The structure of latticed/cellular materials is often designed with the lack of information about macro-material. Material information of each macro-element is realized by reducing the scale, ...homogenizing the microstructure, and calculating the properties of an equivalent material for the macro-element. The lattice structure is simultaneously optimized at both the macro- and microstructural levels with additional connectivity constraints, while finite element analysis (FEA) and design variable updates are required twice (at the macro- and micro-levels) for each optimization loop. This approach requires significant storage and has a substantial computational cost. In addition, when the size of the unit cell is quite large compared to the macrostructure, the homogenization method could fail to provide sufficient accuracy. To deal with these issues, in this work, we propose a new multiscale topology optimization approach for the direct and simultaneous design of lattice materials, without material homogenization at the microscale, using adaptive geometric components. The adaptive geometric components are projected onto macro- and micro-element density fields to calculate the effective densities of grid elements. Macro-and microstructures are simultaneously optimized, considering the load and boundary conditions of the overall structure without any additional constraints. FEA and design variable updates are required only once for each optimization loop. Furthermore, the minimum length scales of the macrostructure and the length scales of microstructures can be simultaneously controlled explicitly by simply adjusting the bounds of the size parameters. Some benchmark structures are topologically optimized with different types of lattice materials (such as square, diamond, and triangle) to verify the effectiveness of the proposed method.
This paper addresses the load restoration problem after power outage events. Our primary proposed methodology is using multi-agent deep reinforcement learning to optimize the load restoration process ...in distribution systems, modeled as networked microgrids, via determining the optimal operational sequence of circuit breakers (switches). An innovative invalid action masking technique is incorporated into the multi-agent method to handle both the physical constraints in the restoration process and the curse of dimensionality as the action space of operational decisions grows exponentially with the number of circuit breakers. The features of our proposed method include centralized training for multi-agents to overcome non-stationary environment problems, decentralized execution to ease the deployment, and zero constraint violations to prevent harmful actions. Our simulations are performed in OpenDSS and Python environments to demonstrate the effectiveness of the proposed approach using the IEEE 13, 123, and 8500-node distribution test feeders. The results show that the proposed algorithm can achieve a significantly better learning curve and stability than the conventional methods.
The blossom of renewable energy worldwide and its uncertain nature have driven the need for a more intelligent power system with the deep integration of smart power electronics. The smart inverter is ...one of the most critical components for the optimal operation of Smart Grid. However, due to the deep information and communication technology (ICT) infrastructure implementation that most inverter-based smart power systems tend to have, they are vulnerable to severe external threats such as cyberattacks by hackers. This paper presents a comprehensive review of the system structure and vulnerabilities of typical inverter-based power system with distributed energy resources (DERs) integration, nature of several types of cyberattacks, state-of-the-art defense strategies including several detection and mitigation techniques, and an overview and comparison of testbed and simulation tools applicable for cyber-physical research. Finally, challenges, unsolved problems, and future direction of the field are discussed and concluded at the end of the journal. This paper provides an all-inclusive survey at the state of the art smart grid cybersecurity research and paves the path for potential research topics in the future.
Electrical weapons and combat systems integrated into ships create challenges for their power systems. The main challenge is operation under high-power ramp rate loads, such as rail-guns and radar ...systems. When operated, these load devices may exceed the ships generators in terms of power ramp rate, which may drive the system to instability. Thus, electric ships require integration of energy storage devices in coordination with the power generators to maintain the power balance between distributed resources and load devices. In order to support the generators by using energy storage systems, an energy management scheme must be deployed to ensure load demand is met. This paper proposes and implements an energy management scheme based on model predictive control to optimize the coordination between the energy storage and the power generators under high-power ramp rate conditions. The simulation and experimental results validate the proposed technique in a reduced scale, notional electric ship power system.
•Comprehensive linear time-varying models for DC microgrids are derived.•Model reference adaptive control is enhanced for DC microgrids for the first time.•Experiments validate the capability of the ...proposed method in a 400VDC microgrid.•Comparative study using ISE index verifies the efficacy of the proposed algorithm.
There are tradeoffs between current sharing among distributed resources and DC bus voltage stability when conventional droop control is used in DC microgrids. As current sharing approaches the setpoint, bus voltage deviation increases. Previous studies have suggested using secondary control utilizing linear controllers to overcome drawbacks of droop control. However, linear control design depends on an accurate model of the system. The derivation of such a model is challenging because the noise and disturbances caused by the coupling between sources, loads, and switches in microgrids are under-represented. This under-representation makes linear modeling and control insufficient. Hence, in this paper, we propose a robust adaptive control to adjust droop characteristics to satisfy both current sharing and bus voltage stability. First, the time-varying models of DC microgrids are derived. Second, the improvements for the adaptive control method are presented. Third, the application of the enhanced adaptive method to DC microgrids is presented to satisfy the system objective. Fourth, simulation and experimental results on a microgrid show that the adaptive method precisely shares current between two distributed resources and maintains the nominal bus voltage. Last, the comparative study validates the effectiveness of the proposed method over the conventional method.
As the penetration of power-electronics based smart inverters (SIs) is increasing in distribution grids, it adds computational challenges in solving dynamic models of large-scale distribution ...feeders. Voltage and reactive power (Volt/VAr), and voltage and active power (Volt/Watt) dynamics have been analyzed at slower time scales akin to the control of legacy grid devices. However, smart inverters, being power-electronics based devices, can provide dynamic active/reactive power support at a faster time scale, which necessitates Volt/VAr and Volt/Watt dynamics to be analyzed at a faster time scale. The existing dynamic models are overly detailed and computationally intractable for distribution feeders with a large number of inverters. In this context, this proposed work aims towards developing a computationally tractable, scalable, and accurate phasor-based model for dynamic Volt/VAr and Volt/Watt analyses of large distribution systems with high penetration of smart inverters. Case studies demonstrate that the proposed phasor-based model sufficiently captures the Volt/VAr and Volt/Watt dynamics, and is computationally faster by one order of magnitude compared to the average model and by two orders of magnitude compared to the detailed switching model. Case studies also demonstrate the efficacy and scalability of the proposed model in analyzing Volt/VAr and Volt/Watt dynamics of large-scale power networks with hundreds of SIs.
The results of experimental and theoretical studies of crack formation, deformation and failure of monolithic reinforced concrete frames under accidental action caused by the sudden removal of one of ...columns of the first floor. Experimental studies were carried out on the model of structural fragment of a three-story two-span reinforced concrete frame, designed from fine-grained concrete class B40 with reinforcement A500. The test of experimental structures is executed by the gravitational load, with use of specially designed lever loading device. Special effect in the form of sudden removal of central columns of the frame was modelled using a specially designed mechanism in the form of sudden switching-off device. The obtained experimental values of widths of cracks, deflections, pictures of cracking and failure of experimental structures of the frames before and after beyond-design action. The experimental values of these parameters are compared with the results of calculation by the method taking into account the specifics of static-dynamic loading of physically and constructively nonlinear structures under the considered special actions.
This study aims to examine and analyze the effect of corrosion wastage on the ship hull of a double hull very large crude oil carrier. To calculate the ultimate bending moment capacity, along with ...the neutral axis position at the limit state, section modulus, and moment of inertia, the incremental-iterative method is employed. This paper also considered the residual strength checking criteria of ship hull and the ultimate stress behaviors of the representative structural elements. Then, Paik's probabilistic corrosion, which employs two levels of corrosion rate and three different assumptions of coating life time, is applied to assess the corrosion effects. The calculation results obtained through relevant analyses are also presented.
Abstract
Ship hull structures, particularly those with extended service life, contain various imperfections, including initial deformities, residual stresses, and corrosion-induced degradation. Such ...variables critically influence the structural integrity of the hull girder. Consequently, comprehending the residual strength capacity of the hull girder is paramount for guaranteeing the vessel’s operational safety and dependability, especially under severe loading scenarios. The existing methods to determine the hull girder’s ultimate bending moment either ignore imperfections or consider imperfections such as initial deformations or residual stresses but with the assumption of a fixed level. In the current study, the stress distribution method and incremental-iterative method are used; and solutions are proposed to cover factors such as initial deformations, residual stresses, and corrosion. A single skin bulk carrier, 56,000 DWT of deadweight, is employed as the demonstrative example. Eleven cases of imperfections are suggested, and the variations of ultimate bending moment for hogging and sagging conditions under the effect of uncertainties are analyzed and evaluated. The results of the current study are offered and discussed. Several concluding remarks are also presented.
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