مجله مهندسی برق و الکترونیک ایران
سال دوازدهم شماره 3 (زمستان 1394)

With increasing the presence of Microgrids (MGs) in the power systems, investigating the MG stability during transient faults is necessary. This study investigates the transient stability analysis of a MG supplied by multiple inverter interfaced distributed generations (IIDGs) during fault. The transient stability of a MG is highly depends on the IIDGs control strategy. A MG, simulated on Matlab/Simulink, including three IIDGs and two local loads connected to the IEEE 37-node distribution system is considered as test system. Simulation results show that the MG may lose its stability due to the IIDG’s transient over-currents during fault. By adding a transient current control loop to the voltage source inverter (VSI) control strategy, the IIDG’s output currents are limited during fault. In the connected mode, an external fault is applied to the MG. By using an appropriate tuning based on fuzzy logic method, the transient loop controller coefficients is optimally tuned. The simulations results show the good performance and fast dynamic of the proposed control approach; also the transient stability enhancement of the MG, obviously.

In this paper, we investigated the necessity and effects of optimal control methods in the stability and performance of a micro-grid that operates in islanded mode. The micro-gird is consisted of a combine heatand power (CHP), a capacitor bank (CB), photovoltaic panel (PV) as power generation sources. In addition, battery and the building electricity and thermal load are placed on the load side. A simple comprehensive model is assumed for each component of the microgird and the dynamic behavior of micro-grid voltage and frequency is studied in details. Since the micro-grid operates in islanded mode, the fluctuations in the building load act as disturbances and could cause the instability in the voltage and frequency. So it is necessary to design a controller to regulate the voltage and frequency. Particle swarm optimization (PSO) is used as a method to realize the optimal controller in the micro-grid. Simulation results showed the effects of proposed controller on the reduction of voltage and frequency fluctuations.

This paper presents a probabilistic optimal power flow (POPF) algorithm considering different uncertainties in a smart grid. Different uncertainties such as variation of nodal load, change in system configuration, measuring errors, forecasting errors, and etc. can be considered in the proposed algorithm. By increasing the penetration of the renewable energies in power systems, it is more essential to consider the stochastic nature of wind power generation units. In this study, probability density function (PDF) of wind speed has been considered as Weibull. The proposed algorithm is based on point estimation method. In order to show the effectiveness of the proposed algorithm it has been applied to IEEE 14-bus test case power system. Simulation results show the effectiveness and precision of the proposed algorithm.

In methods presented to calculate the voltage collapse point, the transmission system is usually the only part of the power system that is completely modeled. Distribution systems are often replaced by aggregate load models because the use of the detailed model of distribution systems in voltage stability analysis not only increases the computation time, but also decreases the probability of convergence of calculations. But this replacement can cause a considerable error in determining voltage collapse point. In this paper, a method for considering distribution systems in voltage stability studies is presented. In this method, a detailed model of distribution systems is used. The loads are connected to the secondary side of distribution transformers. An iterative method is used to calculate the voltage collapse point. In each iteration, first the value of active and reactive power delivered to transmission buses that supply distribution systems is increased. Then for each given value of the delivered power, the power received at the secondary side of distribution transformers is calculated. With this work, the voltage collapse point is determined using separately solving transmission and distribution system equations. On the other word, instead of solving a large set of equations, some smaller sets of equations are solved. The simulation results show the effectiveness of the proposed method.

This paper presents the application of the matching pursuit method to model synchronous generator. This method isuseful for online analysis. In the proposed method, the field voltage is considered as input signal, while the terminal voltage and active power of the generator are output signals. Usually, the difference equation with a second degree polynomial structure is used to estimate the corresponding parameters of the regressor matrix. This matrix contains the elements of the difference equation for all sampling intervals. However, the number of parameters that should be estimated increases significantly with the increase in the difference equation order. In this paper, the matching pursuit method is implemented to identify important parameters with a very good accuracy and in a shorter time. In order to show the effectiveness of the proposed algorithm, it is applied to a nonlinear generator and the required signals are sampled using a phasor measurement unit (PMU). The simulation results show the effectiveness and precision of the proposed method.

The ultimate goal of power system operation and planning is to increase power system reliability which enforces interconnected operation of power system. As a result of power system interconnection, the inter-area oscillation under different disturbances may cause power system partial or total blackout. DC segmentation of interconnected power systems is a solution in which the topology of the network is changed by dividing it into small segments, and connecting them by DC-links. From the dynamic operation and planning aspects of the power system, the controllability of the system at the presence of multiple HVDC links used for segmented power system is a complicated problem. In this paper a procedure to study the concept of power system DC segmentation using the DC-links as a grid shock absorber is introduced. A well-established evolutionary technique, namely imperialist competitive algorithm (ICA) is used to design a HVDC supplementary controller for a non-segmented interconnected power system. The performance of the controller and the DC-segmented system is investigated after DC segmentation. The obtained results show the potential value of using supplementary controllers in the DC-segmented power networks.

Preventive maintenance scheduling of generating units is addressed as a long-term scheduling in power system studies aiming to increase the reliability incorporating cost reduction. It consists of knowing which generating units should be shut down for regular safety inspection. In this paper, a new formulation of preventive maintenance scheduling associated with cost reduction index (CRI) is presented. Mainly, the purpose of the maintenance problem is minimizing the operation as well as maintenance costs over a specified time horizon. CRI is introduced in such a way to reduce the operation cost along the scheduling time while determining the most proper maintenance scheme. The proposed framework is structured as a mixed integer linear programming (MILP) and solved using CPLEX solver. The suggested model is applied to a standard IEEE reliability test system (RTS) and the promising results show the effectiveness of the proffered model.

A Chaotic Optimization Algorithm (COA) based approach for the robust coordinated design of the UPFC power oscillation damping controller and the conventional power system stabilizer has been investigated in this paper. Chaotic Optimization Algorithms, which have the features of easy implementation, short execution time and robust mechanisms of escaping from local optimum, is a promising tool for engineering applications. The coordinated design problem of PSS and UPFC damping controllers over a wide range of loading conditions and systemconfigurations is formulated as an optimization problem with the time domain -based objective function which is solved by a COA based on Lozi map. Since chaotic mapping enjoys certainty, ergodicity and the stochastic property, the proposed chaotic optimization introduces chaos mapping using Lozi map chaotic sequences which increases its convergence rate and resulting precision. To ensure the robustness of the proposed coordinated controllers tuning, the design process takes into account a wide range of operating conditions and system configurations. The effectiveness of the proposed method is demonstrated through nonlinear time-domain simulation and some performance indices studies under over a wide range of loading conditions. The results of these studies show that the COA based coordinated controller has an excellent capability in damping power system oscillations and enhance greatly the dynamic stability of the power system.

In conventional direct torque control (DTC), the stator flux is usually kept constant by ontrolling the x-axis component f the stator voltage in the stator flux reference frame. The torque is then controlled by he y-axis component of stator oltage. In this scenario, the stator current does not exceed its permissible value. owever, in the so-called optimal fficiency mode, the induction motor operates with reduced flux-level at light loads. The eference flux increases with he increase in the load torque. Consequently, stator current overshoots from its hreshold, and the inverter’s rated ower needs to be increased. When the load torque increases suddenly, speed will drop ignificantly as the flux level ad been initially reduced. In this paper, the origin of this phenomenon and the vershoot current are described using he equivalent circuit model of an induction motor. A current limiting strategy is then roposed for the DTC drive with torque controller. Also to improve dynamic response, an algorithm for optimum istribution of stator current is eveloped that includes flux and torque components. This algorithm produces the aximum feasible output torque and inimizes the speed drop. Numerical simulation verifies the efficiency and efficacy of both roposed methodologies.

Self-affine maps were successfully used for edge detection, image segmentation, and contour extraction. They belong to the general category of patch-based methods. Particularly, each self-affine map is defined by one pair of patches in the image domain. By minimizing the difference between these patches, the optimal translation vector of the self-affine map is obtained. Almost all image processing methods, developed by using self-affine maps, take advantage of either the attracting or repelling behaviors which have been, only, experimentally investigated. In this paper, we analytically study the properties of self-affine maps and prove their attracting and repelling behaviors. Furthermore, the new corner/edge pointing behavior is also proposed for contractive self-affine maps. We show that the conventional cost function of self-affine maps may cause critical uncertainty due to providing multiple equivalent optimal translation vectors. Thus, a new cost function is suggested to effectively tackle this problem. Forevaluation, it is used with the selfaffine snake (SAS) for contour extraction. Experimental results demonstrated that the enhanced SAS provides better performance compared to a number of different active contour methods in terms of both solution quality and CPU time.

The Padé approximation of Tz e has special importance in some engineering problems such as investigation of linear time-delayed systems. In this paper a novel method is introduced to construct Padé approximation form of some practical functions, such as z e, (1 -z)α (1 +z)β with non-integers α and β, ln(1+z) and (1+z)β with non-integer β. This method gives a new closed form of Padé approximant of these functions by orthogonal polynomials. It introduces also a novel method to reduce the order of an ordinary differential equation. In other words, the method of this paper presents a new method to approximate the solution of an n -order ordinary differential equation by an (n-1)- order ordinary differential equation.

Handwritten digit recognition can be categorized as a classification problem. Probabilistic Neural Network (PNN) is one of the most effective and useful classifiers, which works based on Bayesian rule. In this paper, in order to recognize Persian (Farsi) handwritten digit recognition, a combination of intelligent clustering method and PNN has been utilized. Hoda database, which includes 80000 Persian handwritten digit images, has been used to evaluate our proposed classifier. Obtained results show that PNN is a powerful classifier and excellent choice for classification of Persian handwritten digits. Correct recognition rate when training and testing data have been used directly (without clustering) for training data is 100% and for testing data is 96%, but when k-means has been used as cluster tool and clusters'' center have been used as training data, in this case, correct recognition rate for training data is 100% and for testing data is 96.16%. In addition, when Particle Swarm Optimization (PSO) has been used to find optimum clusters for each class of Persian handwritten digits, correct recognition rate in training data is 100% and for the testing data it reaches to 98.18%.

In this Work, ITO/PET (Indium Tin Oxide / Polyethylene Terephthalate) electrode structure which provides biocompatibility, mechanical stability and flexibility is fabricated. Flexible ITO/PET implantable electrode array for a retina has been developed. The electrode array is fabricated on a thin PET/ITO substrate and is encapsulated using, SU- 8, an insulating material. PET substrate and SU-8 polymer make electrodes flexible so that they could shape to contoured tissues. A layer of gold on the stimulation sites serves to reduce the electrode/tissue interface impedance. Prototypes of 4×4 and 12×6 electrode arrays are fabricated for primary and dense configuration of retina prosthesis respectively. The exposed electrode diameters are 125μm for primary and 100μm for dense micro electrode array. The stimulation sites of primary configuration were connected via 50μm interconnects with 5 μm spacing. 40μm width traces with 15 μm spacing connect sites to bonding pads in the other design. To verify the functionality of the micro fabricated electrodes, the electrochemical impedance spectroscopy is measured. The electrode/tissue impedance was 19.4KΩ at 1 KHz for 7854μm2 area.