Browsing by Author "Farhad Hossain, Md."

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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 Hasib
Power 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.
Performance analysis of DS-CDMA wireless and mobile communication system in a Nakagami-m frequency selective multi-path fading channel
(Department of Electrical and Electronic Engineering (EEE), 2005-10) Farhad Hossain, Md.
In the recent years, code division multiplc access (CDMA) draws considerable intcrests of the wireless communication engineers to be used as a multiple access technique because of its some invaluable advantages over time division multiple access (TDMA) and frequency division multiple access (FDMA). Proposed air interface standards for the third generation (3G) mobile communication arc cdma2000-1 xRTT; cdma2000-3xRTT; cdma2000-1xEV, DV, DO; W-CDMA; TD-SCDMA and EDGE. For multiple access communication, direct sequence CDMA (DS-CDMA) is the specified technology for all the above mentioned air-interfaces except for EDGE whose standard is TDMA. The reverse link performance of DS-CDMA wireless and mobile (cellular and non-cellular) communication systems over Nakagami-m distributed frequency selective multi-path fading channel has been investigated in this thesis. Both the conventional correlator type receiver and the RAKE receiver have been considered for the analysis. Noise is taken as additive white Gaussian noise (AWGN) with two sided power spectral density No/2. To model multiple access interference (MAl), standard Gaussian approximation (SGA) has been considered in this work. Three different conditions with perfect, imperfect and absence of power control have been analyzed in this work. The signal-to-interferencenoise- ratio (SINR) and the bit error rate (BER) expressions for the system have been derived and simulated by varying the number of user, the number of fading path, the processing gain, the fading parameter m, the SNR per bit E. / No, the number of interfering cell, cell radius, the number of RAKE receiver finger and the variation in the amplitude of the received signal. The performance of the system is evaluated considering the acceptable BER equal to or less than 10.3. The analysis shows that in all the three cases of perfect power control (PPC), imperfect power control (IPC) and absence of power control (APC) for both cellular and 11011- cellular DS-CDMA systems, as the number of simultaneous active user and the number of fading path increases, the BER of the system increases indicating the performance degradation. For all the above cases, as the value of the processing gain, the r<:tio E,,/ No and the fading parameter m increases, the 13ER of the system improves. In case of IPC both the cellular and the non-cellular (single cell) systems, as the variation of the received signal amplitude with respect to the mean value increases, the SINR decreases and the BER increases. It also reveals that the performance of the cellular system degrades with the increase of the number of the interfering cell. In case of APC, as the cell radius increase, the performance of the system degrades. For the fading parameter m $\, the system performance with a eorrelator type receiver is very poor and impracticable for implementation as the BER remains always greater than 10,3 It is observed that the performance is much bettcr if a RAKE receiver is used. With the increase of the number of RAKE receiver finger, the system performance improves. From the analysis, it can be concluded that the best performance is achieved in case of PPC and the worst in case of APC. For APC, the BER less than 10,J is aehicvable only for very few numbcr of active user and a large value of m, Hence, to gct a satisfactory performance for DS-CDMA wireless mobile communication systems, some form of power control must be implemented.