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Hybrid multilevel converters (HMCs) are more attractive than the traditional multilevel converters, such as modular converters, because they offer all the features needed in a modern voltage source converter-based dc transmission system with reduced size and weight, at a competitive level of semiconductor losses. Therefore this study investigates the viability of a HMC that uses dc side H-bridge chain links, for high-voltage dc and flexible ac transmission systems. In addition, its operating principle, modulation and capacitor voltage balancing, and control are investigated. This study focuses on response of this HMC to ac and dc network faults, with special attention paid to device issues that may arise under extreme network faults. Therefore the HMC with dc side chain links is simulated as one station of point-to-point dc transmission system that operates in an inversion mode, with all the necessary control systems incorporated. The major results and findings of subjecting this version of the hybrid converter to ac and dc networks faults are presented and discussed.

This study explorers the operating principles of the hybrid converter with ac side cascaded H-bridge cells and the H-bridge alternative arm modular multilevel converter, where the main objective is to compare their steady-state and transient performance. This comparison focuses on steady-state aspects such as active and reactive power control and semi-conductor losses and the transient performance of these converters during ac and dc network faults. To facilitate this comparison, detailed switch models of both high-voltage direct current (HVDC) converters are developed and simulated in Matlab-Simulink. The major outcomes are discussed and strengths and weakness associated with each converter are highlighted.

This study examines the possibility of operating different voltage source converter topologies in high-voltage direct current grids. The investigation is motivated by growing concern from the utility companies and transmission system operators regarding the compatibility of these converters, especially the behaviour of resultant multi-vendor dc grids during ac and dc network faults. In an attempt to establish the credibility of the expressed concerns, the transient response of illustrative multi-vendor six-terminal dc grids that consists of four two-level converters, a two-switch modular converter and a H-bridge modular converter are examined during ac and dc network faults. The main results obtained and observations drawn are highlighted and discussed.

The modular multilevel converter (MMC) has become the most attractive voltage source converter (VSC) topology for high‐power applications. AC or dc short‐circuit fault conditions cannot be avoided in high‐voltage direct current (HVDC) schemes, independent of the converter and the transmission link technology (e.g. cables or overhead lines). The probability, frequency, and characteristics of these faults depend on the employed transmission technology, converter technology (e.g. two‐level or MMC), and the grounding topology (e.g. direct, resistive, or high‐impedance). This chapter presents and compares the two‐level VSC‐HVDC and MMC‐HVDC fault characteristics and behaviors under ac and dc fault conditions. It discusses the impacts of different component failures, and presents related control and protection techniques. VSC‐HVDC terminals are designed to fulfill FRT requirements and remain synchronously connected to the network and provide the required ancillary services during the unbalanced ac fault conditions. Different techniques are applied to detect and protect the converter components from ac and dc fault conditions.

Low voltage direct current (LVDC) distribution has gained the significant interest of research due to the advancements in power conversion technologies. However, the use of converters has given rise to several technical issues regarding their protections and controls of such devices under faulty conditions. Post-fault behaviour of converter-fed LVDC system involves both active converter control and passive circuit transient of similar time scale, which makes the protection for LVDC distribution significantly different and more challenging than low voltage AC. These protection and operational issues have handicapped the practical applications of DC distribution. This paper presents state-of-the-art protection schemes developed for DC Microgrids. With a close look at practical limitations such as the dependency on modelling accuracy, requirement on communications and so forth, a comprehensive evaluation is carried out on those system approaches in terms of system configurations, fault detection, location, isolation and restoration.

High voltage direct current (HVDC) transmission is an economical option for transmitting a large amount of power over long distances. Initially, HVDC was developed using thyristor-based current source converters (CSC). With the development of semiconductor devices, a voltage source converter (VSC)-based HVDC system was introduced, and has been widely applied to integrate large-scale renewables and network interconnection. However, the VSC-based HVDC system is vulnerable to DC faults and its protection becomes ever more important with the fast growth in number of installations. In this paper, detailed characteristics of DC faults in the VSC-HVDC system are presented. The DC fault current has a large peak and steady values within a few milliseconds and thus high-speed fault detection and isolation methods are required in an HVDC grid. Therefore, development of the protection scheme for a multi-terminal VSC-based HVDC system is challenging. Various methods have been developed and this paper presents a comprehensive review of the different techniques for DC fault detection, location and isolation in both CSC and VSC-based HVDC transmission systems in two-terminal and multi-terminal network configurations.

The introduction of hybrid alternating current (AC)/direct current (DC) distribution networks led to several developments in smart grid and decentralized power system technology. The paper concentrates on several topics related to the operation of hybrid AC/DC networks. Such as optimization methods, control strategies, energy management, protection issues, and proposed solutions. The implementation of neural network optimization methods has great importance for the successful integration of multiple energy sources, dynamic energy management, establishment of system stability and reliability, power distribution optimization, management of energy storage, and online fault detection and diagnosis in hybrid networks like the hybrid AC–DC microgrids (MG). Taking advantage of renewable energy generation and cost-cutting through the neural network optimization technique holds the key to these progressions. Besides identifying the challenges in the operation of a hybrid system, the paper also compares this system to conventional MGs and shows the benefits of this type of system over different MG structures. This review compares the different topologies, particularly looking at the AC–DC coupled hybrid MGs, and shows the important role of the interlinking of converters that are used for efficient transmission between AC and DC MGs and generally used to implement the different control and optimization techniques. Overall, this review paper can be regarded as a reference, pointing out the pros and cons of integrating hybrid AC/DC distribution networks for future study and improvement paths in this developing area .

A new active impedance estimation based protection strategy which is suitable for utilisation in a DC zonal marine power distribution system is presented. This method uses two triangular current ‘spikes’ injections for system impedance estimation and protection when faults are detected. By comparing the estimated impedance with the pre-calibrated value, the fault location can be predicted and fault can be isolated without requiring communication between two injection units. Using co-ordinated double injections and line current measurement (directional fault detection), faults in the system with same impedance and different fault positions can be distinguished, located and isolated. The proposed method is validated using experimental test results derived from a 30 kW, 400 V, twin bus DC marine power system demonstrator. The experimental tests were applied to both faults during normal operation and faults that occur during system restoration.

Ensuring fault tolerance in systems of multiple sources of energy (SMSE) is crucial for reliable operation. Detecting, localizing, and characterizing faults are essential tasks for system integrity. This paper presents a novel approach utilizing hierarchical Bayesian belief networks (HBBNs) to identify and isolate open-circuit faults in DC–DC power converters commonly employed in SMSE applications. Our method addresses the challenge of fault detection and isolation, significantly enhancing system reliability. In particular, we design a comprehensive system capable of detecting and isolating faults in a system of multiple DC–DC converters, leveraging the interpretability and efficiency of HBBNs. We also utilize measurements from other converters to detect and isolate faults in a single converter, enabling efficient fault management. Our approach utilizes regularly monitored variables of the system, eliminating the need for additional sensors, thereby reducing complexity and cost. Additionally, we generalize HBBNs to be adaptable to any number of converters, providing scalability and flexibility in fault detection and isolation. Notably, the interpretability and simplicity of HBBNs, with a small number of parameters compared to other data-driven methods such as neural networks, contribute to their effectiveness in fault management. Through extensive testing on simulated data generated via a developed state space model, our approach demonstrates its effectiveness in bolstering the robustness of DC–DC power converters against open-circuit faults.

This study focuses on reduced‐order dynamical modelling of droop controlled converter‐based DC sub‐microgrid (MG) in a hybrid AC/DC MG. In hybrid MGs, electrical power is exchanged between the AC and DC sub‐MGs by a bidirectional AC/DC converter. The authors aim to develop a comprehensive reduced‐order dynamical model for the DC side in this part, incorporating standard classes of electrical loads including constant current, constant power, and constant resistance loads. Furthermore, dynamical behaviour of the power exchange between the AC and DC sub‐MGs is modelled, considering that the bidirectional power converter controller aims to equalise the load ratios of AC and DC sub‐MGs in order to facilitate overall decentralised control over the hybrid MG. Analytical derivations of steady‐state values of main variables are given and the overall dynamical and algebraic equations are determined. In order to validate the developed model, a hybrid MG is implemented in PSCAD. Then, the proposed model for the case study is implemented in Matlab/Simulink and the results are compared with the PSCAD outputs. The comparative results show the validity of the developed reduced‐order comprehensive model. The reduced‐order models are preferred in designing observers such as model‐based fault detection and diagnosis observers.