Dissertations/Theses - Department of Electrical and Electronic Engineering

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2-D modeling of subthreshold characteristics of symmetric and asymmetric double gate junctionless field effect transistors
(Department of Electrical and Electronic Engineering (EEE), 2015-08) Imtiaz Ahmed; Khosru, Dr. Quazi Deen Mohd.
The miniaturization of traditional planar Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) becomes quite challenging for very short channel devices as the formation of ultra-sharp source and drain junctions with a high process complexity is needed. Junctionless Field Effect Transistors (JLFETs) are considered promising for the sub-20 nm era due to their constant doping profile from source to drain. They provide a great scalability without the need for rigorously controlled doping and activation techniques as well as reduced short channel effects (SCEs) compared to the conventional MOSFETs. Though a number of analytical and simulation studies based on one-dimensional (1-D) model for Double Gate (DG) JLFETs have been carried out over the past few years but rigorous and accurate two-dimensional (2-D) analytical study on this device is yet to be reported. This work proposes a physically based analytical model of 2-D electrostatic potential in the channel applicable for both symmetric and asymmetric DG JLFETs considering similar and dissimilar gate biased configurations operating in the subthreshold region. The model is derived by solving 2-D Poisson’s equation along the channel while assuming a cubic potential distribution across the silicon thickness using appropriate boundary conditions and 1-D capacitance model. Different device parameters like channel doping concentration, thicknesses of silicon channel, top and bottom gate oxide, gate length, applied drain and gate biases, flat-band voltages of gates etc. are included in this model. To justify the accuracy of the model, Poisson’s equation with the same boundary conditions is solved in the COMSOL Multiphysics using Finite Element Method (FEM) and the obtained results are matched with those of the modeled ones. The obtained results from analytical model is also matched with SILVACO ATLAS simulation results to underpin the validity of this model. The proposed potential model is compared with the reported data available in the literature for short channel symmetric DG JLFET. In order to avoid extensive mathematical complexity numerical techniques by MATLAB is further used to calculate threshold voltage, subthreshold drain current and different SCEs viz. threshold voltage roll-off (TVRO), drain induced barrier lowering (DIBL), subthreshold swing (SS) from the proposed analytical model for different structures of DG JLFETs. A detail comparative study of potential distribution in the channel region for different gate and drain voltages obtained from the proposed analytical model is performed for symmetric and asymmetric DG JLFETs. The effect of bottom gate flat-band voltage of asymmetric structures and bottom gate voltage of dissimilar gate biased structures on the threshold voltage are investigated. Finally a performance comparison of subthreshold behavior i.e. TVRO, DIBL, drain current and SS have been made between symmetric and asymmetric DG JLFETs. Both symmetric and asymmetric DG JLFETs show nearly ideal TVRO (~0V), DIBL (~0mV/V) and SS (~60mV/decade) at the gate length around 50nm which make them good competitors of the conventional MOSFETs for short channel devices.
3-D correction of ring and radiant artifacts in flat panel detector based cone beam volume CT imaging
(Department of Electrical and Electronic Engineering (EEE), 2011-03) Emran Mohammad Abu Anas; Kamrul Hasan, Dr. Md.
The use of °at-panel detectors (FPDs) is becoming increasingly popular in cone beam volume and multi-slice CT imaging. But due to the presence of the defective and insu±cient calibrated detector elements in the FPD, the diagnostic quality of the FPD-based CT images in both CT systems is degraded by ring and radiant artifacts. In this thesis, a detail analysis including the classi¯cation, detection and correction of these ring artifacts is presented. Unlike conventional approaches, we emphasize here on the separate detection and correction schemes for each type of rings for their e®ective removal. Two di®erent ring and radiant artifact correction algorithms are presented here: one is applicable for the fan or parallel beam geometry based multi-slice CT and the other for the cone beam geometry based CT. For the correction of the ring artifacts in multi-slice CT, an artifact strength based sinogram-processing method is presented and a mathematical index is used to isolate the defective and mis-calibrated pixels from the good ones. For the correction of artifacts resulting from the defective detector elements, 2D variable window moving average and weighted moving average ¯lters are proposed. On the other hand, the normalization correction scheme is adopted to correct the responses of the mis- calibrated detector elements. Next, a novel approach is presented to eliminate the ring and radiant artifacts of a 3-D cone beam volume CT image using the proposed sinogram based ring artifact removal method. Finally, an e®ective and fast ring artifact correction technique speci¯cally for 3-D cone beam volume CT image is proposed. Some template images are derived from the responses of the detector pixels using their statistical properties and then, an e®ective non-causal derivative based detection algorithm in 2-D space is presented for the detection of defective and mis- calibrated detector elements separately. An image inpainting based 3-D correction scheme is proposed for the estimation of responses of defective pixels and the responses of the mis-calibrated pixels are corrected using the normalization technique. A simpli¯cation of the proposed method is also suggested for its real time implementation. The overall experimental results demonstrate the superiority of the proposed two correction methods (applicable for the multi-slice and cone beam CTs) over the other methods reported in the literature.
3-D model of wrap around CNTEFT with multiple CNT channel and analytical modeling of its capacitances
(Department of Electrical and Electronic Engineering (EEE), 2013-05) Rakibul Karim Akanda, Md.; Khosru, Dr. Quazi Deen Mohd.
Wrap around CNTFET has gate around it except the bottom portion. Wrap around gate carbon nanotube field effect transistor (CNTFET) has the advantage of more accurately controlled channel than top gated CNTFET. 3-D simulation is performed using Finite Element Method (FEM) to model wrap around CNTFET with multi-CNT channel. For predicting the device speed, capacitance modeling is necessary. Along with gate to channel capacitance, there are fringing capacitance as well. Analytical expressions to model various capacitances of the same device are also derived from capacitances for 1-D FET with multiple cylindrical conducting channels incorporating screening effect due to multiple CNT. Various capacitances thus obtained exhibit remarkable improvement over planar top gate device with multiple conducting channels. This is attributed to the charge enhancement and better charge confinement in the channel of the wrap around CNTFET with multi-CNT channel. Capacitances are also extracted from the proposed FEM model. The extracted capacitances are in excellent agreement with the capacitances obtained from the analytical model presented in this study. Temperature distribution of wrap around CNTFET with multi-cnt is presented in this paper. For predicting the device characteristics, temperature modeling is necessary. Temperature distribution is changed due to multi-cnt as voltage of adjacent cnt affects temperature of mid cnt. Again it is also changed by temperature dependent thermal conductivity of single walled carbon nano tube (SWCNT). Current flow is changed both due to multi-cnt wrap around structure and due to the temperature change. So, temperature has a great impact on current modeling and thus on device characteristics. All these issues are considered in this paper using finite element method.
8085-Based universal EPROM programmer
(Department of Electrical and Electronic Engineering, 1985-01) Khandaker Atwar Hossain; Mahbubur Rahman, Dr. Syed
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AB initio study of the thermoelectric property tunability of strained and doped single-wall carbon nanotubes
(Department of Electrical and Electronic Engineering (EEE), BUET, 2024-08-28) Mafizul Islam, Md.; Zubair, Dr. Ahmed
Carbon nanotubes (CNTs) have garnered significant importance as thermoelectric (TE) ma- terial due to their high TE performance owing to their inherent fascinating property of one-dimensional quantum confinement of charge carriers. Among various methods to tune and enhance TE properties, the impact of strain on the properties of CNT-based TE de- vices is rarely explored, though it is immanently present, specifically on multi-functional CNT fibers. We presented the tunability with enhanced electronic and TE properties of single-wall CNTs (SWCNTs) by strain engineering and doping technique within the frame- work of density functional theory and Boltzmann transport theory. Semiconductor-metallic- semiconductor transition with a zigzag pattern of band gap (Eg) and Seebeck coefficient (S) variation was observed for uniaxial strain due to shifting of the Dirac points. The SWC- NTs exhibited a significant enhancement of their thermoelectric transport properties while underwent tensile and compressive strains. Applying uniaxial stress enhanced the Seebeck coefficient of SWCNTs from 1172 to 1580 µV/K for (11,0) SWCNT. We achieved the en- hancement of thermoelectric power factor (PF) 2.2 mW m−1 K−2 (-9%), 1.3 mW m−1 K−2 (-3%), 1.15 mW m−1 K−2 (-3%), and 1.24 mW m−1 K−2 (-9%) for (6,6), (9,0), (10,0), and (11,0) SWCNTs, respectively. The overall performance parameter thermoelectric figure of merit, zT , was tuned up to 1.56 when the strain varied from -9% to +9%, a few-fold incre- ment compared to its relaxed state. The optimum range of doping for SWCNTs was found to be within 5 × 1019cm−3 – 1 × 1021cm−3 to attain the best thermoelectric PF and zT . The unique advantages associated with CNT’s one-dimensionality can be extracted by tun- ing the Fermi level near singularity points of the density of states at the band edges by appropriately doping the CNT bundle. Though halogen compounds act as excellent dopants among the various doping agents used in the CNT system, the impact of their orientation and placement in the CNT system is rarely explored. In this thesis, we explored the impact of halogen doping on the electronic and thermoelectric properties of of SWCNT bundle. We investigated the impact of halogen (ICl, IBr, and Br2) molecules filling the tube, inter- calation between adjacent SWCNTs, and their orientation on the thermoelectric properties of semiconducting SWCNT network through density functional theory-based ab initio cal- culations and Boltzmann transport theory. Our calculated halogen-doped SWCNT system showed improved thermoelectric performance. Notably, the system’s PF can be increased from 0.28 to 2.4 mW m−1 K−2, more than an eight-fold increment of its value obtained at the Fermi level. On the other hand, ICl intercalation of the SWCNT bundle shifted the peak thermoelectric PF below its Fermi level, and it can be attained by doping the CNT bundle with additional acceptor atoms. We achieved the best enhancement of thermoelectric PF from zero for pristine CNT to 0.3 mW m−1 K−2 when the ICl molecules were horizontally aligned outside the CNT in close spacing with adjacent dopants. It can further be pulled up to 1.82 mW m−1 K−2 by doping the CNT with an acceptor concentration of 7.1×1021 cm−3. The IBr and Br2 doped SWCNT bundle increased the PF to 0.286 mW m−1 K−2 and 0.122 mW m−1 K−2, respectively. We discussed the underlying physics behind the alteration of thermoelectric properties of uniaxially strained and halogen-doped SWCNTs. The insights gained from this investigation would help designing and fabricating substrate-supported and flexible thermoelectric nanogenerators and coolers for applications to power up wearable electronics and IoT-based remote devices where thermal energy is abundant.
AC transmission system stabilisation by DC link
(Department of Electrical and Electronic Engineering, BUET, 1980-05) Quamrul Ahsan, Md.; Rahman, Dr. Syed Fazle
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AC voltage regulation by C^uk switch mode power supply
(Department of Electrical and Electronic Engineering, 2003-07) Alomgir Hossain
High frequency switching power supplies have become part of electronic equipments to provide regulated DC of desired voltages at a low cost and high efficiency. These power supplies have several advantages over their counterpart the linear power supplies. Main advantages are smaller compact size due to elimination of step down transformer and small filters due to high frequency operation. These power supplies have high efficiency because the regulating devices in them works as switches ensuring low device loss. Their output voltage can be controlled for a wide range of input voltage fluctuation by changing the duty cycle of the switching signals. Four common types of switch mode converters are used in DC to DC conversion. They are BUCK, BOOST, BUCK-BOOST and CI\UK converters. Researchers are trying to modifY these DC regulator to regulate AC voltages. Utility AC voltage fluctuation both momentary and prolonged, affect adversely the domestic, industrial and commercial customers. In this thesis an AC Cl\uk regulator made of. ideal switches and practical IGBT switches have investigated. For generation of switching voltage a commercially available IC chip SG l524B is used, thus circuit is compact and more viable. The same chip is utilized to regulate the output voltage by varying the gate signal voltage of the IGBTs if any changes occur in output voltage. As a result the regulator is maintaining constant voltage across the load during any change in supply voltage as well as for variation of load. Freewheeling path and surge voltage across switches create problem in the proposed regulator. Snubbers are used for suppressing surge voltage across switches. Input current of this regulator is greatly reduced to a acceptable limit but still efficiency of this regulator is very poor and needs further research to increase the efficiency to make the regulator commercially viable. In this thesis input filter capacitor of proper size is used for improving the power factor and which decreases the input current.
Accelerated convergence in back propagation learning algorithm
(Department of Electrical and Electronic Engineering, 1996-01) Saha, Arun Kumar; Kamruzzaman, Dr. Joarder
Recent developments in the field of artificial neural networks have provided potential applications in various fields. Artificial neural nets are inspired from the studies of biological nervous system and composed of many simple nonlinear computational elements called neurons which are connected by links of variable weights. These weights are adjusted by a learning process and finally settle down to a set of weights that realizes the task at hand. The most popular learning algorithm for feed-forward connectionist networks is Back-propagation algorithm' This a.lgorithmdefines sumof squared errors measured at the output layer as error function and updates weights to minimizethis error by steepest descent method. . Majordrawb~cks of this algorithm are its slow convergence and possibility of getting stuck1in local minima.These drawbacks hinder its widespread applications , in real-world .problems. Several methods have been proposed to improve its convergence. But these methods require someadditional parameters to be adjusted and fast increase of somelearning rate parameter might cause unstable behavior in the learning process and needs mo're complicated training procedure. Having considered all these problems, a newerror function expressed as exponential of the. sum of squared errors measured at the output layer is defined in the proposed research. Weight update using this modification varies the learning rate parameter dynamically during training as opposed to constant learning rate parameter used in . standard Back-propagation. This adaptation of learning rate during learning is found to significantly improve the convergence speed of Back-propagation algorithm.
Accurate drain current modeling of long channel junctionless double-gate field effect transistor
(Department of Electrical and Electronic Engineering, 2015-01) Shuvro chowdhury; Ziaur Rahman Khan, Dr. Md.
A new model based on linear depletion inside the channel region has been proposed for long channel Junctionless Double Gate (JL DG) MOSFETs. The proposed model shows improved performance over the previous models based on abrupt depletion. This new model provides a different way to determine surface potential of JL DG MOSFETs. The model based on gradual depletion provides different sets of equations for surface potential which are valid for different regions of operations. Thus there is a trade-off between accuracy and complexity in using the proposed model. Having the knowledge of surface potential, the proposed model obtains expressions for current and threshold voltage. The current expressions have been evaluated using numerical techniques. Surface potential characteristics and current voltage characteristics are found to have smooth transition between fully depleted region and partially depleted region. This smoothness may prove useful for SPICE like programs. Capacitance-voltage characteristics, output conductance and transconductance characteristics have also been observed, analyzed and compared against the results obtained from TCAD simulations. Upon comparison, it has been found that the results obtained from proposed model matches quite perfectly with the results provided by TCAD tool. Finally the effects of different device and process parameters on threshold voltage of the JL DG device and the sensitivities of threshold voltage on these parameters have been explored and analyzed. It has been observed that threshold voltage of JL DG MOSFET is very sensitive to parameter variations which would require proper control of geometry and impurity concentration.
Acoustic echo and noise cancellation schemes using time and frequency domain adaptive techniques
(Department of Electrical and Electronic Engineering (EEE), 2011-11) Upal Mahbub; Anowarul Fattah, Dr. Shaikh
Acoustic echo occurs in real life environment when speech signal coming out from a loudspeaker is delayed, attenuated and reflected back to the source microphone. Most communication systems are prone to acoustic echo which can severely degrade the quality and intelligibility of the signals transferred through the communication channels. In conventional acoustic echo cancellation (AEC) methods gradient based adaptive filter algorithms, such as least mean squares (LMS) and normalized LMS are employed where an error function is minimized to obtain the optimal filter coefficients corresponding to the acoustic echo path. The main problem of these methods is the necessity of the dual channels, one for the reference signal and the other for the echo corrupted signal. However in many practical applications only one channel is available, such as a conference hall environment with single microphone and a loudspeaker. Due to the unavailability of separate reference signal in single channel scenario, the task of echo cancellation becomes extremely difficult and is attempted by a few researchers. In this thesis, first a single channel echo cancellation scheme is developed based on the gradient based LMS adaptive filter algorithm, where, unlike conventional dual channel schemes, a delayed version of an estimated echo cancelled signal is utilized as a reference signal. In the proposed formulation, the effect of flat delay, i.e. the time required to produce an echo, is incorporated with a view to reduce the number of unknown parameters of the acoustic echo path, which offers a faster convergence. Moreover, based on energy and cross-correlation coefficients of the reference and current frames, a multi-step stopping criteria is developed, which can efficiently control the updating procedure of the proposed LMS adaptive filter. Extensive experimentation is carried out on real life speech signals corrupted by echoes using the proposed single channel LMS algorithm with and without the multi-step update constraints. It is found that the performance of former one, the controlled LMS algorithm, is far better than that of the later one in terms of (a) the average echo return loss enhancement (ERLE) in dB and (b) the difference between input- and output-signal to distortion ratio (SDR) in dB. In real life applications, inclusion of noise with the speech signals is obvious in most of the cases, which makes the task of single channel echo cancellation even more difficult. In view of handling the challenging task of cancelling the echo in the presence of noise, a two step algorithm is developed where a spectral subtraction based noise reduction scheme is introduced after the single channel echo cancellation. It is shown that even under severe noisy conditions in different acoustic environments the proposed two-step single channel acoustic echo and noise cancellation (AENC) method can significantly reduce the effect of both echo and noise. As an alternate to the gradient based approaches, the problem of dual channel echo and/or noise cancellation can also be realized based on some optimization algorithm driven adaptive filters. However, undoubtedly the problem would be very difficult for the single channel scenario which is the case under consideration. Thus, the single channel AEC problem is formulated as an optimization problem introducing the particle swarm optimization (PSO) algorithm, which offers a quick convergence to the desired solution. For proper operation of the PSO algorithm, a frame by frame processing is required for which the overlap-add method is adopted. In order to estimate the unknown coefficients of the acoustic echo path, the PSO based algorithm is formulated both in the time and frequency domain separately and it is found that the frequency domain approach performs better in comparison to the time domain approach. The performance of the proposed PSO algorithms are also investigated for different controlling parameters, namely number of particles, maximum particle velocity etc. The PSO based algorithm is also extended for the complicated case of adaptive echo and noise cancellation. From detailed simulations it is found that the performance of the proposed PSO based AENC algorithm outperforms that of the proposed gradient based algorithm under different noisy conditions at various acoustic environments.
Active disturbance rejection control for load frequency control of Bangladesh power system
(Department of Electrical and Electronic Engineering (EEE), 2012-11) Sayem, A. H. M.; Abdul Hasib Chowdhury, Dr.
Energy shortage is considered as the most critical infrastructure constraint to the economic growth of Bangladesh. One of the main causes of the supply shortage, among others, is the poor operational efficiency of power plants. For the operation of a large power system maintaining the proper power quality has always been a difficult task. Increasing number of major power grid blackouts that have been experienced around the world including Bangladesh (2007, November and December) in recent years show that today’s power system operation requires more careful consideration of all forms of system instability and control problems and introduction of more effective and robust control strategies. In this thesis an Active disturbance rejection control (ADRC) based decentralized Load Frequency Control (LFC) system for Bangladesh Power System (BPS) is developed and a proposal has been made for feedback connections from various load rich areas to generation rich areas to minimize the area control error. The frequency control system maintains frequency and tieline power flows to specified values in the presence of physical constraints and model uncertainties. As power load demand varies randomly in an interconnected power system, both area frequency and tie-line power interchange also vary. The objectives of LFC are to minimize the transient deviations in these variables (area frequency and tie-line power interchange) and to ensure their steady state errors to be zeros. Unexpected external disturbances, parameter uncertainties and model uncertainties are big challenges for LFC design. As an increasingly popular practical control technique, ADRC has the advantages of requiring little information from the plant model and being robust against disturbances and uncertainties. A solution to the LFC problem based on ADRC is an alternative one. Performance analysis of ADRC is done by comparing with Proportional Integral Derivative (PID) controller and for two area non-reheat and reheat turbine unit. ADRC is also implemented on a large power station. Finally, the design of BPS is done considering its radial connection and proposal is made about feedback connection based on the performance of ADRC. The dynamic model of the power system and the controller design based on the model are elaborated. Simulation results show that ADRC controller is attractive for the LFC problem in terms of its stability and robustness.
Adaptive battery energy storage system controller for enhancing frequency stability of a low inertia grid
(Department of Electrical and Electronic Engineering (EEE), BUET, 2023-07-25) Mahadi Hasan, Md.; Chowdhury, Dr. Abdul Hasib
Due to the rapid increasing demand of grid connected solar photovoltaic, the overall system inertia decreases and hence causes system instability. Virtual inertia emulation is one of the solution to this low inertia problem that can be done through inverter control and energy storage devices. This thesis presents a simple improved adaptive Fuzzy-ANFIS hybrid algorithm-based BESS controller that can emulate virtual inertia to improve frequency stability of a low inertia grid. The proposed controller has been tested on a low inertia grid for different case studies and a comparative analysis has been done in the last chapter of this thesis. The presented controller eliminates the limitations of conventionally used PI controller and hence an improvement of frequency nadir by 0.43% for sudden load variation and 0.61% for transient short circuit fault have been observed. Moreover, improvement in ROCOF and settling time have been noticed for the proposed controller also.
Adaptive clarke transformation based three-phase PLL under amplitude and phase unbalances in presence of harmonics
(Department of Electrical and Electronic Engineering (EEE), BUET, 2021-01-05) Zahidul Islam, Md.; Reza, Dr. Md. Shamim
Precise and fast estimation of the phase information of grid voltages is crucial for grid synchronization of various power electronic devices, which is a research trend in modern smart grid technology. However, the task of instantaneous phase estimation has become difficult as the grid voltages may contain harmonics, DC offset, frequency variations, and voltage unbalances due to an increase in renewable energy penetration to grid, and domestic and industrial non-linear loads. In this dissertation, a fast and accurate instantaneous phase estimation technique is developed to overcome the present limitations in this field. This dissertation proposes a three-phase phase locked loop (PLL) algorithm relying on adaptive Clarke transformation (ACT) for tracking the phase angle of unbalanced grid voltages associated with harmonics and DC offset. A meticulously tuned band pass filter (BPF) is inserted in each phase to remove harmonics and DC offset. Two separate algorithms are proposed to estimate the amplitudes and the phase-angle deviations of three-phase voltages. Using the estimated amplitudes and phase angle deviations, a set of analytical expressions is derived for the coefficients of Clarke transformation (CT) matrix to make them adaptive under both amplitude and phase unbalances, which is named as adaptive CT (ACT). The ACT is capable of generating orthogonal signals from unbalanced three-phase voltages, whereas the conventional CT based on constant matrix fails to do so. After getting the orthogonal voltages, a conventional SRF-PLL is used to track the phase-angles of all three-phases. A phase-correction technique is also developed to make the PLL frequency adaptive without using any frequency feedback loop.
Adaptive resource allocation for multiuser OFDM system based on modified genetic algorithm and particle swarm optimization
(Department of Electrical and Electronic Engineering, 2008-09) Imtiaz Ahmed; Majumder, Dr. Satya Prasad
Like other wireless systems, Orthogonal Frequency Division Multiplexing (OFDM) requires the proper allocation of the limited resources, like total transmit power and available frequency bandwidth, among the users to meet the users' service requirements. As a matter of fact, adaptive resource allocation is one of the most challenging tas~s for multiuser OFDM systems. In this dissertation, two evolutionary approaches, Genetic Algorithm (GA) and Particle Swarm Optimization (PSO) have been applied for adaptive subcarrier and bit allocations to minimize the overall transmit power (margin adaptation) and to maximize the . throughput (rate adaptation) of a multiuser OFDM system. Each user will be assigned a number of subcarriers. This allocation of subcarriers may be done through unconstrained or I fairly scheduled approaches. The number of bits are then calculated according to channel state information and subcarrier arrangements. The transmit power level as well as bit rate for an OFDM symbol are evaluated through these subcarrier and bit information. Simulation results reveal that both the evolutionary approaches outperform the conventional static resource allocation schemes considerably both in unconstrained and constrained cases. The results further assert that both. the algorithms can handle large allocation I of subcarriers without significant performance degradation. However the performance of PSO has been found to be better than the GA in terms of execution time, simplicity and convergence. '. The original versions of GA and PSO have been modified in different manners to provide further improvements. All these modified versions perform relatively better than the original versions. Furthermore the modification of PSO has been done by three different manners where all of them perform relatively better than the original PSO as ~ell as the original and modified versions of GA. Finally all these modified versions have been compared with the existing algorithms. The comparison reveals the fact that the modified versions of PSO perform relatively much better results than the previously best algorithm for higher number of users.
Alternative routing strategy to enhance call handling capacity of a multiexchange telecommunication network system
(Department of Electrical and Electronic Engineering, 1986-09) Abdur Rahman, Md.; Khan, Dr. A.K.M. Mahfuzur Rahman
Multiexchange networks with direct routing (simple), alternate routing (hierarchical) and alternate routing (symmetrical) networks are studied in this work. Network systems are studied by digital simulation. Roulettemodel simulation method is used in this case. The computer program is written in Fortran IV and run in IBM 4331. The validity of the simulation model cheeRed with the similiar results obtained from analytical or exact solution method ( which is derived on the basis of classical probability theory) for small systema. The equations derived for different simple systems in exact solution method are solved with Gaussian-elimination technique. Three, four and.five exchange networks are studied in the present work. Maximum number of links considered here are ninety (90) and the traffic considered from 0.5 to 0.85 erlangs. 'Congestion probability' and 'Traffic handling capacity' are considered as performance indices of the network systems. Results obtained from exact solution and simulation methods are presented graphically. The performance of symmetrical network is the best among the three systems studied followed by direct routing (simple) and alternate routing networks (hierarchical).
An adaptive load shedding methodology for improving voltage and frequency stability of renewable integrated power systems
(Department of Electrical and Electronic Engineering (EEE), BUET, 2024-04-20) Abrar, Sk. Fahim; Alam, Dr. Mohammad Jahangir
Blackouts with disastrous effects have occurred all over the world including Bangladesh in last few years. The inefficient design of current load shedding techniques is one of the primary causes of the widespread effects of these blackouts. But research into recent blackout occurrences shows that voltage collapse and associated problems are equally crucial to preserving the integrity of power networks. Therefore, when taking preventive measures against blackouts, it is imperative to take both frequency and voltage into account. This necessitates a review of the emergency mechanisms in place to prevent blackouts. Achieving a sustainable and ecologically friendly energy mix depends on integrating renewable energy sources into the traditional power grid. Solar, wind, and hydroelectric power are examples of renewable energy sources that can produce clean, plentiful energy without consuming natural resources or adding to greenhouse gas emissions. By reducing reliance on fossil fuels and diversifying energy sources, the incorporation of renewable energy resources can improve energy security. Given that renewable energy is frequently locally accessible and may be used, this lessens vulnerability to price changes and supply disruptions. Traditional load shedding approaches do not incorporate any asynchronous renewable source such as solar photovoltaic (PV). Notably, utility scale PV plants may include Battery Energy Storage System (BESS) to provide supplementary frequency support. In this research an adaptive load shedding methodology is introduced which helps to improve overall blackout protection for renewable integrated power systems. The methodology takes care of frequency and voltage stability in response of combinational disturbances of electric power system. The methodology involves measuring the power mismatch and relative disturbance magnitude with taking advantage from load damping factor. At the same time, it decides the amount of load to be shed from each bus using the voltage and frequency sensitivities combined with the additional inertia support from BESS. The proposed load shedding methodology is scripted in python language and implemented on New England test system (IEEE 39 Bus) to execute in ‘Digsilent PowerFactory’ environment for all illustrative case studies. The scheme is tested on IEEE 39 bus with python scripted simulation. There are four scenarios considering 250 MW, 500 MW and 1000 MW injection of PV based power generation sources with conventional generation loss of 800 MW and 1000 MW. The threshold frequency is considered 49.10 Hz. The total amount of Battery Energy Storage System is 300 MW. For each case, it can be observed that the scheme successfully maintains the system frequency above 49.10 Hz with a minimal amount of load shedding. Hence, it can be stated that the proposed methodology successfully maintains frequency stability for a modern power system with large-scale PV generation through adaptive feeder selection for load shedding.
Analysis and compensation of PMD effects in high speed fiber-optic transmission systems using different modulation and detection scehemes
(Department of Electrical and Electronic Engineering, BUET, 2008-05-27) Saiful Islam, Md.; Majumder, Dr. Satya Prasad
Polarization Mode Dispersion (PMD) is a serious barrier that limits high-speed optical fiber telecommunication systenls. PMD is the result of random birefringence in single-mode fibers along the transmission path. This leads to wavelength-dependent polarization states and phases at the output that eventually results in inter-symbol interference and system performance degradation. In this thesis work, analytical models have been developed to assess and compensate for the effect of PMD considering different modulation and detection schemes; such as, intensity modulation-direct detection (lM-DD), continuous phase frequency shift keying (CPFSK) directand heterodyne detection. Based on the analytical model, the average BER performance of an IMDD optical transmission system is evaluated considering Maxwellian distribution for the differential group delay (DOD). The probability density functions (pdfs) of the random output signal phase fluctuation due to PMD in a CPFSK direct- and heterodyne detection system are analytically developed. The pdf of the signal phase fluctuation is used to evaluate the average BER as a function of mean DOD. As the passive PMD mitigation techniques do not require any dynamically adjusted components, and are bit rate independent, an analytical model of PMD compensation technique is also developed with several Iinecoding to make the single channel optical transmission system more tolerant to PMD. The impact and compensation of PMD in multi-channel optical transmission system is also analytically investigated considering the interaction of cross-phase modulation (XPM) and PMD. XPM changes the state-of-polarization (SOP) of the channels through nonlinear polarization rotation and induces nonlinear time dependent phase shift' for polarization components that leads to amplitude modulation of the propagating waves in a wavelength division multiplexing (WDM) system. The angle between the SOP changes randomly and as a result PMD causes XPM modulation amplitude fluctuation random in the perturbed channel. The pdf of the random angle fluctuation between the SOP of pump and probe due to PMD is determined analytically and the impact of PMD on XPM in a two channel pump-probe configuration is evaluated. To mitigate the PMD in a multi-channel transmission system, a high-birefringent adjustable linear chirped Bragg grating (LCFBO) based PMD compensation scheme is developed and its performance is evaluated using the optimum system parameters. The results of this research work will find applications iri the design of WDM transmission systems in presence of fiber nonlinear effects and PMD.
Analysis and design of a balanced output static converter
(Department of Electrical and Electronic Engineering, BUET, 1989-06) Shafiqul Islam, Md.; Khan, Dr. Shahidul Islam
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Analysis and design of an inverter
(Department of Electrical and Electronic Engineering, 1993-12) Paul, Dilip Kumar; Shahidul Islam Khan, Dr.
A singie phase inverter is designed and constructed to provide regulated output voltage at 220V. The output power stage is designed using two power iransistors coupled to the load ihrough a transformer. The output is a 50 Hz square wave which is obtained by push-pull operation of the two transistors using 50 Hz complementary trigger pulse stream. Feedback control is used for constant output voltage with load variation. A theoretical analysis of the inverter is presented. The circuit is tested and it is found that the waveforms obtained at different stages of the circuit conform well with those obtained by computer simulation. The circuit is found to be efficient and economical for applications in appliances during power failures.
Analysis and implementation of a look up table based hybrid PWM scheme for voltage source inverters
(Department of Electrical and Electronic Engineering, BUET, 2002-05-07) Quamruzzaman, Muhammad; Mujibur Rahman, Dr. Kazi
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