithm can guarantee not only convergence but also bounded transient tracking error. By transforming the constrained system into an unconstrained system, the transient response of dc bus voltage can always stay within user-defined, time-varying bounds. Convergence of the transformed system can meet any transient performance requirement of the original system. Through proper control effort distribution, overall load demand can be shared among the distributed generators (DGs) according to predefined percentages. Accurate load sharing can indirectly minimize the harmful circulating currents among the DGs. Switch-level simulation and hardware experimentation with both single-bus and multiple-bus dc microgrids demonstrate the effectiveness of the proposed control design.

In modern control systems, most control algorithms, especially system-level ones, are implemented in discrete-time (DT) with digital controllers and digital communications. In this paper, a distributed DT algorithm is developed to achieve proportional load current sharing and average bus voltage regulation in DC microgrids. In order to reduce the communication requirement among the controllers, a periodic event-triggered (PET) DT algorithm is proposed by introducing a novel PET condition. Since the PET condition is detected periodically, the Zeno phenomenon existing in continuous-time (CT) event-triggered algorithms can be avoided. The communication burden for detecting the PET condition is also relieved since only the local and neighbors’ triggered information is required. Through the Lyapunov synthesis, the current sharing error is proved to converge to zero asymptotically. Finally, comparative results among different algorithms, based on a detailed switch-level microgrid model, are given to validate the effectiveness of the proposed PET control design.