Modular multilevel converter (MMC)-based high-voltage direct current (HVDC) grid is a promising technology for multiple offshore wind farms (OWFs) to form interconnected offshore grids. However, in the MMC-HVdc grid, the converter terminals connecting the OWFs could only be passively pre-charged by the ones connecting active ac networks via the dc lines during the start-up process. Under this condition, the start-up process of HVdc grid of the passive-side converters becomes significantly complicated. This paper, considering an HVdc grid integrating multiple passive-side converters, investigates its start-up process with three main contributions: 1) a modified nearest level modulation (NLM) method is proposed for the passive-side converters to mitigate the inrush currents after they are deblocked; 2) based on the dc-side equivalent models, the inter-converter resonances and oscillations between the passive-side converters are analyzed after they are deblocked, and; 3) an oscillation damping controller is proposed to provide the electrical damping of inter-converter oscillations between the passive-side converters. The simulation results in PSCAD/EMTDC from a four-terminal MMC-HVDC grid with two terminals connecting the OWFs verify the effectiveness of the proposed start-up control schemes.INDEX TERMS HVDC grid, modular multilevel converter (MMC), start-up control, inrush currents, inter-converter oscillations. NOMENCLATURE T iTerminal4) SM Sub-module HB Half-bridge CB Circuit breaker VSC Voltage source converter MMC Modular multilevel converter HVDC High-voltage direct current PTP Point-to-point NLM Nearest level modulation SCR Short circuit ratio PCC Point of common coupling MTDC Multi-terminal HVDC The associate editor coordinating the review of this manuscript and approving it for publication was Ahmad Elkhateb.

Polymethylsilsesquioxane (PMSQ) nanoparticles with mass percentages of 0, 2.5, 5.0, 7.2, 9.4 wt %, respectively, were constructed by molecular dynamics methods in this paper. Composite molecular models were established using PMSQ and MPIA (poly-metaphenylene isophthalamide) fiber. The influence of different PMSQ contents on the thermal stability of meta-aramid insulation paper was analyzed from the parameters of mechanical property, interaction energy, and mean square displacement. The results showed that the trend of mechanical properties decreased with the increase of PMSQ content. When the PMSQ content was 2.5 wt %, the mechanical properties of the composited model were the best, which was about 24% higher than that of the unmodified model. From an intermolecular bonding and nonbonding point of view, the energy parameters of composite model with the 2.5 wt % content was better than those of the composite model with other contents. Therefore, it is considered that MPIA can interact better with the 2.5 wt % content PMSQ composite model. When the PMSQ content is 2.5 wt %, the overall chain movement in the composite model is slower than that of the unmodified model, which can effectively inhibit the diffusion movement of the MPIA chain. In general, the thermal stability of composite molecular models MPIA and PMSQ (2.5 wt %) was better improved.

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At present, the sparse recovery problem is mainly solved by convx optimization algorithm and greedy tracking method. However, the former has defects in recovery efficiency and the latter in recovery ability, and neither of them can obtain effective recovery under large sparsity or small observation degree. In this paper, we propose a new sparse recovery algorithm based on arithmetic optimization algorithm and combine the ideas of greedy tracking method. The proposed algorithm uses arithmetic optimization algorithm to solve the sparse coefficient of the signal in the transform domain, so as to reconstruct the original signal. At the same time, the greedy tracking technique is combined to design the initial position of the operator before solving, so that it can be searched better. Experiments show that compared with other methods, the proposed algorithm can not only obtain more effective recovery, but also run faster under general conditions of observation number. At the same time, It can also recover the signal better in the presence of noise.

With the construction of firm and intelligent power grid in China, it is difficult for the traditional management method of electrical energy metering device to meet the prospecting requirements. Using the computer and internet techniques to realize the information and intelligentization of the electrical energy metering management has become a necessary guarantee of improving power supply ability, marketing control, and customer service. This paper introduced a kind of large and intelligent condition management system of the gateway electrical energy metering device. The key technologies and realize process were analyzed. Moreover, a detailed description of the application modules such as the GIS smart display of metering point, the condition management of metering devices and the visual monitoring of metering point was presented. The trial operation in the selected transformer substations and the power stations of Chongqing Power Electrical Corp. indicated that, the condition management system is very open, safety and efficient. According to the data exchange with the production and scheduling platform, the system improved the efficient operation of the electrical energy metering devices. Meanwhile, combined with the real-time visual monitoring, the condition management system improved the prevention ability of electricity filching, realized the unified automatic large-scale management of electrical energy metering devices.