Current issue of "International Review of Electrical Engineering (IREE)"

by Srinivasan Pradabane, N. V. Srikanth, B. L. Narasimharaju

Abstract - This paper proposes a new space-vector based Pulse Width Modulation (PWM) scheme for an open-end winding induction motor drive. This PWM scheme requires two isolated power supplies and uses instantaneous phase reference voltages. The circuit configuration requires two isolated power supplies for the dual inverter fed open end winding induction motor drive. By exploiting the rich switching redundancy that is offered by the circuit configuration, all the active states are used in clamping the inverters. The proposed scheme is simulated using MATLAB/Simulink platform and the simulated results are validated using dSPACE experimentation.

Keywords: Open-End Winding, Space Vector Modulation, Fixed-Bias.

by L. Popova, T. Musikka, R. Juntunen, M. Polikarpova, M. Lohtander, J. Pyrh�nen

Abstract - Laminated busbars are extensively used to connect components in hard-switched power inverters due to their low stray inductance. The objective of this paper is to demonstrate the design procedure and modeling of laminated busbars of a three-level three-phase Active Neutral Point Clamped (ANPC) inverter, with special emphasis placed on ensuring low stray inductance at an early design stage. In the paper, the partial inductances of the busbars are estimated using an ANSYS Q3D software tool. The stray inductance of the commutation loops is determined analytically and by cosimulation with ANSYS Simplorer and Q3D. Cosimulation allows analysis of the influence of the stray inductance on the performance of the inverter. The modeling results show that the laminated busbars proposed in this paper have low stray inductance. The stray inductances of the commutation loops were measured before the inverter is assembled to verify the modeling results. The approach used reduces the need for late-stage modifications based on prototype testing.

Keywords: Busbars, Converters, Impedance Measurement, Inductance.

by R. Uthirasamy, U. S. Ragupathy, V. Kumar Chinnaiyan, C. Megha

Abstract - This paper presents the experimentation of Cascaded Multilevel Inverter (CMLI) for Photo Voltaic (PV) generator feeding an isolated load. For the improvement of PV system utilization, boost chopper interfaced CMLI configuration is developed under Multicarrier Sinusoidal Pulse Width Modulation (MCSPWM) control schemes. Boost chopper interfaced CMLI configuration boost up the PV panel voltage and generates high quality of AC power for domestic and drive load applications. The entire systems of PV, boost chopper and CMLI are simulated using SIMULINK and the results are validated. Prototype hardware model of single phase boost chopper interfaced 7-Level CMLI configuration is developed and its output illustrations are validated.

Keywords: Boost Chopper, Cascaded Multilevel Inverter, Photo Voltaic, Pulse Width Modulation .

by A. Savu, A. Radoi, A. Florescu

Abstract - This paper presents the analysis of two different types of transformer structures used in wireless power applications. Conventional litz wire windings are compared to planar windings in terms of AC/DC performance ratio, coupling and inductance. Planar and litz wire transformer arrangements are proposed. A 6W LLC wireless power converter is used to compare the different transfomer structures. Finite element anlysis is used to determine the transformer coupling coefficient and AC/DC ratios. The simulation results are validated by experimental measurements.

Keywords: Wireless Transfomer, LLC Converter, AC/DC Ratio, Coupling Factor, Wireless Charging.

by Srinivasan Pradabane, N. V. Srikanth, B. L. Narasimharaju

Abstract - This paper investigates the rise in junction temperature in power semiconductor devices that occur due to the alternate Fixed-bias inverter switching scheme for an open-end winding induction motor drive. The PWM scheme considered here uses only instantaneous phase reference voltages. The inverter losses are estimated for this drive system with this PWM strategy using an existing thermal model. The motor current is computed using d-q model of an open-end winding induction motor and the simulation study was performed using MATLAB/Simulink.

Keywords: Open-End Winding, Space Vector Modulation, Thermal Evaluation, Junction Temperature.

by K. R. Aravind Britto, R. Vimala, K. Baskaran

Abstract - This paper proposes randomization technique for conducted noise reduction in flyback converter. Flyback converter is the most favorite choice for low power high frequency application among the variety of Switching Power Converters (SPC). These SPC�s are the main sources of conducted noise for neighboring circuits. The SPC should operate in accordance with the Electro-Magnetic Compatibility (EMC) rules so as to facilitate the Electro-Magnetic Interference (EMI) produced by switching the SPC. A unique combination of randomized Pulse Width Modulation (PWM) and Sigma Delta (∑-Δ) PWM technologies has been created in order to overcome the problems associated with the harmonic noise generation. Pseudo-Random Sequence Generator (PRSG) is used to provide automatic dynamic dithering for removing undesired idle tones in the output of the Sigma Delta Modulator (SDM). Field Programmable Gate Array (FPGA) based Randomized ∑-Δ PWM is used to achieve Conducted EMI reduction. Carrier frequency, duty ratio and pulse position are the three randomized parameters used to generate the triggering signals. This scheme is implemented using SPARTAN 3AN FPGA based controller for switching the flyback converter in which the experimentally investigated result shows reduction of conducted EMI in the range of 10 to 20dB�V over random PWM modulators.

Keywords: EMC, EMI, Flyback Converter, FPGA, Randomized Switching Control.

by Mohamed Benbouzid, Brice Beltran, Yassine Amirat, Sylvie Breton

Abstract - This paper deals with the sensorless control of a doubly-fed induction generator based wind turbine. The sensorless control scheme is based on a high-order sliding mode observer to estimate the generator rotational speed. Indeed, high-order sliding mode observers provide theoretically finite time exact state observation and estimation of absolutely continuous unknown inputs. The proposed global control strategy combines an MPPT using a high-order sliding mode speed observer and a high-order sliding mode for the generator control. This strategy presents attractive features such as chattering-free behavior, finite reaching time, robustness and unmodeled dynamics (generator and turbine). Simulations using the wind turbine simulator FAST on a 1.5-MW three-blade wind turbine are carried out for the validation of the proposed sensorless control strategy.

Keywords: Wind Turbine, Doubly-Fed Induction Generator, Sensorless Control, Observer, High-Order Sliding Mode.

by Jani E. Heikkinen, Tatiana A. Minav, Heikki M. Handroos

Abstract - A Hardware-in-the-loop (HIL) setup using multi-body-dynamics-based software by MeVea Oy as a real-time simulator of the mechanical parts was built. Setup demonstrated the possibility to test an electric drive with a hydraulic load which was emulated with another electric drive. This enables one to construct different system identification, parameter estimation, control tuning and state monitoring algorithms based on testing in real-time simulation environment without real hydraulic components. The effects of delays in data transfer inside the HIL on the stability of the system were evaluated.

Keywords: Drives, Pump, Permanent Magnet Machines, Hardware In Loop (HIL), Real-Time Simulation, Emulator, Hydraulics.

by Krzysztof Ludwinek

Abstract - This paper presents a method for determining internal generated voltages induced in a salient pole synchronous generator in the 3-phase armature winding, both with and without a rotor skew in the no-load steady state. The induced phase stator voltages are determined using a linear model in the natural reference frame of the stator and rotor without considering damping circuits during supplying field winding from DC voltage or DC current source. In the circuital model, the stator to field mutual inductances and the field winding self-inductance are determined in the FEMM program. Simulations made in the circuital model and in the program Flux 2D Skew of the waveforms of the induced phase stator voltages during supplying field winding from DC voltage, are verified experimentally in a laboratory stand.

Keywords: No-Load State, Rotor Skew, Salient Pole Synchronous Generator, Self- and Mutual Inductances.

by Kai Sun, Yuchao Shi, Lipei Huang, Yongdong Li

Abstract - In order to estimate the fluctuating rotor speed of sensorless interior permanent magnet synchronous motor (IPMSM) with compressor load accurately, an improved model reference adaptive system (MRAS) method is proposed, in which the motor parameter variations are taken into account. A rotor flux oriented vector control is employed as the basic control strategy for the IPMSM-compressor drive. The sensitivity of motor parameters on estimated rotor speed in MRAS is analyzed, which shows that motor inductances have the greatest influence on the rotor speed estimation. Based on these analysis, a finite element method (FEM)-based model of IPMSM is built by the magnetic field analysis using JMAG software to obtain the stator inductance variations caused by different phase currents and rotor position. The stator inductance variations are used in the reference model of MRAS for accurate speed estimation. In addition, the rotor speed fluctuation caused by the compressor load could be lowered by the load matching motor output torque based on precise rotor position estimation. Simulation and experimental results demonstrate the feasibility and effectiveness of the proposed control method, which shows that the error between the actual rotor speed and the estimated speed is limited within a very low level.

Keywords: Interior Permanent Magnet Synchronous Motor (IPMSM), Compressor, Sensorless Control, Model Reference Adaptive System (MRAS), Finite Element Method (FEM).

by A. Djahbar, A. Bouhani Benziane, B. Mazari

Abstract - Multi-phase converters are required mainly for feeding variable speed multi-phase drive systems. This paper discusses one such solution by using direct ac-ac converter (matrix converter) that can be used to supply a six-phase and three-phase two-connected motor drive system. Matrix converter based two-motor drive system is presented for the first time in this paper. Two stator windings of six-phase and three-phase machines when connected in series while the rotors may be connected to different loads. Appropriate phase transposition is introduced while connecting the series stator winding to obtain decoupled control of the two machines. This paper proposes a scalar modulation scheme with three intervals technique of three-phase to six-phase matrix converter supplying series-connected six-phase and three-phase induction machine drive system. The dynamic decoupling of each machine from the group is obtained using the direct vector control algorithm. Mathematical modeling of the series-connected two-machine showed the possibility of independent control of each machine in the group. The independent control is demonstrated by analyzing the characteristics of the stator current references and speed of each machine under direct vector control scheme. The viability of the proposed control techniques is proved using analytically and simulation approach.

Keywords: Matrix Converter, Induction Machines, Vector Control.

by Vikas Sharma, Shubhi Purwar

Abstract - In this paper, the Chebyshev Neural Network based discrete-time adaptive speed control is investigated for a light weighted all-electric vehicle. The electric vehicle (EV) is driven by DC motor. Firstly, the continuous-time EV system model is transformed by using appropriate state-transformation into the strict feedback form, from which the discrete-time model is derived by the approximation technique. Secondly, the state feedback control is presented via backstepping, applied to a discrete-time EV system. Here, Chebyshev Neural Network (CNN) is used to estimate the unknown nonlinearities and also to avoid the causality contradiction problem which is encountered in the discrete-time backstepping controller design. The unknown nonlinearities arise due to varying mass of passengers. Stability analysis is presented through Lyapunov function, so that system-tracking stability and error convergence can be assured in the closed-loop system. The CNN based control algorithm for the EV system is developed and a new European driving cycle (NEDC) test is performed to test the control performance. Through simulation results the effectiveness of the proposed control schemes are shown.

Keywords: Backstepping Control, Chebyshev Neural Network, Discrete-Time EV System, DC Motor, New European Drive Cycle.

by Sun Kyo Kim, Young Gyu Jin, Yong Tae Yoon

Abstract - Energy storage systems (ESSs) are envisioned to be more popular during the next decades as part of Smart Grid expansion. Especially, as uncertainty with intermittent resource penetration increases the importance of frequency control reserves (FCRs), ESSs has considered as one of potential resources in FCR provision. ESSs can be very efficient resources for frequency control, which activation time is shorter and more flexible than traditional resources. In this work, we provide a valuation method for ESSs in tertiary reserve resources. We will also explain a brief overview of frequency control services and characteristics of ESSs. An indicative study is presented where the proposed method assesses the value of ESSs from provision in the competitive reserve market.

Keywords: Energy Storage System, Ancillary Service, Frequency Control Reserve, Secondary Reserve, Tertiary Reserve; Activation Time.

by Guan Chun, Zhu Qingyong

Abstract - This paper presents a novel approach using multi-label classification to predict the classes of multiple disturbances for a power quality (PQ) event in power systems. A novel multi-label lazy learning approach named KNN-Bayesian is designed to recognize the multiple power quality disturbances based on k-nearest neighbor and Bayesian methods. In terms of the especial multi-label evaluation metrics, the proposed approach is tested on powerline signals that contain single and multiple disturbances. Experimental results indicate that KNN-Bayesian can effectively classify multiple PQ disturbances under different noise conditions.

Keywords: Power Quality, Multi-Label Classification, K-Nearest Neighbor, Bayesian Rule.

by Guesmi Tawfik, Farah Anouar, Hadj Abdallah Hsan, Ouali Abderrazak

Abstract - This paper deals with an adaptive power system stabilizers (PSS) and thyristor controlled series capacitor (TCSC). The proposed approach employs the second version of non-dominated sorting genetic algorithms (NSGA-II) to tune stabilizer parameters on a wide range of loading conditions to create a data base. Two eigenvalue-based objective functions are considered to place the closed-loop system eigenvalues in the D-shape sector. Then, adaptive network based fuzzy inference systems (ANFIS) is used to establish the relationship between these operating points and the corresponding stabilizer parameters. The approach has been examined on a power system test. Eigenvalue analysis and nonlinear simulations show the effectiveness and the capability of the proposed stabilizer to enhance system stability in real-time.

Keywords: Power System Stability, PSS, TCSC, NSGA-II, ANFIS.

by Alma Bijedic, Hidajet Salkic

Abstract - The paper presents the analysis of the results of research implemented on a solar plant in trial period, maximum power of 10 kW, performed in order to assess the impact of a solar plant on distribution network and the quality of electrical energy. The electrical energy quality parameters values are collected in the common coupling point and compared to criteria set in EN 50160 standard. The analysis is made for the case before and after connecting a solar plant, both with minimum and maximum consumption. Also, the same analysis is performed for the case of 50% of installed capacity of solar plant production. The calculation is performed in Power CAD software package.

Keywords: Solar Radiation, Solar Power Plant, Norm EN50160, Electrical Energy Quality.

by Mahdi M. M. El-Arini, Ahmed M. Othman, Taghreed Said

Abstract - Economic Dispatch (ED) problem plays an important role in the operation of the power system. This problem has been tackled and solved by numerous methods. In this paper, Particle Swarm Optimization (PSO) and Genetic Algorithms (GA) are used to solve both convex and non-convex economic dispatch problem considering the linear and non-linear constraints. Using these optimization techniques the simulation results for IEEE 14-bus and 30-bus system are presented and comparison is made between their performances.

Keywords: Economic Dispatch, Genetic Algorithms, Particle Swarm Optimization, Valve Point Effects.

by B. Bahani, M. Ferfra, M. Chraygane, M. Bousseta N. El Ghazal, A. Belhaiba

Abstract - This paper deals with modeling and optimization of a new type of HV power supply with three magnetrons. The design of this new power supply uses a new single-phase high voltage transformer with magnetic leakage flux supplying in the same time three cells. Each cell multiples the voltage, stabilizes the current and supplies in its turn a single magnetron. The π equivalent model of the transformer is developed, taking into account the saturation phenomenon and the stabilization process of each magnetron current. This model is based on the determination of the analytical expressions of its non-linear inductances, that can be determined from the establishment of the B(H) magnetization curve of the used material. This way facilites the optimization strategy of this new HV power supply for the microwave generators with N=3 magnetrons, to envisage the possibility of obtaining gains in section, space, and weight, and to reduce the transformer cost, and, therefore, that of all the installation. This system presents multiple advantages relative to the current device: gains of volume, weight and the cost of the whole power supply. The simulation results, upbraided by Matlab-Simulink code, are in good agreement with those found by experimental measurement for one magnetron, while respecting the conditions recommended by the manufacturer of magnetron: Ipeak<1.2 A, Imean≈ 300 mA.

Keywords: Modeling, Optimization, Magnetron, New Power Suply, Matlab-SIMULINK .

by Rania Gamal Mohamed, Doaa Khalil Ibrahim Hosam Kamal Youssef, Hassan H. Rakha

Abstract - This paper presents a comparative study between different configurations of photovoltaic (PV) systems which are the stand-alone (PV) system, the PV-diesel- battery hybrid system and the grid connected PV system. The main objective is to select the optimal system configuration for supplying the electrical load of an Egyptian isolated village. Design calculations are carried out using three different techniques which are: Heuristic approach, loss of power supply probability indices, and HOMER software analysis. The life cycle cost (LCC) analysis is also conducted to assess the economic viability of designed system. As for determining the optimal PV panels tilt angle, three cases are studied. The paper also shows the effect of electricity price on PV standalone system, and grid extension.

Keywords: Grid Connected System, HOMER, Life Cycle Cost, Stand-Alone System, Optimal Sizing, And Photovoltaic (PV) Systems .

by Ni Ketut Aryani, Mat Syai�in, Adi Soeprijanto, I. Made Yulistya Negara

Abstract - This work exploits Quantum Genetic Algorithm (QGA) to optimize the placement and sizing of Distributed Generators (DGs) in unbalance radial distribution systems. The goal is to minimize losses and at the same time still maintain acceptable voltage profiles. DGs may be placed at any load buses. The proposed method determines which load buses are to have DGs and of what sizes they are respectively. In the method, QGA randomly generates initial population, find network losses and bus voltages of each population member by means of power flow calculations, keep the member with the best configuration as a reference and re-generate the new population for the next iteration based on the current population and based on the reference. Observations were performed based on a modified IEEE Standard 399-1997. All loads in the standard are increased gradually until the point where at least one bus voltage trips below 0.95 pu. Finally, the network with DGs installed as suggested by the proposed method is then compared to this network.

Keywords: Unbalance Radial Distribution System, Power Losses, Voltage Profile, Quantum Genetic Algorithm.

by A. C. Adewole, R. Tzoneva

Abstract - This paper develops a hybrid fault detection and diagnosis method using Discrete Wavelet Transform (DWT) to extract characteristic features from transient waveforms obtained from disturbance recorders in electric power distribution networks. Entropy per unit indices are computed from the DWT decomposition of substation measurements made up of three phase and zero sequence currents, and are used as input to rule-based decision-taking algorithms and multi-layer Artificial Neural Networks (ANNs). Different learning algorithms and architectures were experimented upon to obtain the structure of the ANNs. Comparisons, verification, and analysis made of the results obtained from the application of this method have shown good performance for different fault types, fault locations, fault inception angles, and fault resistances. The proposed method is distinct because of the processing stage done with DWT/wavelet energy entropy per unit formulation, and the use of practical equipment such as the Real-Time Digital Simulator (RTDS) and an Intelligent Electronic Device (IED) configured as a disturbance recorder.

Keywords: Artificial Neural Network, Discrete Wavelet Transform, Distribution Networks, Fault Diagnosis, Signal Processing.

by K. D. Kerrouche, A. Mezouar, L. Boumediene, K. Belgacem

Abstract - This paper proposes the modeling and control of Wind Energy Conversion System (WECS) based on the Double Fed Induction Generator (DFIG). In order to improve the effectiveness of the WECS, two independent control objectives can be stated. Therefore, a control is designed to capture a maximum energy at certain wind speed range and to regulate the stator reactive power, contributing to the compensation of the power factor according to grid requirements. A cascade control structure based on Fuzzy Logic Controller (FLC) has been applied to perform these two main objectives. The control algorithm tracks the maximum power for wind speeds at rated speed of wind turbines and ensures that the power will not go over the rated power for wind speeds over the rated value. Then, the Rotor Side Converter (RSC) is controlled to follow the optimal torque for a given maximum wind power, based on stator flux-oriented vector control. The control algorithm employs fuzzy logic controller to effectively achieve a smooth control of both stator active and reactive powers quantities under fault conditions. Some simulation results are presented to show the effectiveness of the WECS based on DFIG with the proposed control strategy.

Keywords: Wind Turbine, DFIG, MPPT, Flux Orientation, Fuzzy Logic Controller.

by Ismayil C., Sreerama Kumar R., Sindhu T. K.

Abstract - This paper illustrates the automatic generation control (AGC) of an interconnected two-area power system under deregulated environment. Two controllers viz., a Hybrid Fuzzy-PI (HFPI) controller and Genetic Algorithm based PI (GAPI) controller are proposed to improve the dynamic response of the power systems. The conventional two-area AGC system is modified to take into account the effects of bilateral contracts on the dynamics. In the HFPI controller, the parameters of the conventional PI controller are modiﬁed according to the principles of fuzzy logic whereas in GAPI controller, tuning parameter design criteria is reformulated as an optimization problem and the general rules of genetic algorithm (GA) are applied. The convergence and performance of the system are studied for different operating cases and a comparison among conventional integral (CI) controller, HFPI controller and GAPI controller are presented. The simulation results show that the HFPI controller proposed in this paper perform better than the conventional integral and GAPI controllers in terms of settling time and overshoot.

Keywords: Automatic Generation Control, Deregulation, Hybrid Fuzzy controller, Genetic Algorithm, Optimization, Power System Control.

by C. S. Boopathi, Subhransu Sekhar Dash, A. Venkadesan, C. Subramani, G. V. Anilkumar

Abstract - This paper identifies suitable learning algorithm for neural network based Economic Load Dispatch (ELD). ELD is an important problem in real time power system planning and Operation. The conventional methods used for economic load dispatch are iterative techniques & takes longer time for computation. Neural Network (NN) provides an alternate solution for on-line Load Dispatch. The on-line Load Dispatch requires the NN model to be accurate, simple and structurally compact to ensure faster execution time for effective load dispatch. This in turn to a large extent depends on the type of Neural Learning algorithms used to train the Neural Architecture. The feed forward neural architecture is trained off-line using three types of learning algorithms to solve economic load dispatch problem. Their performance is compared in terms of accuracy, computational complexity and structural compactness. The results are extensively validated for IEEE 26 Bus system. The promising results obtained are presented.

Keywords: Learning Algorithms, Feed Forward Neural Architecture, Artificial Neural Network, Economic Load Dispatch.

by N. Chaiyabut, P. Damrongkulkamjorn

Abstract - This paper presents the optimal spinning reserve for wind power uncertainty which calculated by the security-constrained unit commitment with wind power reliability constraint. The wind power uncertainty is considered a normal distribution with seven sections and takes into calculation the wind power reliability index. The wind power reliability index introduced in this paper is the expected energy not supplied for wind power generation which is calculated from the model of wind power uncertainty and the spinning reserve due to wind power uncertainty. In order to effectively solve this mixed-integer nonlinear problem, the proposed algorithm is divided into two parts: the DC security-constrained unit commitment (DC-SCUC) and the AC optimal power flow (ACOPF). Therefore, the optimal results from this algorithm, including the optimal spinning reserve, guarantee that the output powers of committed generators satisfy the AC power flow. This algorithm is applied to the modified IEEE 30 bus with six thermal generators for three case studies. In the case where the loss of load due to wind power uncertainty is not allowed to occur, the sensitivity of spinning reserve for wind power uncertainty to the wind power increment can be found by the algorithm.

Keywords: Optimal Spinning Reserve, Wind Power Uncertainty, Security-Constrained Unit Commitment, Reliability, EENS.

by C. S. Boopathi, Subhransu Sekhar Dash, K. Selvakumar, A. Venkadesan C. Subramani, D. VamsiKrishna

Abstract - In this paper, unit commitment (UC) problem with POZ constraint is proposed. A simple dynamic programming method is used to UC. Various constraints such as power balance, minimum up time & down time, ramp up & down, next hour load and most specifically prohibited operating zone constraint is also considered. The results are extensively validated for IEEE 10 units and 6 units. The results obtained are compared without POZ constraint. The promising results obtained are presented.

Keywords: Unit Commitment (UC), Prohibited Operating zone (POZ), Dynamic Programming (DP).

by Atabak Njafi, İres İskender

Abstract - Transformers are generally designed in order to deliver the required power to the connected loads with minimum losses at nominal fequency. Nowadays, power quality affected by non-linear loads and unbalanced condition and causes deflection from a perfect sinusoidal waveform. This condition leads to additional losses, increase temperature and reduction of the useful life of the transformers. In order to avoid of these difficulties, the nominal capacity of transformer should be reduced. This paper studies the increase of losses and Derating of Distribution transformer, working under Different harmonic load and unbalanced supply voltage conditions. In this paper, the three dimensional finite element method (3-D) is utilized as a Instruments for viewing magnetic flux density on core and windings of distribution transformers. The computer simulation according to the use of the FEM has been improved in Ansoft - Maxwell. In section 2, a 3-D time stepping finite element method (TSFEM) is used in order to modeling the transformer under different harmonic load with (THDI =12% and 23%). The maximum flux density in the transformer windings under sinusoidal and different harmonic load condition has been investigated in this section. In section 3, the effect of harmonic load on transformer load losses Has been examined. The result shows that under harmonic load current, copper loss an eddy current losses significantly increases. In section 4, derating of transformer under harmonic load based on IEEE c57.110 standard and TSFEM by using k-factor and harmonic loss factor (HFL) Has been done. The result shows that IEEE standard is a Conservative method for transformer derating under high harmonic current (THDI ) and TSFEM is a Convenient method for transformer derating in this condition. In section 5, The Performance of the transformer when the supply voltage are balanced compared to the case in which supply voltage is unbalance , the result shows that , the unbalanced voltage Causes increase in the both copper loss and core loss. Afterwards, Based on the data obtained from the TSFEM, a distribution transformer under different unbalanced condition has been derated.

Keywords: Distribution Transformer, Derating, Time Stepping Finite Element (TSFEM), Harmonic Load, FHL.

by S. Vujević, T. Modrić, B. Vukić

Abstract - Computation of the internal impedance per unit length of solid and tubular cylindrical conductors, energized by time-harmonic current, needs to take into account the skin effect. Computation algorithm involves direct numerical manipulations with Bessel functions. These functions are defined by infinite series, which yield unstable and often erroneous results, particularly for large magnitudes of the function argument at high frequencies. Therefore, different approximate formulas are used in order to alleviate this problem. In this paper, exact formulas, with scaled Bessel and Neumann functions and scaled coefficients, for internal impedance of single-layer and two-layer cylindrical conductors are presented. In two numerical examples presented, results for internal impedances per unit length of two-layer conductors computed by exact formulas are compared with results based on Amos subroutines, which are used in program package MATLAB.

Keywords: Internal Impedance, Two-Layer Solid Cylindrical Conductor, Two-Layer Tubular Cylindrical Conductor, Skin Effect, Bessel Functions, Approximate Formulas.

by R. C. Mart�nez-Montejano, R. Castillo Meraz, R. E. Lozoya-Ponce, I. Campos-Cant�n, M. F. Mart�nez-Montejano, R. O. Lozoya Ponce

Abstract - A phase Locked Loop is a control system that generates an alternating signal with a bounded phase respect to an input signal, a feedback loop makes a comparison between these two signals and modifies its output signal frequency. This paper presents a PLL scheme which is aimed to estimate the angular frequency of the input voltage as well as a low pass filter function for cleaning this distorted signal. A prototype based on operational amplifiers has been built and simulations and experimental results are presented to prove its performance.

Keywords: Adaptive Law, Harmonic Oscillator, Lyapunov Approach, PLL, Operational Amplifier.