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.

To suppress the influence of power fluctuation in the DC microgrid system, virtual DC motor (VDM) control is applied to the energy storage converter for improving the stability of the power system. Due to the fixed parameters adopted in the traditional VDM control strategy, the dynamic response of the system cannot be taken into account. Based on the traditional VDM control strategy, a small signal model is established to analyze the influence of the moment of inertia J and the damping coefficient D on the control performance. Moreover, the effects of J and D on the dynamic response of DC bus voltage under power fluctuation are thoroughly analyzed. Consequently, a novel parameter-adaptation-based VDM control strategy is proposed by realizing the adaptive adjustment of J and D. Furthermore, the comparative simulation of three kinds of operating conditions are carried out, and the adaptive parameters are determined through comparative simulation under three kinds of operating conditions. The results show that the proposed control strategy has great superiorities in control effect and flexibility, and the anti-interference and dynamic response abilities of the system are significantly improved, by comparing with the traditional control strategies.