The neutral grounding method is one of the crucial technologies used to ensure the safe and stable operation of distribution networks. With the development of power electronics-based devices, the application of highly controllable inverters to grounding systems has become a new research direction. In this review paper, based on the summary and analysis for the research status, the flexible grounding system is systematically classified according to its control strategy and structure. And then, the following core technologies are summarized: faulted line selection, current component detection, high-resistance ground identification, ground parameter measurement, neutral point voltage control, and fault location. Subsequently, deficiencies of flexible grounding systems are summarized from the following four perspectives: reliability, accuracy, speed, and stability. Finally, some discussions and recommendations for future development and research direction are presented. Especially, the reliability-enhanced hybrid grounding system that combines active inverters and low resistance for high-resistance ground faults is recommended. This paper provides a systematic literature review of the crucial technologies for flexible grounding systems and can be used as a technical reference for future research.

In modern micro-grid systems, the control of power electronic converters faces numerous challenges, including the uncertainty of parameters of the controlled objects, variations in the operating environment, and the increased complexity of models for controlled objects. Meanwhile, traditional control strategies for power electronic converters are characterized by high model dependency, slow dynamic response, and static control parameters, which make them inappropriate for the development of modern micro-grid systems. In response to this problem, researchers have proposed reinforcement learning (RL) control strategies to achieve fast adaptive control through real-time interaction with the environment. However, there is currently a lack of comprehensive literature reviews specifically focusing on RL control for power electronic converters. To fill this gap, this article provides a summary of existing published research papers. Specifically, the mainstream RL algorithms are listed. The existing RL control strategies for various power electronic converter topologies are introduced. The sim-to-real methods for practical implementation are illustrated. The underlying design considerations are discussed. Moreover, the future research prospects are explored.

Shunt active power filter is widely used for compensating harmonic current produced by nonlinear loads. As one of the critical technologies, the current control strategy has an important influence on the compensation precision and dynamic response performance. In this paper, the essential of the current-limiting demands for the comprehensive protection of SAPF are revealed, namely the limiting control objects: 1) RMS of the compensation current (mainly for the overheat protection of IGBTs and inductors); 2) the instantaneous wave of compensating current (mainly for the accurate current control and IGBT $I_{cnom}$ specification) and 3) the instantaneous wave of PWM-VSC modulation voltage (for the need of liner close-loop control). To avoid the over-current fault and over-modulation fault of SAPF, this paper proposes an improved current-limiting scheme based on particle swarm optimization (PSO) to achieve the two optimization targets: 1) the minimization THD for the grid-side current; 2) the maximization utilization ratio for the capacity of SAPF. The main advantage lies on the optimum limiting ratios of each harmonic order calculated in real time respectively to achieve the flexible and liner limiting control. In addition, taking the control structure consisting of selective harmonic detection under the multiple rotating reference frame and the single PI current loop for example, this paper gives the method for steady state error correction qualitatively and quantitatively, which employs the method based on the frequency response characteristics analysis. Finally, simulation and experimental results verify the effectiveness of the proposed strategy.

A matrix converter (MC) with model predictive control (MPC) based on the source reactive power control usually fails to show sinusoidal source currents. The analysis presented in this paper shows that this common combination of converter and control has the inherent inability to eliminate some harmonics in the source currents, even with additional passive or active damping control. Direct source current control can be implemented to give sinusoidal source currents and intrinsic active damping. However, the issue of steady-state error in output currents then arises, as the MC topology does not allow of the independent control of source and output currents. Therefore, feedback control of load active power is proposed to address this issue without degrading the fast dynamic performance. Benefiting from the direct source current control, a simplified implementation is also proposed to decrease the number of candidate switching states from 27 to 5, which significantly reduces the computational burden. Experimental results have verified the theoretical analysis and the effectiveness of the proposed control scheme.

For the direct matrix converter (DMC) applied to PMSM drives, a novel modulation scheme is proposed in this paper, which could reduce the control complexity and improve the efficiency while achieving zero common-mode voltage. Only three counter-clockwise or clockwise rotating vectors are adopted to operate the DMC. The proposed scheme has the inherent ability to generate sinusoidal input currents without explicit control in the modulation. The duty cycle expressions for the three vectors are quite simple without any trigonometric operations. The switching pattern is fixed, saving the need of sector and sequence determination. Therefore, compared with the conventional modulation scheme which also uses only rotating vectors, the proposed scheme could reduce the execution time by over 80%. In addition, it achieves higher efficiency benefiting from the reduced switching actions. Its validity is verified by experimental results on a 2.4-kW DMC-fed PMSM drive prototype.

The DC distribution technology brings challenges and opportunities to the penetration of large‐scale distributed renewable energy sources. In a medium‐voltage DC (MVDC) distribution network, operation mode switch occurs owing to some large disturbances. To manage these disturbances and achieve seamless switch of the operation mode, a hierarchical control strategy with two layers, namely local and global layers, is proposed in this work. The novel local layer controller called P–U–I controller is designed to enhance the voltage stability, improve system controllability and suppress overcurrent under large disturbances. This controller only needs local information and does not rely on fast communication. Furthermore, as the scale of the DC distribution network expands continuously, the number of system operation mode increases in geometric progression. Therefore, a global layer controller based on breadth‐first search algorithm is proposed. This controller can automatically identify the system topology and adjust the control mode of converters to optimise the system operating characteristics. Thus, this hierarchical control strategy can accurately control the system power in steady state, suppress overcurrent under large disturbances and suit large‐scale DC distribution networks. Finally, a three‐terminal MVDC distribution network simulation model established on power systems computer‐aided design and RT‐LAB validates the proposed strategy.