IRANIAN JOURNAL OF SCIENCE AND TECHNOLOGY TRANSACTION B- ENGINEERING
Year:2004 | Volume:28 | Issue:B3|Papers Count:12

This paper presents a variable structure based approach to the load frequency control system problem in electric power generation systems. This approach combines the salient features of both variable structure and fuzzy systems to achieve high-performance and robustness. The control signal consists of equivalent control, switching control and fuzzy control components. The influence of system nonlinearities such as generation rate constraint and governor dead band is considered. In addition, a decentralized form of this controller is applied to a two-area power system. Simulation results show that the system responses are strongly robust for parameter variations.

Railway power system planners and operators need a tool for testing their designs or new timetables. This paper presents a tool for the DC traction systems, an algorithm called decoupled AC/DC load flow. The main feature of this algorithm is to separate the solution of AC and DC equations. Therefore, a minimum number of iterations is needed to avoid any convergence problems. Electrical variables of AC and DC systems as a function of time and location can be determined. In this paper, for a better presentation of the real operating conditions, i.e. the uncertainties in the timetable, the trains departure time has been considered as a random variable. The Monte Carlo simulation method has then been implemented to the AC/DC load flow algorithm in order to determine the worst operating conditions of the system. Simulation results indicate the effectiveness of the suggested AC/DC load flow algorithm, and the importance of the mentioned stochastic simulation method.

Transformers are normally designed and built for use at rated frequency and prefect sinusoidal load current. Non-linear loads on a transformer lead to higher losses, early fatigue of insulation, premature failure and reduction of the useful life of the transformer. To prevent these problems, the rated capacity of a transformer, which supply non-linear loads must be reduced.In this paper a 50-kVA three-phase distribution transformer is modeled using the finite element (FE) method and its losses are estimated under rated frequency and load conditions, as well as under non-linear loads. An equivalent rating of the transformer is estimated based on the harmonic loss factor and is compared to the recommended standard rating. This comparison shows that the estimation of derating of the transformer supplying non-linear loads using the standard recommendations is acceptable, but slightly conservative.

The purpose of this paper is to introduce a new type of a load commutated voltage-forced synchronous machine drive which is capable of controlling the stator current waveforms, an approximate to sine wave. Then, Schawarz-Chirstoffel transformation is utilized to obtain the harmonic series of the air gap magnetic conductances of the mentioned machine due to the stator slotting with smooth rotor and vice versa. These harmonic series are used to calculate the rotor to stator mutual inductances for both types of the skewed and unskewed rotor machines. The drive system equations are derived and used to develop a C++ computer program to model the machine drive system on PC. Finally, the system simulation results obtained are compared to some available practical results.

This study addresses the design and properties of a sliding-mode neural-network control (SMNNC) system for a nonlinear flexible arm that is driven by a permanent magnet (PM) synchronous servo motor. First, the dynamic model of a flexible arm system with a tip mass is introduced. When the tip mass of the flexible arm is a rigid body, not only bending vibration but also torsional vibration occurr. In this study, the vibration states of the nonlinear system are assumed to be unmeasurable, i.e., only the actuator position can be acquired to feed into a suitable control system for stabilizing the vibration states indirectly. Then, a SMNNC scheme without the feedback of the vibration measure is proposed to control the motor-mechanism coupling system for periodic motion. All adaptive learning algorithms in the SMNNC system are derived in the sense of Lyapunov stability analysis, so that the system-tracking stability can be guaranteed in the closed-loop system. The effectiveness of the proposed control scheme is verified by both numerical simulation and experimental results.

Two dimensional periodic arrays of dipole and patch elements can be used as frequency selective surfaces (FSS) for polarization or frequency filtering at microwave frequencies. Numerous papers dealing rigorously with the study of FSS in the frequency domain have been published. All those methods directly give the characteristic of the FSS frequency by frequency. Computation time becomes important if these structures have to be studied over a very large bandwidth. Analysis methods in the time domain, followed by Fourier transform, have the advantage of directly given characteristics versus frequency. In this paper we introduce a new way for modeling and analysis of different FSS structures in the time domain using the 3D-transmission line matrix (TLM) method. All numerical results predicted by the TLM method are compared with experimental results, and good agreements are obtained.

In this paper, a new kind of coupled line called circular symmetric coupled strip or microstrip transmission line is introduced. These kind of coupled lines are less sensitive to external electromagnetic interference and can also be designed for large scale and low crosstalk interconnects. First, an exact expression of capacitance and inductance matrices (both for strip and microstrip structures) is derived using the Quasi-TEM approach and Laplace equations. Then, these matrices have been diagonalized to get to a set of single partial differential equations describing the system of coupled circular transmission lines and also a simple method for diagonalizing these matrices is presented. Finally some time domain examples derived from the frequency domain are given to show the properties of the structure.

With limited frequency spectrum and a tremendous growth in the demand for mobile communication services, the problem of channel assignment becomes increasingly important. Efficient channel assignment allows for an increased utilization of the limited frequency spectrum available for land mobile radio-telephone service.In this paper, a new traffic-adaptive non-uniform compact pattern based dynamic assignment algorithm is presented, which can be exploited in a PCS cellular environment with highly mobile users. The proposed dynamic channel assignment scheme copes with the handoff originated call demands problem, as well as focusing on reducing the blocking probability of the new calls. Computer simulation results on a 49-cell network conducted in a different manner and environment compared to previous studies indicate superior teletraffic performance of the proposed strategy over its predecessors. By using a discrete time step model traffic simulator, the effect of user mobility on the grade of service also comes under study in this research.

This paper presents a new non-iterative method for the determination of the equivalent circuit parameters of single–sided linear induction motors. In the proposed method, the relative permeability of the back iron is calculated by a simple function. Then using electromagnetic analysis, the parameters of the equivalent circuit are obtained. Next, a few approximations are proposed to develop an individual expression for calculation of each of the equivalent circuit parameters. The obtained expressions show the relation between the parameters and different features of motors such as pole pitch, input current, and slip.

We treat the problem of encoding the outputs from an information source, and the class of encoding schemes considered is restricted to those in which each output from the source is encoded as a separate, uniquely decodable codeword. We present a uniquely decodable scheme in this class which minimizes the ensemble average length of the transmitted message. The efficiency of this method in some cases is higher than conventional methods like block coding (in this paper we call it "block to variable length coding" relative to the title of the paper) by using the Huffman method. Although the efficiency of the encoding scheme can be improved by using the variable length to variable length technique, this topic is not pursued.

Traffic data and statistics (as density) are among the most important and crucial tools for the study of traffic, optimization of urban transportation and traffic programming. For this reason, the Tehran Traffic Control Company (TTCC) has built some data acquisition stations in various highway locations in Tehran. There are TTCC 10 data acquisition stations in use at the moment and more stations are to be designed and implemented in the future. The implementation of each station consists of three stages: field study, equipment installation and setting up magnetic rings, which require a long design time and high cost for data acquisition equipment, as well as induction sensors and heavy work load for installing magnetic rings. The final stage comes with many problems, particularly in highways, and needs high safety standards. Considering problems such as cost and safety of the implementation, the design of virtual stations seem necessary. A virtual station does not exist physically, but it can calculate traffic data. This proposal is known as Spatial Forecasting, which provides forecasting in space dimension using statistical data from present stations. The present article introduces an intelligent neuro-fuzzy structure which is trained over statistical traffic density data of the Modarres highway and is used for spatial forecasting. Results of simulation show that a neuro-fuzzy network can be used successfully for this purpose.

In this study, the time-course of the recovery from inactivation of molluscan ionic currents is examined. Molluscan voltage-gated ionic currents are described in Hodgkin-Huxley-like equations. The peak value function of the recovering conductance is derived from the mathematical equivalent of an experimental procedure of the recovery process in a general form by including the number of inactivation gates. Then the curves of the recovery and its approximation for the molluscan ionic currents are obtained. It is shown that recovering conductance of molluscan ionic currents is asymptotically exponential.