• Journal of Electrical Engineering and Technology
• /
• v.10 no.2
• /
• pp.452-464
• /
• 2015

This paper proposes a DC microgrid operational strategy and control method for improved service reliability. The objective is to supply power to as many non-critical loads as possible, while providing an uninterrupted power supply to critical loads. The DC bus signaling method, in which DC voltage is an information carrier, is employed to implement the operational strategy in a decentralized manner. During grid-connected operation, a grid-tied converter balances the power of the microgrid by controlling the DC voltage. All loads are connected to the microgrid, and operate normally. During islanded operation, distributed generators (DGs), a backup generator, or an energy storage system balances the power. However, some non-critical loads may be disconnected from the microgrid to ensure the uninterrupted power supply to critical loads. For enhanced service reliability, disconnected loads can be automatically reconnected if certain conditions are satisfied. Control rules are proposed for all devices, and detailed microgrid operational modes and transition conditions are then discussed. Additionally, methods to determine control parameter settings are proposed. PSCAD/EMTDC simulation results demonstrate the performance and effectiveness of the proposed operational strategy and control method.

• Computers and Concrete
• /
• v.19 no.3
• /
• pp.243-256
• /
• 2017

Epoxy-coated reinforcing bars are widely used to protect the corrosion of the reinforcing bars in the RC elements under their in-service environments and external loads. In most field surveys, it was reported that the corrosion resistance of the epoxy-coated reinforcing bars is typically better than the uncoated bars. However, from the experimental tests conducted in the labs, it was reported that, under the same loads, the RC elements with epoxy-coated reinforcing bars had wider cracks than the elements reinforced with the ordinary bars. Although this conclusion may be true considering the bond reduction of the reinforcing bar due to the epoxy coating, the maximum service loads used in the experimental research may be a main reason. To answer these two phenomena, service performance of 15 RC beam specimens with uncoated and epoxy-coated reinforcements under different fatigue loads was experimentally studied. Influences of different coating thicknesses of the reinforcing bars, the fatigue load range and load upper limit as well as fatigue load cycles on the mechanical performance of RC test specimens are discussed. It is concluded that, for the test specimens subjected to the comparatively lower load range and load upper limit, adverse effect on the service performance of test specimens with thicker epoxy-coated reinforcing bars is negligible. With the increments of the coating thickness and the in-service loading level, i.e., fatigue load range, load upper limit and fatigue cycles, the adverse factor resulting from the thicker coating becomes noticeable.

• Proceedings of the Korea Concrete Institute Conference
• /
• 2003.11a
• /
• pp.396-401
• /
• 2003

For a realistic and reasonable determination of condition and status, assessment system should include load modeling for the loads acting on slab bridges due to traffic loads, evaluation system for fatigue, and determination of remaining service life. The purpose of the present study is to develop a realistic expert system which can estimate reasonably the fatigue and service life of slab bridges, based on the load models that are derived from the traffic loads.

• Nuclear Engineering and Technology
• /
• v.47 no.3
• /
• pp.340-350
• /
• 2015

This paper proposes strain-based acceptance criteria for assessing plastic instability of the safety class 1 nuclear components made of ferritic steel during level D service loads. The strain-based criteria were proposed with two approaches: (1) a section average approach and (2) a critical location approach. Both approaches were based on the damage initiation point corresponding to the maximum load-carrying capability point instead of the fracture point via tensile tests and finite element analysis (FEA) for the notched specimen under uni-axial tensile loading. The two proposed criteria were reviewed from the viewpoint of design practice and philosophy to select a more appropriate criterion. As a result of the review, it was found that the section average approach is more appropriate than the critical location approach from the viewpoint of design practice and philosophy. Finally, the criterion based on the section average approach was applied to a simplified reactor pressure vessel (RPV) outlet nozzle subject to SSE loads. The application shows that the strain-based acceptance criteria can consider cumulative damages caused by the sequential loads unlike the stress-based acceptance criteria and can reduce the overconservatism of the stress-based acceptance criteria, which often occurs for level D service loads.

• Proceedings of the KSME Conference
• /
• 2001.06a
• /
• pp.424-429
• /
• 2001

Residual stresses can be produced during manufacturing processes, eg. welding, machining and plastic working, and also in service. It can be superimposed with externally applied loads, so that unexpected deformations and failures of members will be occurred. Especially, the strength and the life of welded components are affected extensively by the residual stresses distributed around their weldments not only under static loads, but also fatigue loads. These residual stresses are not kept constant, but relaxed or redistributed during service. Under static loads the relaxation takes place when the residual stress superimposed with external stress exceeds locally the yield stress of material used. It is shown that under fatigue loads the residual stress is considerably relieved by the first or few cycle loading, and then gradually relaxed with increasing loading cycles. Although many investigations in this field have been carried out, the phenomenon and mechanism of the stress relaxation by mechanical means are still not clear, and there are few comprehensive models for predicting specific effects on the stress relaxation. In this study, the effects of applied static and fatigue loads on the residual stress relaxation were Investigated, and a model to predict quantitatively the residual stress relaxation was proposed.

• Computers and Concrete
• /
• v.6 no.5
• /
• pp.421-435
• /
• 2009

Chloride induced corrosion is a concern that governs the durability of concrete structures in marine environments, especially in tidal environments. During the service lives of concrete structures, internal cracks in the concrete cover may appear due to imposed loads, accelerating chloride penetration because of the simultaneous action of environmental and service structural loads. This paper investigated the effects of cyclic flexural loads on chloride diffusion characteristics of plain concretes, and proposed a model to predict the chloride penetration into plain concretes subjected to both tidal environments and different cyclic flexural load levels. Further, a new experiment was performed to verify the model. Results of the model using Finite Difference Method (FDM) showed that the durability of concretes in tidal environments was reduced as cyclic flexural load levels, SR, increased, and the modeling results fitted well with the experimental results.

• Proceedings of the Computational Structural Engineering Institute Conference
• /
• 2001.04a
• /
• pp.414-421
• /
• 2001

Recently, the damping effect of building structures are greatly reduced because the use of non-structures members as like curtain wall are decreased and large open space are in need for the service of buildings. Assembly and office buildings with a lower natural frequency have a higher possibility of experiencing excessive vibration induced by human activities as like jumping, running and walking. These excessive vibration make the occupants uncomfortable and the serviceability deterioration. The common method of application of walking loads for the vibration analysis of structures subjected to walking loads is to inflict a series unit walking load and a periodic function at a node. But this method could not consider the moving effect of walking. In this study, natural frequency and damping ratio of plate structure are evaluated by heel drop tests. And new application of equivalent walking loads are introduced for vibration analysis of real slab system subjected to walking loads. The response obtained from the numerical analysis are compared well to the results measured by experimental tests. It is possible to efficiently analyze the vibration of floor which is subjected to walking loads by applying equivalent walking loads.

• Journal of Welding and Joining
• /
• v.20 no.3
• /
• pp.84-90
• /
• 2002

The strength and the life of welded components are affected extensively by the residual stresses distributed around their weldments not only under static loads, but also fatigue loads. The residual stress can be superimposed with externally applied loads, so that unexpected deformations and failures of members will be occurred. These residual stresses are not kept constant, but relaxed or redistributed during in service. Under static loads the relaxation takes place when the external stress superimposed with the residual stress exceeds locally the yield stress of material used. It is shown that under fatigue loads the residual stress is considerably relieved by the first or flew cycle loading, and then gradually relaxed with increasing loading cycles. In this study the phenomenon and mechanism of the stress relaxation by mechanical means were investigated and a model to predict quantitatively the residual stress relaxation for the case of static and fatigue loading condition was proposed.

• The Transactions of the Korean Institute of Electrical Engineers A
• /
• v.49 no.1
• /
• pp.13-18
• /
• 2000

The paper proposes a simple method to calculate the portion of real power losses of transmission lines allocated to individual loads. The method is implemented by defining loss distribution factors, and analyses the loads shares in the transmission line losses. The paper also performs calculation of sensitivities of the line losses with respect to load changes so as to be easily employed in on-line applications. Effectiveness of the algorithm is verified by computer simulations and it is estimated that the results can be used to compute the cost of one ancillary service under deregulated environment in electric power industries.

• International Journal of Control, Automation, and Systems
• /
• v.3 no.spc2
• /
• pp.376-385
• /
• 2005

The proliferation of technology has made global conduction of business increasingly dependent upon the availability of reliable power. As a result, alternate power systems are being installed and expanded to protect the broadening scope of critical electrical loads. Bus transfer restores designated critical loads to an alternate source when utility derived service becomes inadequate or goes out of service due to any contingency. This paper describes the practices, requirements and implementation of bus transfer of motor loads to an alternate source of power. A new high-speed automatic bus transfer scheme is proposed which includes the development of a new algorithm for determining the type of bus transfer required and the realization of the scheme by using modem protection devices and intra-substation communication facilities.