Robust control techniques for DFIG driven WECS with improved efficiency

Wind energy has emerged as one of the fastest growing renewable energy sources since mid-80‘s due to its low cost and environmentally friendly nature compared to conventional fossil fuel based power generation. Current technologies for the design and implementation of wind energy conversion systems (WECSs) include induction generator and synchronous generator based units. The doubly fed induction generator (DFIG) is chosen in this thesis because of its economic operation, ability to regulate in sub-synchronous or super-synchronous speed and decoupled control of active and reactive powers. Among the major challenges of wind energy conversion system, extraction of maximum power from intermittent generation and supervision on nonlinear system dynamics of DFIG-WECS are of critical importance. Maximization of the power produced by wind turbine is possible by optimizing tip-speed ratio (TSR), turbine rotor speed or torque and blade angle. The literature reports that a vast number of investigations have been conducted on the maximum power point tracking (MPPT) of wind turbines. Among the reported MPPT control algorithms, the hill climb search (HCS) method is typically preferred because of its simple implementation and turbine parameter-independent scheme. Since the conventional HCS algorithm has few drawbacks such as power fluctuation and speed-efficiency trade-off, a new adaptive step size based HCS controller is developed in this thesis to mitigate its deficiencies by incorporating wind speed measurement in the controller. In addition, a common practice of using linear state-feedback controllers is prevalent in speed and current control of DFIG-based WECS. Traditional feedback linearization controllers are sensitive to system parameter variations and disturbances on grid-connected WECS, which demands advanced control techniques for stable and efficient performance considering the nonlinear system dynamics. An adaptive backstepping based nonlinear control (ABNC) scheme with iron-loss minimization algorithm for RSC control of DFIG is developed in this research work to obtain improved dynamic performance and reduced power loss. The performance of the proposed controller is tested and compared with the benchmark tuned proportional-integral (PI) controller under different operating conditions including variable wind speed, grid voltage disturbance and parameter uncertainties. Test results demonstrate that the proposed method exhibits excellent performance on the rotor side and grid side converter control. In addition, the compliance with the modern grid-code requirements is achieved by featuring a novel controller with disturbance rejection mechanism. In order to reduce the dependency on system‘s mathematical model, a low computational adaptive network fuzzy interference system (ANFIS) based neuro-fuzzy logic controller (NFC) scheme is developed for DFIG based WECS. The performance of the proposed NFC based DFIG-WECS is tested in simulation to regulate both grid and rotor side converters under normal and voltage dip conditions. Furthermore, a new optimization technique known as grey wolf optimization (GWO) is also designed to regulate the battery power for DFIG driven wind energy system operating in standalone mode. In order to verify the effectiveness of the proposed control schemes, simulation models are designed using Matlab/Simulink. The proposed model for MPPT and nonlinear control of grid-connected mode and GWO based power control of standalone DFIG-WECS has been successfully implemented in the real-time environment using DSP controller board DS1104 for a laboratory 480 VA DFIG. The comparison among different controllers suggests that each control technique has its own specialty in wind power control application with specific merits and shortcomings. However, the PI controller provides fast convergence, the ANFIS based NFC controller has better adaptability under grid disturbances and ABNC has moderate performance. Overall, the thesis provides a detailed overview of different robust control techniques for DFIG driven WECS in grid-connected and standalone operation mode with practical implementation

Application of Power Electronics Converters in Smart Grids and Renewable Energy Systems

This book focuses on the applications of Power Electronics Converters in smart grids and renewable energy systems. The topics covered include methods to CO2 emission control, schemes for electric vehicle charging, reliable renewable energy forecasting methods, and various power electronics converters. The converters include the quasi neutral point clamped inverter, MPPT algorithms, the bidirectional DC-DC converter, and the push–pull converter with a fuzzy logic controller

Advanced modeling and analysis of the doubly-fed induction generator based wind turbines

The doubly fed induction generator based wind turbine (DFIG-WT) with a partially rated converter, which is currently the dominating concept in the market, suffers from high short-circuit current and DC voltage magnitudes during symmetrical and unsymmetrical grid faults. The incorporated frequency converter is sensitive against high current and voltage magnitudes. Therefore, a proper protection is essential. This can be achieved through protective devices like chopper and/or crowbar or through full disconnection from the grid under severe fault conditions. However, this may not subject the DFIG-WT to the grid codes requirements. Furthermore, the high short-circuit current magnitudes will eventually elevate the fault current levels in the grid. Therefore, a deep understanding of the dynamic response of the DFIG-WT during different types of fault is essential in, assessing fault ride through (FRT) capability, proper design of the electric power system component and right settings of the protective relays for selective disconnection. In this thesis a detailed modelling of the individual components of the DFIG-WT considering all the system non-linarites was made. A controller was developed for each of the turbine, the machine side converter and the line side converter. The controller allows for maximum energy yield, fast and accurate response, and a separate control of the positive and negative sequence components as well as selective frequency components. A new criterion was proposed to assess the stability of the DFIG-WT output currents and voltages when connected to the grid with the consideration of the system’s non-linarites. A detailed analysis of the DFIG-WT response to symmetrical and unsymmetrical faults was carried out. The analysis has considered the complete system considering the controller influence as well as the system’s non linarites. Based on the analysis a new set of mathematical equations describing the short-circuit current, transient impedance, time constants and eigen frequencies were proposed and validated against the actual behavior, and the results showed a very high accuracy. From the provided analysis new methods for reduction of the peak short-circuit current was developed. The new methods result in the highest peak current reduction without exploiting the converter voltage limits, or operating in over modulation or disconnection of the wind turbine. Finally but not last, a method to estimate the equivalent parameters of the DFIG-WT for fault current calculation in the same manner as for IEC 60909 was proposed. The new method requires the aid of parameter identification and no load FRT test to estimate the equivalent parameters without any knowledge of the controller configuration. Furthermore, a new method was proposed to estimate the equivalent R X ratio in meshed networks. The new method leads to better accuracy in comparison to the methods found in IEC-60909.Die doppelgespeiste Asynchrongenerator basierte Windenergieanlage (DASM-WEA) leidet an hohen Kurzschlussströmen und hohen Gleichstromspannungen bei symmetrischen und asymmetrischen Netzfehlern. Die WEA Umrichter sind empfindlich gegenüber hohen Strömen und Spannungen, deswegen sind Schutzeinrichtungen wichtig. Dies kann durch den Einsatz des Bremschoppers und/oder Überspannungsschutzsystemen, oder durch die vollständige Trennung vom Netz bei schwerwiegenden Störfällen erreicht werden. Des Weiteren propagieren die hohen Kurzschlussströme unter bestimmten Umständen ins Netz. Tiefes Verständnis des dynamischen Verhaltens von DASM-WEAs während verschiedener Störfälle ist wichtig für die Beurteilung der FRT Fähigkeiten, richtige Auslegung der Komponenten des elektrischen Systems und die richtigen Einstellungen der Schutzrelais zur selektiven Abschaltung. Als Teil dieser Doktorarbeit sind detaillierte Modelle einzelner Komponenten von DASM-WEAs unter Berücksichtigung von Nichtlinearitäten gemacht worden. Ein Regler wurde für jeweils für die WEA, für den Generatorseitigen- und den Netzseitigen-Umrichter entwickelt. Der Regler erlaubt eine maximale Energieausbeute, eine schnelle und genaue Reaktion sowie separate Regelung für das Mitsystem und das Gegensystem als auch selektive Frequenzkomponenten. Ein neues Kriterium wird vorgestellt, um die Stabilität der Ausgangsströme und -spannungen der DASM-WEA unter Berücksichtigung von Nichtlinearitäten zu bewerten. Eine detaillierte Analyse der DASM-WEA Reaktion auf symmetrische und asymmetrische Störungen wurde ausgeführt. Die Analyse berücksichtigt das gesamte System, sowohl den Einfluss des Reglers als auch die Nichtlinearitäten des Systems. Basierend auf dieser Analyse wurde ein neuer Satz mathematischer Formeln vorgestellt, welche Kurzschlussströme, transiente Impedanzen, Zeitkonstanten und Eigenfrequenzen beschreiben. Diese wurden dem echten Verhalten gegenüber validiert, welches eine hohe Genauigkeit aufzeigte. Basierend auf dieser Analyse wurden neue Methoden zur Reduzierung des hohen Kurzschlussstromes entwickelt. Die neuen Methoden resultieren in der höchsten Reduzierung des Kurzschlussstromes ohne die Umrichter Spannungsgrenzen zu überschreiten, die Trennung der WEA oder den Betrieb in Übermodulation. Als letztes wurde noch eine Methode zur Schätzung der äquivalenten Parameter des DASM-WEA für Fehlerstromberechnungen vorgestellt, in ähnlicher Weise wie in der IEC-60909. Die neue Methode benötigt nur Parameter Identifikation und Lastfreie FRT Tests um die äquivalenten Parameter ohne die Kenntnisse über die Reglerkonfiguration zu schätzen. Des Weiteren wurde eine neue Methode vorgeschlagen das R/X Verhältnis in vermaschten Netzen zu schätzen. Die neue Methode führt zur höheren Genauigkeit als die der Methoden aus der IEC-60909

Inter-Microgrid Operation: Power Sharing, Frequency Restoration, Seamless Reconnection and Stability Analysis

Electrification in the rural areas sometimes become very challenging due to area accessibility and economic concern. Standalone Microgrids (MGs) play a very crucial role in these kinds of a rural area where a large power grid is not available. The intermittent nature of distributed energy sources and the load uncertainties can create a power mismatch and can lead to frequency and voltage drop in rural isolated community MG. In order to avoid this, various intelligent load shedding techniques, installation of micro storage systems and coupling of neighbouring MGs can be adopted. Among these, the coupling of neighbouring MGs is the most feasible in the rural area where large grid power is not available. The interconnection of neighbouring MGs has raised concerns about the safety of operation, protection of critical infrastructure, the efficiency of power-sharing and most importantly, stable mode of operation. Many advanced control techniques have been proposed to enhance the load sharing and stability of the microgrid. Droop control is the most commonly used control technique for parallel operation of converters in order to share the load among the MGs. But most of them are in the presence of large grid power, where system voltage and frequency are controlled by the stiff grid. In a rural area, where grid power is not available, the frequency and voltage control become a fundamental issue to be addressed. Moreover, for accurate load sharing a high value of droop gain should be chosen as the R/X ratio of the rural network is very high, which makes the system unstable. Therefore, the choice of droop gains is often a trade-off between power-sharing and stability. In the context, the main focus of this PhD thesis is the fundamental investigations into control techniques of inverter-based standalone neighbouring microgrids for available power sharing. It aims to develop new and improved control techniques to enhance performance and power-sharing reliability of remote standalone Microgrids. In this thesis, a power management-based droop control is proposed for accurate power sharing according to the power availability in a particular MG. Inverters can have different power setpoints during the grid-connected mode, but in the standalone mode, they all need their power setpoints to be adjusted according to their power ratings. On the basis of this, a power management-based droop control strategy is developed to achieve the power-sharing among the neighbouring microgrids. The proposed method helps the MG inverters to share the power according to its ratings and availability, which does not restrict the inverters for equal power-sharing. The paralleled inverters in coupled MGs need to work in both interconnected mode and standalone mode and should be able to transfer between modes seamlessly. An enhanced droop control is proposed to maintain the frequency and voltage of the MGs to their nominal value, which also helps the neighbouring MGs for seamless (de)coupling. This thesis also presents a mathematical model of the interconnected neighbouring microgrid for stability and robustness analysis. Finally, a laboratory prototype model of two MGs is developed to test the effectiveness of the proposed control strategies

Control of grid-connected three-phase three-wire voltage-sourced inverters under voltage disturbances

Tesi per compendi de publicacions, amb una secció retallada per drets de l'editorThe present doctoral thesis focuses on designing control schemes for three-phase three-wire voltage-sourced inverters connected to the grid under voltage disturbances. The research recognizes the large-scale integration of distributed power generation systems into the network and takes advantage of this circumstance to investigate and develop new control strategies in order to provide better support to the modern power grid. As a first contribution, a new algorithm to maximize power delivery capability of the inverter has been developed and experimentally tested under voltage imbalance conditions, i.e., during slight/shallow and deep asymmetrical sags. The algorithm of this control strategy meets grid code requirements, performs active power control, limits the maximum current injected by the inverter, and eliminates active power oscillations. As a result, six different cases of current injection were identified in this work, considering restrictions imposed by grid codes as well as different active-power production scenarios. The second contribution of this research work has provided an experimental analysis of a low-voltage ride-through strategy whose voltage support capability had not been tested when voltage sags occur. This study was performed considering a scenario of multiple grid-connected inverters, different profiles of active power injection, and the equivalent grid impedance seen from the output side of each converter. In the third contribution has been proposed a closed-loop controller for low-power distributed inverters that maximizes the current injection when voltage sag occurs. The control algorithm has been designed to meet grid code requirements and avoid overvoltage in non-faulty phases during grid faults. The controller is responsible for meeting coordinately several objectives and addressing the interactions that appear among them. In the last two chapters, the argument of this doctoral thesis is complemented, the obtained experimental results are globally analyzed, finally, the present research work is concluded.Esta tesis doctoral, presentada en la modalidad de compendio de publicaciones en cumplimiento parcial de los requisitos para optar al título de Doctor en Ingeniería Electrónica de la Universidad Politécnica de Cataluña, se centra en el diseño de esquemas de control para inversores trifásicos conectados a la red eléctrica durante perturbaciones de voltaje. La investigación reconoce la integración a gran escala de los sistemas de generación distribuida en la red y aprovecha esta circunstancia para estudiar y desarrollar nuevas estrategias de control con el propósito de brindar un mejor soporte a la red eléctrica moderna. Como primera contribución, se desarrolló un nuevo algoritmo para maximizar la capacidad de suministro de potencia del inversor en condiciones de desequilibrio de voltaje, es decir, durante caídas asimétricas de tensión leves, poco profundas y severas. El algoritmo de esta estrategia de control fue diseñado para cumplir los requerimientos de los vigentes códigos de red (grid codes), realizar control de la potencia activa, limitar la corriente máxima inyectada por el inversor y eliminar las oscilaciones de la potencia activa instantánea. Como resultado, en esta investigación se identificaron y validaron experimentalmente seis casos diferentes de inyección de corriente en la red, trabajo que tuvo en cuenta no solo las restricciones impuestas por los códigos de red, sino también los diferentes escenarios de producción de potencia activa. La segunda contribución de este trabajo de investigación ha proporcionado el análisis experimental de una estrategia de inyección de corriente cuya capacidad de soporte de voltaje no se había probado durante fallos de red. Este estudio se realizó sobre un escenario de múltiples inversores conectados a la red eléctrica, utilizando diferentes perfiles de inyección de potencia activa y considerando, como aspecto fundamental para el análisis experimental, la impedancia de red equivalente vista desde el lado de salida de cada convertidor. En la tercera contribución se diseñó un controlador en lazo cerrado para inversores distribuidos de baja potencia que maximiza la inyección de corriente cuando se produce una caída de tensión. Este algoritmo de control también satisface los requerimientos de los actuales códigos de red en cuanto a inyección de corriente reactiva durante fallos de red, pero cuenta con la capacidad adicional de evitar sobretensiones en las fases no defectuosas. De igual forma, este controlador es responsable de acometer coordinadamente varios objetivos y gestionar las interacciones que aparecen entre ellos. En los últimos dos capítulos se complementa la unidad temática de esta tesis doctoral, se analizan globalmente los resultados experimentales obtenidos y, finalmente, se concluye el presente trabajo de investigación agregando, también, futuros campos de estudio.Postprint (published version

Control of grid-connected three-phase three-wire voltage-sourced inverters under voltage disturbances

The present doctoral thesis focuses on designing control schemes for three-phase three-wire voltage-sourced inverters connected to the grid under voltage disturbances. The research recognizes the large-scale integration of distributed power generation systems into the network and takes advantage of this circumstance to investigate and develop new control strategies in order to provide better support to the modern power grid. As a first contribution, a new algorithm to maximize power delivery capability of the inverter has been developed and experimentally tested under voltage imbalance conditions, i.e., during slight/shallow and deep asymmetrical sags. The algorithm of this control strategy meets grid code requirements, performs active power control, limits the maximum current injected by the inverter, and eliminates active power oscillations. As a result, six different cases of current injection were identified in this work, considering restrictions imposed by grid codes as well as different active-power production scenarios. The second contribution of this research work has provided an experimental analysis of a low-voltage ride-through strategy whose voltage support capability had not been tested when voltage sags occur. This study was performed considering a scenario of multiple grid-connected inverters, different profiles of active power injection, and the equivalent grid impedance seen from the output side of each converter. In the third contribution has been proposed a closed-loop controller for low-power distributed inverters that maximizes the current injection when voltage sag occurs. The control algorithm has been designed to meet grid code requirements and avoid overvoltage in non-faulty phases during grid faults. The controller is responsible for meeting coordinately several objectives and addressing the interactions that appear among them. In the last two chapters, the argument of this doctoral thesis is complemented, the obtained experimental results are globally analyzed, finally, the present research work is concluded.Esta tesis doctoral, presentada en la modalidad de compendio de publicaciones en cumplimiento parcial de los requisitos para optar al título de Doctor en Ingeniería Electrónica de la Universidad Politécnica de Cataluña, se centra en el diseño de esquemas de control para inversores trifásicos conectados a la red eléctrica durante perturbaciones de voltaje. La investigación reconoce la integración a gran escala de los sistemas de generación distribuida en la red y aprovecha esta circunstancia para estudiar y desarrollar nuevas estrategias de control con el propósito de brindar un mejor soporte a la red eléctrica moderna. Como primera contribución, se desarrolló un nuevo algoritmo para maximizar la capacidad de suministro de potencia del inversor en condiciones de desequilibrio de voltaje, es decir, durante caídas asimétricas de tensión leves, poco profundas y severas. El algoritmo de esta estrategia de control fue diseñado para cumplir los requerimientos de los vigentes códigos de red (grid codes), realizar control de la potencia activa, limitar la corriente máxima inyectada por el inversor y eliminar las oscilaciones de la potencia activa instantánea. Como resultado, en esta investigación se identificaron y validaron experimentalmente seis casos diferentes de inyección de corriente en la red, trabajo que tuvo en cuenta no solo las restricciones impuestas por los códigos de red, sino también los diferentes escenarios de producción de potencia activa. La segunda contribución de este trabajo de investigación ha proporcionado el análisis experimental de una estrategia de inyección de corriente cuya capacidad de soporte de voltaje no se había probado durante fallos de red. Este estudio se realizó sobre un escenario de múltiples inversores conectados a la red eléctrica, utilizando diferentes perfiles de inyección de potencia activa y considerando, como aspecto fundamental para el análisis experimental, la impedancia de red equivalente vista desde el lado de salida de cada convertidor. En la tercera contribución se diseñó un controlador en lazo cerrado para inversores distribuidos de baja potencia que maximiza la inyección de corriente cuando se produce una caída de tensión. Este algoritmo de control también satisface los requerimientos de los actuales códigos de red en cuanto a inyección de corriente reactiva durante fallos de red, pero cuenta con la capacidad adicional de evitar sobretensiones en las fases no defectuosas. De igual forma, este controlador es responsable de acometer coordinadamente varios objetivos y gestionar las interacciones que aparecen entre ellos. En los últimos dos capítulos se complementa la unidad temática de esta tesis doctoral, se analizan globalmente los resultados experimentales obtenidos y, finalmente, se concluye el presente trabajo de investigación agregando, también, futuros campos de estudio

Contributions to the multi-frequency control of gridtied voltage source converters

El interés por la producción de energía limpia está en aumento y la generación de este tipo de energía se puede fomentar mediante la instalación de generadores locales. Dichos generadores son conectados a la red de distribución a través de convertidores de potencia. Al mismo tiempo el número de cargas conectadas a la red está incrementando y con ello el número de cargas no lineales. Estas últimas consumen corrientes armónicas y esto provoca distorsión armónica a la red. En esta tesis se estudia y se presentan contribuciones en el control de los convertidores de potencia para que al mismo tiempo que se inyecta potencia, el convertidor sea capaz de actuar adecuadamente frente a la distorsión armónica del voltaje de la red (control multifrecuencial). En primer lugar, esta tesis cubrirá el estudio de las diferentes técnicas de control de corrientes armónicas y también de las diferentes técnicas de sincronización y detección de componentes armónicas de tensión presentes en la red. En cuanto al cálculo de referencias de corrientes armónicas, se explican las principales variantes dependiendo de la funcionalidad deseada y se estudia la entrega de potencia instantánea constante incluso con red distorsionada. Además, se propone un nuevo método de cálculo para eliminar las principales oscilaciones de potencia sin exceder las limitaciones de distorsión de corrientes. También se describen las limitaciones del convertidor cuando se trabaja con componentes fundamentales y armónicas. Se analizan los principales saturadores multifrecuenciales para evitar la sobremodulación y se propone un nuevo saturador que no empeora la dinámica total del sistema y siempre consigue el mínimo THD de corriente. Por último, se aborda la problemática de la distorsión armónica del voltaje de red. Primero se estudia la compensación de las corrientes consumidas por cargas locales y después se propone la compensación directa de la tensión del PCC. En esta tesis se intenta incrementar el número de funcionalidades que puede desempeñar el convertidor para que además de entregar potencia, sea capaz de mejorar la calidad de la red, sin exceder las limitaciones físicas del convertidor. Cada una de las contribuciones es validada mediante resultados de simulación y experimentales

Multilevel Monte Carlo approach for estimating reliability of electric distribution systems

Most of the power outages experienced by the customers are due to the failures in the electric distribution systems. However, the ultimate goal of a distribution system is to meet customer electricity demand by maintaining a satisfactory level of reliability with less interruption frequency and duration as well as less outage costs. Quantitative evaluation of reliability is, therefore, a significant aspect of the decision-making process in planning and designing for future expansion of network or reinforcement. Simulation approach of reliability evaluation is generally based on sequential Monte Carlo (MC) method which can consider the random nature of system components. Use of MC method for obtaining accurate estimates of the reliability can be computationally costly particularly when dealing with rare events (i.e. when high accuracy is required). This thesis proposes a simple and effective methodology for accelerating MC simulation in distribution systems reliability evaluation. The proposed method is based on a novel Multilevel Monte Carlo (MLMC) simulation approach. MLMC approach is a variance reduction technique for MC simulation which can reduce the computational burden of the MC method dramatically while both sampling and discretisation errors are considered for converging to a controllable accuracy level. The idea of MLMC is to consider a hierarchy of computational meshes (levels) instead of using single time discretisation level in MC method. Most of the computational effort in MLMC method is transferred from the finest level to the coarsest one, leading to substantial computational saving. As the simulations are conducted using multiple approximations, therefore the less accurate estimate on the preceding coarse level can be sequentially corrected by averages of the differences of the estimations of two consecutive levels in the hierarchy. In this dissertation, we will find the answers to the following questions: can MLMC method be used for reliability evaluation? If so, how MLMC estimators for reliability evaluation are constructed? Finally, how much computational savings can we expect through MLMC method over MC method? MLMC approach is implemented through solving the stochastic differential equations of random variables related to the reliability indices. The differential equations are solved using different discretisation schemes. In this work, the performance of two different discretisation schemes, Euler-Maruyama and Milstein are investigated for this purpose. We use the benchmark Roy Billinton Test System as the test system. Based on the proposed MLMC method, a number of reliability studies of distribution systems have been carried out in this thesis including customer interruption frequency and duration based reliability assessment, cost/benefits estimation, reliability evaluation incorporating different time-varying factors such as weather-dependent failure rate and restoration time of components, time-varying load and cost models of supply points. The numerical results that demonstrate the computational performances of the proposed method are presented. The performances of the MLMC and MC methods are compared. The results prove that MLMC method is computationally efficient compared to those derived from standard MC method and it can retain an acceptable level of accuracy. The novel computational tool including examples presented in this thesis will help system planners and utility managers to provide useful information of reliability of distribution networks. With the help of such tool they can take necessary steps to speed up the decision-making process of reliability improvement.Thesis (Ph.D.) -- University of Adelaide, School of Electrical and Electronic Engineering, 201

Advanced Operation and Maintenance in Solar Plants, Wind Farms and Microgrids

This reprint presents advances in operation and maintenance in solar plants, wind farms and microgrids. This compendium of scientific articles will help clarify the current advances in this subject, so it is expected that it will please the reader