In a high-voltage flexible DC power transmission system, the modular multilevel converter (MMC) has unique advantages. Compared with those of the ordinary MMC, the output voltage levels of a hybrid-leg MMC (HL-MMC) consisting of submodules (SMs) and IGBT are doubled, the harmonic content of output voltage is reduced, and the DC bus voltage utilization is also doubled under the condition of the same number of SMs. The series-connected HL-MMC (SC-HL-MMC) is composed of three single-phase HL-MMCs, and its number of SMs is reduced by 2/3 under the condition of the same DC bus voltage. To be equipped with a DC short-circuit fault-tolerant capability, a hybrid SC-HL-MMC consisting of half bridge SMs (HBSMs) and full bridge SMs (FBSMs) is applied, and the scheme of optimal ratio between FBSMs and HBSMs is analyzed to satisfy the short-circuit fault ride-through requirement. To verify the performance of the proposed topology, a simulation model is built. Simulation results show that the stable three-phase output voltage from the SC-HL-MMC contains less harmonic under the rated operation condition, and it can block the short-circuit current rapidly under the DC short-circuit fault condition, thereby verify the validity of this topology.

Under the partial shade condition of photovoltaic (PV) arrays, multiple peaks will appear on the P-U curve, and the traditional particle swarm optimization (PSO) algorithm cannot search for the maximum power point quickly and accurately. In view of this issue, a maximum power point tracking (MPPT) control strategy based on improved PSO algorithm using chaotic mapping and Gaussian perturbation is put forward. First, a chaotic Sine mapping was introduced to construct a nonlinear random incremental inertia weight, and Gaussian perturbation was introduced in the “individual cognition” part of the particle swarm. Meanwhile, a logarithmic function was used to construct a learning factor, thus forming an improved PSO algorithm based on chaotic mapping and Gaussian perturbation. Through the test of six typical unimodal and multimodal functions, it was verified that this algorithm converged faster and was less susceptible to local maxima. Then, the algorithm was applied to MPPT control, and comparative simulations of different algorithms on MPPT control were performed. The comparative simulation results show that in three cases of uniform light intensity, static partial shading and dynamic shading, the MPPT control strategy based on improved PSO algorithm using chaotic mapping and Gaussian perturbation has a faster convergence speed and a smaller search oscillation amplitude, and it can find the maximum power point with a higher searching precision, thereby improving the power generation efficiency of the MPPT system.

Aimed at the issue that the implementation of maximum power point tracking (MPPT) using group heuristic algorithms for photovoltaic arrays under shading conditions depends on algorithm-specific parameter settings, which leads to problems such as difficulty in the design of algorithms, a low tracking efficiency and large tracking errors, the tracking control of MPPT under shading conditions is implemented based on the Jaya algorithm. The proposed algorithm requires only general control parameters without any specific control parameters, and the solution of maximum power always moves towards the best solution during iterations. In addition, the worst solution is discarded to avoid local solutions, so that the system always tracks the maximum power accurately and maintains efficient operation. A simulation model of photovoltaic arrays under shading conditions is built in MATLAB/Simulink, and the Jaya algorithm is compared with traditional PSO and GA algorithms under the same model. Experimental results show that during the process of optimization, the use of the Jaya algorithm can track the maximum power in the shortest time while keeping the smallest tracking error and oscillation. At the same time, the global maximum power point can also be re-located accurately and quickly when the light intensity changes abruptly.

To reduce the dependence of a control system on the communication network, a DC microgrid (DC-MG) usually adopts the distributed control architecture. However, due to the lack of global information, this control mode is highly vulnerable to cyber-attacks, which leads to greater security risks in the DC-MG. There are many types of cyber-attacks, among which the false data injection attacks (FDIA) account for the highest proportion and pose the biggest danger. On this basis, aimed at the instability problems such as interruption of DC-MG operation caused by FDIA, a DC-MG cyber-attack detection and control method based on feedforward neural network (FNN) is proposed. First, the influence mechanism of FDIA causing the instability of the control system is studied, and a mathematical model of instability caused by FDIA is established. Second, an intelligent estimator is constructed by using FNN to monitor the output current from a distributed power converter in the DC-MG, and the error data values of FDIA are calculated according to the output from the estimator. Third, aimed at the calculated values of error data, a reference tracking method based on a PI controller is introduced to reduce the error data in the attacked converter. In addition, the proposed method can eliminate all the high-domain unba-lanced attacks. Finally, the proposed method was simulated and verified by experiments carried out in MATLAB/Simulink and OPALRT, respectively, and results proved its effectiveness.

As a kind of clean, pollution-free and renewable energy, photovoltaic (PV) power has been vigorously developed and utilized worldwide. However, PV power generation systems are likely to suffer from lightning strikes due to long-term exposure to open areas. Since the existing researches on the lightning receiving of PV modules are all based on the condition that the module surfaces are free of fouling, they do not consider the effect of fouling surface on the lightning receiving characteristics. Aimed at the deficiency in the existing researches on the lightning receiving characteristics of PV modules with fouling surface, a simplified fouling model of PV module suitable for lightning strike is established. Then, through the simulation calculation of electric field intensity and charge density of the PV module with and without fouling, it is found that compared with those of the module without fouling, the electric field intensities at the head and end of the fouled side of the PV module increase by 41.1% and 33.8%, respectively. Meanwhile, a lightning strike rod-plate model of PV module with fouling surface was built in a high-voltage laboratory to carry out long-gap discharge tests, and it was found that the PV modules with fouling surface were more susceptible to lightning strike, thus verifying the accuracy of the simulation model. Moreover, the influence of the installation angle of PV modules on the lightning receiving characteristics in the case of fouling surface was also discussed. Results show that at any angle, the lightning strike probability and electric field intensity of the fouled side were higher than those of the unfouled side. Compared with that in the case of an installation angle of 8°, the electric field intensity at the end of the fouled side decreased by 24.8% when the installation angle was 45°, indicating a deceased probability of lightning strike.

To study the temperature evolution law in the hydrogen fuel cell diffusion layer and analyze the factors affecting the temperature distribution, the cathode and anode diffusion layers and their internal fluid were taken as the research objects, and a correlation method for the heat and mass transfer between diffusion layer Z-Y profile and X-Y profile was established. Considering the fluid Soret effect, Dufour effect, adsorption effect and fluid-solid conjugate heat transfer effect in the diffusion layer, and combined with the N-S equation, Brinkman-Darcy extended model, Langmuir adsorption equation and interfacial capture Cahn-Hilliard theory, a heat and mass double-diffusion model of gas-liquid two-phase flow in porous media was constructed. The Lattice Boltzmann method was used for numerical solution to demonstrate the temperature evolution law in the diffusion layer and elucidate the mechanism of fluid state influence on the temperature distribution. Results show that the diffusion layer's thermal conductivity is closely related to its internal fluid state. The thermal conductivity of hydrogen-water vapor atomized flow in the anode diffusion layer is better than that of the separated flow composed of air-liquid water in the cathode diffusion layer. Therefore, the anode diffusion layer has obvious advantages over the cathode diffusion layer in terms of the equilibrium of temperature field evolution, continuity of isotherm distribution and utilization of heat transfer area. The research results can provide a theoretical basis for improving the thermal management level of hydrogen fuel cells.

With the increasing grid-connected operation capacity of wind turbines, the research on key technologies for their fault ride through (FRT) is becoming more and more urgent. Since the wind turbine with a doubly-fed induction generator (DFIG) is one of the dominant types on the wind power market, researchers have extensively studied the key technologies for the FRT capability of DFIG-based wind turbines. The status quo of researches on the related FRT technologies is reviewed. First, the basic theory of DFIG-based wind turbines is introduced. Then, the analysis methods for the FRT mechanism at present are summed up, and the advantages and disadvantages of different FRT solutions are classified. Finally, the analysis methods and technologies for the FRT of DFIG-based wind turbines are forecasted and summarized.

To further optimize the resource structure, reduce the energy gap and alleviate environmental pollution in China, a scientific evaluation and decision-making mechanism of photovoltaic (PV) power generation projects is in urgent need of establishment, and the comprehensive performance evaluation system of PV power stations should to be improved to promote the sustainable development of the whole social energy system. Focused on the comprehensive evaluation technology for PV power generation units. Based on the basic principle of “multi-criteria, multi-level and multi-factor” decision-making, a comprehensive evaluation system and an evaluation method suitable for PV power generation units are proposed, which comprehensively consider the advantages and disadvantages of objective and subjective weight methods. The principle of minimum discriminant information is used to calculate the combined weight, and a comprehensive evaluation model for PV power generation units is established to compare and analyze the comprehensive evaluation results of a novel PV medium-voltage power generation unit and a conventional PV power generation unit, thus proving the superiority of the design indexes for the novel unit.

A lightweight modular multilevel converter-type fuel cells (MMC-FCs) system based on the partial power conversion (PPC) of FCs is proposed, so as to realize the power control of the FC system which is incorporated into a hybrid medium- and low-voltage network. The MMC-FCs on the AC side based on the synchronous switching network of a high-frequency link (HFL) can realize the coupling cancellation of the sub-module (SM) fundamental frequency and double-frequency ripple power, and reduce the demand of SM capacitance by [2(1–m²/4)^3/2f_SM]/[(1+2m²)πf_F] times. As a result, the volumes of SMs are reduced, and the lightweight of SMs are realized by improving the power density. The FC system uses partial power control to incorporate the DC low-voltage bus, which reduces the partial power capacity and loss of the FC converter. Meanwhile, the FC converter is reused with the AC HFL square wave power signal to reduce the power conversion stages, thus further improving the lightweight of the system. The structure of MMC-FCs, SM filtering principle, FC PPC mechanism, SM capacitance, FC PPC voltage and other parameters are analyzed and designed in detail. Finally, through an evaluation on volume and losses, the effectiveness of the proposed MMC-FCs scheme was verified by simulation and experimental results.

To meet the requirements of fast performance and reliability for multi-terminal modular multi-level converter based high voltage direct current (MMC-HVDC) grid protection, a fault identification scheme based on the characteristics of fault current component is proposed. First, the currents under faults inside and outside of a region are analyzed by using the propagation process of the traveling wave component of fault current, and the inside and outside faults are identified through the difference in changes in fault current. Second, the line-mode component is obtained by decoupling the bipolar line, and the polarity of the line-mode component of fault current can be taken as a direction criterion. Third, the calculated ratio between the averages of positive and negative fault currents is used to realize the fault pole selection. A simulation model of multi-terminal MMC-HVDC transmission line is established in PSCAD/EMTDC, and simulation results show that the proposed fault identification method can identify all kinds of faults quickly and accurately only by using the single-terminal fault current characteristics without the need of two-terminal communication, and it also has the capability of anti-overresistance, thereby satisfying the requirements for MMC-HVDC transmission fault protection.