Dissertations/Theses - Department of Electrical and Electronic Engineering
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Item Performance Analysis of Direct Sequence CDMA Power Line Communication Systems over the Frequency Selective Channel(Department of Electrical and Electronic Engineering (EEE), BUET, 2005-11-20) Hasan, Md. Mahmud; Rahman, Dr. Md. SaifurOutdoor power line communications (PLC) systems represent an additional and alternative for classical and communications. In this dissertation an analytical model of Direct Sequence CDMA power line communications systems has been developed for frequency-selective fading channel based on .the concepts of wireless communication. Binary phase shift keying (BPSK) modulation scheme is considered. The fading in power line is considered to follow Rayleigh distribution. During the analysis it is considered that power line channel can carry frequencies from I to 21.48 MHz (bandwidth occupation 20.48 MHz) and the chip rate would be 10.24 Mchips/s for DS-CDMA. The current research of power line channel modeling has been concentrated on home automation, broadband indoor communications and broadband data transfer in a low voltage distribution network between home and transformer station. A frequency selective multipath channel is modeled whose path gain is found to be Rayleigh distributed. A noise term has been introduced in the developed model, which consists of all type of noises in power line. To estimate impulse noises the statistical probability density functions have been used. On the basis of the analytical model, the performance of the DS-CDMA technique has been evaluated. The impact of power line noise and multipath propagation on signal to interference ratio (SIR) and bit error rate (BER) are then analyzed under various conditions. The performance parameters are evaluated as a function of number of simultaneous users.PLC with DS-CDMA is found to be a promising option for future telecommunication in a LAN and MAN.Item Assessment of Torsional Resonance in Turbine-Generator Shaft Due to Multiple EAF Operation(Department of Electrical and Electronic Engineering (EEE), BUET, 2013-07-13) Minarul Islam, Md.; Chowdhury, Dr. Abdul HasibThe oscillating torques between different mass sections of turbine-generator shaft are amplified at resonance, which indicates that the oscillating torques absorb more energy from the system. This may cause violent swing motion creating torsional stress and even catastrophic failure in the mechanical system. The torsional stressing may arise as a result of torsional resonance induced by pulsating components in the electrical torque of the generator. Hence, the interaction between the electrical system and the turbine-generator has to be analysed in advance to assess the severity of torsional stress to the shaft system to ensure undisturbed operation. The frequencies of torsional oscillations are in the subsynchronous range. There are many sources of subsynchronous frequencies in the electrical network such as capacitor compensated transmission line, EAF loads, load-commutated inverters, PWM voltage-source inverters etc. These equipments induce pulsating components in the air-gap torque of synchronous machine and through which the torsional oscillating torques are amplified during torsional resonance. The EAF load causes power quality problems such as flicker, voltage imbalances, harmonics and interharmonics. If the subsynchronous frequencies created by EAF leads to resonance, the respective matched torsional modes of the turbine-generator system are excited causing torsional oscillating torques amplification. The magnitude of torsional resonance depends on the proximity of stimulating frequency and natural frequency, on the magnitude and duration of the excitation and on the damping of the excited natural mode. Torsional oscillating torques amplification due to multiple EAF operations at the same point of common coupling (PCC) or at close proximity has not been studied yet. Investigation of torsional excitation due to multiple EAF operations is the main focus of this thesis. In this work, the three-phase EAFs are modeled using MATLAB/Simulink from mathematical model of electric arc, where the resistances of the EAFs varied deterministically in the frquency range of 1.2-45.5 Hz to meet the actual characteristics of EAF.Item Design of a one-way communication based control scheme for power management unit in scalable DC microgrid architecture(Department of Electrical and Electronic Engineering,( EEE), 2019-08-24) Hasan., Abu Shahir Md.Khalid; Khan, Dr. Md. Ziaur RahmanRenewable energy has gained much interest in recent years to keep the environment safe from ongoing climate change and become less dependent on fossil fuels. DC microgrids adapts nicely to distributed control ofrenewable energy sources to supply electricity in the remote regions. They ensure efficacious instantaneouspower sharing among differentdomestic Power Management Units (PMUs) along with maintaining stability of the grid voltage. Here design metricsand performance evaluation of a scalable DC microgrid aredocumented where asource converter initially complies with the efficient powersharing phenomenon among a set of two home PMUs. Thesource converter is connected with a Photovoltaic panel of 300W and uses Perturb and Observation (PO) method for executing Maximum Power Point Tracking (MPPT). A boost average DC-DC converter topology is used to enhance the voltage level ofthe source converter before transmission. The load converter consists of parallel PMUs. Each PMU isconstructed with high switching frequency based Full Bridge (FB)converter to charge an integrated Energy Storage System (ESS). The overall system is modeled and simulated onMATLAB/Simulink platform with ESSs in the form of Lead Acidbatteries connected to the load side of the FB converter circuitsand these batteries yield to support marginalized power utilities.The behavior of the system is tested in different solar insolationlevels along with several battery charging levels of 12 V and 36V to assess the power efficiency. In each testbed the efficiency isfound to be more than 93% which affirm the reliability of theframework and a look-up table is generated comprising the gridand load quantities for effective control of power transmission. Next the full bridge (FB) converter is used as the fanout node to lower the source-end voltage and to generate a dc bus of 48 V. Buck converter topology is used to design the PMUs and each PMU is controlled in a way to generate an output voltage suitable for a 12 V battery charging application. A one-way communication control based interface is designed to maintain the change in the solar irradiation level so that the more important units receive more power in a crisis situation. To make the system more sensible in case of long duration drop in solar irradiance level, a time based control scheme has been proposed which ensures that the turn off time is distributed evenly among the PMUs. The system works successfully for different irradiation pattern. The one-way communication based approach represent less circuit complexity and a cost effective control scheme for DC microgrid in the perspective of underprivileged people of Bangladesh.Item Comprehensive dynamic security assessment of Bangladesh power system(Department of Electrical and Electronic Engineering (EEE), BUET, 2019-03-24) Farhad Hossain, Md.; Chowdhury, Dr. Abdul HasibPower system security analysis is a key to reliable operation at maximum efficiency. Security analysis in this context refers to the ability of a power system to withstand pre-specified disturbances called contingencies. A power system must be able to survive dynamic events, and hence dynamic security assessment is more computationally intensive as it requires the electro-mechanical transient stability analysis of the system which concerns the transient behavior of the power system when moving from the pre- to post- contingency operating point. Dynamic security assessment is an evaluation of the ability of a certain power system to withstand a defined set of contingencies and to survive the transition to an acceptable steady-state condition. This is dependent on the transient stability evaluation which provides information in relation to the ability of a power system to retain stable operation during major disturbances resulting from either the loss of generation or transmission facilities, sudden or sustained load changes, or momentary faults. In the event of disturbances, the electro-mechanical oscillation of synchronous generator will be used to measure the transient stability. It is determined by observing the variation of the rotor angle as a function of time throughout the duration of the fault. The transient stability depends on the magnitude of the fault, duration of the fault and the speed of the protective devices. If the system is transiently stable, the oscillation of the rotor angle will damp down to a safe operating limit. Dynamic security assessment identifies those disturbances that cause instability and the results of the transient stability analysis are used to determine the system’s security level. This thesis demonstrates a methodical approach to dynamic security assessment. The method is based on a combination of voltage and angular criterion, N-1 and N-2 contingency analysis, transient voltage dip, transient stability analysis, and use of a performance index. A combined contingency ranking based on bus phase voltage and phase angle, and stability margin for all machines in a power system is used. The method is very effective for secured planning and operation of a power system, and for protection scheme design. The method is applied to the Bangladesh power system network (BPSN) for dynamic security assessment of the system. Critical buses and the paths connecting them are identified. These forms the backbone of BPSN and the system is most vulnerable at these locations as any serious contingency involving these nodes may lead the system to partial or complete blackout. Additional protective measures should be taken for the most critical buses of the system to prevent sudden system collapse or blackout.Item Design and analysis of photonic structures for spectra dependent energy harvesting of photovoltaic devices(Department of Electrical and Electronic Engineering, BUET, 2019-12-01) Saha, Ajanta; Baten, Dr. Md. ZunaidIn this thesis, we present a detailed study to design spectra selective photonic structures for harvesting energy efficiently from ambient light sources employing photovoltaic (PV) energy conversion technology. To choose the appropriate absorber material of the photonic structure, efficiency limit of single-junction PV devices for different absorber materials have been evaluated and optimum absorber bandgaps have been identified for natural and different artificial light sources. Due to widespread use of Red-Green-Blue (RGB) white LEDs, performance characteristics of ideal and practical PV devices have been studied in detail under white LED sources having a wide range of correlated color temperatures (CCTs) and fraction of blue in their corresponding spectrum and a relation has been established between maximum efficiency of the PV device and photometric features of the sources. Depending on bandgap of the absorber material, both positive and negative correlations are observed between photon conversion efficiency of PV devices and CCT values of the white LED sources. For material bandgaps of ~1.5eV or lower, higher photon conversion efficiencies are obtained for warm glow white LEDs. On the contrary, white LEDs characterized to emit cool light are found to be more conducive for PV devices having absorber layer bandgaps of ~2eV or higher. Upon selecting the absorber material we have designed the photonic structure having periodically arranged dielectric filled circular holes in the window layer of a practical PV device and evaluated optical absorption along with conversion efficiency of the device under solar irradiation and different white LEDs for a wide range of diameters and filling factors of the circular arrays. Based on the obtained results, a relation between optimum conversion efficiency of the PV device, filling factor of dielectric holes and photometric features of LED sources has been established. For absorber material bandgap of ~1.5eV, warm glow white LEDs outperform the cool light white LEDs and peak efficiency is obtained at relatively low filling factor (~40%) of dielectric holes. On the contrary, photonic structures with relatively high filling factor of holes (~70%) exhibit optimum conversion efficiency under white LEDs emitting cool light. Conversion efficiency of the photonic structures with filling factor in between 50% -70% is found to be optimum for operation under solar irradiation. These results in effect provide the necessary guidelines for designing homojunction, heterojunction or patterned PV devices suitable for operation under different light sources.Item Design of millimeter wave based sag measurement and monitoring system for overhead transmission lines in a smart grid(Department of Electrical and Electronic Engineering, BUET, 2019-11-03) Ayman Uddin Mahin; Hossain, Dr. Md. FarhadIn smart grid, power transmission monitoring and control is a crucial part. Electrical power transmission capacity of an overhead transmission line is dependent on the sag of that line. Power transmission through a transmission line can be increased with the decrease in sag. Thus, real-time sag monitoring is required for effectively using the transmission lines for transferring power. In this thesis, two simple, yet effective millimeter wave (mmWave) based techniques of measuring sag of overhead transmission line are proposed. These techniques can be used for measurement and monitoring of real-time sag of overhead transmission lines. The first technique only uses a single transmitter in each transmission line and a single transceiver. The transmitter sends mmWave signal to the transceiver. Simulation results demonstrate that based on the received power at the transceiver, sag of overhead transmission line can be measured effectively. Impact of different system parameters, namely, shadow fading, number of samples, horizontal distance between transmitter and transceiver, and horizontal displacement on the accuracy of the calculated sag for practical 132 kV, 230 kV and 400 kV transmission lines is comprehensively investigated. However, this technique suffers from large error, requires ground wire and strict alignment between the transmitter and the transceiver. To overcome the drawbacks of the first technique, another mmWave based technique is proposed that uses parabolic approximation. In the second technique, a transmitter and a transceiver along with an angle of arrival (AoA) sensor are used for each transmission line. Impact of system parameters on the accuracy of the calculated sag is also investigated for 132 kV, 230 kV and 400 kV lines. This technique shows significant improvement in accuracy of sag calculation compared to the first technique, though the accuracy degrades with error in AoA measurement. Network parameters for establishing communications among the devices in the proposed techniques are rigorously studied. Trade-offs between latency and sensitivity with bandwidth, and latency and percentage average error with number of samples are also investigated. Moreover, the proposed techniques are compared with the existing techniques of measuring sag of overhead transmission lines in terms of accuracy, hardware requirements and so forth.Item Energy efficient hybrid powered C-Ran architectures with dynamic user association(Department of Electrical and Electronic Engineering, BUET, 2019-07-30) Rubina Aktar, Mst.; Hossain, Dr. Md. FarhadDuring the last decade, there has been remarkable development in cellular networks market due to the ubiquitous availability of internet access in worldwide. The number of users and corresponding cellular traffic has escalated astronomically. To cope with the tremendous growth of data demand across the globe, cellular networks are deploying an increasing number of base stations (BSs) which leads to a voluminous inflation in energy consumption. Cloud radio access network (C-RAN) is new born mobile network architecture has the potential to reduce the power consumption compared to the traditional RAN network architecture. But, network densification in C-RAN places an extensive burden on the electric grid system. Concerns about global warming and increasing number of base stations (BSs) leading to rising energy consumption have prompted extensive research effort focusing on energy efficiency (EE) issue for cellular networks. The integration of renewable energy harvesting (REH) technology is expected to be pervasively utilized by telecom operators aiming to reduce carbon foot-prints and gird energy consumption. However, the dynamic nature of RE generation could lead to energy outage and service quality deterioration. Thus utilization of commercial grid supply in conjunction with RE generators is a more realistic option for sustainable network operations. In this thesis we propose hybrid powered C-RAN architectures and required energy usage algorithms to enhance EE. Each RRH is equipped with renewable energy generators, such as solar panel along with a set of batteries as an energy storage device and also connected to grid energy supply. Afterward, dynamic user association policies are integrated with the proposed model for further improving EE. The prime goal is to quantify the EE of various user association schemes, namely distance-based, SINR-based, green energy availability-based and traffic aware-based under the proposed network model. An extensive simulation-based study is carried out for evaluating the EE performance of the proposed framework varying different system parameters. Numerical results validate the proposed network models compared to other counterparts.Item Phonon scattered electron transport calculation of junctionless nanowire transistor by a computationally efficient atomistic approach(Department of Electrical and Electronic Engineering (EEE), BUET, 2019-08-31) Samzid Bin Hafiz, Md.; Khosru, Dr. Quazi Deen MohdIn this thesis, electronic transport characteristics of Junctionless Nanowire Transistor (JNT) in presence of phonon scattering has been studied. JNT is a novel electronic device, which has no distinct p-n junction and no doping concentration gradients in its structure. As the channel length of MOSFETs scales down, the formation of ultra-shallow source/drain junctions poses difficult fabrication challenge. The JNT overcomes this difficulty as it has no p-n junction. The device contains a doped Silicon channel region surrounded by two Poly gate structures on opposite directions, separated by an oxide layer on each side. As gate voltage is increased, the induced depletion region reduces, and as voltage exceeds threshold, the channel conductance commences. For voltage over flat-band, the device operation switches from depletion to accumulation mode. The current-voltage characteristics of the device closely resemble that of MOSFET. The device, free from doping gradient optimization constraints, can support further scaling down of the device structure than traditional FETs. The device also exhibits better non-ideal short channel and subthreshold characteristics, along with superior high temperature operations. Instead of conventional effective mass approach the quantum mechanical atomistic tight binding method has been used in this thesis. The effect of phonon scattering on electronic current has also been studied. In this thesis, the atomic scale calculations were performed in Python scripting via QuantumATK software. Electronic band structure, transmission spectrum and projected local density of states were observed. The phonon band structure, phonon density of states and phonon transmission spectrum were also observed. The elastic and inelastic component of current were analyzed. The inelastic current distribution due to various phonon modes was explained. Then the total current of the device with varying gate and drain voltage was calculated. The drain current vs gate voltage was evaluated by varying oxide thickness, channel thickness, channel length, doping concentration, and channel material crystal orientation. Finally, the simulation approach was verified by comparing with other results.Item Bit error rate performance evaluation of an ultra wide band communication system with dual carrier modulation and space diversity(Department of Electrical and Electronic Engineering (EEE), BUET, 2019-08-21) Abdur Rahman Ferdous, A K M; Majumder, Dr. Satya PrasadDual carrier modulation (DCM) is taken as a global standard for ultra wide band (UWB) wireless communication system in establishing wireless personal area network (WPAN) such as wireless universal serial bus (W-USB) and wireless high definition media interface (WHDMI), where a data rate of ≥320 Mbps is desired. In practice, DCM transforms 2 Quadrature Phase Shift Keying (QPSK) symbols into two 16 Quadrature Amplitude Modulation (16QAM) like DCM symbols using DCM matrix. Constellation rearrangement (CoRe) is an inherent part of DCM and thereby DCM achieves coding gain by constellation rearrangement (CoRe) and diversity gain by symbol repetition. A number of researches are carried out on the BER performance of a DCM MB-OFDM in an UWB wireless communication system but without space diversity. In these works, the authors claim that DCM provides a considerable amount of BER reduction in comparison to QPSK and improves the overall data handling capacity in short range wireless communication. In view of the success of space diversity in reducing BER in numerous wireless communication studies, it is envisaged that incorporation of space diversity with DCM may further reduce the BER at the receiver and thereby enhance the overall performance of a system. Hence, an analytical approach is adopted to evaluate the BER performance of DCM MB-OFDM UWB system with space diversity over a Rayleigh fading channel. Analytical expression for BER is deduced and evaluated through numerical computation where, the graphical results are compared with the previous works to validate its effectiveness. Analyzing the results, it is found that with the increment of number of receiving antenna, receiver sensitivity increases and the overall performance improves. However, using multiple transmitting antennas without incorporating any transmission diversity coding technique causes performance deterioration. Therefore, in a DCM system, the number of transmitting antenna should not be more than that of the receiving antenna in absence of transmission diversity coding technique. The outcome of the research is expected to open a new horizon in the arena of DCM to carry out further research in quest of enhancing the performance of a DCM UWB wireless communication system in future.Item Design and analysis of reconfigurable photonic crystals and whispering gallery mode resonators based on optical force(Department of Electrical and Electronic Engineering (EEE), BUET, 2019-08-06) Nusrat Jahan; Baten, Dr. Md. ZunaidPhotonic crystals (PCs) and whispering gallery mode (WGM) resonators are two of the most versatile photonic structures in the field of photonics and cavity quantum electrodynamics. A long standing challenge in the design of these structures has been their dynamic tunability, which is the capability of varying their resonant characteristics without going through the fabrication process. In this work the prospect of dynamic tunability of polystyrene (PS) microparticle based PCs and WGM resonators is studied by spatially reconfiguring these structures employing optical force. The behavior of optical force exerted by monochromatic illumination via dual counter-propagating Gaussian beams is analyzed using Maxwell Stress Tensor and Finite Difference Time Domain (FDTD) analysis techniques. Both one dimensional (1D) and two dimensional (2D) water submerged periodic PS microsphere arrays are found to exhibit spatial re-orientation in the presence of optical force. Detailed analysis shows that such optical force induced reconfiguration can be controlled by varying the illumination intensity, incident wavelength, and also the surrounding medium of the PS microparticles. For illumination with 980 nm wavelength, 1D array results in an inhomogeneous stable spacing of microspheres whereas periodically reconfigured arrays can be attained by illuminating in direct correlation with the resonant wavelength of the structure. Changing illumination intensity enahnces the magnitude of optical force acting on the particles, though it does not alter their stable locations. On the other hand, variation of refractive index of the background medium alters the stable interparticle separation of particles. Band structures and transmission characteristics of the PS array are observed to evolve in accordance with the spatial reconfiguration. A complete bandgap with nearly 100% reflection is attained for the 1D array whereas nearly 94% reflectivity is obtained for the 2D array. Besides photonic crystals, WGM resonators comprising of polystyrene (PS) microspheres are also analyzed and their Q factors are compared with those of single solid monospherical WGM resonators. The highest Q-factor of 91224 is obtained for a water submerged 20μm monosphere WGM micro resonator whereas for multiple microsphere based circularly symmetric structures no WGM is observed. By changing the surrounding medium, an exponential rise in Q factor is achieved which peaks for air with a WGM mode having Q factor of 408. For water suspended hexagonal configurations of the microspheres, the WGM mode disappears in the presence of optical force, thereby resulting transition from a highly reflective to a nearly transparent medium. Based on such optical force mediated reconfiguration, conveniently tunable photonic crystals and whispering gallery mode resonators can be designed and implemented for novel sensing, detection and lasing applications.