Enhancement of Transient Stability of DFIG Based Variable Speed Wind Generator Using Diode-bridge-type Non-superconducting Fault Current Limiter and Resistive Solid State Fault Current Limiter

The application of doubly-fed induction generator (DFIG) is very effective in the fast-growing wind generator (WG) market. The foremost concern for the DFIG based WG system is to maintain the transient stability during fault, as the stator of the DFIG is directly connected to the grid. Therefore, transient stability enhancement of the DFIG is very important. In this work, a diode-bridge-type nonsuperconducting fault current limiter (NSFCL) and resistive solid-state fault current limiter (R-type SSFCL) are examined to augment the transient stability of the DFIG based WG system.In simulations, temporary balanced and unbalanced faults were applied in the test system to investigate the proposed NSFCL and the R-type SSFCL transient stability performance. Besides a DC resistive superconducting fault current limiter (SFCL), bridge-type fault current limiter (BFCL) and series dynamic braking resistor (SDBR) are also considered to compare their performance with the proposed NSFCL and R-type SSFCL. These simulations were performed with Matlab/Simulink software. Simulation results clearly indicate that the NSFCL and R-type SSFCL enhances the transient stability of the DFIG based WG. Moreover, the NSFCL works better than the DC resistive SFCL, BFCL and SDBR in every aspect and R-type SSFCL works better than the SDBR in all aspect

Comparison of low voltage ride through capabilities of synchronous generator with STATCOM and DFIG based wind farms

Includes bibliography.Increase in wind generation and grid-integration of wind energy technologies has resulted from an increasing demand of cheap and clean electricity across the globe. Wind generators are available as small, medium and large scale electric generators, usually in the range of 1kW to 100MW and are usually installed in areas rich in wind resource which may or may not be located close to the load centres. Wind energy penetration has increased since the 1970s with the total worldwide capacity of installed wind power reaching about 282,275 MW. Apart from technical issues of grid-integration, research is also being done to investigate the participation of wind energy systems to enhance grid performance through fault ride-through capabilities, providing voltage control and power quality improvement etc. The goal of a Fault Ride Through (FRT) or Low Voltage Ride Through (LVRT) system is to enable a wind farm (WF) to withstand a severe voltage dip at the connection point and still stay connected to the power system as long as the fault persists. Wind turbine designs are required to incorporate LVRT capability as per Grid Code’s requirements only if they are technically needed for a reliable and secure power system operation. The basic requirement for LVRT is that the wind turbines must maximise their reactive power injections to the network without exceeding the turbine limits. The maximisation of reactive current must continue for at least 150msafter the fault clearance or until the grid voltage is recovered within the normal operation range. It is important here to discuss here the immediate impact of the voltage dip on the wind farm (WF) operation. During the voltage dip caused by the fault, the active power provided to the grid by the WF is instantaneously reduced. This power becomes at least temporarily lower than the mechanical power available at the rotor hence the rotor speed of the wind generator increases. It is required for the LVRT capability of the WF, that the wind generators of the WF must not disconnect from the grid during fault persistence, either due to over-speeding or under voltage protections. After the clearing of the fault that led to the voltage dip, the voltage at the wind turbine bus would increase. It is also required that the wind generators should resume their power supply to the network without losing stability

Large Grid-Connected Wind Turbines

This book covers the technological progress and developments of a large-scale wind energy conversion system along with its future trends, with each chapter constituting a contribution by a different leader in the wind energy arena. Recent developments in wind energy conversion systems, system optimization, stability augmentation, power smoothing, and many other fascinating topics are included in this book. Chapters are supported through modeling, control, and simulation analysis. This book contains both technical and review articles

Fault ride-through improvement of DFIG-WT by integrating a two-degrees-of-freedom internal model control

A novel two-degree-of-freedom internal model control (IMC) controller that improves the fault ride-through (FRT) capabilities and crowbar dynamics of doubly fed induction generator (DFIG) wind turbines is presented. As opposed to other control strategies available in the open literature, the proposed IMC controller takes into account the power limit characteristic of the DFIG back-to-back converters and their dc-link voltage response in the event of a fault and consequent crowbar operation. Results from a digital model implemented in Matlab/Simulink and verified by a laboratory scale-down prototype demonstrate the improved DFIG FRT performance with the proposed controller

Performance analysis of doubly-fed induction generator (DFIG)- based wind turbine with sensored and sensorless vector control

PhD ThesisConventional energy sources are limited and pollute the environment. Therefore more attention has been paid to utilizing renewable energy resources. Wind energy is the fastest growing and most promising renewable energy source due to its economically viability. Wind turbine generator systems (WTGSs) are being widely manufactured and their number is rising dramatically day by day. There are different generator technologies adopted in wind turbine generator systems, but the most promising type of wind turbine for the future market is investigated in the present study, namely the doubly-fed induction generator wind turbine (DFIG). This has distinct advantages, such as cost effectiveness, efficiency, less acoustic noise, and reliability and in addition this machine can operate either in grid-connected or standalone mode. This investigation considers the analysis, modeling, control, rotor position estimation and impact of grid disturbances in DFIG systems in order to optimally extract power from wind and to accurately predict performance. In this study, the dynamic performance evaluation of the DFIG system is depicted the power quantities (active and reactive power) are succeed to track its command signals. This means that the decouple controllers able to regulating the impact of coupling effect in the tracking of command signals that verify the robust of the PI rotor active power even in disturbance condition. One of the main objectives of this study is to investigate the comparative estimation analysis of DFIG-based wind turbines with two types of PI vector control using PWM. The first is indirect sensor vector control and the other type includes two schemes using model reference adaptive system (MRAS) estimators to validate the ability to detect rotor position when the generator is connected to the grid. The results for the DFIG-based on reactive power MRAS (QRMRAS) are compared with those of the rotor current-based MRAS (RCMRAS) and the former scheme proved to be better and less sensitive to parameter deviations, its required few mathematical computations and was more accurate. During the set of tests using MATLAB®/SMULINK® in adjusting the error between the reference and adaptive models, the estimated rotor position can be obtained with the objective of achieving accurate rotor position information, which is usually measured by rotary encoders or resolvers. The use of these encoders will conventionally lead to increased cost, size, weight, and wiring ii complexity and reduced the mechanical robustness and reliability of the overall DFIG drive systems. However the use of rotor position estimation represents a backup function in sensor vector control systems when sensor failure occurs. The behavioral response of the DFIG-based wind turbine system to grid disturbances is analyzed and simulated with the proposed control strategies and protection scheme in order to maintain the connection to the network during grid faults. Moreover, the use of the null active and reactive reference set scheme control strategy, which modifies the vector control in the rotor side converter (RSC) contributes to limiting the over-current in the rotor windings and over-voltage in the DC bus during voltage dips, which can improve the Low Voltage Ride-through (LVRT) ability of the DFIG-based wind turbine system.my home country of Iraq and its Ministry of Planning for providing a scholarship for my study

Unified Power Quality Conditioner for Grid Integration of Wind Generators

A Unified Power Quality Conditioner (UPQC) is relatively a new member of the custom power device family. It is a comprehensive custom power device, with integrated shunt and series active filters. The cost of the device, which is higher than other custom power/FACTS devices, because of twin inverter structure and control complexity, will have to be justified by exploring new areas of application where the cost of saving power quality events outweighs the initial cost of installation. Distributed generation (such as wind generation) is one field where the UPQC can find its potential application. There has been a considerable increase in the power generation from wind farms. This has created the necessity for wind farms connectivity with the grid during power system faults, voltage sags and frequency variations. The application of active filters/custom power devices in the field of wind generation to provide reactive power compensation, additional fault ride through capability and to maintain Power Quality (PQ) at the point of common coupling is gaining popularity. Wind generation like other forms of distributed generation often relies on power electronics technology for flexible interconnection to the power grid. The application of power electronics in wind generation has resulted in improved power quality and increased energy capture. The rapid development in power electronics, which has resulted in high kVA rating of the devices and low price per kVA, encourages the application of such devices at distribution level. This work focuses on development of a laboratory prototype of a UPQC, and investigation of its application for the flexible grid integration of fixed and variable speed wind generators through dynamic simulation studies. A DSP based fully digital controller and interfacing hardware has been developed for a 24 kVA (12 kVA-shunt compensator and 12 kVA-series compensator) laboratory prototype of UPQC. The modular control approach facilitates the operation of the device either as individual series or shunt compensator or as a UPQC. Different laboratory tests have been carried out to demonstrate the effectiveness of developed control schemes.A simulation-based analysis is carried out to investigate the suitability of application of a UPQC to achieve Irish grid code compliance of a 2 MW Fixed Speed Induction Generator (FSIG). The rating requirement of the UPQC for the wind generation application has been investigated. A general principle is proposed to choose the practical and economical rating of the UPQC for this type of application. A concept of UPQC integrated Wind Generator (UPQC-WG) has been proposed. The UPQC-WG is a doubly fed induction machine with converters integrated in the stator and rotor circuits and is capable of adjustable speed operation. The operation of UPQC-WG under sub and super-synchronous speed range has been demonstrated. The Irish grid code compliance of the same has been demonstrated with a detailed dynamic simulation

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

The impact of wind generators on a Powe system's transient stability

This thesis discusses the investigations carried out on the different types of wind generators and how these would affect the transient stability of a hypothetical power network as presented in this report. Focus was on the transient responses of the conventional synchronous generator’s rotor angle and terminal voltage when connected to different types of wind generators. The three different wind generator technologies explored were the squirrel cage induction generator (SCIG), doubly-fed induction generator (DFIG) and the converter driven synchronous generator (CDSG)

Data-driven Protection of Transformers, Phase Angle Regulators, and Transmission Lines in Interconnected Power Systems

This dissertation highlights the growing interest in and adoption of machine learning approaches for fault detection in modern electric power grids. Once a fault has occurred, it must be identified quickly and a variety of preventative steps must be taken to remove or insulate it. As a result, detecting, locating, and classifying faults early and accurately can improve safety and dependability while reducing downtime and hardware damage. Machine learning-based solutions and tools to carry out effective data processing and analysis to aid power system operations and decision-making are becoming preeminent with better system condition awareness and data availability. Power transformers, Phase Shift Transformers or Phase Angle Regulators, and transmission lines are critical components in power systems, and ensuring their safety is a primary issue. Differential relays are commonly employed to protect transformers, whereas distance relays are utilized to protect transmission lines. Magnetizing inrush, overexcitation, and current transformer saturation make transformer protection a challenge. Furthermore, non-standard phase shift, series core saturation, low turn-to-turn, and turn-to-ground fault currents are non-traditional problems associated with Phase Angle Regulators. Faults during symmetrical power swings and unstable power swings may cause mal-operation of distance relays, and unintentional and uncontrolled islanding. The distance relays also mal-operate for transmission lines connected to type-3 wind farms. The conventional protection techniques would no longer be adequate to address the above-mentioned challenges due to their limitations in handling and analyzing the massive amount of data, limited generalizability of conventional models, incapability to model non-linear systems, etc. These limitations of conventional differential and distance protection methods bring forward the motivation of using machine learning techniques in addressing various protection challenges. The power transformers and Phase Angle Regulators are modeled to simulate and analyze the transients accurately. Appropriate time and frequency domain features are selected using different selection algorithms to train the machine learning algorithms. The boosting algorithms outperformed the other classifiers for detection of faults with balanced accuracies of above 99% and computational time of about one and a half cycles. The case studies on transmission lines show that the developed methods distinguish power swings and faults, and determine the correct fault zone. The proposed data-driven protection algorithms can work together with conventional differential and distance relays and offer supervisory control over their operation and thus improve the dependability and security of protection systems