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In this paper the performance of a neutral point clamped (NPC) three-level voltage source inverter is investigated. The inverter feeds an electric drive which is controlled by field oriented control (FOC). The improvement of the control scheme is achieved using Mamdani’s fuzzy logic controller for both rotor speed and inverter current. The main issue of this work is neutral point (NP) balancing for a three-level NPC inverter combined with a new Space Vector Pulse Width Modulation (SVPWM) algorithm. The performance of the electric system is verified via Matlab/Simulink © . The operation of the electric system is investigated in steady state and transient responses in both cases whether using or not the NP balancing algorithm. Finally, the ability of the proposed switching pattern generation algorithm for NP balancing is established.

In this paper the control scheme of a three-level three-phase inverter for grid integration of distributed generation (DG) units is investigated. Furthermore, the inverter compensates the reactive power of the load. The presented control scheme is based on instantaneous power theory in order to generate the appropriate inverter reference currents. In the outer DC bus voltage control loop and in the inner current control loop, the fuzzy logic (FL) control and the Proportional-Resonant (PR) control is applied respectively. The switching pattern generation is achieved using Space Vector Pulse Width Modulation technique. The steady state and dynamic response of the proposed electric power system has been evaluated under various operating conditions. Simulation results verify the effectiveness of the proposed control system.

This paper presents a novel control scheme for a distribution static compensator (DSTATCOM) aimed at mitigating voltage sag at the Point of Common Coupling (PCC) or eliminating current harmonics of the load. The proposed control scheme uses the d – q reference frame for the decoupling of active and reactive current components. Furthermore a selective algorithm is integrated to the control scheme in order to choose between the voltage support mode or the current harmonics elimination mode. Finally, the DSTATCOM is based on a three phase three level flying capacitor topology. The proposed control scheme outperforms the conventional one due to the ability of the suggested control system to combine multiple functions, allowing DSTATCOM to respond in various power quality issues. The effectiveness of the proposed control scheme to enhance the multifunction ability of the DSTATCOM is verified via Matlab/Simulink© in both steady state and transient response.

In this paper a grid connected photovoltaic (PV) system is presented. The grid integration of the PV system is carried out via a three phase three level neutral point clamped (NPC) inverter. To control the inverter a modified version of voltage oriented control (VOC) method and the space vector pulse width modulation (SVPWM) technique have been applied. With the proposed modification the PV system operates as a shunt active power filter (SAPF), a reactive power compensator, and a load’s current balancer simultaneously. In this way the PV system operates more efficiently compared to the conventional PV systems and offers ancillary services to electric power system. The effectiveness of the proposed control scheme is established through simulation results with Matlab/Simulink in steady state and transient response of the electric power distribution system.

In this article, the control technique of an AC variable-speed drive is presented using a modified direct torque control method. The AC motor of the drive is an induction motor, and the modified scheme of the direct torque control uses the space vector modulation technique to create the switching pattern of the inverter. The steady state of operation is investigated in depth as well as the dynamic performance of the system. The results of this modified control scheme are compared to the corresponding results of the classical direct torque control method, leading to the conclusion that the modified scheme is more robust and smoother in comparison to the classical one.

In this paper is analyzed and simulated on the computer the operation of a three level H-bridge converter as well as its parallel operations. Based on the simulation results the operating behavior between a) a three level H-bridge neutral point clamped converter, b) a three level H-bridge neutral point clamped converter with parallel elements and c) two three level H-bridge neutral point clamped converters parallel connected is being compared. From the simulation results is obvious that in the first two cases the ripples, the distortion in primary and secondary winding currents, and the power factor are quite satisfactory and almost identical to each other. In the third case as compared with the first two, we observe that current harmonics with higher amplitude appear in the primary winding of the transformer.

In this paper, a photovoltaic (PV) system, with maximum power point tracking (MPPT), connected to a three phase grid is presented. The connection of photovoltaic system on the grid takes place in one stage using voltage source inverter (VSI). For a better utilization of the photovoltaic system, the control strategy applied is based on p–q theory. According to this strategy during sunlight the system sends active power to the grid and at the same time compensates the reactive power of the load. In case there is no sunlight (during the night for instance), the inverter only compensates the reactive power of the load. In this paper the use of p–q theory to supply the grid with active power and compensate the reactive power of the load is investigated. The advantage of this control strategy is that the photovoltaic system is operated the whole day. Furthermore, the p–q theory uses simple algebraic calculations without demanding the use of PLL to synchronize the inverter with the grid.

In this paper, a photovoltaic (P/V) system, with maximum power point tracking (MPPT), connected to a three phase grid is presented. The connection of photovoltaic system on the grid takes place in one stage using voltage source inverter (VSI). For a better utilization of the photovoltaic system, the control strategy applied is based on p-q theory. According to this strategy during sunlight the system sends active power to the grid and at the same time compensates the reactive power of the load. In case there is no sunlight (during the night for instance), the inverter only compensates the reactive power of the load. The advantage of this strategy is the operation of the photovoltaic system the whole day. In this paper the p-q theory is proposed, which does not demand the use of PLL and introduces simple algebraic calculations.

This paper investigates the control scheme of a multilevel cascade single phase converter -consisted of two cell cascaded full-bridge converters-, where the modified p-q theory with ramp comparison has been applied for the proper single-phase interconnection of a 5 kW photovoltaic park with the grid. With the proposed method the converter is able to generate a five level ac-voltage, to utilize maximum power point tracking operation and achieve both active power control of the grid connected photovoltaic park system along with reactive power control for the correction of the reactive power of the load. The switching pattern of the topology -where IGBTs can be used as switches- is realized with the use of the ramp comparison method and two PI controllers. In addition, this pattern has been implemented taking into consideration the two-cell cascaded full-bridge multilevel converter of the topology. On the other hand, the PI controllers have been selected based on the requirements of the system, while the control strategy was built taking into account the maximum power point tracking. Finally, for the dimensioning of the converter suitable waveforms are presented - based on the derivation of appropriate mathematical equations.

In dieser Arbeit wird die Untersuchung des Betriebsverhaltens eines Synchronmotors sowohl beim Anlauf als auch im stationaren Zustand durchgefuhrt. Dieser wird durch einen Stromrichter bestehend aus einer Thyristogleichrichterbrucke, einem Zwischenkreis mit grosser Induktivitat und einem Stromwechselrichter mit einer einfachen Zwangsloscheinrichtung gespeist. Zunachst wird ein mathematisches Modell fur das ganze System aufgestellt und das Betriebsverhalten durch Simulation mit der Methode der veranderlichen Topologie behandelt. Besonders wird auf das Modell zur Berechnung der Spannungs-belastung der Halbleiterelemente und auf die Simulationslogik geachtet. Aus den Simulationsergebnissen ergeben sich Spannungsraumzeigerdiagramme und Zeitfunktionen der elektrischen Grossen, die zur Erklarung des Systemverhaltens dienen.

Partial discharges take place, among others, inside voids enclosed in insulations. The partial discharge behaviour of epoxy resin samples is studied with the aid of a recently developed streamer partial discharge model (Pedersen's model) and its relation to the modem Pulse Height Analysis and Phase Resolved Measurement techniques is investigated. Furthermore, some relations based on Pedersen's model are deduced. The advantages and disadvantages of the aforementioned model are analysed. Scanning electron microscope photographs show the nature of the deterioration of the void walls due to the partial discharges. The question whether initiation of the treeing phenomenon leading to breakdown from an enclosed spherical void is possible, is also investigated.

In this paper two different conversion systems of wind energy into electricity are investigated. In the first system studied the wind turbine is connected to a three phase diode rectifier. At the output of the non-controlled rectifier a DC/DC boost converter is connected. Controlling the switching pattern for the DC/DC converter the maximum power point tracking (MPPT) is achieved. In the second system the wind turbine is connected to a three-level rectifier consisting of IGBTs switches. The three-level rectifier is triggered with the space vector modulation method (SVPWM). The maximum power is achieved through and algorithm which modifies the switching pattern. On the grid side a three level inverter is connected. The controllers applied to both systems are fuzzy controllers. For the study of these two systems Matlab/ Simulink was used in steady state and transient response.

In this paper the impact of photovoltaic (PV) system connected to the medium voltage distribution grid using three-phase three-level inverter and step-up transformer will be investigated. The PV inverter is capable to control active and reactive power independently. The impact to the grid voltage profile of both active and reactive power regulation of the PV system will be examined. In order to simplify the control scheme and improve the performance of the converters the p-q theory has been chosen due to the ability to compensate the reactive power of the load. The proposed electric power system was investigated via simulation results during steady state and transient response. The analysis will be focused on the grid voltage profile improvement regarding of the of the inverter's active and reactive power injection.

© 2012 Tsengenes and Adamidis, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Performance Evaluation of PI and Fuzzy Controlled Power Electronic Inverters for Power Quality Improvement

This paper discusses a new control scheme of a three-phase grid-connected wind energy conversion system (WECS). The control uses maximum power point tracker (MPPT) for optimal use of the wind energy. The wind turbine generator (WTG) consists of a variable speed wind turbine and the MPPT is implemented via a dc-dc boost converter. A three-level voltage source inverter (VSI) is used as interface with the ac power grid. The switching pattern generation for the VSI is achieved using SVM. Mamdani's FLC is applied in DC bus voltage control and current control. Validation of models and control schemes is performed by using the MATLAB/Simulink.

In this paper a new Space Vector Pulse Width Modulation (SVPWM) algorithm is proposed for multilevel voltage source inverters. The proposed algorithm directly identifies the two level subhexagon in which the reference voltage vector lies. The switching sequence corresponding to a two level inverter is determined. The proposed method can be extended to any m-level inverter and a generalized algorithm is proposed. Based on the mathematical model presented, a “five level inverter-induction motor” system is investigated in steady state and transient responses. The induction motor is controlled using Field Oriented Control (F.O.C.). The improvement of the control scheme is achieved using Mamdani's fuzzy logic controller for both rotor speed and inverter current. The electric drive system is simulated in case of a five level inverter and the results presented verify the good performance of the proposed SVPWM algorithm.

In this paper an Electric Drive system is presented. The system consists of a Five Level Neutral Point Clamped inverter and an electrical drive. The electrical drive is controlled by the Five Level NPC inverter using sensorless Field Oriented Control (F.O.C). The switching pattern applied to the inverter is Space Vector Pulse Width Modulation (SVPWM). The reference frame of the pulse algorithm applied is g-h coordinate system. The electric drive system is investigated in steady and transient state and simulation results are presented.

In this paper the performance of a neutral point clamped three level voltage source inverter is investigated. The inverter feeds an electrical drive which is controlled by field oriented control (F.O.C). The modulation technique used is Space Vector Pulse Width Modulation (SVPWM). Specifically, a new approach is proposed for SVPWM a three level inverter which can be extended to multilevel inverters. The neutral point balancing of a three level inverter is controlled and a possible expansion for multilevel inverters is investigated. The proposed SVPWM algorithm is simulated using Matlab/Simulink software and the performance of the ‘inverter-electrical drive’ system is investigated for steady and transient state.

This paper presents a shunt active filter which eliminates current harmonics and compensates the reactive power of the load. The majority of shunt active power filter control methods which have been presented require a complex and sophisticated mathematical model, especially for the tuning of the PI controllers. To simplify and improve the shunt active filter control system, fuzzy logic theory is applied. In this paper, by the way of controlling the capacitor voltage of the dc bus a fuzzy logic algorithm is implemented. One such algorithm is also implemented to control the output current of the inverter. The performance of the capacitor voltage fuzzy logic control system was investigated, and the improvement of the dynamic response of the fuzzy controller compared to the conventional PI controller became evident. For the reduction of the grid current THDi index, a fuzzy logic hysteresis current controller is investigated. In order to improve the performance of shunt active filter under non ideal grid voltages, a modified version of the p-q theory is proposed. The simulation results are presented and discussed showing the advantages of fuzzy logic theory in the dc bus voltage control and current control.

In this paper the connection in single stage single phase of a photovoltaic system (PV) to the electric power system is presented. A modified version of p-q theory is proposed in order to implement this strategy in a single phase inverter. To extract maximum power from the PV array an algorithm Maximum Power Point Tracking (MPPT) is used. For the switching pattern generation the hysteresis controller is used. The behavior of both the permanent and the transient state, which includes changing the radiation and the load is analyzed using a computer.

In this paper a new Space Vector Pulse Width Modulation method is proposed for multilevel voltage source inverters. This method is based on Space Vector Modulation and is compared to other similar methods. According to the method presented the multiplicity of the switching states of a multilevel inverter is also considered. This means that the new pulse method takes into account the multiplicity of the switching states which means a symmetrical distress of the inverter's elements while the number of transitions in a switching time is 7. This method can be applied in a multilevel inverter who feeds a high power three-phase passive load or a high power three-phase motor drive. The new method was used in order to pulse a five-level inverter, which feeds a high power passive load using Matlab-Simulink, and the results are shown in this paper.

This paper investigates the operation of a Distributed Generation (DG) unit which is connected via a three-phase voltage-source inverter to the distribution grid. The current control method with Space Vector pulse width modulation (SVPWM) is examined and compared to other current control methods. The strategy, which has been developed to control the reactive power and have a better utilization of DG unit, uses the p-q theory to achieve uninterrupted compensation of the reactive power of the system. A PI controller is used to control the active power and regulate the voltage of the capacitor. Finally, the proposed electrical power system is simulated on the computer and the steady-state and dynamic response of the system is evaluated.

In this paper two methods of speed estimation of a three phase induction motor system are analyzed. The method of direct torque control with no sensors was used to control the motor system. The mathematical models of the above mentioned methods are analyzed for every case. Following the estimation method analysis of open and closed loop the observation method of closed loop is expanded with the use of deterministic sets. The simulation on the computer of the whole motor system is carried out based on the mathematic models with Matlab/Simulink software. The results of the simulation bring forward the advantages of the expanded method in nonlinear systems.

In this paper the problems created by non linear load as well as the solutions having been applied so far are briefly reviewed. A new simple and effective reference current generation method of a shunt active power filter (SAPF) is proposed. This method aims at eliminating harmonics from the mains currents. It also compensates the reactive power which is created from the linear and non linear loads. The new method is applied both in the case that mains voltages are ideal and in the case that mains voltages are non ideal. The new method is compared to the conventional p-q theory and the results are analyzed and commented on. From the analysis and the commentary the advantages of the new technique are concluded.

This paper initially refers to the devices used to compensate upper harmonics. Subsequently, it examines the principal methods for controlling an active filter which is connected to the electricity grid in order to compensate reactive power and to eliminate the electricity harmonics. The mathematic model following the p-q theory is analyzed for the creation of a reference current, aiming at the elimination of the upper harmonics and at reactive power compensation. An improved control method is presented regarding current control through the use of a Hysteresis Band with an altering tolerance band. In this way, current control is adjusted onto the load current.

Manche Antriebsprobleme werden am besten durch den Einsatz eines Synchronmotors gelost, der uber einen Spannungswechselrichter gespeist wird. Das bekannte Anlaufproblem macht zunachst Schwierigkeiten, die mittels geeigneter methoden uberwunden werden konnen. Bis die Drehzahl einen kritischen Wert erreicht hat, kann eine zwangskommutierte Anlaufschaltung in Funktion gesetzt werden, die den Betrieb des Wechselrichters und somit das Hochlaufen der Maschine ermoglicht. Um eine ausfuhrliche Untersuchung beliebiger Betriebszustande auszufuhren, ist die digitale Simulation unentbehrlich. In dieser Arbeit wird eine Simulationsmethode behandelt, die auf der Erfassung der veranderlichen Netzwerktopologie beruht und durch Flexibilitat, Genauigkeit und geringen Rechenaufwand charakterisiert ist.

This paper presents a new method for MPPT in photovoltaic systems based on "Hill Climbing" algorithm. According to the proposed method cube of slope of power vs. voltage (P-V) curve is calculated in each calculation step. Through this calculation both the direction of photovoltaic's (PV) voltage change and the step size of this change are resulted without making any slope or sign control. Moreover, a way to apply this method in PV systems which are consisted of Ns in series and Np in parallel connection PV arrays with high installed nominal power is proposed. A DC to DC buck-boost converter is used in connection with PV arrays for achieving operation in maximum power point and keeping output voltage constant. The whole system is simulated using Matlab/Simulink and simulation results are presented and analyzed.

This paper introduces the control of a squirrel cage induction motor which is fed from an inverter. The inverter is driven using the method SVPWM. The central processing unit during the development must be very fast in order to give the pulses in appropriate time into the gates of power semiconductors, compatible in EMI, cheap, with big flash memory. The consumer power electronics, which are used for motor drive, must be reliable and cheap. The central processing unit which is selected in consumer power electronics is cheap with small flash memory in order to reduce the cost of implementation. In this paper is tried to be done a code for a DSP, which can be contained in very small flash memory of a DSP in order to drive an induction motor with very good performance. The new method which is presented in this paper is compared with the classical SVPWM with look up tables. The practical results of these methods are represented and compared.

In this paper, solar radiation simulation using clearness index kt, hour of day omegas and day of year n is studied. Daily distributions of solar radiation are presented for various clearness indexes kt. Additionally, taking into account the studies about solar radiation, a photovoltaic array system and a DC/DC buck - boost converter are studied. Simulation of the whole system is presented focusing on DC/DC converterpsilas control strategy so that the system operates in maximum power point (MPP) and converterpsilas output voltage remains constant. Incremental conductance algorithm is used for maximum power point tracker (MPPT) implementation. A simplest method for controlling duty cycle D and photovoltaic arraypsilas voltage by using a new variable (d = D/1-D) is proposed. Simulation results are shown and analyzed. For the implementation of this system, the dsPIC30F2010 Microchippsilas microcontroller was programmed to provide pulses to the semiconductor element of the DC-DC converter in order to track the Maximum Power Point (MPP) of the photovoltaic array.

Der Spannungsrichtermotor zeichnet sich durch gewisse wichtige Vorteile aus, wie hohe Drehzahl und fast sinusformige Standerstrome, und bietet eine gute Losung fur manche Antriebsprobleme. Es ist naturlich das bekannte Anlaufproblem zu losen. Durch eine zusatzliche Schaltung kann der Wechselrichter solange zwangskommutiert betrieben werden, bis die Drehzahl einen geeigneten Wert erreicht hat und die Standerspannungen zur Lastfuhrung ausreichen. In dieser Arbeit wird eine derartige Kommutierungsschaltung beschrieben und die Funktionsweise des gesamten Systems bestehend aus Gleichspannungsquelle-Anlaufschaltung-Wechselrichter und Synchronmaschine erlautert. Zur ausfuhrlichen Untersuchung dieses Systems dient eine digitale Simulation, die Gegenstand einer weiteren Arbeit sein wird.

This paper presents a method of controlling the Torque of an asynchronous motor that is fed by a PWM-Inverter. The control method that is used is a modified direct torque control (DTC) scheme with constant switching frequency, using space vector modulation (SVM). In this scheme a closed loop of torque control is applied. Small and rapid changes on motor's torque are achieved by applying an appropriate voltage vector, which is responsible for the jerky movement of the stator's flux vector. The voltage vector can be of any magnitude and angle and is produced using Space Vector Pulse Width Modulation. The modified method of DTC using SVM improves the electrical magnitudes of asynchronous machine, such as reduction of current and flux ripple and also fading out the edges of torque ripple which are occurred during the sector changes of stator flux vector. Another point of this paper is the presentation of speed control in dynamic situations. The production of the reference torque value is achieved by using a fuzzy logic speed controller. The response of the fuzzy PI is compared to the response of a classic PI speed controller. Results, shows that the fuzzy PI speed controller has a better response in a wide range area of motor speed. Combining the characteristics of DTC-SVM scheme and the fuzzy logic speed controller we can have a high performance vector controlled asynchronous motor drive.

Instrumentelle Weiterentwicklung und Erfahrungszuwachs in der laparoskopischen Operationstechnik haben fur die Magenchirurgie neue Akzente gesetzt. Derzeit werden erprobt: Kardio-Myotomie, refluxverhutende Operationen, Pyloro-Myotomie, verschiedene Vagotomieverfahren, inkl. Ubernahung perforierter Ulcera u. als Palliativeingriffe Gastrostomie u. Gastro-Jejunostomie. Nur wenige Literaturberichte uber kleine Serien ohne Langzeitverlaufe konnen die Wertigkeit dieser Verfahren nicht belegen, wenngleich die ersten Ergebnisse positiv zu sein scheinen. Hinsichtlich Indikation und Methodenwahl sollten zu Gunsten der laparoskopischen Durchfuhrbarkeit keine Kompromisse gemacht werden. Die eigenen, sehr guten Erfahrungen beschranken sich vorwiegend auf Ulkusubernahungen und Gastro-Jejunostomien.

Die Komplikationen bei 250 laparoskopischen Cholezystektomien wurden in 3 Kategorien eingeteilt: I. Intraoperativ beherrschbare Komphkationen (23,6%), welche fur die Komphkationsrate nicht relevant sind. IL Komplikationen, die eine Notfall-Laparotomie (einmal wegen Aortenverletzung: 0,4% bzw. ein „Umsteigen“ wegen Verwachsungen: 0,4%) erforderten. III. Komplikationen, die postoperativ konservativ, laparoskopisch oder operativ (n = 2) zu behandeln waren: (n = 11; 4,4%), (Pankreatitis, Nachblutung, Choledochusstenose, Thrombose). Die Komplikationsrate sinkt mit zunehmender Erfahrung (OP-Nr. 0–30: 13,3%; 31–70: 7,6%; 71–250: 2,7%) aufwerte, die denen der konventionellen Cholezystektomien entsprechen.