Year:2006 | Volume:40 | Issue:3 (97)|Papers Count:17

The aim of this paper is to investigate the methods of representation of a generator in power systems and a new design for a power system stabilizer based on genetic algorithms and artificial neural networks. For this purpose the most important models which are usually used to simulate the generator and power system are introduced. These models are applied in a multimachine system. A new simple genetic algorithm is applied to a power system stabilizer to find the optimum parameters of a PID controller at different fixed operating points. Then an artificial neural network is proposed to adapt the parameters of the stabilizer to different operating points and different conditions such as a three phase short circuit. This stabilizer is applied to the multimachine systems with and without infinite bus.

The multilevel AC/AC converters have found various applications in high powers and voltages, nominated as the potential third generation of FACTS controllers. These devices have become more popular among researchers, engaging both half-bridge and full-bridge sub-modules. This paper presents a new control and modulation scheme for these converters that provide control of output ac voltage independent of the input ac voltage and the load current as well as input/output reactive power control. Like the advanced UPFC, it is also possible to control the transferred active power between the two sides. Moreover, the dc-link voltages are balanced using the proposed modulation scheme, while a closed-loop control adjust the dc levels at a reference value to secure the stable operation of the AC/AC converter. The whole circuit is simulated with PSCAD and a single-phase laboratory prototype (2kVA, 100V) was implemented, including the power circuits along with the control and modulation blocks to confirm the validity of the proposed schemes.

This paper is aimed at the sensrorless control of capacitor in single phase induction motors. This is done towards three objectives i.e. maximization of average torque, minimization of torque pulsations and optimization of torque. The objectives are achieved by first obtaining the average torque and pulsation torque in terms of a variable capacitance and motor speed. The objective functions are then defined mathematically. Later the appropriate capacitance is calculated in a wide range of motor speed according to each objective. The motor speed is estimated by three methods using electrical sensors. The estimated speed is used for the switching of a power electronic switch in parallel with a constant capacitor to provide an appropriate capacitance in each case. Extensive simulation results are presented to verify the effectiveness of the sensorless capacitor switching in achieving the torque control objectives.

In text classification, words are usually used as features. Since text classification methods are faced to too many features, here we propose several methods to improve feature selection in text classification, and we show merits of the proposed methods in comparison to the relevant ones. First, we propose two methods that just concentrate on all positive and negative correlation factors between features and document categories. The first method uses all correlation factors with a positive effect while the second one uses positive factors with a positive effect and negative factors with a negative effect in feature selection criteria. Our experiments show that the positive factors are more effective than the negative factors in feature selection criteria. Next, we propose a hybrid method. First, it uses Relief-F filtering method to select a number of features spending a low computation cost. Then, the selected features are further reduced using an SFS or SBS wrapper method. The hybrid method has a better performance in comparison to filtering methods. Our experiment results show that applying this method to SVM Light classifier on Reuters-21578 corpora we can remove up to 94% of features while improving classification accuracy. Moreover, it is worth to mention that Relief-F by itself has shown a good result where it is the first time it is applied to text domain.

In this paper, we consider nonlinear control system of the form: dx/dt= Ax- bξ, dξ/dt=f(σ), σ=cTx- ρξ Using liapounov function and liapounov method, we first establish the stability of system (obtaining some relation between control parameters). Secondly, we give an example which clarifies our method.

The concept of "strict positive realness" of the rational transfer functions extensively used in various field of control. The basic definition of strict positive realness is motivated by Popov’s hyper stability theory and is stated in the frequency domain. With expiry fourth decade, still there is not unique statement which states the necessary and sufficient conditions for strict positive realness in the control literatures. In this paper four related common statements in the strict positive realness literature which is appeared in the control theory are discussed. The drawback of these common statements is analyzed through some counter examples. Moreover a new necessary condition for strict positive realness is obtained from high frequency behavior of the Nyquist diagram of the transfer function. Finally a more simplified and completed conditions for strict positive realness of single-input single-output linear time-invariant systems are presented based on the complex functions analysis approach.

Technical and economical assessment of distributed generation (DG) installation in power systems is one of the main stages in related studies. Because some capabilities of sub-transmission substations such as presence of operators and existing monitoring and control facilities, they are suitable candidate for DGs installation. This paper presents a new method for economical evaluation of DG installation in a sub-transmission substation. In this approach all cost items (including investment, operation, and maintenance costs of DG) and benefit items (including substation capacity upgrade deferral, reduction in annual cost of purchased active/reactive power and reliability improvement) are mathematically modeled. Then, these items are aggregated as an objective function. Also operational constraints of DG are considered in the formulation. The effectiveness of the proposed approach is evaluated by some numerical studies and sensitivity analysis.

The subject of 4NF, but not 5NF, relation is introduced in the textbooks, but no theoretical study is achieved on their decomposition algorithm. In this kind of normal relation, there exists a special semantic constraint so called "cyclic nature constraint" which needs yet a theoretical analysis. On the other hand, in the practice of database design it is sometimes necessary to decompose a relation into two or more relations. In this paper, we f11st mention the reason for decomposing a relation, and then give a rigorous defmition of cyclically constraint relation. Then by providing five Lemmas and their proofs, we describe in detail the characteristics of this kind of relation. We also introduce tow theorems and their proofs and provide the algorithms, based on these theorems to decompose the cyclically constrained relation, with a very high degree of flexibility. Finally, we discuss the results of this research.

Switched reluctance motors (SRM) are attracting much attention because of their special advantages. Generated heat by losses can reduce the life time of SRM. Therefore, optimal design based on thermal modeling helps to improve their performance and increases their life time. In this paper, a lumped thermal model of SRM based on the analogue of conductive and convective thermal resistances is proposed. On the other hand, using fans or blowers on motors cause some technical and economical problems. Therefore, an innovative design is proposed in which the rotor itself operates as an axial fan and reduces the temperature rises within the motor. Finally the stator dimensions are optimized using obtained thermal model.

This paper presents the solution of VAr planning problem using an accelerated ant colony algorithm (AAC). This intelligent technique has been applied to solve some problems in power systems so far, but not for solving the problem of VAr planning. In this paper the VAr planning is considered as a multi-objective function weighted by Analytical Hierarchy Process (AHP) and solved by AAC technique while all already existing VAr sources are taken into account to enable the system to be operated under economically conditions. The proposed method has been applied on the modified IEEE 14 bus and IEEE 30 bus systems. To show the effectiveness of the proposed method the simulation results of the IEEE 30 bus system are compared with the results of Ref. which is based on Simulated Annealing method (SA).

Most useful information in different kind of signals is usually carried by such singularities as the edges and peaks. Examples of such signals include Radar signals, the signals generated by the digital communication systems and biological signals (such as ECG, EEG and even medical images). Therefore, extraction and locating these singularities is a main and an initial common step in most of the signal and image processing application. In this paper a new multi-resolution based method for automatically extraction of singular points within the signals is presented where the information at various signal resolutions is combined together in a novel fuzzy manner. In the proposed algorithm, first, the multiresolution description of the signal is obtained using the wavelet transform. The information at each wavelet transform scale is next transformed into a fuzzy subset of the signal by means of appropriate fuzzy flying functions for each kind of the singularities (edges or peaks). The resulting fuzzy subsets describe to what degree any sample point from the signal domain can represent a singularity at that particular scale. Finally, combining the information at various fuzzy subsets of the signal by means of the fuzzy operators, the sample points with the highest possibility of coincidence with a singularity are identified. The superiority of the proposed algorithm in comparison to the commonly used techniques is shown using various synthetic and real signals.

A hybrid method based on time domain simulation technique and the Boundary of stability region based Controlling Unstable equilibrium point method (BCU method) for direct analysis of power system transient stability using the Center of Inertia (COI) formulation is presented. The BCU method is based on the relationship between the boundary of stability region of a power system and that of a reduced system. This method finds the Controlling Unstable Equilibrium Point (CUEP) of the original system via a reduced system whose CUEP is easier to compute. Features distinguishing the BCU method from the existing direct methods are that it finds the exact controlling UEP relative to a fault-on trajectory. Having obtained the closest point to the controlling UEP using the BCU method, we calculate the exact controlling UEP by employing a modified Newton-Raphson technique. The proposed method has been successfully applied on two test systems and the results are given.

Transmission open access is one of the essential issues in restructuring of electric industry. In this way, pricing of transmission system should be performed in a fair and reasonable procedure. This paper describes the principle and implementation of a reliability based methodology for allocating the cost of transmission network. A probabilistic index is defined which measures the relative importance or reliability benefit of the unused (additional) capacity of each particular line for each transaction. The index is calculated based on contingency conditions analysis corresponding to line outages. Transmission embedded costs are allocated based on the contribution of each transaction to base capacity in normal condition and also to the additional capacity in contingency conditions using the defined index. Furthermore, the split between the capacity use and reliability margin costs is performed on a reasonable decision. As a result, the proposed methodology provides an equitable means of allocating the cost of transmission network among its users. The methodology is illustrated in a small power system and its conceptual and computational feasibility are shown.

By increasing use of concurrent systems, especially in critical applications, we need a proper model for specifying these systems in a reliable way. In this paper, we propose an actor-based language for modeling concurrent systems and present its operational semantics in basic transition system. The model consists of independent reactive objects which communicate via asynchronous message passing. Dynamic creation of reactive objects and dynamic changing topology are two features of the model. The proposed language and its formal semantics have been a basis for developing an automated tool for modeling and verifying reactive systems.

High rates of building change in urban area, particularly in the developing countries, call for an efficient and fast technique for mapping the spatial changes for updating and monitoring of existing topographic urban geospatial databases. Most of the existing automatic methods for change detection process are optimized to employ parametric approaches and fuzziness behaviors of objects are neglected. So these methods fail to take the complexities of change detection into account and encounter some problems to achieve the acceptable reliability level in complex cases. In this research an efficient method presented in the form of a strategic framework based on a neuro-fuzzy reasoning system which uses different dataset of the test area to deal with the problem. The potential of proposed method which was based on information fusion technique was evaluated based on a dataset of city of Rasht in Iran.