DSpace Angular ::Πλοήγηση Πολυτεχνείο Κρήτης ανά Ημερομηνία έκδοσης

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Power Optimization of Wind Electric Conversion Systems Integrated into the Utility Grid

(Wind Engineering, 1982) Kalaitzakis K.C; Vachtsevanos, G.J

This paper is concerned with the interconnected operation of WECS with the utility grid.Specifically ,it addresses problems of stability , protection and power flow optimization from the WECS to the power lines. The approach followed contains both a theoritical and a experimental component.A variable-speed constant-frequency wind electric system utilizing a synchronous generator and line-commutated inverter provides experimental verification of computer simulation studies for the dynamic and steady-state performance of the interconnected system. Simulation results indicate that the transient bahavior of the wind electric-power grid system is satisfactory with appropriate devices providing adequate protection to both the wind generator and the utility grid from a variety of fault conditions. A maximum power tracking mechanism has been designed for the continuous matching of the wind-electric generator to the grid impedance characteristics.As a result,maximum electric power is transferred from the wind machine to the power lines.Computer simulation studies of this type of operation indicate the substantial improvement in power transfer that is achieved. The proposed scheme tends to minimize equipment and maintenance costs while maximizing the energy transfer capabilities of the wind-electric conversion system and maintaining a high degree of reliability in overall system performance.

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Design of a Power Conditioning Unit for Nonlinear Source-Load Systems

(Institute of Electrical and Electronics Engineers, 1984) Kalaitzakis K.C; Vachtsevanos, G.J

This paper describes the development, design, and operation of an electronic device which performs dynamic matching between electric energy sources and their respective loads. This matching is achieved by monitoring the output power of the energy source and modulating a control signal which, in turn, regulates appropriately the operation of a source-load interface device. This device is characterized by a simple structure and an ability to continuously match the characteristics of the source to those of the load, for maximum power transfer, even when these characteristics are randomly time-varying. The device may be used with existing energy systems, without significant changes, and operates effectively even when the utility grid substitutes for the load. Its operation is described in terms of a configuration involving renewable energy sources, such as wind electric conversion systems and photovoltaic arrays, as the primary energy generators. The latter are attracting the growing interest of many investigators recently.

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Maximum Power Transfer in Non-Linear Source-Load Systems

(John Wiley and Sons, 1984) Vachtsevanos, G.J; Kalaitzakis K.C

This paper is concerned with the conceptual design and realization of matching networks for the continuous transfer of maximum power from a non-linear source with randomly varying characteristics to a load. Such sources are commonly encountered in the use of photovoltaic arrays or wind energy conversion systems for the production of electric power. Experimental studies as well as computer simulation results verify the validity of the design and point to methods for its practical implementation.

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Correlation Study of Arterial Blood Pressure Level to the Amplitude of the Pressure Pulse Waveform

(Butterworth & Co Ltd, 1984) Vachtsevanos, G.J; Kalaitzakis K.C; Papamarkos, N; Ziakas G.; Economou K.; Gemitzis K.

Automated blood pressure measurements are usually characterized by poor operational reliability and a considerable degree of complexity in performing the measurement. This paper introduces a new technique for the indirect measurement of the systolic and diastolic blood pressure of an individual. The technique is based upon a statistically consistent relationship between the amplitude of the pulsative pressure waveform at the systolic and diastolic points, and the amplitude of pulse signals detected when the artery is fully occluded. Clinical testing and statistical analysis techniques are used to derive appropriate numerical values for these relationships. The proposed procedure thus incorporates an adaptive measurement philosophy achieving minimum observer involvement and consequently high instrument accuracy. Overall measurement errors are maintained well within proposed standards for automated sphygmomanometers.

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Penetration of Wind Electric Conversion Systems into the Utility Grid

(Institute of Electrical and Electronics Engineers, 1985) Vachtsevanos, G.J; Kalaitzakis K.C

This paper is concerned with the development of appropriate models for the interconnected operation of wind generator clusters with an autonomous power system and simulation techniques for the study of the degree of penetration of such wind electric conversion devices when operating in parallel with the utility grid. The quality of the interconnected system performance is specified in terms of operational constraints and the resultant penetration strategy is implemented via a microprocessor-based control scheme. The strategy assures a satisfactory level of system performance while optimizing the available energy transfer from the wind generators to the utility grid.

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Design and development of a new electronic sphygmomanometer

(Kluwer Academic Publishers, 1985) Vachtsevanos, G.J; Kalaitzakis K.C; Papamarkos, N

The paper introduces a new technique for the indirect measurement of the systolic and diastolic blood pressure in humans. The technique is based upon a statistically consistent relationship between the amplitude of the pulsatile pressure waveform at the systolic and diastolic points and the amplitude of pulse signals detected when the artery is fully occluded. An adaptive measurement philosophy has been implemented in the design of an electronic sphygmomanometer which, in addition to a pressure transducer, contains suitable electronic instrumentation for processing and displaying the electronic signals. Verification of overall system accuracy is accomplished with direct comparison with manual auscultatory measurements. Clinical testing of a prototype indicates a satisfactory performance; measurement errors are maintained well within proposed standards for automated sphygmomanometers.

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On the control and stability of grid connected photovoltaic sources

(Institute of Electrical and Electronics Engineers, 1987) Kalaitzakis Kostas; Καλαϊτζακης Κωστας; Vachtsevanos, G.J

A methodology is proposed for the effective integration of photovoltaic (PV) devices into the electric utility distribution network operations. The dispersed PV generator is viewed as an active device used to improve system stability by appropriately modulating the power conditioning unit's output power. Disturbances on the utility system can be damped out by injecting this power into the grid in such a way so that the net effect is a cancellation of undesirable oscillations. The approach is implemented by monitoring the oscillating power and generating control signals which shape accordingly the interface unit's output power. Successful implementation of the scheme relies heavily on the speed and flexibility with which the electronic inverter moves power from the primary source/storage facility to the utility lines. Simulation studies, using the proposed control approach, indicate that application of these policies may result in reduced load following requirements for conventional power generating units, increase the value and acceptability of new energy technologies, and improve power quality and stability of the interconnected system.

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A Methodology for Dynamic Utility Interactive Operation of Dispersed Storage and Generation Devices

(Institute of Electrical and Electronics Engineers, 1987) Vachtsevanos, G.J; Kalaitzakis K.C

This paper is introducing a new methodology for the dynamic integration of dispersed storage and generation devices into the electric utility distribution network operations. The dispersed source is viewed as an active device contributing towards the regulation of real and reactive power flows while improving the stability of the power system. Conceptual means are developed for an effective DSG-utility grid interface. Computer models of appropriate interconnection and control equipment are used in simulation studies to test the effectiveness of control strategies and optimize design parameters. Simulation results indicate that load frequency control and voltage regulation may be effectively accomplished with dispersed generators within a fraction of the time required for conventional regulating units. Appropriate modulation and conditioning of the DSG-output power can assist in damping out undesirable power oscillations. Implementation of the proposed policies may result in reduced load following requirements for conventional power generating units, increase the value and acceptability of new energy technologies, and improve power quality and stability of the interconnected system.

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A Hybrid Photovoltaic Simulator for Utility Interactive Studies

(Institute of Electrical and Electronics Engineers, 1987) Vachtsevanos, G.J; Kalaitzakis K.C

An analog-digital photovoltaic (PV) array simulator is considered. The analog section is designed on the basis of an equivalent solar cell model while the digital section is constructed realizing the mathematical representation of the array. Fast time responses achieved by the analog section make this part suitable for the study of transient phenomena associated with the interconnected operation of PVs and the utility grid. Its digital counterpart is more appropriate for long-term experimental investigations due to its inherent accuracy and reliability. The combined hybrid simulator offers a versatile and flexible piece of apparatus capable of simulating the performance of any PV array under a variety of operating conditions. The device can be constructed with low-cost components in a compact arrangement offering transportability and ease of operation. Experimental results derived from a laboratory constructed prototype match closely the theoretically computed characteristics.

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Σύστημα υποστήριξης αποφάσεων για την αξιολόγηση και διάταξη επενδυτικών προγραμμάτων

(Πολυτεχνείο Κρήτης, 1989) Χαραλαμπίδης Κωνσταντίνος; Skiadas Christos; Σκιαδας Χρηστος

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Development of a microprocessor-based adaptive technique for blood pressure measurements.

(Kluwer Academic Publishers, 1989) Kalaitzakis Kostas; Καλαϊτζακης Κωστας; Papamarkos, N; Vachtsevanos, G.J

The paper introduces a new microprocessor-based adaptive technique for the indirect measurement of the systolic and diastolic pressure in humans. The technique is based upon a statistically consistent relationship between the amplitude of the pulsative pressure waveform at the systolic and diastolic points and the amplitude of pulse signals detected when the artery is fully occluded. An adaptive measurement philosophy has been implemented in the design of an electronic analog-digital sphygmomanometer which, in addition to a pressure transducer, contains suitable electronic instrumentation for processing and displaying the electronic signals. A dedicated microprocessor is used to store statistical relations and control the operation of the device. Verification of overall system accuracy is accomplished via direct comparison with manual auscultatory measurements. Clinical testing of a prototype indicates satisfactory performance; measurement errors are maintained well within proposed standards for automated sphygmomanometers.

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Κατασκευή λογισμικού 5ης γενιάς για την επίλυση προβλημάτων γραμμικού προγραμματισμού

(Πολυτεχνείο Κρήτης, 1990) Αντωνακάκης Παναγιώτης Ι.; Zopounidis Konstantinos; Ζοπουνιδης Κωνσταντινος; Skiadas Christos; Σκιαδας Χρηστος

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Πρόβλεψη της πτώχευσης των επιχειρήσεων. Εφαρμογή του μοντέλου της πολυμεταβλητής διαφοροποίησης (discriminant analysis) σε ελληνικές επιχειρήσεις

(Technical University of Crete, 1990) Λούρος Σ.

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Analysis of one-dimensional solute transport through porous media with spatially variable retardation factor

(1990) Chrysikopoulos Constantinos; Χρυσικοπουλος Κωνσταντινος; Peter K. Kitanidis

A closed-form analytical small-perturbation (or first-order) solution to the one-dimensional advection-dispersion equation with spatially variable retardation factor is derived to investigate the transport of sorbing but otherwise nonreacting solutes in hydraulically homogeneous but geochemically heterogeneous porous formations. The solution is developed for a third- or flux-type inlet boundary condition, which is applicable when considering resident (volume-averaged) solute concentrations, and a semi-infinite porous medium. For mathematical simplicity it is hypothesized that the sorption processes are based on linear equilibrium isotherms and that the local chemical equilibrium assumption is valid. The results from several simulations, compared with predictions based on the classical advection-dispersion equation with constant coefficients, indicate that at early times, spatially variable retardation affects the transport behavior of sorbing solutes. The zeroth moments corresponding to constant and variable retardation are not necessarily equal. The impact of spatially variable retardation increases with increasing Péclet number.

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Design of a Microprocessor-Based Sphygmomanometer.

(Biomedical Instrumentation and Technology, 1990) Kalaitzakis Kostas; Καλαϊτζακης Κωστας; Papamarkos, N; Vachtsevanos, G.J

This paper describes the implementation on a microprocessor of a new method for the indirect measurement and recording of the systolic and diastolic blood pressure in humans. The technique is based on a statistical analysis of the cardiac pulse pressure signal. Polynomial relations are derived between the amplitude of the pulsatile pressure waveforms at the systolic and diastolic points and the amplitude of pulse signals detected when the artery is fully occluded. With the dual objective of automating the measurement procedure and minimizing errors, an electronic analog-digital sphygmomanometer that contains suitable electronic instrumentation was developed. The functions of processing the pressure signal, automating the measurement, and recording the results are performed and controlled by a microprocessor. A laboratory prototype embodying this approach was constructed and its performance and reliability were verified using a series of clinical tests. The test results indicate that the device is accurate within acceptable bounds for automated blood pressure instruments.

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One-dimensional solute transport in porous media with partial well-to-well recirculation: Application to field experiments

(1990) Chrysikopoulos Constantinos; Χρυσικοπουλος Κωνσταντινος; Paul V. Roberts

A solute transport model incorporating well-to-well recirculation was developed to facilitate the interpretation of pilot-scale field experiments conducted for the evaluation of a test zone chosen for in situ restoration studies of contaminated aquifers, where flow was induced by recirculation of the extracted fluid. A semianalytical and an approximate analytical solution were derived to the one-dimensional advection-dispersion equation for a semi-infinite medium under local equilibrium conditions, with a flux-type inlet boundary condition accounting for solute recirculation between the extraction-injection well pair. Solutions were obtained by taking Laplace transforms to the equations with respect to time and space. The semianalytical solution is presented in Laplace domain and requires numerical inversion, while the approximate analytical solution is given in terms of a series of simple nested convolution integrals which are easily determined by numerical integration techniques. The applicability of the well-to-well recirculation model is limited to field situations where the actual flow field is one dimensional or where an induced flow field is obtained such that the streamlines in the neighborhood of the monitoring wells are nearly parallel. However, the model is fully applicable to studies of solute transport through packed columns with recirculation under controlled laboratory conditions.

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A Simulation Model for the Reliable Integration of a 4.5MW Wind Farm into the Power Grid of the Crete Island

(Taylor & Francis, 1990) Kalaitzakis Kostas; Καλαϊτζακης Κωστας; Stavrakakis Georgios; Σταυρακακης Γεωργιος

This paper describes a computer simulation model which has been developed to study the effects of integrating wind energy conversion systems (WECS) into autonomous power systems. The quality of the interconnected system performance is specified in terms of operational constraints. Load-flow and power-system-stability studies were preformed in order to evaluate the penetration impact of a 4·5 MW wind farm on the local grid fo Sitia-Crete. The results show that the above wind energy penetration level will not cause any problem to the existing electric power grid.