Showing papers in "IEEE Transactions on Energy Conversion in 1999"
TL;DR: In this article, the authors describe the development of a general probabilistic model of an autonomous solar-wind energy conversion system (SWECS) composed of several wind turbines (wind farm), several photovoltaic (PV) modules (solar park), and a battery storage feeding a load.
Abstract: This paper describes the development of a general probabilistic model of an autonomous solar-wind energy conversion system (SWECS) composed of several wind turbines (wind farm), several photovoltaic (PV) modules (solar park), and a battery storage feeding a load. The model takes into consideration outages due to the primary energy fluctuations and hardware failure. It allows the simulation of wind farms and solar parks containing either identical or different types of wind turbines and PV modules with the load being fed from either the renewable sources, or the battery storage, or both. A methodology is also presented to determine an upper limit on the size of the battery storage required to satisfy a given load profile taking into consideration the charging/discharging of the batteries.
404 citations
TL;DR: In this article, the active frequency drift method (AFD), also called the frequency bias method, enables islanding detection by forcing the frequency of the voltage in the island to drift up or down.
Abstract: Islanding of photovoltaic (PV) systems can cause a variety of problems and must be prevented. However, if the real and reactive powers of the load and PV system are closely matched, islanding detection by passive methods becomes difficult. Also, most active methods lose effectiveness when there are several PV systems feeding the same island. The active frequency drift method (AFD), also called the frequency bias method, enables islanding detection by forcing the frequency of the voltage in the island to drift up or down. In this paper, AFD is studied analytically, using the describing function analysis technique, and by simulation, using MATLAB. It is shown that AFD has a nondetection zone (NDZ) in which it fails to detect islanding, and that this NDZ includes a range of unity-power-factor loads. Finally, the paper describes a novel method using positive feedback which significantly shrinks the size of the AFD NDZ.
353 citations
TL;DR: In this paper, the authors developed an artificial neural network (ANN) that significantly improves forecasting accuracy comparing to the persistence forecasting model, which is tested at different sites over a year.
Abstract: Wind energy conversion systems (WECS) cannot be dispatched like conventional generators. This can pose problems for power system schedulers and dispatchers, especially if the schedule of wind power availability is not known in advance. However, if the wind speed can be reliably forecasted up to several hours ahead, the generating schedule can efficiently accommodate the wind generation. This paper illustrates a technique for forecasting wind speed and power output up to several hours ahead, based on cross correlation at neighboring sites. The authors develop an artificial neural network (ANN) that significantly improves forecasting accuracy comparing to the persistence forecasting model. The method is tested at different sites over a year.
282 citations
TL;DR: In this paper, a general approach is presented to develop and to interpret the sizing and power density equations for axial flux machines with various topologies, different waveforms of back EMF and current.
Abstract: Based on the concept of the converter fed machine (CFM), an optimal machine design can be considered as the best match of the machine topology, the power electronic converter and the performance specification. To compare power production potential of axial flux machines with various topologies, different waveforms of back EMF and current, general purpose sizing and power density equations for such machines are needed. In this paper, a general approach is presented to develop and to interpret these equations. Sample applications of the sizing and power density equations are the axial flux toroidal permanent magnet utilized to compare the axial flux toroidal permanent magnet (AFTPM) machine and the axial flux two-stator permanent magnet (AF2SPM) machine.
275 citations
TL;DR: A real load test is used to investigate the effects of an unbalanced voltage supply on an induction motor's performance and it is strongly suggested that the related regulations, and a motor's derating factor and temperature rise curves should be based on not only a voltage unbalance factor, but also the magnitude of the positive-sequence voltage.
Abstract: This paper uses a real load test to investigate the effects of an unbalanced voltage supply on an induction motor's performance. Based upon various experiments, including: (1) cases with the same unbalance voltage factor but different unbalanced voltages; (2) cases with only one unbalanced voltage but different degrees of unbalance; and (3) cases with the same positive-sequence voltage but different negative-sequence voltages, the importance of the positive-sequence voltage in the motor's apparent performance and of the negative-sequence voltage in the hidden damage are pointed out. Finally, it is strongly suggested that the related regulations, and a motor's derating factor and temperature rise curves should be based on not only a voltage unbalance factor, but also the magnitude of the positive-sequence voltage.
266 citations
TL;DR: A comprehensive list of books, workshops, conferences, and journal papers related to induction motors faults detection and diagnosis can be found in this article, where the authors provide a detailed survey.
Abstract: This paper provides a comprehensive list of books, workshops, conferences, and journal papers related to induction motors faults detection and diagnosis.
238 citations
TL;DR: In this paper, a two-level power converter is proposed which utilizes two six-transistor inverters and is capable of producing voltages which are identical to those of three-level and four-level converters.
Abstract: A cascaded two-level power converter is proposed which utilizes two six-transistor inverters and is capable of producing voltages which are identical to those of three-level and four-level converters. Since the machine voltages are the same, the converter performance is the same as is verified through laboratory tests. The advantages and disadvantages of the proposed cascaded converter are explored. The proposed converter is simpler to construct and offers more nonredundant switching states per number of active semiconductors than standard multi-level converters.
231 citations
TL;DR: In this paper, the authors describe the design, implementation and testing of a prototype version of a peak power tracking system for small wind turbines in battery charging applications, and show that one configuration consisting of reactive compensation by capacitors and a DC/DC converter is able to optimally load the wind turbine and thus obtain maximum energy capture over the whole range of wind speeds.
Abstract: This paper describes the design, implementation and testing of a prototype version of a peak power tracking system for small wind turbines in battery charging applications. The causes for the poor performance of small wind turbines in battery charging applications are explained and previously proposed configurations to increase the power output of the wind turbines are discussed. Through computer modeling of the steady-state operation the potential performance gain of the proposed system in comparison with existing systems is calculated. It is shown that one configuration consisting of reactive compensation by capacitors and a DC/DC converter is able to optimally load the wind turbine and thus obtain maximum energy capture over the whole range of wind speeds. A proof of concept of the peak power tracking system is provided by building and testing a prototype version. The peak power tracking system is tested in combination with a typical small wind turbine generator on a dynamometer. Steady-state operating curves confirming the performance improvement predicted by calculations are presented.
229 citations
TL;DR: In this article, the authors describe the development of a computer model for simulating the transient operation of a tubular solid oxide fuel cell (SOFC), which includes the electrochemical, thermal, and mass flow elements that affect SOFC electrical output.
Abstract: This paper describes the development of a computer model for simulating the transient operation of a tubular solid oxide fuel cell (SOFC). The model includes the electrochemical, thermal, and mass flow elements that affect SOFC electrical output. The electrochemical and thermal parts of the model were developed and verified separately before they were combined to form the transient model. The results of model verification tests are presented. Transient simulations were conducted with constant reactant flows and constant inlet temperatures. The transient electrical response of the cell to a load change is described.
227 citations
TL;DR: In this article, the capacity factors computed from the Weibull statistical model using cubic mean of wind speed data fairly match the actual capacity factors obtained from Kappadagudda wind power station.
Abstract: Site matching of wind turbine generators is investigated based on the appropriate selection of statistical models and means of wind speed data. The wind speed means are computed using arithmetic mean, root mean square and cubic mean cuberoot. Wind speed frequency distributions are modelled using Weibull and Rayleigh probability density functions. Wind speed data of an existing wind power station, located at Kappadagudda, Karnataka, India, is used for computational purposes. The analytically obtained capacity factors are validated by comparing with the actual capacity factors obtained at Kappadagudda. It is observed that the capacity factors computed from the Weibull statistical model using cubic mean of wind speed data fairly match the actual capacity factors obtained from Kappadagudda wind power station. Various commercially available wind turbine generators are used for site matching studies. The model described in the paper is useful for planning of wind power stations as it can be applied for the accurate assessment of wind power potential at a site.
204 citations
TL;DR: In this paper, the development of an axial-flux permanent-magnet generator for a gearless wind energy system is discussed, which aims to demonstrate the feasibility of integrating wind and photovoltaic energy converters for the generation of electricity and to achieve optimum exploitation of the two energy sources.
Abstract: The paper discuses the development of an axial-flux permanent-magnet generator for a gearless wind energy system which aims to demonstrate the feasibility of integrating wind and photovoltaic energy converters for the generation of electricity and to achieve optimum exploitation of the two energy sources. The merits of an axial-flux generator topology are discussed with reference to the particular requirements of an electrical generator for a direct-coupled wind turbine application. The design, construction and test results of a 5 kW, 200 rev/min permanent-magnet generator, to form a 10 kW pilot power plant with a 5 kW photovoltaic array, are presented.
TL;DR: In this article, a simple PI conventional controller for the exciter loop is carried out by using a first-order model and compared four control algorithms based on conventional and intelligent control theories.
Abstract: This paper discusses the control problem for a class of wind energy conversion systems (WECS). It first develops a detailed model and then compares four control algorithms based on conventional and intelligent control theories. A simple PI conventional controller for the exciter loop is carried out by using a first order model. When the system operating points change, the PI controller fails to provide sufficient damping or acceptable performance. Therefore both fuzzy voltage and fuzzy power regulators are introduced. Also a conventional adaptive pitch controller is proposed to adjust the pitch angle of the rotor blades in order to maximize the energy capture and reduce the mechanical loads. As an alternative to this controller, a neural network controller is also designed. Using the existing nonlinear wind model and the different control algorithms, the dynamic behavior of the controlled (WECS) is simulated. By selecting convenient wind data, the system characteristics such as its tracking performance, its robustness and its ability to recover from large disturbances are studied and discussed.
TL;DR: In this article, an algorithm combining multi-pass dynamic programming (MPDP) with a time-shift technique has been developed for two purposes: economic dispatch of BES; and finding optimal BES power and energy capacities in a power system.
Abstract: Battery energy storage (BES) systems show promise of savings for both the utility and the customer. An algorithm combining multi-pass dynamic programming (MPDP) with a time-shift technique has been developed for two purposes: economic dispatch of BES; and finding optimal BES power and energy capacities in a power system. Due to the daily cyclic nature of the load curve, a 168-hour load curve is decomposed into seven 24-hour subsections for easy management. Each subsection is treated independently. Decomposition can save computation time and computer memory. A time-shift technique provides better starting state values and improves the finding process of the MPDP approach. The Kansas City Power and Light system is used as an example to test this algorithm. The results obtained in this research have been compared with those from EPRI's DYNASTORE with reasonable discrepancy.
TL;DR: In this paper, a nonlinear mathematical model of an internal reforming molten carbonate fuel cell stack is developed for control system applications to fuel cell power plants based on principles of energy and mass component balances and thermochemical properties.
Abstract: A nonlinear mathematical model of an internal reforming molten carbonate fuel cell stack is developed for control system applications to fuel cell power plants. The model is based on principles of energy and mass component balances and thermochemical properties. Physical data for this model is obtained from a 2 MW system design that is a precursor to a demonstration fuel cell power plant running on natural gas at the City of Santa Clara, CA. The model can be used to provide realistic evaluations of the responses to varying load demands on the fuel cell stack and to define transient limitations and control requirements. Simulation results are presented for a transient response to a power plant trip at full load.
TL;DR: In this paper, an appraisal of on-line monitoring techniques to detect airgap eccentricity in three-phase induction motors is presented, and the results verify that the interpretation of the current spectrum proposed in this paper was successful in diagnosing air gap eccentricity problems in large, high voltage, three phase induction motors.
Abstract: An appraisal of on-line monitoring techniques to detect airgap eccentricity in three-phase induction motors is presented. On-line current monitoring is proposed as the most applicable method in the industrial environment and a novel analysis strategy is explained. The practical details of on-site case histories and analysis of the current spectra for each motor are presented. The results verify that the interpretation of the current spectrum proposed in this paper was successful in diagnosing airgap eccentricity problems in large, high voltage, three-phase induction motors.
TL;DR: In this article, a new strategy for controlling voltage and frequency of a self-excited induction generator (SEIG) is presented, which operates in the linear region of the core magnetizing curve, so that efficiency and performance are upgraded.
Abstract: A new strategy for controlling voltage and frequency of a self excited induction generator (SEIG) is presented. The SEIG operates in the linear region of the core magnetizing curve, so that efficiency and performance are upgraded. An external excitation circuit, comprising permanently connected capacitors and electronically switched inductances is used. The external circuit allows to compensate for the generator reactive demand. A detailed analysis is performed, showing some salient aspects related to the connection of the external excitation circuit on the control performance. Asynchronous switching is used, but some important considerations must be taken into account related to the instantaneous phase angle between stator voltage and external inductor current at the switching instant, if good transient response is desired. Sliding mode controllers are proposed, showing good dynamic response and robust behavior upon changes in load and generator parameters. Computer simulations are used to demonstrate the validity of the proposed scheme.
TL;DR: In this paper, two methods are proposed to assess the impact of multiple wind farms on a large electrical power system, based on the Monte Carlo simulation of the wind speed occurring in each wind farm, taking into account measurements of mean values and correlations.
Abstract: Two methods are proposed to assess the impact of multiple wind farms on a large electrical power system. The methods are based on the Monte Carlo simulation of the wind speed occurring in each wind farm, taking into account measurements of mean values and correlations. The first method is based on the wind speed distribution, assumed to be Rayleigh, and on the correlation matrix, obtained from previously known simultaneous wind speed series. The second method is based on the application of the simulation to the wind speed series, taking into account the previously obtained conditional probabilities of the wind speeds as functions of other wind speeds in wind farms.
TL;DR: In this article, the authors used a real load test to investigate the effects of each order of harmonic from 2 to 13 under various voltage distortion factors (VDF) on the performance of a three-phase induction motor.
Abstract: This paper uses a real load test to investigate the effects of each order of harmonic from 2 to 13 under various voltage distortion factors (VDF) on the performance of a three-phase induction motor. The investigation includes input current, power factor, efficiency, temperature rise and their impacts on the consumers and utility companies. Since the life span of the motors is dramatically affected by the temperature rise, a new derating factor is proposed in this paper. Besides, the impacts of harmonics on electricity energy, consumers and the life span of a motor are also discussed, respectively. Finally, it is strongly suggested that even order harmonics and harmonics having an order below 5 should be considered in related regulations of harmonics control and limits.
TL;DR: In this paper, a number of novel models where state-space variables are selected in various different ways are presented, and the new models, presented in the paper, yield the same accuracy as the existing models that are based on single saturation factor approach.
Abstract: Presently available transient d-q axis models of saturated salient pole synchronous machines are derived by selecting either all the winding currents or all the winding flux linkages as state-space variables. This paper presents a number of novel models where state-space variables are selected in various different ways. The idea originates in the 'generalised flux' and 'generalised inductance' concept, which has recently been successfully developed and applied in the derivation of numerous models of saturated smooth air-gap AC machines. The concept is here extended to salient pole synchronous machines. Saturation itself is accounted for by means of a single saturation factor approach and conversion of an anisotropic to an isotropic machine is executed in the usual way. The new models, presented in the paper, yield the same accuracy as the existing models that are based on single saturation factor approach. This statement is confirmed by a simulation study and a sample of simulation results is included. However, a number of new models are considerably simpler than the existing ones, and thus more convenient for application in simulations.
TL;DR: In this paper, the current waveforms for optimal excitation of surface-mounted permanent magnet synchronous machines are presented, involving varying degrees of minimization of RMS current and torque ripple.
Abstract: The current waveforms for optimal excitation of surface-mounted permanent-magnet synchronous machines are set forth. Four different modes are considered, involving varying degrees of minimization of RMS current and torque ripple. The optimized waveforms are markedly different than the traditional sinusoidal or rectangular excitation schemes. Inclusion of cogging torque and arbitrary degree of torque ripple minimization generalize this work over that of previous authors. An experimental drive and a detailed computer simulation verify the proposed control schemes.
TL;DR: In this paper, a combined RBF and backpropagation network is used to predict the effects of passing clouds on a utility-interactive photovoltaic (PV) system with battery storage.
Abstract: In this paper, a combined radial-basis-functions (RBF) and backpropagation network is used to predict the effects of passing clouds on a utility-interactive photovoltaic (PV) system with battery storage. Using the irradiance as input signal, the network models the effects of random cloud movement on the electrical variables of the maximum power point tracker (MPPT) and the variables of the utility-linked inverter over a short period of time. During short time intervals, the irradiance is considered as the only varying input parameter affecting the electrical variables of the system. The advantages of artificial neural network (ANN) simulation over standard linear models is that it does not require the knowledge of internal system parameters, involves less computational effort, and offers a compact solution for multiple-variable problems. The model can easily integrated into a typical utility system and resulting system behavior can be determined. The viability of the battery-supported PV power system as a dispatchable unit is also investigated. The simulated results are compared with the experimental results captured during cloudy days. This model can be a useful tool in solar energy engineering design and in PV-integrated utility operation.
TL;DR: In this paper, simplified electrical models of a fuel cell generation system for system control are proposed, and using the electrical models, system performance of a FCL generation system in which power is boosted by step-up choppers is analyzed.
Abstract: The fuel cell generation system consists of a stack, a reformer, and converters. The stack generates DC power by electrochemical reaction. For system design and analysis, it is necessary to obtain electrical models. Simplified electrical models of a fuel cell generation system for system control are proposed. Then using the electrical models, system performance of a fuel cell generation system in which power is boosted by step-up choppers is analyzed. A fuzzy controller is designed for improved system performance. Simulation and experimental results confirmed the high performance capability of the designed system.
TL;DR: In this paper, alternative configurations of the electrical part of a variable speed wind turbine are considered, using a squirrel cage induction generator and voltage or current source converters, as well as a double output induction generator with a rotor converter cascade.
Abstract: It is recognized that the most important advantage of the variable speed wind turbines (VS WTs) over the conventional constant speed (CS) machines are the improved dynamic characteristics, resulting in the reduction of the drive train mechanical stresses and output power fluctuations. In this paper alternative configurations of the electrical part of a VS WT are considered, using a squirrel cage induction generator and voltage or current source converters, as well as a double output induction generator with a rotor converter cascade. The WT operation is simulated for typical wind speed time series and the examined schemes are comparatively assessed. It is shown that, using suitable converters and controls, a great reduction of the mechanical stresses and output power fluctuations can be achieved, compared to the CS mode of operation of the WT.
TL;DR: In this paper, a time-stepping coupled finite element-state space (FESSA) model was used to diagnose and characterize the effects of rotor (cage) broken bars and connectors in squirrel-cage induction motors.
Abstract: In this paper, a comprehensive presentation of a time-stepping coupled finite element-state space modeling approach for diagnosis and characterization of effects of rotor (cage) broken bars and connectors in squirrel-cage induction motors is given. The model is used to compute/predict the characteristic frequency components which are indicative of rotor bar and connector breakages in the armature current waveforms and developed torque profiles, respectively. It was found that failures due to rotor connector breakages tend to degrade the developed torque of the machine considerably more than failures due to rotor bar breakages. Also, the effects and implications of rotor breakages on core loss distributions are quantified and described. Furthermore, this model has great potential in future applications to generate databases for use in overall noninvasive diagnostics of faults or troubleshooting in relation to quality assessments of the state of the motor and the overall drive.
TL;DR: In this paper, the authors presented dynamic performances of an isolated self-excited induction generator under different power-factor loading conditions, including sudden connection and disconnection of various power factor loads.
Abstract: This paper presents dynamic performances of an isolated self-excited induction generator (SEIG) under different power-factor loading conditions. Both simulated results based on computer simulations and experimental tests based on a laboratory 1.1 kW induction motor driven by a DC motor are clearly compared. Characteristics of both terminal voltage and magnetizing reactance of the studied SEIG subject to sudden connection and disconnection of various power-factor loads are investigated.
TL;DR: In this article, a general probabilistic model of an autonomous wind energy conversion system (WECS) composed of several wind turbines (wind farm) connected to a load and a battery storage is presented.
Abstract: This paper describes the development of a general probabilistic model of an autonomous wind energy conversion system (WECS) composed of several wind turbines (wind farm) connected to a load and a battery storage. The proposed technique allows the simulation of wind farms containing identical or different wind turbines types and considers a bidirectional flow of power in and out of the battery. The model is based upon a simple procedure to estimate the joint probability distribution function of the total available wind power and that of the turbines operating modes due to hardware failure. A methodology is also developed to use the proposed model to determine an upper limit on the size of the battery storage required for a given number of turbines to satisfy the load with a certain expected energy not supplied (EENS). The model can also be used to evaluate the energy purchased from or injected to the grid in the case of grid-connected systems.
TL;DR: In this paper, a method of calculation of the inductances of an induction machine is proposed, based on the winding function approach, which allows for all harmonics of the MMF to be taken into account.
Abstract: A method of calculation of the inductances of an induction machine is proposed. The skewing of the rotor bars of the machine and the resultant linear rise of MMF across the slot, together with the nonsinusoidal distribution of the stator winding are taken into account. The method is based on the winding function approach, which allows for all harmonics of the MMF to be taken into account. The results obtained by the proposed method have been compared with those obtained by a conventional manner, which take into account the above phenomena by means of scale factors applied to the harmonics. A very good agreement between them is demonstrated.
TL;DR: In this paper, the frequency signature of some asymmetrical motor faults are well identified using the fast Fourier transform (FFT), leading to a better interpretation of the motor current spectra.
Abstract: Preventive maintenance of electric drive systems with induction motors involves monitoring of their operation for detection of abnormal electrical and mechanical conditions that indicate, or may lead to, a failure of the system Intensive research effort has been for sometime focused on the motor current signature analysis This technique utilizes the results of spectral analysis of the stator current Reliable interpretation of the spectra is difficult, since distortions of the current waveform caused by the abnormalities in the drive system are usually minute In the present investigation, the frequency signature of some asymmetrical motor faults are well identified using the fast Fourier transform (FFT), leading to a better interpretation of the motor current spectra Laboratory experiments indicate that the motor current signature FFT-based analysis, with the proposed approach, is still a reliable tool for induction motor asymmetrical faults detection
TL;DR: In this article, a three level control system for a variable speed wind energy conversion scheme (VSWECS) supplying a weak AC power system is presented, where the objective of the control strategy is to maximize energy capture and simultaneously to support the voltage of the bus where the VSW ECS is connected.
Abstract: A three level control system for a variable speed wind energy conversion scheme (VSWECS) supplying a weak AC power system is presented. The objective of the control strategy is to maximize energy capture and simultaneously to support the voltage of the bus where the VSWECS is connected. Using an insulated gate bipolar transistor (IGBT) inverter, both control of active and reactive power supplied to the grid and reduction of harmonic distortion can be achieved. The response of the proposed scheme has been tested and evaluated in a test system using a developed computer program simulating in detail the system operation.
TL;DR: In this paper, an analytical model is introduced to describe switched reluctance machines (SRM), where the nonlinear function of flux linkage depending on the winding current and rotor position is considered to be composed by three components: aligned and unaligned flux linkages, and an angular function, where the two flux linkage are only determined by phase current, and the angular function depends only on the rotor position.
Abstract: An analytical model is introduced to describe switched reluctance machines (SRM). The nonlinear function of flux linkage depending on the winding current and rotor position is considered to be composed by three components: aligned and unaligned flux linkages, and an angular function, where the two flux linkages are only determined by phase current, and the angular function depends only on the rotor position. Examples of the application of this model in simulating the phase current, calculating the instantaneous torque and estimating the rotor position are presented. The results are compared with measurements.