Sharif Digital Repository

An important issue in power system planning is maintenance scheduling of generating units. In a traditional power system, the problem of maintenance scheduling is of high importance and has various technical and economical constraints. Different methods have been used to solve the problem. On the other hand, changes in legal environment of electricity industry poses new bound and requirements on the maintenance scheduling problem. This paper presents a new approach on maintenance scheduling of generating units in a generating company (GENCO). The proposed approach uses Markov Decision Process (MDP) to minimize total costs of unserved energy and reserve. The impacts on maintenance scheduling... 

The electric power industry around the world is presently undergoing considerable changes with respect to structure, operation and regulation. Distributed generation is expected to play a major role in this new environment One the main benefits of DG is to enhance system reliability as it can restore power to the separated or bottle-necked part of distribution system in case of a contingency occurs. This paper presents an approach to examine and evaluates the applicability and customer interruption cost analysis in DG-connected distribution system in a specified planning horizon. The impacts on customer interruption cost reduction of different load growth rate, DG location and different... 

Distributed generation (DG) portion of power production is increasing more and more. DG poses different benefits and difficulties in a power system. Inadvertent islanding is a problem with DG which may cause health danger to line crew and can damage both DG and power system equipments. This paper investigates the effect of reactive power on ROCOF islanding detection method. Impacts of load power factor and load combination are studied using a sample test system. The paper offers a corrective method to cope with impacts of load reactive power. A way is suggested to locate DG such that the role of reactive power becomes a positive factor in islanding detection. © 2004 IEEE  

Distributed generation is expected to play an increasing role in emerging power systems. They use different type of resources and technologies to serve energy to power system. DG has positive and negative effects for both the utilities and customers. System and load point reliability indices are normally improved if DG is installed in a distribution network. On the other hand, DG has some specific characteristics which distinguish it from conventional generating units. Therefore, they should not be treated as conventional generating units to perform reliability evaluation. An appropriate model is required to be developed in order to assess impacts of DG on reliability indices. Impacts of DG... 

Large scale Internet services are expected to only increase in complexity and popularity. Their energy consumption is also a major concern in data centers. Smart scheduling of their sub-services on data center Physical Machines (PM) can effectively improve their energy as well as performance. Since today servers are not energy-proportional yet, a major and traditionally neglected source of inefficiency in them is the utilization level of PMs. We present two scheduling algorithms for precedence-constrained parallel Virtual Machines (VM) in a virtualized data center where each VM represents a sub-service of the Internet-scale service. Our algorithms use virtualization technology to increase... 

Al-20Si-5Fe-2X (X=Cu, Ni, Cr) ribbons were produced by melt-spinning and consolidated by hot pressing at 400°C for 60 min. Optical and transmission electron microscopy (TEM), X-ray diffractometry (XRD), and hardness test were used to evaluate the microstructure and mechanical strength of the ribbons and hot consolidated specimens. The compressive strength of the specimens at ambient and elevated temperatures was also examined. The microstructure of the ribbons exhibited featureless and dendritic zones, and contains spherically-shaped Si particles with an average diameter of 20 nm. After hot-pressing, ultrafine Si (110-150 nm) and iron-containing intermetallic particles were formed. The... 

Al-20Si-5Fe-2X (X = Cu, Ni and Cr) ribbons were produced by melt-spinning and consolidated by hot pressing at 400 °C for 60 min. The microstructure of the ribbons and the consolidated alloys was investigated using optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffractometry (XRD) method, and transmission electron microscopy (TEM). The hardness and compressive strength of the specimens at ambient and elevated temperatures were examined. The microstructure of the ribbons exhibited featureless and dendritic zones. Results of XRD and TEM showed formation of spherically shaped Si particles with an average diameter of 20 nm. Ultrafine Si (110-150 nm) and iron-containing... 

This paper presents an approach for optimal operation of a grid-connected hybrid system to maximize the system expected generated energy while maximizing the electricity sale revenue. The hybrid system consists of a photovoltaic (PV) module, an electrolyser (EL), hydrogen storage tank (HS) and a proton exchange membrane (PEM) fuel cell (FC). The generated energy is sold to the distribution grid. During the grid interruptions, the hybrid system output is used as an emergency supply to serve the local load, hence increasing the supply reliability. The proposed approach splits the whole problem in two sub problems: revenue maximizing problem (RMP) and expected generation maximizing problem... 

In this paper, we propose a novel link-tracing sampling algorithm, based on the concepts from PageRank vectors, to sample from networks with high community structures. Our method has two phases; (1) Sampling the closest nodes to the initial nodes by approximating personalized PageRank vectors and (2) Jumping to a new community by using PageRank vectors and unknown neighbors. Empirical studies on several synthetic and real-world networks show that the proposed method improves the performance of network sampling compared to the popular link-based sampling methods in terms of accuracy and visited communities  

This paper describes the photoluminescence study of titanium dioxide (TiO2) nanorods grown by a hydrothermal synthesis method on the surface of fluorine doped tin oxide (FTO) coated glass. The effects of growth conditions including: reaction time, precursor concentration and adding NaCl to hydrothermal solution on the structural and optical properties of productions are examined by using SEM, XRD, TEM and room temperature photoluminescence measurements. Also, the performance of the TiO2 nanorods as a photoanode of dye sensitized solar cells is investigated. The different excitation energies and intensities are chosen to verify the discrete electronic state of radiative recombination centers... 

DG application has received increasing attention during recent years. The impact of DG on various aspects of distribution system operation, such as reliability and energy loss, depend highly on DG location in distribution feeder. Optimal DG placement is an important subject which has not been fully discussed yet. This paper presents an optimization method to determine optimal DG placement, based on a cost/worth analysis approach. This method considers technical and economical factors such as energy loss, load point reliability indices and DG costs, and particularly, portability of DG. The proposed method is applied to a test system and the impacts of different parameters such as load growth... 

We have investigated an oxidation of substrate effect on structural morphology of zinc oxide (ZnO) rods. ZnO rods are grown on porous silicon (PS) and on thermally oxidized porous silicon substrates by carbothermal reduction of ZnO powder through chemical vapour transport and condensation. Porous silicon is fabricated by electrochemical etching of silicon in hydrofluoric acid solution. The effects of substrates on morphology and structure of ZnO nanostructures have been studied. The morphology of substrates is studied by atomic force microscopy in contact mode. The texture coefficient of each sample is calculated from X-ray diffraction data that demonstrate random orientation of ZnO rods on... 

Here, the UV photodetection of ZnO rods grown on porous silicon substrates are reported. Laterally interconnected ZnO rods have been synthesized by chemical vapor transport and condensation method on porous silicon substrates. As characterized by current-voltage measurements the I-V characteristics have linear behavior, indicating space charge effect. The device exhibits photocurrent response of 0.027 A/W for 325 nm UV light under -5 V bias. The rise and decay time constants under these conditions are 19 and 62 s, respectively  

ZnO tetrapods and rods were grown on silicon and thermally oxidized porous silicon substrates with and without Au catalyst layer by carbothermal reduction of ZnO powder through chemical vapor transport and condensation method (CVTC). Porous silicon was fabricated by electrochemical etching of silicon in HF solution. The effect of substrates on morphology, structure and photoluminescence spectra of ZnO nanostructures has been studied. The texture coefficient (TC) of each sample was calculated from XRD data that demonstrated random orientation of ZnO nanostructures on the oxidized porous silicon substrate. Moreover, TC indicates the effect of Au catalyst layer on formation of more highly... 

The aim of this work is to study the effect of cooling rate and subsequent hot consolidation on the microstructural features and mechanical strength of Al-20Si-5Fe-2X (X = Cu, Ni and Cr) alloys. Powder and ribbons were produced by gas atomization and melt spinning processes at two different cooling rates of 1 × 105 K/s and 5 × 107 K/s. The microstructure of the products was examined using optical microscopy, scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. The particles were consolidated by hot pressing at 400 °C/250 MPa/1 h under a high purity argon atmosphere and the microstructure, hardness and compressive strength of the compacts were evaluated.... 

Al-20Si-5Fe melt was rapidly solidified into particles and ribbons and then consolidated to near full density by hot pressing at 400°C/250 MPa/1 h. According to the eutectic-growth and dendritic-growth velocity models, the solidification front velocity and the amount of undercooling were estimated for the particles with different sizes. Values of 0.43-1.2 cm/s and 15-28 K were obtained. The secondary dendrite arm spacing revealed a cooling rate of 6 × 105 K/s for the particles with an average size of 20 μm. Solidification models for the ribbons yielded a cooling rate of 5 × 107 K/s. As a result of the higher cooling rate, the melt-spun ribbons exhibited considerable microstructural... 

The effects of particle size and alloying elements on the size and morphology of secondary/intermetallic phases in gas atomized Al-20Si-5Fe-2X (X = Cu, Ni, Cr) powders were studied. The microstructure composed of particle-like primary Si with average diameter of 0.5-2.7 μm, δ-Al4FeSi2 intermetallic compound containing small amount of dissolved alloying elements with the size of 1.4-7.7 μm, and fine Al-Si eutectic matrix with 0.25-0.4 μm spacing. As the powder particle size decreases, the primary silicon becomes finer while its morphology is not affected. For instance, 58-75% reduction in the size of primary silicon was observed when decreasing the average particle size from 89 to 18 μm. A... 

The size and morphology of secondary phase and intermetallic compounds in gas atomized Al- 20Si-5Fe-2X (X=Cu, Ni, Cr) powders were studied. It was shown that the microstructure compromised of fine particle-like primary Si, aciculate δ-Al9FeSi3 intermetallic compound containing small amount of dissolved alloying elements, and fine Al-Si eutectic matrix. Decreasing the powder particle size yields finer primary silicon but its morphology is not affected. Also, significant refinement in the size of δ-Al9FeSi3 intermetallic was observed as the particles become finer. This particle size effect on the microstructure of Al-20Si-5Fe alloy was found to be influenced by the presence of chromium. Here,... 

A uniformly distributed ZnO nanowire array has been grown on silicon (100) substrates by catalyst-free chemical vapor transport and condensation. The effect of growth conditions including source heating temperature, substrate temperature, and gas flow rate on growth properties of ZnO nanowire arrays are studied. Scanning electron microscopy, X-ray diffraction, and room temperature photoluminescence are employed to study the structural features and optical properties of the samples. The results show a correlation among experimental growth parameters. There is a zone for substrate temperature, by controlling gas flow rate, that uniformly distributed and well aligned ZnO nanowire arrays can be...