Year:2009 | Volume:42 | Issue:7 (117)|Papers Count:12

In this paper, we propose an approach for automatic generation of novel intrusion signatures. This approach can be used in the signature-based Network Intrusion Detection Systems (NIDSs) and for the automation of the process of intrusion detection in these systems. In the proposed approach, first, by using several one-class classifiers, the profile of the normal network traffic is established. Using an inductive learning method, the signature of this novel intrusion is generated and the signature database is automatically updated. We evaluate our approach by performing experiments on the dataset provided by the DARPA Intrusion Detection Evaluation Program. The results of experiments show that our proposed approach can be successfully used for automatic generation of novel intrusion signatures.

Vacuum circuit breakers are able to interrupt the high frequency currents. But these characteristics may have some consequences in power systems when a power frequency current is interrupted using a vacuum circuit breaker. If the contacts open just before the zero current, the vacuum chamber can not withstand the recovery voltage on the breaker and the breakdown occurs in the chamber. As a result, high frequency currents are generated in the circuit and because of the capability of the vacuum circuit breaker to interrupt such high frequency currents, these currents are interrupted, but as the gap is still small, this leads to a series of interruption and breakdown known as multiple re-ignitions. Very high and steep over-voltages can be produced as the result of the multiple re-ignitions in the breaker. In this paper, a high frequency model for the vacuum circuit breakers is implemented in the ATP Software using the MODELS programming language to study the influences of the multiple re-ignitions in the vacuum circuit breakers on the other components of the power systems. It has been shown that depending on the contact separation time and circuit parameters, not only very high over-voltages are produced on different components but also high mechanical stresses are applied to the rotating machines like motors during the multiple re-ignitions of the vacuum circuit breakers.

Signaling is the most important part of telecommunication networks and Signaling Number 7 is the most used signaling protocol for telephone calls all over the world. Planning of signaling network is very important for reliable PSTN (Public Switch Telephone Network) operation. One of the important issues in planning signaling networks is the distribution of signaling load on signaling links and linksets. Loadsharing of signaling links in SS7 network is done according to ITU-T standards by four bits of SLS (Signaling Link Selection) field of SS7 messaging unit. Maximum number of simultaneous SS7 links between two SPs (Signaling Point) is 16 links. For having redundancy there is two linksets connecting two SPs through STPs (Signal Transfer Point).In this case each linkset can have maximum 8 SS7 links. This paper at first explains the fundamental of signaling loadsharing according to ITU-T standards and its limitation then recommends a new method for overcoming 8 links limit in each linkset by dividing DPCs (Destination Point Codes) into two groups. By reversing the order of SS7 signal routing of two DPC groups in each linkset, it is possible to equally distribute signaling load to 16 links in each linkset (totally 32 equally loadhsared links in two linksets). The advantage of new suggested method is to increase efficiency and to overcome existing ITU-T standards limitation. By using this method, there is no need to upgrade to MPC (Multiple Point Code) or any other software patch. The results of practical implementation of mentioned method are included in paper.

Hysteresis motors are exciter-less and brushless machines with outstanding characteristics such as constant torque and speed, robust structure, low and constant starting current and smooth operation. On the contrary, hysteresis machines have some usage limitations such as low power factor, efficiency and torque density. In this paper, calculation of electrical equivalent circuit parameters versus design parameters of high frequency disk-type hysteresis motors are presented in details for steady state operating conditions. It is demonstrated that some of the equivalent circuit parameters alter significantly with input voltage and different operating conditions. Therefore, an equivalent circuit with fixed elements may give an inaccurate prediction of steady state performance of a machine in many circumstances. Furthermore, in this paper for a disk-type hysteresis motor which is designed and manufactured by use of variable parameter model, a sensitivity analysis is presented and influences of some design parameters on machine performances are discussed. Many experiences show that W10, tr, g, Ro, Ri ( respectively, Opening of stator slot, Thickness of rotor disk, Axial air gap between stator and rotor, Outer radius of rotor and stator, Inner radius of rotor and stator) are the most important structural and effective design parameters of the disk type hysteresis machines. Because of special applications of these machines and existing limitations, values of g, Ro, Ri are supposed constant concerning the initial design, and performance of the machine including the efficiency is studied when two other parameters vary. By an optimization program, optimum values of W10 and tr as two important design parameters are calculated. Based on these parameters and by their values other operation parameters such as efficiency and power factor are calculated.

In recent years several catastrophic power systems blackouts have occurred worldwide. Various reasons have been declared for these failures. Economical limitations due to power system restructuring restrictions, inadvertent operation of protective relays and inefficient design of conventional load shedding schemes are of the most important reasons causing these blackouts. In fact, due to both economical and technological restrictions, it is not possible to completely prevent these blackouts. However, with the aid of some protection and control strategies, frequency and intensity of these blackouts may be reduced. One of the important protection strategies used for this purpose is a class of protection schemes known as System Protection Schemes or Wide Area Protection Schemes. One of the most commonly used types of system protection schemes, generally accepted after the north-eastern blackout of 1965, is Under Frequency Load Shedding (UFLS) scheme. Conventional under frequency load shedding scheme is designed to retrieve the balance of generation and consumption following a disturbance. In the conventional load shedding method frequency settings, time delay settings and the amount of load to be shed in each step are constant values. The loads to be shed by this scheme are also constant load feeders and are not selected adaptively. Using this constant non-adaptive load shedding algorithm is not the most efficient scheme for all power system disturbances. In some combinational disturbances, events causing frequency to drop are followed by other events causing voltage drop. In these cases, since loads are voltage dependent, total system load is reduced and system frequency might not decrease so much to activate UFLS relays. However, the system could eventually collapse due to voltage instability. In many such cases the system would survive if the load shedding relays operate adaptively and appropriately. For example if for large disturbances, higher frequency settings and lower time delays are used adaptively, a faster load shedding response is obtained and as a result system collapses may be prevented. In this paper a new UFLS algorithm is proposed. The purpose of this algorithm is to adaptively adjust speed of load shedding based on the magnitude of disturbance. In this method rate of frequency decline is used as a criterion to determine intensity of disturbance. Thereby, for large disturbances higher frequency settings and lower time delays are used. Application of the proposed algorithm to the simulated model of Khorasan network in Iran confirms its satisfactory performance. As the results of simulations show, several voltage collapse instabilities may be prevented by using the proposed adaptive UFLS method.

With the rapid growth of indoor wireless communication systems, the need to accurately model radio wave propagation inside the building environments has increased. Many site-specific methods have been proposed for modeling indoor radio channels. Among these methods, the ray tracing algorithm and the finite-difference time domain (FDTD) method are the most popular ones. The ray tracing approach as a high frequency technique is efficient for calculating the received field at a small number of receiver locations. Application of FDTD method as a full wave technique for indoor propagation modeling is time consuming and requires large amounts of memory. The parabolic equation method (PEM) is a fast full-wave technique which allows accurate modeling of the propagation environment and its electrical parameters. This paraxial version of the wave equation can be solved by marching techniques which need far less computation resources than a full elliptic equation. The PEM has been extensively used as an efficient algorithm for outdoor propagation modeling. In this paper we propose an unprecedented application of PEM for indoor propagation problems. Depending on the required speed and accuracy of computations, two and three-dimensional versions of the PEM can be used for indoor problems. Without loss of generality, we restrict ourselves to the two-dimensional problems and use two-dimensional approximation of the parabolic equation for fast and accurate radio wave propagation modeling in indoor environments. The parabolic equation has been derived for lossless media where the refractive index is very close to unity. To the authors' best knowledge the paraxial version of the wave equation has not yet been derived for propagation in general lossy dielectric media. In this paper, we first derive the general form of the parabolic wave equation for lossy dielectric media where it can be used for modeling the radio wave propagation through walls. The special form of the parabolic equation for modeling wave propagation in free space can be derived from this general form. We then apply PEM to model propagation of radio waves through a row of windows, reinforced concrete walls and typical corridors inside buildings. As windows are one of the most prevailing architectural elements in buildings, the phenomenon of plane wave transmission through them is of interest. In this paper PEM is used to model the radio wave propagation through windows. The numerical simulation results are presented for both normal and oblique incidence and compared with some reported results. The transmission and reflection characteristics of inhomogeneous walls have been studied by many numerical and analytical methods such as the finite-element method (FEM) and FDTD. In this paper, we use PEM to characterize reflection and transmission properties of reinforced concrete walls under plane wave incidence. The effect of several parameters namely wall thickness, bar diameter and spacing on the transmission coefficients of reinforced concrete walls will be analyzed. Corridors are also popular elements of buildings, so that the analysis of radio wave propagation in corridors has involved many researchers. The PEM is an effective method for modeling wave propagation in these environments. The effect of obstacles such as cupboards and cabinets inside a corridor can be modeled by PEM. This method is also able to model the effects of variations of the corridor direction on the wave propagation. The numerical simulation results will be presented and compared with the available data in the literature.

Although many mathematics-based and heuristic approaches have been recently developed on optimally allocation of TCSCs for lines overloads reduction and buses voltage stability enhancement during fault conditions, the works on the TCSCs efficiency to achieve the abovementioned goals are rare. This idea that TCSCs can surely enhance the systems security must be comprehensively investigated. In this paper, after the optimal allocation of TCSCs, their efficiencies on lines overloads reduction and buses voltage stability enhancement during fault conditions are investigated using two new indices named transmission index and voltage stability index. The numerical results show TCSCs can remarkably enhance the buses voltage stability, but they can not significantly reduce the lines overloads. In addition, the impact of TCSCs installation on mitigation of load shedding aim to enhance the buses voltage stability is presented as a new work. The IEEE standard 14 and 30 buses systems are selected as case studies.

This paper presents a new method to find the optimum switching angles in voltage source multilevel converters in order to minimize specific higher order harmonics and decrease the total harmonic distortion (THD) of their output voltage waveform. The output voltage waveform of inverter can either be in the form of staircase or PWM. In order to increase the degrees of freedom and elimination of more harmonics, one can increase the number of levels or use PWM waveform which has more switching than once at each level. However, both of them result in complexity of nonlinear transcendental equations which solving them by using conventional numerical iterative techniques such as Newton-Raphson method is not feasible. In this paper, particle swarm optimization algorithm is presented to find optimum switching angles of PWM waveform in multilevel converters. These angles must be determined so that the desired fundamental harmonic is maintained and at the same time undesired higher order harmonics are suppressed. Theoretical and simulation results for an eleven-level converter show the efficiency of the proposed algorithm to determine the optimum angles in order to decrease the undesired harmonics and produce very high quality output voltage waveform.

There are fewer problems encountered with optical voltage transducers in comparison with their inductive and capacitive counterparts. Although capacitive and inductive transformers are used vastly for the purpose of measurement and protection in the networks, they cause problems such as core saturation and improper transient responses which decrease the accuracy of measurement and efficiency of the protection schemes. As a result, optical voltage transducers are considered as proper candidates for replacing the conventional inductive and capacitive transformers. Lighter weight, smaller size, larger dynamic range, wider bandwidth, insensitivity to electromagnetic interference, stability over temperature change, absence of iron core saturation, low maintenance and replacement cost, etc. are some of the advantages of optical voltage transducers over inductive and capacitive transformers. In this paper, the Modified Adaptive Method is introduced in order to measure the integral of electric field with minimum number of sensors. Using this algorithm makes the effects of other fields minimum. One more advantage of this method is the possibility of improving the accuracy of the measurement by using a correction factor. The correction factor is determined by considering the level of accuracy, environmental conditions and the number of sensors. Simulation results show the great validity and effectiveness of this method over a wide range of variations.

The parallel machine scheduling problem is an important and difficult problem to be considered in the real-world situations. Traditionally, this problem consists of the scheduling of a set of independent jobs on parallel machines with the aim of minimizing the maximum job completion. In today's manufacturing systems, in which both early and tardy finishing of job processing are undesired, the objectives related to earliness and tardiness penalties have become increasingly popular. In this paper, two major goals are considered as follows: (1) total weighted earliness; (2) total weighted tardiness. Due to the complexity of such a hard problem, a new multi-objective meta-heuristic method, i.e. multi-objective scatter search (MOSS), is proposed to obtain the locally Pareto-optimal frontier where the simultaneous minimization of the above-mentioned objectives is desired. In order to validate the performance of the proposed MOSS method, in terms of solution quality and diversity level, various test problems are considered and the reliability of this method, based on different comparison metrics, is compared with the Elite Tabu Search (ETS) devised in this paper. The computational results show the high capability of the proposed MOSS method.

Development and expansion in information and communication tools in last years had caused universal organizations and companies expand their markets in the entire world. Thereinafter, traditional local markets have modified to world wide rivalry markets. Therefore, in order to be successful in the new market, organizations need to focus on core process of organization. For this reason organizations would like to outsource the noncore processes to outside service provider companies. Selection of the most suitable supplier/s is the vital decision with wide domain of inferences in the supply chain management. The suppliers play key role in the successes of supply chain management targets. In this article the supplier selection techniques and models had been studied. Results had been appeared that none of the published models can could support the all outsourcing problems in the general form. In this study the Analytic Network Process (ANP) model which has initiated by Saaty (1996), had selected for base model to expand and make the final model. Analytic network process is the developed face of Analytic Hierarchy Process (AHP) model. ANP with have all attributes of AHP can enter all dependences and feedbacks between elements of decision making to final rating of alternatives. Since, a part of this model involves paired comparisons of element, so in vague conditions of judgments, this model seems to be none complete model to cover all problems of outsourcing. Therefore, the Analytic Network Process (ANP) is adopted and developed by fuzzy set theory to cover the indeterminacy of decisions made in this field. The authors have developed a linear programming model to elicit eigenvectors from fuzzy comparison matrices. Hybridization of these two concepts can model supplier selection problem in all circumstances and reaches the optimal choice. Finally, a numerical example is used to validate the proposed model.

In today's world, economic climate changes more quickly, and countries realize that globalization has made the world smaller and more competitive. Also, customers seek products and services that can respond to their specific needs and firms make effort to create competitive advantages to keep their profit and market share. All of the above trends lead firms and countries to focus on supply chain and integrated logistics. Making supply chain activities more effective and efficient is a sustainable competitive advantage for countries. One of the important parts of these activities is logistics activities, which can make a significant reduction in costs. Efficient management of logistics activities is a perfect source for creating competitive advantages. Besides, it allows firms to respond to their customers' specific needs, which in turn, results in customer satisfaction. Because designing the forward and reverse logistics network separately leads to sub-optimality, one of the proper areas to achieve more efficiency in logistics network is to design forward and reverse logistics network simultaneously. This paper proposes a MINLP model for integrated logistics network design to avoid the sub-optimality caused by a separate, sequential design of the forward and reverse networks. The proposed model could support multi-level capacity for each facility and also consider saving costs associated with locating the distribution centers and collection/inspection centers in the same places. To escape from the complexity of the proposed MINLP model, the model is made linear by defining a new variable and adding a constraint to the model. Since such network design problems belong to a class of NP-hard problems, we propose an efficient memetic algorithm with dynamic local search mechanism and we compare the associated numerical results by exact solutions in a set of problems to present the high-quality performance of the proposed algorithm. Future research could be aimed robust models to accommodate the changing parameters of the business environment during the life-time of the logistics network. Also addressing the demand uncertainty and the supply of return products in a multi-product integrated logistics network is a promising research avenue with significant practical relevance. Although our memetic dynamic search strategy proved to be competitive for the IFRLN network problem under consideration, other meta-heuristics algorithms such as tabu search or scatter search could offer promising avenues for developing richer integrated logistics networks as well.