Optimal-regulator-based control of NPC boost rectifiers for unity power factor and reduced neutral-point-potential variations

Neutral-point-clamped pulsewidth modulation rectifiers (NPCRs) are suitable for high-voltage systems because of their circuit structure. The NPCRs, however, have a problem, in that the neutral point potential (NPP) varies when the current flows into or out of the neutral point. The variations cause voltage deviations in the input waveforms, as well as unbalanced voltage stress on the devices. This paper describes a controlling method for NPCRs based on a state-space model. There are three control objectives: (1) to keep the power factor at unity; (2) to keep the DC-link voltage at a reference value; and (3) to keep the neutral point potential at 0 V. The neutral point current is treated as one of the inputs. The controller is designed based on the optimal regulator theory in order to achieve the three control objectives simultaneously. The validity of the proposed method is demonstrated by experimental results

Wind power drive laboratory test platform

Uusiutuvien energianlähteiden ja etenkin tuulivoiman tutkimiseen on käytetty paljon resursseja lähivuosina. Tämän diplomityön tavoitteena oli rakentaa Tampereen teknillisen yliopiston Sähköenergiatekniikan laitokselle muun muassa muuttuvanopeuksisen tuulivoimakäytön tutkimiseen soveltuva laboratoriotestausjärjestelmä. Laitteiston pohjana käytettiin laitokselle aiemmin tasasähkönjakelun tutkimiseen kehitettyä laitteistoa. Kyseinen laitteisto koostuu verkkosuuntaajasta ja kuormavaihtosuuntaajasta. Tuulivoimakäytön laboratoriotestausjärjestelmää varten olemassa olevaan laitteistoon täytyi siis lisätä vielä kestomagneettikonekäyttö ja säätöjärjestelmä koko laitteiston ohjausta varten. Lisäksi kestomagneettikoneeseen kytkettiin dc-konekäyttö simuloimaan tuuliturbiinin tai mekaanisen kuorman synnyttämää momenttia. Koko laitteiston ohjaamisesta huolehtii dSPACE-reaaliaikasimulaattori. Järjestelmän äly on kuitenkin pyritty pitämään mahdollisimman hajautettuna, ja varsinkin aikakriittiset virta- ja jännitesäädöt on toteutettu jo suuntaajissa. Työn kirjallisessa osuudessa selvitetään perusteet tuulivoimakäytöistä, vektorimoduloidun kolmitasoisen suuntaajan toiminnasta ja säädöstä sekä kestomagneettikonekäytöstä ja sen säädöstä. Lisäksi työssä käydään läpi rakennetun laitteiston rakenne ja toiminta. Viimeiseksi esitellään simulointi- ja mittaustuloksia kahdesta eri testitilanteesta. Työssä päästiin tavoitteisiin, eli laitteisto saatiin toimintaan ilman suuria ongelmia, ja sen toiminta oli odotetun mukaista. /Kir1

Convertisseurs multiniveaux à bus continu avec point milieu. Nouvelles topologies et stratégies de contrôles

Dans cette thèse, un nouveau concept de représentation, d'analyse et de synthèse des convertisseurs MN est introduit pour mieux comparer, comprendre et classifier les différentes topologies. La famille des convertisseurs, basée sur un bus d'alimentation DC à trois niveaux et des circuits de conversion multicellulaires, fait l'objet d'une analyse approfondie. Les structures de cette famille permettent de générer des niveaux de tension de sortie supplémentaires en gardant une bonne contrôlabilité et des contraintes acceptables au niveau des divers composants. Outre les structures SMC ("stacked multicell converter") et ANPC MN ("active neutral point clamped converter") déjà connues, de nouvelles topologies sont proposées et les limites de fonctionnement sont étudiées en détail. Des concepts de réglage pour le courant PN basés sur une variation de tension homopolaire et basés sur l'utilisation des états redondants sont proposés. Les performances peuvent être nettement améliorées. ABSTRACT :In this thesis, a new concept for the representation, the analysis and synthesis of ML (multi level) converters is introduced. This concept allows simple comprehension, comparison and classification of topologies. The family of ML converters with 3 level split DC link supply has been studied more indepth. The converter structures of this family allow for ML output generation while providing good controllability and reasonable components dimensioning. In addition to the known SMC (stacked multi cell converter) and ANPC (active neutral point clamped converter), new topologies are introduced and the functional limitations are studied. Control concepts based on CM (common mode) voltage injection and based on the use of redundant states are introduced. The performance can be significantly improved compered to state of the art control schemes

Investigation and control of a hybrid asymmetric multi-level inverter for medium-voltage applications

Power-electronic inverters are becoming popular for various industrial drives applications. In recent years also high-power and medium-voltage drive applications have been installed. However, the existing solutions suffer from some important drawbacks. Hybrid asymmetric multi-level inverters promise significant improvements for medium-voltage applications. This dissertation investigates such a hybrid inverter. To simplify the topology, some inverter parts are deprived of their feeding from the net and can only supply reactive power. The non-supplied intermediate-circuit capacitor voltages are inherently unstable and require a suitable control method for converter operation, preferably without influence on the load. Apart from normal operation, also converter start-up is an issue to consider, for which it is desirable to limit additional equipment. In this dissertation, we investigate the behaviour of this new inverter, and develop methods to obtain its reliable operation for the considered applications. These methods include modulation, voltage stabilization and start-up. We establish suitable models for their foundation. The principle achievement of this work is the development of a control method to stabilize a multitude of capacitor voltages which have no equilibrium state. Power balancing is performed by varying the common-mode output voltage, using a non-linear model-predictive controller. This method, which is new to power electronics, is applied to our hybrid asymmetric nine-level inverter driving an asynchronous motor. Computer simulations and measurements on an experimental drive system demonstrate stable behaviour in steady-state and during transients over the whole operating range. The obtained results prove the possible implementation of such a complex control algorithm for fast real-time operation. As second important accomplishment, this thesis proposes a start-up method that charges the non-supplied intermediate-circuit capacitors in parallel with the supplied ones, without additional equipment. Measurements show its successful application in the investigated drive system

Development of Robust Control Schemes with New Estimation Algorithms for Shunt Active Power Filter

The widespread use of power electronics in industrial, commercial and even residential electrical equipments causes deterioration of the quality of the electric power supply with distortion of the supply voltage. This has led to the development of more stringent requirements regarding harmonic current generation, as are found in standards such as IEEE-519. Power Quality is generally meant to measure of an ideal power supply system. Shunt active power filter (SAPF) is a viable solution for Power Quality enhancement, in order to comply with the standard recommendations. The dynamic performance of SAPF is mainly dependent on how quickly and how accurately the harmonic components are extracted from the load current. Therefore, a fast and accurate estimation algorithm for the detection of reference current signal along with an effective current control technique is needed in order for a SAPF to perform the harmonic elimination successfully. Several control strategies of SAPF have been proposed and implemented. But, still there is a lot of scope on designing new estimation algorithms to achieve fast and accurate generation of reference current signal in SAPF. Further, there is a need of development of efficient robust control algorithms that can be robust in face parametric uncertainties in the power system yielding improvement in power quality more effectively in terms of tracking error reduction and efficient current harmonics mitigation. The work described in the thesis involves development of a number of new current control techniques along with new reference current generation schemes in SAPF. Two current control techniques namely a hysteresis current control (HCC) and sliding mode control (SMC) implemented with a new reference current generation scheme are proposed. This reference generation approach involves a Proportional Integral (PI) controller loop and exploits the estimation of the in phase fundamental components of distorted point of common coupling (PCC) voltages by using Kalman Filter (KF) algorithm. The KF-HCC based SAPF is found to be very simple in realization and performs well even under grid perturbations. But the slow convergence rate of KF leads towards an ineffective reference generation and hence harmonics cancellation is not perfect. Therefore, a SMC based SAPF is implemented with a faster reference scheme based on the proposed Robust Extended Complex Kalman Filter (RECKF) algorithm and the efficacy of this RECKF-SMC is compared with other variants of Kalman Filter such as KF, Extended Kalman Filter (EKF) and Extended Complex Kalman Filter (ECKF) employing simulations as well as real-time simulations using an Opal-RT Real-Time digital Simulator. The RECKF-SMC based SAPF is found to be more effective as compared to the KF-HCC, KF-SMC, EKF-SMC and ECKF-SMC. Subsequently, predictive control techniques namely Dead Beat Control (DBC) and Model Predictive Control (MPC) are proposed in SAPF along with an improved reference current generation scheme based on the proposed RECKF. This reference scheme is devoid of PI controller loop and can self-regulate the dc-link voltage. Both RECKF-DBC and RECKF-MPC approaches use a model of the SAPF system to predict its future behavior and select the most appropriate control action based on an optimality criterion. However, RECKF-DBC is more sensitive to load uncertainties. Also, a better compensation performance of RECKF-MPC is observed from the simulation as well as real-time simulation results. Moreover, to study the efficacy of this RECKF-MPC over PI-MPC, a comparative assessment has been performed using both steady state as well as transient state conditions. From the simulation and real-time simulation results, it is observed that the proposed RECKF-MPC outperforms PI-MPC. The thesis also proposed an optimal Linear Quadratic Regulator (LQR) with an advanced reference current generation strategy based on RECKF. This RECKF-LQR based SAPF has better tracking and disturbance rejection capability and hence RECKF-LQR is found to be more efficient as compared to RECKF-SMC, RECKF-DBC and RECKF-MPC approaches. Subsequently, two robust control approaches namely Linear Quadratic Gaussian (LQG) servo control and H∞ control are proposed in SAPF with highly improved reference generation schemes based on RECKF. These control strategies are designed with the purpose of achieving stability, high disturbance rejection and high level of harmonics cancellation. From simulation results, they are not only found to be robust against different load parameters, but also satisfactory THD results have been achieved in SAPF. A prototype experimental set up has been developed in the Laboratory with a dSPACE-1104 computing platform to verify their robustness. From both the simulation and experimentation, it is observed that the proposed RECKF-H∞ control approach to design a SAPF is found to be more robust as compared to the RECKF-LQG servo control approach in face parametric uncertainties due to load perturbations yielding improvement in power quality in terms of tracking error reduction and efficient current harmonics mitigation. Further, there is no involvement of any voltage sensor in this realization of RECKF-H∞ based SAPF resulting a more reliable and inexpensive SAPF system. Therefore, superiority of proposed RECKF-H∞ is proved amongst all the proposed control strategies of SAPF

Modulation strategies for the neutral-point-clamped converter and control of a wind turbine system

Els convertidors multinivell són topologies de convertidors d’electrònica de potència que poden generar tres o més nivells de voltatge en cadascuna de les fases de sortida. Com a resultat, els voltatges i corrents generats per aquestes topologies presenten una distorsió harmònica baixa. Hi ha diferents configuracions de convertidors multinivell, les quals es basen en connectar dispositius de potència o convertidors en sèrie. El resultat d’aquestes connexions permet obtenir voltatges alts, tant en la part de corrent continu com en la de corrent altern del convertidor. A més, cada dispositiu sols ha de suportar una fracció del voltatge total del bus de corrent continu. Per aquestes raons, els convertidors multinivell són generalment utilitzats en aplicacions d’alta potència. El convertidor de tres nivells amb connexió a punt neutre (neutral-point-clamped) és el més utilitzat. La recerca d’aquesta tesis doctoral està focalitzada en aquesta topologia de convertidor, i el principal objectiu és l’aportació de noves tècniques de modulació. Aquestes tenen en compte diferents aspectes: la velocitat computacional dels algorismes, l’equilibrat de les tensions dels condensadors del bus de contínua, les pèrdues de commutació i les oscil·lacions de baixa freqüència en el punt neutre del convertidor. Totes les estratègies de modulació proposades en aquesta tesis són modulacions d’amplada de polses basades en portadora. En la primera modulació que es presenta, s’injecta un senyal comú (seqüència zero) a totes les moduladores, que es basa en els patrons de la modulació vectorial que utilitza tres vectors dels més propers al de referència (nearest-three-vector modulation). S’estudien i es comparen els resultats d’aquesta modulació amb la seva homòloga, basada en perspectiva vectorial. Una segona proposta és l’anomenada modulació d’amplada de polses de doble senyal (double-signal pulse-width modulation). Aquesta modulació és capaç d’eliminar completament les oscil·lacions de voltatge en el punt neutre del convertidor. No obstant això, es produeix un increment de les pèrdues de commutació en els dispositius de potència i, a més, no hi ha un equilibrat natural de les tensions en els condensadors del bus. Una última estratègia de modulació, anomenada modulació híbrida (hybrid pulse-width modulation), es basa en la combinació de la modulació sinusoïdal (sinusoidal pulse-width modulation) i la de doble senyal. Aquesta presenta una solució de compromís entre reduir les pèrdues de commutació, en detriment d’un augment de l’amplitud de les oscil·lacions de voltatge en el punt neutre. Una segona part d’aquesta tesis es centra en les aplicacions a generació eòlica, ja que els convertidors multinivell estan començant a ser utilitzats en aquest camp. Això es produeix fonamentalment per l’augment continu de les dimensions de les turbines eòliques. En aquesta part de la recerca s’ha considerat la configuració de dos convertidors multinivell connectats a un mateix bus de contínua (back-to-back), tot i que els convertidors han estat estudiats independentment. Inicialment s’ha estudiat el convertidor que va connectat a la xarxa elèctrica i s’ha aplicat l’estratègia de control coneguda com a control orientat a tensió (voltage-oriented control). S’han utilitzat controladors estàndard (proporcional-integral), als quals s’ha afegit un control difús que supervisa i modifica els valors de les constants dels controladors. Aquest supervisor difús millora la dinàmica de la tensió del bus de contínua davant canvis de càrrega quan el convertidor treballa com a rectificador. Per una altra part, s’ha estudiat el control d’una turbina eòlica basada en un generador d’imants permanents. En aquest cas, s’ha aplicat l’estratègia de control coneguda com a control orientat a camp (field-oriented control). S’han avaluat i comparat els avantatges i inconvenients de diferents formes de sintonitzar els controladors.Multilevel converters are power electronic topologies that can generate three or more voltage levels in each output phase. As a result, the voltage and current waveforms generated have lower total harmonic distortion. Multilevel topologies are based on connecting power devices or converters in a series. Consequently, high voltages can be handled on the dc and ac sides of the converter, while each device stands only a fraction of the total dc-link voltage. For these reasons multilevel converters are generally applied to high-power applications. The three-level neutral-point-clamped converter is the most extensively used multilevel topology. This topology is the main focus of research in this dissertation. The main objective is to propose new modulation strategies that are able to meet a compromise solution while considering computational algorithm speed, voltage balance in the dc-link capacitors, switching losses and low frequency voltage oscillations at the neutral point. All the modulation strategies proposed here are based on carrier-based pulsewidth modulation. A new modulation strategy has been implemented using a proper zero-sequence signal injected into the modulation signals. The zero sequence is determined from a space-vector modulation standpoint, particularly the nearest-threevector modulation strategy. The proposed carrier-based technique is compared with its space-vector modulation counterpart. It shows some advantages, such as easier implementation and reduced switching events; however, it still produces oscillations in the neutral-point voltage for some operating conditions. A new modulation strategy able to completely remove such voltage oscillations is also presented. It is called double-signal pulse-width modulation. The main drawback of this strategy is that it increases the switching frequency of the power devices and has no natural capacitor voltage balance. Some balancing strategies are proposed in this dissertation for this specific modulation. Furthermore, a hybrid pulse-width modulation approach is presented which is able to combine sinusoidal pulse-width modulation with doublesignal pulse-width modulation; this represents a compromise solution between switching losses and neutral-point voltage oscillation amplitudes. The second part of this thesis is focused on wind generation applications. Multilevel converters are starting to be used in such a field nowadays, and are expected to be further applied in the near future as the sizes of wind turbines grow. Two back-toback-connected power converters are considered in this application, although they are analyzed independently. First of all, the control of the grid-connected converter is studied. A voltage-oriented control is used with standard proportional-integral controllers. The originality of the method is that a fuzzy supervisor is designed and included in the structure; the fuzzy supervisor is able to modify the proportionalintegral parameters online. It is shown how the control of the total dc-link voltage improves significantly under load changes when the converter is working as a rectifier. On the other hand, a control study is performed on the wind turbine side. The variable speed wind turbine is based on a permanent magnet synchronous generator. A field-oriented control strategy is applied. The controllers are evaluated and compared using different tuning strategies which highlight the advantages and drawbacks of each

Sistema de conversão estática CA-CC bidirecional aplicado à microrredes CC bipolares

Tese (doutorado) - Universidade Federal de Santa Catarina, Centro Tecnológico,. Programa de Pós-Graduação em Engenharia Elétrica, Florianópolis, 2016.Neste trabalho são apresentados os estudos de retificadores bidirecionais aplicados para as redes de distribuição ativas com barramento em corrente contínua (CC) bipolar, assim como outros conversores, os quais desempenham importantes funções em tais redes. Possíveis arranjos destes em redes ativas de distribuição, incluindo microrredes em CC,são investigados e comparados. Além disto, neste trabalho é apresentado o estudo e a implementação prática de uma rede ativa em CC bipolar,centrado principalmente em rendimento da microrrede, ou seja, do ponto de vista das perdas globais do sistema de distribuição. Baseado nestes estudos será realizada a escolha da topologia de retificador que fará aconexão entre a rede principal e a microrrede em corrente contínua. Ao longo do trabalho, será mostrado que o conversor escolhido é o retificador NPC operando em três níveis, devido ao melhor rendimento apresentado. Buscando trazer melhorias para o conversor NPC, nesta tese será proposta uma técnica de modulação para o cancelamento da ondulação de baixa frequência da tensão do ponto central do referido conversor e, consequentemente sua melhor adaptação ao fornecimento de energia em barramento CC bipolar. Resultados de simulação e experimentais serão apresentados para demonstrar que a técnica para o cancelamento da ondulação de tensão do ponto central é efetiva. Por fim, será apresentada a proposta de incluir um circuito adicional ao NPC, para executar as funções de cancelamento da corrente de baixa frequência do ponto central da microrrede em toda a faixa de operação do retificador, assim como balancear as tensões dos barramentos parciais, em operação normal. Isto resolve algumas limitações inerente são conversor NPC no que se refere à sua capacidade de balanceamento do barramento CC em condições de carregamento desbalanceado. Além disto, em caso de falha de algum braço do retificador, este circuito poderá prover tolerância a falhas.Abstract: This work presents analysis of bidirectional power rectifiers applied tofuture dc bipolar active distribution networks, as well as other converters that perform important functions in such systems. Possible arrangements of these converters in active distribution networks, including dc microgrids, are investigated. Moreover, study and practical implementation of a dc bipolar active network, mainly focused on the system-wide efficiency, i.e., from the perspective of the overall system losses is discussed. Based on these losses the rectifier topology ischosen. This rectifier provides the interface between the main network to the dc active network. In the course of the work it is shown that thechosen converter is the three-level/-phase NPC, due to the achievable higher overall efficiency. In order to introduce improvements to theNPC converter, this work proposes a modulation technique for the cancellation of the dc-link center point low frequency current components. Simulation and experiment results are presented to show that the proposed center point cancellation technique is effective. Finally, an additional circuit is presented to be added to the originalNPC converter with the function to perform the cancellation of the dclink center point low frequency current in all operating range of rectifierand provide balance of the dc-link partial voltages, during regularoperation. This circuit solves the inherent limitations of the NPCrectifier regarding its ability to control the partial dc-link voltages underhighly unbalanced conditions. In case of failure of a rectifier leg this additional circuit can provide fault-tolerant operation