Sharif Digital Repository

Due date assignment for customer orders has been studied in various production environments using different modeling approaches. In this paper the researchers consider a production system in which the orders of several customers are produced in a single batch because of the economy of scale. If a batch is completed before receiving customer orders, inventory carrying cost is incurred but if it is completed after receiving customer orders, shortage cost is incurred and finally if the order is delivered after its due date, tardiness cost is charged. The problem is to decide on batch size, due date of batch (imposed on supply process) and lead time (to be quoted to the next coming customers) so... 

In this paper, a new scheme for position control of a mechanical load driven by a permanent magnet DC motor and a gearbox is presented. The gearbox is assumed to have some amount of backlash. The paper derives a model for the backlash nonlinear behavior. It then proposes a new control structure composed of feedback and feed forward control systems. The feedback control system is designed based on zero backlash. The feed forward part is employed to minimize the backlash effects. The new method is evaluated via simulations in MATLAB/SEVIULINK environment. Simulations results are provided to show the response of the proposed technique to various input references. © 2006 IEEE  

Sub-harmonic stability issues due to the interaction between a load-commutated motor drive and a synchronous turbo-generator train in a Liquefied Natural Gas (LNG) station are studied in this paper. The sub-harmonic current injected by motor drives may excite the mechanical torsional modes of turbogenerator train. In this paper, the electro-mechanical interaction phenomenon is investigated and an active damper is proposed to absorb the sub-harmonic currents of motor drives and increase the damping of the electro-mechanical system. Simulations and experiments validate the instability problem and effectiveness of the proposed solution. © 2018 EPE Association  

Power sharing control among grid forming dc sources employing a conventional voltage droop approach meets inaccurate load sharing and unacceptable voltage regulation performance. Thereby, communication-based secondary and supervisory controllers have been presented to overcome the aforementioned issues. Furthermore, with the aim of eliminating communication system, frequency-based droop approaches have been introduced for low-voltage dc grids where the frequency of the superimposed ac signal onto the dc voltage is proportional to the output power. However, in reality, dc grid structures can be applied to medium and high voltage applications with different X/R ratios. This paper generalizes... 

This paper presents the results of a thorough analysis of applying power quality standards to a granite factory. It explains the procedure of determining power quality indices in a power system. The paper presents the field data and provides the results of statistical analysis in order to extract power quality indices. A comparison against standard limits is also given. The case study gives an overview of power quality in such customers and brings up some practical limitations when applying power quality standards. ©2006 IEEE  

This article proposes two novel approaches to improve the superimposed frequency droop scheme for the control of dc microgrids (MGs). Conventional voltage-based control strategies suffer from issues such as undesirable voltage regulations, poor power sharing among the sources, and negative effects of line resistances on the equivalent droop characteristics. To overcome these challenges, a superimposed frequency droop scheme has been introduced. However, this method suffers from three major issues: 1) instability in terms of load variation, which is due to the location of system dominant poles; 2) limitation in system loading due to the limitation in the transferred reactive power; and 3)... 

In this paper, we analyze the tradeoff between the profits gained by mobile virtual network operators (MVNOs) in an orthogonal frequency division multiple access (OFDMA)-based virtualized wireless network (VWN). In this respect, MVNOs rent the network resources from a mobile network operator (MNO) to create virtual resources based on allocated rates and the cost due to allocated transmit powers in two different strategies: resource-based isolation strategy and rate-based isolation strategy. In resource-based isolation strategy, it is assumed that the whole bandwidth in each base station is divided equally between MVNOs whereas in rate-based isolation strategy, the whole bandwidth in each... 

In this paper, we investigate the resource allocation in an underlay cognitive radio network when multiple primary and secondary users are served via multi-carrier non-orthogonal multiple access (MC-NOMA) scheme. Our main objective is to maximize the sum rate subject to a minimum guaranteed rate for the primary users. The performance of the system is highly affected by the power allocation and subchannel assignment for all users. Hence, our optimization problem is formulated as a nonconvex mixed integer non-linear program whose global optimum can be found through a computationally infeasible exhaustive search. To overcome this challenge, we propose an efficient iterative algorithm to jointly... 

In this paper, we analyze the tradeoff between the profits gained by mobile virtual network operators (MVNOs) in an orthogonal frequency division multiple access (OFDMA)-based virtualized wireless networks (VWNs). In this respect, MVNOs rent the network resources from a mobile network operator (MNO) to create virtual resources based on allocated rates and the cost due to allocated transmit powers in two different strategies: resource-based isolation strategy and rate-based isolation strategy. In resource-based isolation strategy, it is assumed that the whole bandwidth in each base station is divided equally between MVNOs whereas in rate-based isolation strategy, the whole bandwidth in each... 

In this paper, a distributed secondary power sharing approach with low bandwidth communication network is proposed for low voltage direct current (LVDC) microgrids. Conventional droop control causes voltage drop in the grid and also a mismatch on the current of converters in the case of consideration of the line resistances. Proposed control system carry out the current value of the other converters to reach the accurate current sharing and suitable voltage regulation as well. Voltage and current controllers locally regulate the voltage and current of converters as a secondary controller. Secondary controller is realized locally and the communication network is only used to transfer the data... 

The increasing use of power electronic equipment and the importance of their continuous and correct operation have attracted remarkable attention to the condition monitoring of power electronic converters. Since capacitors are the leading cause of power electronic converters failure, this paper attempts to attain a solution to monitor the condition of the DC-link capacitor in a three-phase back-to-back converter using the stimulated measurable transients. The proposed method works based on the temporary change of switching logic and the measurement of the DC-link voltage and converter input phase current transients. By processing the measured signals, the capacitance of the capacitor is... 

Capacitors are one of the most critical components in power electronic converters, yet they are notoriously susceptible to failure. Avoiding unforeseen outages caused by capacitor failures is one of the most effective approaches to increase system availability. Therefore, a variety of methods have been proposed to monitor a capacitor health condition based on different degradation indicators. This paper proposes a new approach based on the accurate measurement of an electrolytic capacitor dissipation factor to detect its end-of-life. Since the dissipation factor is affected by both the capacitor resistance and capacitance simultaneously, it can provide more information about the health... 

The influence of particle size and surface modification on sonocatalytic activity of TiO2 nanoparticles was investigated by measuring the degradation efficiency of methyl orange (MO) as a model pollutant. Crystalline TiO2 nanoparticles with different particles and aggregate size were prepared through solution-phase method with varying synthesis temperatures. Coating with polyvinyl alcohol was performed to enhance colloidal stability of the particles over a wide range of pH values (1.5–7.5). Characterization was carried out using X-ray diffraction, high-resolution transmission electron microscopy, dynamic light scattering, and Fourier-transformed infrared techniques. It was found that... 

In a classical ac microgrid (MG), a common frequency exists for coordinating active power sharing among droop-controlled sources. Like the frequency-droop method, a voltage-based droop approach has been employed to control the converters in low voltage direct current (LVDC) MGs. However, voltage variation due to the droop gains and line resistances causes poor power sharing and voltage regulation in dc MG, which in most cases are solved by a secondary controller by using a communication network. To avoid such an infrastructure and its accompanied complications, this paper proposes a new droop scheme to control dc sources by introducing a small ac voltage superimposed onto the output dc... 

Providing clean, continuous and reliable power to many industrial customers has become a necessity. Where possible, sensitive customers are supplied by two medium-voltage utility feeders one of which is used as back up. Thyristor-based Static Transfer Switches (STSs) are able to provide the loads with a fast transfer to the back up feeder when the main one fails. In addition to being fast, the transfer must be performed with no source paralleling which creates a cross current flowing between the main feeder and the back up one. Due to practical limitations, however, in some cases, cross current cannot be avoided. This paper investigates the performance of thyristor-based STSs with respect to... 

Non-orthogonal multiple access (NOMA) is a promising scheme for the fifth generation (5G) of mobile communication systems. In this scheme, transmission to multiple users is performed on the same subchannel using superposition coding and successive interference cancellation. In this paper, we focus on a multi-cell network with two users' data traffic models, namely elastic and streaming. We exploit the NOMA scheme in order to maximize the download elastic traffic rate at cells, without degrading the download streaming traffic rates. Since elastic traffic rates at different cells are interactive, we maximize the total elastic traffic rates assuming either perfect or partial channel state... 

Non-orthogonal multiple access (NOMA) is a promising scheme for the fifth generation (5G) of mobile communication systems. In this scheme, transmission to multiple users is performed on the same subchannel using superposition coding and successive interference cancellation. In this paper, we focus on a multi-cell network with two users' data traffic models, namely elastic and streaming. We exploit the NOMA scheme in order to maximize the download elastic traffic rate at cells, without degrading the download streaming traffic rates. Since elastic traffic rates at different cells are interactive, we maximize the total elastic traffic rates assuming either perfect or partial channel state... 

Voltage unbalance and short circuits in distribution networks adversely affect the performance of grid-tied voltage source inverters (VSIs). Consequently, the dc-link voltage ripple may significantly increase leading to operation of VSI in an unsatisfactory manner. Conventionally in order to maintain the negative sequence current under unbalanced conditions, low-pass/trap filter in the current control loop are required which significantly reduce the controller bandwidth. In order to minimize the dc-link voltage ripple without impairing the controller bandwidth, this paper investigates the design and performance of Proportional-Integral-Resonant (PIR) controller in improving the performance...