This manuscript presents a comprehensive review of recent advancements in electrical distribution networks, with a specific focus on the incorporation of direct current (DC) applications. The research aims to comprehensively address the current and future aspects of DC, spanning from the distribution level to the utilization level. The renewed interest in DC power systems has led to the investigation of several transitional challenges in recent years. A significant portion of these efforts has been dedicated to determining the feasibility of applying DC to specific use cases. Additionally, the literature has explored design considerations such as system architecture and voltage levels, the integration of DC into existing distribution networks, load flow (LF) computations, and the distinct safety concerns associated with DC power systems. In this paper, the various research endeavors are categorized, evaluated, and scrutinized to assess the current state of the transition from a purely alternating current (AC) distribution system to a solely DC or hybrid AC/DC distribution system. A bibliometric analysis is conducted, constructing a network of co-occurrence based on author-provided keywords, which reveals the primary research foci in this domain. The barriers hindering the widespread adoption of DC distribution systems and potential solutions are also discussed. Moreover, this article synthesizes ongoing efforts to address these obstacles and delineates future research directions by emphasizing the existing knowledge gaps.

The steady-state performance of modular multilevel converter (MMC) can be optimized by the following methods: circulating current injection, third-harmonic signal injection, and average capacitor voltage control. However, most of the papers, due to the deficiencies of their MMC models, are limited to the study of just adopting one method and only optimizing a single-aspect performance. Thus, this paper proposes a general MMC model for the comprehensive study of the steady-state performance optimization. The general model is built based on an equilibrium equation between the internal quantities and the external quantities of the MMC. This general model is capable of analyzing the optimization where the optimizing methods are used in combination to improve the multi-aspect performance of the MMC. In addition, more influence factors are taken into account; hence the calculation error can be smaller than 1% and even the minor effect of each optimizing method can be reflected. The proposed model is compared with traditional models; and it is found that both dc and fundamental components in modulation signals will be affected by the injected circulating current, rather than constant values in the traditional models. Finally, the proposed MMC model is verified by both simulation and experiment.

Hybrid ac/dc distribution grids are gaining popularity due to their potential to conflate the benefits of both ac and dc grids. However, the complex structure of these grids imposes new challenges in power flow (PF) calculations due to multiple ac/dc interconnections, variety of converter operations, and constant dc power loads. This paper proposes a unified PF algorithm incorporating different types of ac/dc and dc/dc converters. The proposed approach is based on the Newton Raphson (NR) algorithm and uses the Modified-Augmented-Nodal-Analysis (MANA) formulation. The proposed formulation can handle arbitrary hybrid ac/dc configurations. The modular structure allows incorporating arbitrary devices and converter controllers. The formulation is built on the summation of line flows and their associated derivatives instead of bus power injections to handle connections with different types of buses equipped with various converters in a straightforward manner. The proposed algorithm is implemented in MATLAB and tested on the modified IEEE 33 bus test system for various configurations. Electromagnetic Transient (EMT) simulations confirmed the accuracy of the proposed method.Another original contribution of the paper is the demonstration of the improved convergence characteristics of MANA based formulation over classical NR formulation through comparisons of conditioning number.

In this paper, a model is proposed for the optimal operation of multi-energy microgrids (MEMGs) in the presence of solar photovoltaics (PV), heterogeneous energy storage (HES) and integrated demand response (IDR), considering technical and economic ties among the resources. Uncertainty of solar power as well as the flexibility of electrical, cooling and heat load demand are taken into account. A p-efficient point method is applied to compute PV power at different confidence levels based on historical data. This method converts the uncertain PV energy from stochastic to deterministic to be included in the optimization model. The concept of demand response is extended and mathematically modeled using a linear function based on the quantized flexibility interval of multi-energy load demand. As a result, the overall model is formulated as a mixed-integer linear program, which can be effectively solved by the commercial solvers. The proposed model is implemented on two typical summer and winter days for various cases. Results of case studies show the important benefits for maximum PV utilization, energy efficiency and economic system operation. Moreover, the influence of the different confidence levels of PV power and effectiveness of IDR in the stochastic circumstances are addressed in the optimization-based operation.