The five-level nested neutral-point-pilot (NPP) topology, as a new structure for converters, bears the advantages of a high power density, robustness and flexibility and is therefore suitable for high-voltage and high-power applications. For a multilevel converter, as the number of power electronic switches increases, the risk of switch failure increases, together with the complexity of fault detection and tolerance control. The requirements for a higher operational stability and reliability continue to grow. However, studies on fault tolerance for multilevel converters are limited. In this paper, a fault diagnosis and tolerance solution for a five-level nested NPP converter is proposed. For the fault diagnosis, a deep learning method integrating the wavelet packet transform (WPT) and long short-term memory (LSTM) is presented. Both open- and short-circuit switch failures can be precisely detected and located without the requirement of a large sample set. Two software-based control strategies for fault tolerance are adopted, and low-cost hardware reconfigurations are also implemented to prevent failure expansion and ensure continuous operation. Furthermore, the voltages of DC-link capacitors and flying capacitors are effectively balanced with the improved algorithm even when a failure occurs. Finally, the effectiveness of the fault-tolerant strategy are proven by simulations and experiments.

In this article, a novel Γ-source circuit breaker (CB) that can offer automatic fault isolation with no reflected fault current in the feeding source is proposed for the protection of dc microgrids. Compared with conventional Z-source CBs, this topology utilizes fewer elements through the magnetic coupling, thus allowing a considerably reduced volume and weight. To accurately quantify the breaker elements, detailed modeling over both short-circuit and overload faults is carried out. The derived equations are significantly valuable for guiding the implementation of the novel breaker. A soft-starting circuit is also introduced to limit the capacitor charging current when reenergizing the breaker. In addition, a bidirectional Γ-source CB extended from the unidirectional topology with the same impedance network is discussed. If required, a crowbar-type switch, which allows the breaker to be manually tripped, can be conveniently integrated into the Γ-source CB. This manual tripping switch is useful for clearing steady-state overload faults. Moreover, experimental waveforms of fault-clearing and manual tripping operations are included to validate the effectiveness of the novel CB.

DC microgrids that feature a simple and efficient integration with renewable energy sources and energy storage elements have drawn increasing attention in industrial applications. In this paper, a bidirectional Z-source circuit breaker with an O-shaped impedance network (abbreviated as O-Z-source circuit breaker) is proposed to guarantee the reliable operation of dc microgrids. By a simplified structure, the O-Z-source circuit breaker allows bidirectional power flow with fewer components compared with conventional Z-source circuit breakers. Increased operating power efficiency can also be obtained from the novel topology. Based on detailed mathematical models, design equations are derived for sizing the components in the O-Z-source circuit breaker. Moreover, a new concept of equivalent capacitance that is useful for estimating the minimum detectable fault ramp rate for Z-source circuit breakers is presented. A discussion over snubber circuits of the proposed breaker is also included. Finally, a laboratory prototype has been built to verify the analysis and design of the O-Z-source circuit breaker.

DC microgrids have attracted increasingly concern in industrial applications due to a simple and efficient integration with renewable energy sources, battery energy storage, and variable speed generators and motors. However, extensive application of dc architecture is still restricted by fault protection challenges posed by the absence of natural current zero-crossing. This paper has proposed a novel thyristor-based dc circuit breaker with a one-shot triggering commutating circuit, which maintains a compact size as no additional power supply is required to precharge the commutating capacitors. Considering di/dt limitation of the triggering thyristor in the commutating circuit, a saturable inductor is utilized to avoid hot spot failure in the thyristor. Moreover, saturation characteristic of the inductor allows a sharp-rising commutating current which can provide zero-crossing for the main thyristor with a shorter delay. A real-time analog protecting unit is also introduced to interrupt fault current accurately and rapidly. Furthermore, detailed breaker topology analysis and circuit design guidelines are provided through mathematical modeling. Finally, the proposed breaker design mothed is verified by simulation studies over two cases and further validated by a 400 V/4 kW laboratory prototype.

Moving average filter (MAF)-based phase-locked loops (PLLs) have received considerable attention in recent years due to their attractive features. Indeed, they are able to completely eliminate the unwanted effect of harmonics, dc offset, and unbalanced voltages. Unfortunately, these advantages come at the cost of open-loop bandwidth reduction, which worsens the system’s dynamic response. The main chal-lenge is to estimate the phase and frequency timely and precisely from an imbalanced and distorted voltage. In addition, optimized parameter design is also a difficult issue. The main aim of this paper is to present an enhanced PLL based on a moving average filter (EMAF-PLL) and a control method with a novel compensa-tion algorithm. The EMAF-PLL can maintain high performance even under harsh grid conditions, and the novel control method enables the designer to set the controller parameters simply and effectively. The design method compensates the PLL small-signal model to form a type-II classic system and then optimizes the control-loop design by the rules of type-II systems. Finally, experimental investigations are performed to validate the effectiveness of the EMAF-PLL. The experimental results are also presented, and comparison with conventional PLLs verifies that the dynamic performance can be significantly improved.

Multiactive bridge (MAB) converter has played an important role in the power conversion of renewable-based smart grids, electrical vehicles, and more/all electrical aircraft. However, the increase in MAB submodules greatly complicates the control architecture. In this regard, the conventional centralized control strategies, which highly rely on the central controller, are limited by the computation burden and flexibility. To overcome this problem, this article proposes a decentralized control for MAB converters. The MAB submodules switching frequencies are adaptively regulated based on local information. Through this effort, flexible electrical power routing can be realized without the need for communications. The proposed control strategy not only relieves the computation burden of MAB control system, but also improves its modularity, flexibility, and expandability. Finally, hardware experimental results of a typical MAB converter are presented for verification.