In recent decades there has been a rapidly growing demand for high quality, uninterrupted power. In light of this fact, this study has addressed some of the causes of poor power quality and control strategies to ensure a high performance level in inverter-fed power systems. In particular, specific loading conditions present interesting challenges to inverter-fed, high power systems. No-load, unbalanced loading, and non-linear loading each have unique characteristics that negatively influence the performance of the Voltage Source Inverter (VSI). Ideal, infinitely stiff power systems are uninfluenced by loading conditions; however, realistic systems, with finite output impedances, encounter stability issues, unbalanced phase voltage, and harmonic distortion. Each of the loading conditions is presented in detail with a proposed control strategy in order to ensure superior inverter performance. Simulation results are presented for a 90 kVA, 400 Hz VSI under challenging loading conditions to demonstrate the merits of the proposed control strategies. Unloaded or lightly loaded conditions can cause instabilities in inverter-fed power systems, because of the lightly damped characteristic of the output filter. An inner current loop is demonstrated to damp the filter poles at light load and therefore enable an increase in the control bandwidth by an order of magnitude. Unbalanced loading causes unequal phase currents, and consequently negative sequence and zero sequence (in four-wire systems) distortion. A proposed control strategy based on synchronous and stationary frame controllers is shown to reduce the phase voltage unbalance from 4.2% to 0.23% for a 100%-100%-85% load imbalance over fundamental positive sequence control alone. Non-linear loads draw harmonic currents, and likewise cause harmonic distortion in power systems. A proposed harmonic control scheme is demonstrated to achieve near steady state errors for the low order harmonics due to non-linear loads. In particular, the THD is reduced from 22.3% to 5.2% for full three-phase diode rectifier loading, and from 11.3% to 1.5% for full balanced single-phase diode rectifier loading, over fundamental control alone. / Master of Science

Fuel cells have the potential to provide clean power for a variety of uses including stand-alone residential power. But to increase the acceptance of fuel cells for off-grid generation, the cost of the energy management system must be greatly reduced. Of the many ways to accomplish this, this paper looks at reducing cost through topology changes and elimination of current sensors. A dual 2.5kW non-isolated bidirectional boost converter is designed and analyzed. The various bidirectional boost topologies are compared on cost and ability to meet the specifications. A sensorless average current mode is designed, implemented and verified through testing in a low-cost fixed-point DSP. Both boost and buck modes are accurately modeled and voltage and current controllers are designed for good closed-loop response. The accuracy of the sensorless average current measurement is investigated in both modes of operation. A classical dual-loop controller is implemented in boost mode with the sensorless average current and in buck mode, a dual controller operating in either current or voltage mode is implemented. The design is verified through testing in boost and buck mode and it is shown that the results are acceptable. / Master of Science

As ondulações na corrente do estator e o forte impacto de erros em parâmetros são apontados como desvantagens do controle preditivo finite control-set (FCS) quando aplicado no controle de máquinas elétricas trifásicas. Neste contexto, o objetivo deste trabalho é propor métodos para aprimorar tanto o desempenho em regime permanente quanto à robustez frente a erros paramétricos do controle preditivo de corrente, conhecido como model predictive current control (MPCC). No presente trabalho são propostas as abordagens MPCC baseada na aplicação de dois e três vetores de tensão durante o mesmo período de controle com intuito de aprimorar o desempenho do controle preditivo de corrente, sem a necessidade de elevar a frequência de amostragem do sistema. Os princípios para escolha dos vetores ótimos e seus respectivos tempos de aplicação são baseados nas expressões para controle desacoplado de torque e fluxo do FOC e no princípio de funcionamento do deadbeat. A robustez paramétrica é garantida por meio de um observador de distúrbios discreto incorporado ao método de controle MPCC. Com a estimação dos impactos causados por variações de parâmetros e dinâmicas não modeladas é possível corrigir os erros dos sistema de controle, mas mantendo-se as suas características transitórias e de regime permanente. Resultados de simulação computacional bem como resultados obtidos em bancada de ensaios experimentais são apresentados para avaliação e comprovação das propostas. / Ripples in the stator current and the strong impact of errors in parameters are pointed as disadvantages of the predictive control when applied in the control of three-phase electric machines. In this context, the objective of this work is to propose methods to improve both the steady-state performance and the robustness against parametric errors of the current predictive control, known as MPCC. In the present work the MPCC approaches are proposed based on the application of two and three voltage vectors during the same control period in order to improve the performance of the current predictive control, without the need to raise the sampling frequency of the system. The principles for choosing the optimal vectors and their respective duration are based on the expressions for FOC method and on the concept of deadbeat operation. In addition, the impact of errors on parameters is minimized by means of a discrete disturbance observer embedded in the MPCC control method. With the estimation of the impacts caused by changes of parameters and not modeled dynamics it is possible to correct the errors of the control system, but keeping their transient and steady state characteristics. Results of computational simulation as well as results obtained in experimental tests are presented for evaluation and proof of the proposals.

In conventional three-phase PWM (Pulse Width Modulation) current source converter based STATCOM (Static Synchronous Compensator) applications, DC link current is kept constant at a predefined value and the reactive power of STATCOM is controlled by varying modulation index. This control strategy causes unnecessary power losses especially when the reactive power of STATCOM is low. For this purpose, in order to reduce the active power drawn by STATCOM, the modulation index can be maximized by adjusting DC link current. Within the scope of this thesis, an adaptive control of DC link current will be designed and applied to a 0.4kV 50kVAr three phase current source converter based STATCOM so that the power losses can be reduced. The theoretical work will be compared and discussed with the experimental results.

Recent advances in power electronics have lead to the wide spread adoption of advanced power supplies and energy efficient devices. This has lead to increased levels of harmonic currents in power systems, degrading the performance of electrical machinery and interfering with telecommunication services. Active filters provide a solution to these problems by compensating for the distorted currents drawn by non-linear loads. Optimal methods for controlling these active filters have been determined by computer simulation and experimental implementation. Methods used for isolating the harmonic content of an unbalanced three phase load current were compared by computer simulations. A technique based on the Fast Fourier Transform (FFT) was developed as part of this work and shown to perform favourably. Notch Filtering, Sinusoidal Subtraction, Instantaneous Reactive Power Theory, Synchronous Reference Frame and Fast Fourier Transform methods were simulated. The methods shown to be suitable for compensation of three phase unbalanced loads were implemented in a Digital Signal Processor to evaluate true performance. These methods were Notch Filtering, Sinusoidal Subtraction, Fast Fourier Transform, and a High Pass Filter based method. A completely digital hysteresis current controller for a three phase active filter inverter has been developed and implemented with a Field Programmable Gate Array. This controller interfaces directly to a digital signal processor and is resistant to electromagnetic interference. Results from the experimental hardware verified that the active filter model used for simulation is accurate, and may be used for further development of harmonic isolation methods. A technique using notch filtering gives the best performance for steady loads, with the FFT based technique giving the most flexible operation for a range of load current characteristics. Novel use of the FFT based harmonic isolation technique allows selective cancellation of individual harmonics, with particular application to multiple shunt filters connected in parallel.

Current control is an important topic in modern power electronics system. For voltage source inverters, current control loop ensures the waveform quality at steady state and the fast response at transient state. To improve the current control performance, quite a few nonlinear control strategies have been presented and one well-known strategy is the hysteresis current control. It achieves fast response without stability issue and it has high control precision. However, for voltage source inverter applications, hysteresis current control has a wide switching frequency range, which introduces additional switching loss and impacts the design of harmonic filter. Other nonlinear current control strategies include one-cycle control, non-linear carrier control, peak current control, charge control, and so on. However, these control strategies are just suitable for specific topologies and it cannot be directly used by voltage source inverters. The recently proposed parabolic current control solves the frequency variation problem of hysteresis current control by employing a pair of parabolic carriers as the control band. By the use of parabolic current control, approximate-constant switching frequency can be achieved. Due to the cycle-by-cycle control structure, it inherently has fast response speed and high precision. These advantages make it suitable for voltage source inverters, including stand-alone inverters, grid connected inverters, active power filters, and power factor correction applications. However, parabolic current control has some limitations, such as dead-time effects, only working as bipolar PWM, complex hardware implementation, non-ideal converging speed. These problems are respectively solved in this dissertation and solutions include dead-time compensation, the implementation on dual-carrier unipolar PWM, sensorless parabolic current control, single-step current control. With the proposed dead-time compensation strategy, current control precision is improved and stable duty-cycle range are extended. Dual-carrier PWM implementation of parabolic current control has smaller harmonic filter size and lower power loss. Sensorless parabolic current control decreases the cost of system and enhances the noise immunity capability. Single-step current control pushes the convergence speed to one switching operation with simple implementation. High switching frequency is allowed and power density can be improved. Detailed analysis, motivation and experimental verification of all these innovations are covered in this dissertation. In addition, the duality phenomenon exists in electrical circuits, such as Thevenin's theorem and Norton's theorem, capacitance and inductance. These associated pairs are called duals. The dual of parabolic current control is derived and named parabolic voltage control. Parabolic voltage control solves the audible noise problem of burst mode power converters and maintains high efficiency in the designed boost converter. / Ph. D.

The permanent magnet switched reluctance motor (PMSRM) is hybrid dc motor which has the potential to be more effect than the switched reluctance (SRM) and permanent magnet (PM) motors. The PMSRM has a both a salient rotor and stator with permanent magnets placed directly onto the face of common pole stators. The PMSRM is wound like the SRM and can be controlled by the same family of converters. The addition of permanent magnets creates nonlinearities in both the governing electrical and mechanical equations which differentiate the PMSRM from all other classes of electric motors. The primary goal of this thesis is to develop a cohesive and comprehensive control strategy for the PMSRM so as to demonstrate its operation and highlight its efficiency. The control of the PMSRM starts with understanding its region of operation and the underlying torque production of the motor. The selection of operating region is followed by a both linear and nonlinear electrical modeling of the motor and the design of current controllers for the PMSRM. The electromechanical model of the motor is dynamically simulated with the addition of a closed loop speed controller. The speed controller is extended to add an efficiency searching algorithm which finds the operating condition with the highest efficiency online. / Master of Science

Within the past decade, talk has arisen of shifting the utility grid from centralized, radial sources to a distributed network of sources, also known as distributed generation (DG); in the wake of deregulation, the California energy crisis, and northeastern blackouts. Existing control techniques for DG systems are designed to operate a system either in the connected or disconnected (islanding) mode to the utility; thus not allowing for both modes to be implemented and transitioned between. Existing detection and re-closure algorithms can also be improved upon. Dependent upon the method implemented, detection algorithms can either cause distortions in the output or completely miss a disturbance. The present re-closure process to reconnect to the utility is to completely shutdown and wait five minutes. The proposed methods of this study improve upon existing methods, via simulation and hardware experimentation, for DG systems that can intentionally islanding themselves. The proposed, "switched-mode", control allows for continuous operation of the system during disturbances by transitioning the mode of control to reflect the change in the system mode (grid-connected or islanding). This allows for zero downtimes without detrimental transients. The proposed detection method can sense disturbances that other methods cannot; and within 25 ms (approximately 1.5 line-cycles at 60 Hz). This method is an improvement over other methods because it eliminates the need to purposely distort the outputs to sense a disturbance. The proposed re-closure method is an improvement over the existing method due to the fact that it does not require the system to de-energize before re-synchronizing and reconnecting to the utility. This allows for DGs to continuously supply power to the system without having to shut down. Results show that the system is generally ready to reconnect after 2 to 5 line cycles. / Master of Science

Hysteresis current control has been widely used in power electronics with the advantages of fast dynamic response under parameter, line and load variation and ensured stability. However, a main disadvantage of hysteresis current control is the uncertain and varying switching frequency which makes it difficult to form an average-value model. The changing switching frequency and unspecified switching duty cycle make conventional average-value models based on PWM control difficult to apply directly to converters that are controlled by hysteresis current control. In this work, a new method for average-value modeling of hysteresis current control in boost converters, three-phase inverters, and brushless dc motor drives is proposed. It incorporates a slew-rate limitation on the inductor current that occurs naturally in the circuit during large system transients. This new method is compared with existing methods in terms of simulation run time and rms error. The performance is evaluated based on a variety of scenarios, and the simulation results are compared with the results of detailed models. The simulation results show that the proposed model represents the detailed model well and is faster and more accurate than existing methods. The slew-rate limitation model of hysteresis current control accurately captures the salient detail of converter performance while maintaining the computational efficiency of average-value models. Validations in hardware are also presented.

Dans un contexte de réduction des émissions de CO2 dans les systèmes industriels et d’amélioration de leur efficacité énergétique, les chauffages par induction répondent à ces critères. Le procédé consiste à plonger un corps conducteur d’électricité dans un champ magnétique variable, induisant ainsi des courants au sein de la pièce qui se met alors à chauffer par effet Joule. Cette technologie présente l’avantage d’atteindre des températures élevées de manière précise et rapide, sans contact avec la source d’énergie. Si aujourd’hui le contrôle de tels procédés passe par la structure mécanique du chauffage, de nombreuses études ont montré que l’association de plusieurs bobines alimentées par des systèmes d’électronique de puissance fournit une souplesse dans le contrôle des profils de température et une flexibilité vis-à-vis des pièces traitées. Ce travail présente deux systèmes de chauffage par induction. Le premier est un prototype réalisé par EDF considérant trois inducteurs alimentés par trois onduleurs à résonance. Nous nous y intéresserons plus particulièrement par la suite. Le second, en cours de réalisation, met en jeu six inducteurs pour du chauffage au défilé. Leurs caractéristiques, leurs alimentations et leurs fonctionnements sont abordés pour ensuite mettre en place deux modélisations des phénomènes électriques, l’une sous forme de fonctions de transfert, la seconde dans l’espace d’état. L’objectif final est le développement d’une alimentation auto-adaptative capable de contrôler en temps réel le profil de puissance injecté dans les pièces à chauffer afin d’atteindre un profil de température requis avec le minimum d’énergie consommée. L’étude se poursuit avec un inventaire des solutions existantes en termes de commande en boucle ouverte et en boucle fermée des systèmes mono-inducteurs, ainsi que des travaux, plus rares, pour les chauffages multi-inducteurs. De plus, des études ont montré que le contrôle de la température à la surface d’une pièce à chauffer passait par le contrôle des courants dans les inducteurs, en amplitude mais également en phase. C’est pourquoi nous proposons des lois de commande pour asservir les courants dans les inducteurs. Une routine d’optimisation fournit les amplitudes et les phases des courants dans les trois inducteurs permettant d’obtenir le profil de température le plus proche d’un profil de référence. Nous l’avons modifiée de façon à tenir compte de solutions énergétiquement optimisées en intégrant les pertes dans les interrupteurs et les inducteurs. Deux lois de commande originales sont proposées. La première étudie une commande robuste dans l’espace d’état par placement de vecteurs propres. La seconde met en jeu des correcteurs résonants qui confèrent à la boucle du système un module de gain infiniment grand à la fréquence de résonance de sorte qu’ils effacent les effets des perturbations agissant à cette même fréquence. Les méthodes de synthèse de ces correcteurs sont échantillonnées avec un très faible rapport fréquence d’échantillonnage/fréquence de résonance ; les performances sont détaillées et analysées pour le prototype à trois inducteurs. Pour tester plus facilement et valider en partie les lois mises en place, un simulateur analogique a été conçu, sorte d’émulateur reproduisant le comportement des courants dans les inducteurs et des tensions à leurs bornes. Il se base sur la modélisation sous forme de fonctions de transfert et ne comprend donc pas d’onduleur. Par l’intermédiaire d’une carte dSPACE intégrant un DSP, nous avons implanté et validé les deux lois de commande robustes sur le simulateur analogique. Par la suite, pour se rapprocher du système réel, nous avons mis en place une commande rapprochée transformant les trois signaux de commande en sortie des correcteurs vers les rapports cycliques des douze interrupteurs des convertisseurs alimentant les inducteurs. Cette étape a été validée par des essais expérimentaux réalisés en parallèle de nos études. / In the context of reducing carbon dioxide emissions in industrial systems and improving their energy efficiency, induction heaters meet these criteria. The method consists in immersing an electrically conductive body in a variable magnetic field, inducing currents within the piece which then begins to heat by Joule effect. This technology has the advantage of reaching high temperatures accurately and quickly, without contact with the power source. Today the control of such works with mechanical structures. But many studies have shown that the combination of several coils supplied by electronic power systems provides flexibility in controlling temperature profiles and flexibility towards the treated parts. This paper presents two systems of induction heating. The first is a prototype made by EDF considering three inductors supplied by three resonant inverters. We will focus more specifically later on this prototype. The second one, in progress, involves six inductors for scrolling heating. Their characteristics, their power supplies and their functions are discussed and then set up two models of electric phenomena, one form of transfer functions and one form in the state space. The final objective is the development of a self-adaptive power able to control real-time power profile injected into parts of heating to achieve a temperature profile required with minimum energy consumption. The study goes on with a list of existing solutions in terms of open-loop control and closed-loop control for single inductor system, as well as few rarer works for multi-inductors heaters. In addition, studies have shown that control of the surface temperature of the piece to be heated passes by the control of currents in the inductors, in magnitude but also in phase. That is why we propose control laws for controlling inductor currents. An optimization routine provides the magnitudes and phases of the currents in the three inductors to obtain the temperature profile closer to a reference profile. We modified it to take into account energy optimized solutions by integrating the losses in the switches and inductors. Two original control laws are proposed. The first one studies a robust control in the state space by placing eigenvalues and eigenvectors. The second one involves resonant markers that give the system loop gain module infinitely large at the resonant frequency so that they erase the effects of disturbances acting at the same frequency. The methods of synthesis of these markers are sampled with a very low sampling frequency / resonant frequency ratio; performance are detailed and analyzed for the prototype with three inductors. So as to test and validate easily some of the studied laws, an analog simulator was designed, a kind of an emulator, which reproduces the behaviour of the currents in the coils and the voltages at their terminals. It is based on the transfer function model and therefore does not include inverters. Through a dSPACE card integrating a DSP, we have implemented and validated the two robust control laws on the analog simulator. Thereafter, in order to be closer to the real system, we have established a close control which turns the three control signals toward the duty cycles of the twelve switches of the converters supplying the inductors. This step was validated by experimental tests carried out in parallel with our studies.