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This paper introduces a distributed secondary control scheme for achieving current sharing and average voltage regulation objectives in a DC microgrid. The proposed scheme employs a dynamic diffusion algorithm (DDA) instead of the consensus algorithm to enable distributed communication among converters. To help understand DDA, the relation of DDA and other diffusion algorithms is discussed in detail and its superiority is shown by comparison with diffusion and consensus algorithms. Furthermore, considering the discrete nature and different sampling time of the digital controller and communication network, a z-domain model of the entire DC microgrid is established. The influence of communication and secondary control parameters on the system stability is investigated. Based on the established model, the tolerable communication rates are obtained. Real-time simulations conducted on the OPAL-RT platform validate the effectiveness of the proposed scheme, showcasing its advantages in terms of convergence speed and stability.

The accessible and convenient hydrogen supply is the foundation of successful materialization for hydrogen-powered vehicles (HVs). This paper proposes a novel optimal scheduling model for gaseous-liquid hydrogen generation and storage plants powered by renewable energy to enhance the economic feasibility of investment. The gaseous-liquid hydrogen generation and storage plant can be regarded as an energy hub to supply concurrent service to both the transportation sector and ancillary market. In the proposed model, the power to multi-state hydrogen (P2MH) process is analyzed in detail to model the branched hydrogen flow constraints and the corresponding energy conversion relationship during hydrogen generation, processing, and storage. To model the coupling and interaction of diverse modules in the system, the multi-energy coupling matrix is developed, which can exhibit the mapping of power from the input to the output. Based on this, a multi-product optimal scheduling (MPOS) algorithm considering complementarity of different hydrogen products is further formulated to optimize dispatch factors of the energy hub system to maximize the profit within limited resources. The demand response signals are incorporated in the algorithm to further enhance the operation revenue and the scenario-based method is deployed to consider the uncertainty. The proposed methodology has been fully tested and the results demonstrate that the proposed MPOS can lead to a higher rate of return for the gaseous-liquid hydrogen generation and storage plant.

With the increasing scale of power systems and the large-scale application of new energy technologies, the planning and construction of future power grids and the stable operation of systems face severe challenges. Among them, transient stability and short-circuit current are two key factors that determine the safe and stable operation of a power system. However, in traditional power-grid planning, there is little research on the strategy of coordinating the solution between transient stability, short-circuit current problems and planning forecasts. Based on the perspective of power-grid planning, this study proposes a method for the optimal planning of a power system’s comprehensive stability margin that uses the external penalty function method. The coupling relationship between the transient stability margin and short-circuit current margin is developed through the system impedance in power-grid planning, and the line impedance optimization planning problem based on the comprehensive stability margin is formulated as a multiobjective optimization model. Finally, the optimal system impedance and the optimal comprehensive stability margin are determined by the external penalty function method. The simulation results of an IEEE 10-generator 39-bus system verify the effectiveness of the proposed method, and the system with the optimal planned impedance determined by this method yields the best comprehensive stability margin. Further simulation verification is conducted in a real power grid in China, and the strategy proposed in this article exhibits good economy and scalability. It provides a theoretical basis for the planning and construction of the future power grid, which is conducive to the safe and stable operation of the future power grid.

Urban multi-energy systems (UMESs) are integrated energy systems (IESs) which can alleviate the current energy crisis, improve energy utilization and realize multi-energy complementarity of modern. Various subsystems involved in the coupling of UMESs, including power grids, gas pipeline networks, cold/heat networks, transportation networks, and energy cyber-physical system, exhibit coupling characteristics such as multi-energy source modeling, complex uncertainty factor modeling, mutual influence of information and physical coupling. At present, the modeling and reliability assessments of UMESs are the most urgent tasks. In this paper, the latest research results on the reliability modeling and evaluation of UMESs are analyzed by considering their coupling components. In addition, the reliability modeling methods and the evaluation indexes of UMESs, including power-gas systems, power-thermal systems, power-traffic systems, and energy cyber-physical systems, are presented in this paper, with specific UMES modeling methods divided into model-driven modeling and data-driven modeling. Finally, future challenges for the reliability modeling and evaluation of UMESs are proposed, such as the dispatch strategy of the coupling components needs to be developed and continuously optimized to improve the reliability of UMESs, and the resilience of UMESs under the extreme scenarios needs to be further studied.

With the rapid development of society, global energy is in short supply. China has put forward an integrated energy system focusing on interconnecting energy resources and harnessing their complementary advantages to address the energy crisis, thus no longer pursuing the production, transportation and supply of a single source of energy. In the continuous development of integrated energy systems, the elements of participation and interaction are becoming more complex. Game theory, which can effectively solve the problems arising from multi-agent trading, is naturally introduced to the integrated energy system. This paper provides a comprehensive overview of the introduction of game theory into the integrated energy system. First, the development of integrated energy is briefly introduced, game scenarios in integrated energy systems is proposed, and game scenarios considering the energy supply side, distribution network, demand side and common planning and dispatching problems in integrated energy systems are summarized. Secondly, main game theory models in the integrated energy system are summarized, such as cooperative game theory model, the non-cooperative game theory model and the Stackelberg game theory model. Finally, the future prospect and challenge for the application of game theory in integrated energy systems is proposed. The new game models are introduced to the integrated energy system, and a mixed game is considered to solve related problems. It is hoped that this work can serve as a reference for the researchers in this field.

Human emotions recognition (HERO) is considered as one of the important techniques for realizing the intelligent Internet of Things. The demand for a robust and precise facial expression recognition algorithm is urgent for the HERO. In this paper, we propose a deep recognition algorithm based on the ensemble deep learning model. The proposed algorithm consists of three sub-networks with different depths. Each sub-network is comprised of convolutional neutral networks and trained independently. The sub-network with more convolutional layers recognizes emotions by extracting local details such as the features of eyes and mouth, while the sub-network with less convolutional layers focuses on the macrostructure of the input image. The three sub-networks are assembled together to constitute the whole model. The experiment is based on the Kaggle facial expression recognition challenge database (FER2013), the Japanese female facial expression database, and the AffectNet database. The experimental results show that the proposed algorithm achieves a test accuracy of 71.91%, 96.44%, and 62.11% better than other competitors, and increases the test accuracy by approximately 2-3% than unique sub-networks.

Three time-reversal-based fault location methods are derived from the theoretical part of this paper, which is named currents-in-time-domain method, forward-currents-in-frequency-domain method, and forward-currents-in-frequency-domain-with-compensation method, respectively. The first proposed method utilizes only the decomposed transient currents in the time domain to calculate the RMS values of the fault currents of assumed fault in a lossless mirror line. The location where the peak value is located is the calculated fault location. The second method uses the forward current in the frequency domain to calculate the RMS values of the fault currents of assumed fault in a loss mirror line. The location where the minimum value is located is the calculated fault location. The third method first calculates the RMS values of fault currents of assumed fault in lossless mirror line, and the fault location is selected from the peak value. Then the final fault location result is compensated by the pre-calculated error for each assumed fault location. A two-terminal AC transmission system is established for testing these three methods. The frequency-dependent lines including homogeneous and inhomogeneous lines are taken into the simulated system for a large number of tests.

This paper proposes a theory for locating line fault based on the basic principle of time reversal. The proof has five steps. First, the theoretical voltage and current at the ends of the line are calculated based on the transmission equation of the line in the additional network after line fault occurred. Second, the loss and lossless mirror line are established according to the parameters of the actual line. Third, the values of the implemented sources at the ends of the mirror line are calculated based on the conjugate complex of the theoretical values of voltage and current in the first step. Fourth, the RMS value of the fault current is calculated by assuming that fault occurs at every point of the mirror line. Finally, we should identify that there is only one peak value among all the RMS values and it exists at the actual fault location. In this paper, the fault location theory is proved with and without reflected waves by using the transmission equation of a single line. It is further proved in the mixed line that consists of two lines of different parameters considering the reflections.

Isotropy, end effect suppression, calculation efficiency and robustness are basic requirements of areal filters for surface metrology. However, current areal filters cause problems in different aspects, which cripples the operation of surface metrology like roughness evaluation. In this paper, a high-order thin-plate spline (TPS) filter is proposed to construct a real isotropic areal spline filter with minimum end effects and high efficiency, aiming to provide a more universal and efficient topography filtration method. The new filter is structured by approximating Gaussian filter to ensure uniform transmission characteristics. The robust algorithm is derived to process surface data with outliers and defects. A fast implementation is also provided in this paper. Performances of various areal metrology filters are illustrated by processing both simulated and practical surfaces, through which the proposed high-order TPS filter herein manifests real isotropy, higher calculation efficiency, slighter end effect as well as robustness comparing to current areal surface metrology filters. Comparing with the ISO 16610-61, the new areal filter can obtain more ideal results, therefore it can take the place of ISO 16610-61 on most occasions.

Abstract The DC microgrid has become a typical distribution network due to its excellent performance. However, a well-designed protection scheme still remains a challenge for DC microgrids. At present, researches on DC microgrids primarily focus on the topology structure, control method and energy control, while researches on fault analysis, detection and isolation have not drawn enough attention. Therefore, this paper intends to depict the current research status in different relative areas and review the proposed protection strategies in order to help researchers to have a clear understanding on DC microgrid protection. Meanwhile, to solve the protection issues and promote the development of the DC microgrid, this paper points out the key areas of future research. The future protection research directions lie in the development of novel protection devices, which are based on electronic technology to provide loose protection constraints and the improvement of suitable protection schemes. In addition, the novel concept of coordinated strategy of control and protection of the DC microgrids is explained.