Sharif Digital Repository

This paper proposes effective approach for determining appropriate crowbar resistance value to be able to improve the brushless doubly fed induction generator ride through capability during any asymmetrical voltage dip scenarios. The brushless DFIG has great potential for wind power plants particularly in offshore applications where maintenance is a major concern. Dynamic behavior of the machine is studied using two axis model and a more precise equivalent circuit model is extracted for analyzing machine behavior under fault conditions. Important limits and constraints in the use of crowbar are identified and discussed in detail. It is shown that large crowbar values can lead to considerable... 

Brushless Doubly Fed Induction Generator (BDFIG) has been recently proposed to be used in variable speed wind turbines. This paper intends to investigate the influence of certain model simplifications to obtain a reduced-order model of a wind turbine with BDFIG suitable for transient studies. To achieve this goal, small signal analysis is performed for a recently manufactured 250 kVA BDFIG to obtain system modes and their participation factors. Identifying highly damped modes, influences of neglecting dynamics of their participated states are studied through time-domain simulation in MATLAB/Simulink  

In this paper we propose and analyze the application of generalized optical orthogonal codes (GOOC) in the optical wireless local area networks (OWLAN). A system deploying GOOC can support more users and achieve better BER performance. We evaluate the system performance of GOOC noting practical constraints of OWLAN applications. We consider average and peak power limitations of free space infrared sources, arising from eye safety and device non-linearity restriction. We include the impact of various noises including background-light induced noise in system evaluation. Proper system design requires appropriate selection of GOOC code parameters. We analyze the influence of code parameters on... 

In the presented paper, an analytical model is developed for calculation of the air gap magnetic flux density in the axial-flux surface-mounted PM machines. The slotting effect is taken into account in the air gap magnetic flux distribution, accurately. The main novelty of this study is replacing the stator teeth by some surface currents at the border of the removed stator teeth. The uniqueness theorem is applied to find the surface currents. The two-dimensional (2D) field solution in the slotless machine is solved easily by separation of variables method. The multi-slice quasi-3D method is applied for taking 3D nature of field distribution into account. In addition, the back-EMF, armature... 

Electric motors represent the largest single load in most power systems. With continuing increase in nonlinear loads, the effects of harmonic contents of power systems on various components is becoming more important. This paper is concerned with the effect of power system harmonics and voltage unbalance on the useful life of electric motors. A simple equivalent circuit is used to calculate the additional heat losses caused by supply distortions. A lumped-parameter thermal network is proposed to calculate the resulting heat distribution within the machine. An Arrhenius chart is then used to estimate motor insulation remaining life. The proposed approach is used to determine the effect of... 

In this paper, harmonic sources in a BDFM are studied, and a method for specifying rotor and stator space harmonic components is established. A criterion for evaluating different rotor designs is introduced, based on space harmonics. Then the equivalent space and time harmonic circuits are obtained, using basic equivalent circuit. Finally, the effects of these harmonics on the torque-speed characteristic are studied  

Wireless infrared optical code-division multiple access (W-OCDMA) is a new developing technique with many useful applications. Noting the limitation on power consumption and eye-safety requirements, wireless optical systems are power limited. Therefore control and efficient use of optical power is a key issue in analysis and design of these systems. Also, multi-user interference is the major source of impairment in these systems and power control is required to control and reduce this interference. Power control and the inevitable errors in its algorithms play an important role in design and implementation of these systems. In this article the authors study the uplink performance of W-OCDMA... 

In this paper, a new structure for the rotor of the Brushless Doubly-Fed Machine (BDFM) is introduced to reduce the machine's torque ripple. The ripple has a considerable effect on the machine start up. At the first section of the paper, analytical expressions for the air gap permeance distribution, the air gap MMF distribution, cogging torque and back EMF are presented. Then, the effects of skewing the rotor loops are analyzed. Finally, the relation between non-skewed quantities and their values in skewed coordinate are introduced  

The application of the brushless doubly-fed machine (BDFM) is in wind power generation in harsh conditions, where reliability is important. In this paper, eddy current losses in a BDFM in four categories: a) the rotor classic eddy current losses, b) stator classic eddy current losses, c) rotor excessive eddy current losses, and d) stator excessive eddy current losses, are calculated for single frequency of the rotor and double frequency of the stator magnetic fields  

The cause of about 10% of faults in induction machines is related to the rotor part. In this paper, an analytical method for general modeling of brushless doubly-fed machines (BDFMs) with broken rotor loops and end ring is presented. The approach is based on calculation of resistance and inductance matrices through the winding function of the machine, so the assumption of sinusoidal MMF is not needed. It is also shown that a fault in a loop can lead to faults in other parts of the rotor. To validate the method a simulation in MATLAB/Simulink is performed based on the D180-frame prototype BDFM  

With the increasing proportion of nonlinear devices in most power systems, the need for a thorough consideration of the impact of harmonics on various components and sub-systems is becoming more important. This paper presents the results of an investigation into the relative impact of harmonics of different orders on the useful life of electric motors insulation system. Thermal modeling of motors is briefly addressed and the overall modeling procedure is presented. The paper is concluded by presenting the results obtained for a particular case as well as a discussion of practical implications of the problem  

With the increasing proportion of nonlinear devices in most power systems, the need for a thorough consideration of the impact of harmonics on various components and sub-systems is becoming more important. This paper presents the results of an investigation into the relative impact of harmonics of different orders on the useful life of electric motors insulation system. Thermal modeling of motors is briefly addressed and the overall modeling procedure is presented. The paper is concluded by presenting the results obtained for a particular case as well as a discussion of practical implications of the problem  

With the increasing proportion of nonlinear devices in most power systems, the need for a thorough consideration of the impact of harmonics on various components and sub-systems is becoming more important. This paper presents the results of an investigation into the relative impact of harmonics of different orders on the useful life of electric motors insulation system. Thermal modeling of motors is briefly addressed and the overall modeling procedure is presented. The paper is concluded by presenting the results obtained for a particular case as well as a discussion of practical implications of the problem  

With the increasing proportion of nonlinear devices in most power systems, the need for a thorough consideration of the impact of harmonics on various components and sub-systems is becoming more important. This paper presents the results of an investigation into the relative impact of harmonics of different orders on the useful life of electric motors insulation system. Thermal modeling of motors is briefly addressed and the overall modeling procedure is presented. The paper is concluded by presenting the results obtained for a particular case as well as a discussion of practical implications of the problem  

In this paper we study slotted ALOHA random access optical code division multiple access packet-switching networks with a chip-level receiver structure. We use generalized optical orthogonal codes (GOOCs) and the overlapping pulse position modulation (OPPM) signaling technique to improve system performance. The impact of physical layer parameters such as the GOOC cross-correlation value and the OPPM overlapping index on key performance benchmarks such as network throughput and delay are analyzed. We also study the stability of the network based on the number of backlogged users, using the expected state drift. It has been shown that using GOOC instead of strict optical orthogonal codes...