نشریه مهندسی برق و مهندسی کامپیوتر ایران
سال هشتم شماره 1 (پیاپی 21، بهار 1389)

In this paper three combinational adaptive load shedding schemes are proposed to enhance the power system stability especially voltage stability margins of the system following severe events. Nowadays, the security margin of power systems against various instabilities is decreased due to the developments, deregulation and competitions in the power industry. In this situation, traditional system protection schemes can not offer adequate protection especially against combinational events. In some combinational disturbances, after initial frequency drop the conventional protection schemes returns back the system frequency to its permissible values; however, the system eventually collapses due to severe voltage declines which result in voltage instability. In some other disturbances, severe voltage declines cause troubles in appropriate operation of the under frequency load shedding relays. In this paper three adaptive combinational load shedding schemes are proposed to counteract such disturbances. The proposed schemes use locally measured frequency and voltage signals and do not need any communication link. In the proposed algorithms, during under frequency condition, load shedding is started from the locations which have higher voltage decay and for longer period of time. The speed, location and amount of load shedding are changed adaptively depending on the disturbance location, voltage status of the system, and the rate of frequency decline. In the second part of this paper using model of a real network, various simulation studies are performed and performance of the proposed schemes is investigated.

paper is part II of a two-part paper. In the first part, several adaptive combinational load shedding schemes were proposed to enhance the power system voltage stability. The main objective of the proposed schemes is to improve the voltage stability margin of the system following large and combinational disturbances. For major disturbances the frequency and voltage stability of the system are jeopardized simultaneously and the conventional schemes might fail to operate correctly. In this part, the proposed methods are simulated in a real network to evaluate their performance. To achieve realistic results, dynamic model of generators, automatic voltage regulators, governors and loads are considered in the simulations. Considering the importance of load modeling in these studies, the frequency and voltage dependence of static loads have been modeled accurately. Dynamic motor loads have also been modeled using aggregate equivalent induction motors at load buses. Performance of the proposed schemes is compared with each other also with performance of the conventional scheme for various combinational disturbances. Considering the obtained simulation results it is concluded that by using the proposed algorithms the power system becomes more robust against large disturbances and the probability of the power system instability is decreased.

In image inpainting, distorted and damaged parts of image or selected objects are removed or replaced with the appropriate information. In this article, image inpainting is performed by using frequency information of wavelet transform. The fill-in is done by diffusion of information of intact pixels into the damaged regions, which is begun from the outermost pixels and gradually the damaged region is reconstructed. To determine direction and the amount of diffusion, the geodesic path based image inpainting method is generalized by incorporating information of wavelet domain. The experimental results confirm superiority of the proposed method over the geodesic path based image inpainting method.

In this paper, an online signature verification method using extended regression in stationary wavelet domain is presented. To calculate the similarity between two signatures by extended regression, we should equalize the time length of the corresponding signals in two signatures. Using all points of the signals to equalize their time length will decrease the difference between a genuine signature and its forgery. Here a new approach based on the extreme points warping of the signals is presented. This approach equalizes the time length of two signals without degrading the differences between them. Also we calculated the similarity of signatures by using the details of the signals in stationary wavelet transform, SWT, domain, which showed very good results. The proposed system was tested on SVC2004 signature database. The results were compared with the results of participant teams in the first international signature verification competition. We have gained EER=6% for skilled forgery signatures. Comparing the result, it shows that we stand in the second rank between all the participants. This system has no verification error for random forgery signatures and stands in the first rank. Our experimental results show that using SWT domain instead of time domain decreases the verification error rate by 35%.

In this paper, we present a contrast independent algorithm for binarization of degraded document images. The proposed algorithm does not require any parameter setting by user. Therefore, it can handle document images with variable foreground and background intensities and low contrast documents. The proposed algorithm involves three consecutive stages. At the first stage, independent of contrast between foreground and background, sensible parts of each character are extracted using the modified water flow model, which is designed for the extraction of sensible part of each character and the drawbacks of water flow model are solved in this algorithm. In the second stage, the gray levels of foreground are estimated using the extracted text pixels and the gray levels of background are locally estimated by averaging the original image. At the third stage, for each pixel of image, the average of estimated foreground and background gray levels is defined as local threshold. After extensive experiments, the proposed binarization algorithm demonstrates superior performance against conventional binarization algorithms on a set of degraded document images captured with camera. Proposed algorithm efficiently extracts the low contrast texts.

The subject of this paper is to read carpet pattern automatically by computer. This is composed of two steps: detection of vertical and horizontal lines in the pattern and color reduction. Color reduction is essential because of limitation of the number of colors that is used in a carpet.To accomplish of this process, we must detect the grid lines on the carpet pattern automatically. These lines are two types: thin lines and thick lines. At the first stage, the distance between thin lines is obtained. Having the first thin line detected, the other thin lines are drawn using this distance. We use a Comb method for detection of thick lines. The major problem in line detection is lagging or leading of the lines due to the mismatch between sampling frequency of the scanner and image resolution. We compensate this distortion in various steps in our algorithm.In the second step, we want all the pixels in the same square, to have the same color. This is obtained by mapping colors to the best color in the palette. We propose three methods. In first method the user selects two selections per any colors. Palette is obtained from some processes in these selections. Those pixels that are in the middle of the squares are mapped to the palette. Then color histogram is computed. The color that has the maximum histogram value is assigned to the square. In order to decrease user’s interference, C-means clustering algorithm is used in two types. The centers of initial clusters are determined once with user’s interference and once randomly. Results of these three methods are compared.We tested our methods on 20 samples of carpet patterns, and the error rate was variable from 0.07% to 0.5% between samples.

In this paper, we propose a new approach for selection of subsets of active sensors with some constraints on energy consumption and estimation error for tracking of a target. The proposed approach exploits the decentralized estimation by using the extended information filter for target tracking. Furthermore, a cost function is defined using spatial correlation for sensor selection. Consequently, the Spatial Split algorithm is proposed based on spatial correlation coefficients for sensor selection. At last, for high speed targets, we propose a modification on spatial split algorithm by changing the sensing range with respect to the target speed. Simulation results show that the tracking accuracy is analogous to those of optimal estimation methods. It is also found that energy consumption decreases due to activating only necessary sensors.

We propose a scheme for improving QoS and reducing transmit power in wireless ad hoc networks by utilizing the signal-to-interference-plus-noise-ratio (SINR) and a pricing function that is proportional to the transmit power of each user. The performance of our proposed method is analyzed by using game theory, where each user’s quality of service is a function of its SINR. The utility function for each user is defined by its desired SINR minus a pricing to provide adequate incentive for each user to choose its power level in such a way to maximize the aggregate of all users’ utilities (total network utility) instead of selfishly maximizing its own SINR. Simulation results show that the performance of the network is improved while the total power consumption is reduced.

In this paper, at first, a new model will be attained for unified power flow controller (UPFC) using state space equation of bilinear systems. Then, a complete novel method of designing UPFC controllers will be represented by the use of variant structure systems. In this method, input signals of UPFC are designed through sliding mode controller. In order to design these kinds of controllers, at first, control rules are obtained by the use of designing four different slide levels and then setting their derivatives (which express dynamics of the flows of axes d and q, in serial and parallel transformers) to zero. In fact, applying the outputs of this controller to UPFC is equal to bring the flows of axes d and q to the reference value in both UPFC. In the other hand, internal dynamics (DC capacitor voltage) will be stabilized in UPFC by means of PI controller. The stability of the system is obtained through Lyapunov function.