Modeling and Control of a Doubly-Fed Induction Generator for Wind Turbine-Generator Systems

Wind energy plays an increasingly important role in the world because it is friendly to the environment. During the last decades, the concept of a variable-speed wind turbine (WT) has been receiving increasing attention due to the fact that it is more controllable and efficient, and has good power quality. As the demand of controllability of variable speed WTs increases, it is therefore important and necessary to investigate the modeling for wind turbine-generator systems (WTGS) that are capable of accurately simulating the behavior of each component in the WTGS. Therefore, this thesis will provide detailed models of a grid-connected wind turbine system equipped with a doubly-fed induction generator (DFIG), which includes the aerodynamic models of the wind turbine, the models of the mechanical transmission system, the DFIG models and the three-phase two-level PWM voltage source converter models. In order to obtain satisfying output power from the WTGS, control strategies are also necessary to be developed based on the previously obtained WTGS models. These control schemes include the grid-side converter control, the generator-side converter control, the maximum power point tracking control and the pitch angle control. The grid-side converter controller is used to keep the DC-link voltage constant and yield a unity power factor looking into the WTGS from the grid-side. The generator-side converter controller has the ability of regulating the torque, active power and reactive power. The maximum power point tracking control is used to provide the reference values for the active power at the stator terminals. The pitch angle control scheme is used to regulate the pitch angle and thus keep the output power at rated value even when the wind speed experiences gusts. Various studies in the literature have reported that two-level converters have several disadvantages compared with three-level converters. Among the disadvantages are high switching losses, high dv/dt, and high total harmonic distortion (THD). Hence, the models and field oriented control schemes for three-level neutral-point-clamped (NPC) converters are also investigated and applied to a WTGS. Besides, an advanced modulation technology, namely, space vector PWM (SVPWM), is also investigated and compared to traditional sinusoidal PWM in a WTGS

Advanced Converter Control Techniques for Improving the Performances of DFIG based Wind Turbines

Unter den alternativen erneuerbaren Energiequellen hat Windenergie in den letzten zehn Jahren den größten Stellenwert im Energieerzeugungssystem erlangt. Die Erhaltung bzw. die Verbesserung der Zuverlässigkeit des Stromversorgungsnetzes mit zunehmenden Windenergieanlagen und deren optimale Nutzung ist eine der wichtigsten Aufgaben. Die Windenergieanlagen sind im Netzbetrieb bestimmten in Netzanschlussrichtlinien angegebenen Anschlussregeln unterworfen. Dies erfordert eine detaillierte Untersuchung von Windenergieanlagen in verschiedenen operativen Szenarien, so dass geeignete Lösungen empfohlen werden können, insbesondere bezüglich Umrichter-Regelung, die die Hauptrolle im Gesamtsystem spielen. In dieser Forschung wurde der doppelt-gespeiste Asynchrongenerator, der immer noch am häufigsten verwendeter Windturbinentyp ist, für eine detaillierte Untersuchung ausgewählt. Sowohl das Betriebsverhalten im stationären Betrieb allgemein als auch unter Berücksichtigung von zwei alternativen Pulsweiten-Modulation (PWM)-Typen und verschiedenen Umrichter-Topologie, untersucht. Vergleichskriterien sind die erzeugte maschinenseitige „common mode“ Spannung, Gesamtverzerrung der Stromwelle im Niederspannungsnetz, Umrichter-Leistungsverluste und Blindleistung-Einspeisefähigkeit. Zusätzlich werden die Anzahl der Komponenten im kompletten Umrichter-System und die geschätzten Kosten als Vergleichskriterien herangezogen. Bezüglich des ersten Szenarios, der Einfluss unterschiedlicher PWM-Typen auf Umrichter Verlustleistung, die Blindleistung-Einspeisefähigkeit und die gesamte harmonische Verzerrung wurden im Detail untersucht, und der am besten geeignete PWM-Typ bezüglich optimaler Leistungskriterien sowie Drehzahlbereiche vorgeschlagen. Im zweiten Szenario wurden zwei verschiedene Umrichter-Topologie, nämlich zweistufiger „Back-to-Back“ Umrichter und dreistufiger „Neutral-Point-Clamped (NPC)“ „Back-to-Back“ Umrichter wurden im Simulationsmodell implementiert, und auf der Grundlage der Simulation Ergebnisse ihre Eignung in Bezug auf Kosten gegen die anfallenden Betriebsvorteile verglichen. Schließlich wurde ein neues Schutzschema für „Fault-Ride-Through“ im dreistufigen „Backto- Back“ NPC-Umrichter als Alternative zum konventionellen Schutzschema mit Chopper im Gleichspannung-Zwischenkreis vorgeschlagen. Das vorgeschlagene Schema zeigt ein sehr ähnliches dynamisches Verhalten wie das konventionelle Schema, wenn die inneren IGBTs des maschinenseitigen Wechselrichters (MWR) für etwa zweifachen Nennstrom der Überstromschutzgrenze ausgelegt werden. Außerdem ermöglicht eine einfachere Bedienung ohne höhere Anzahl von Komponenten. Die Verwendung von inneren IGBTs mit höherem Nennstrom erhöht die Kosten der MWR. Jedoch werden die Gesamt Kosten um etwa 15% weniger, da der Chopper im Gleichspannung-Zwischenkreis dadurch überflüssig gemacht wird.Among the renewable energy alternatives, wind energy has made the biggest impact on the total energy production in the last decade. Maintaining or improving the reliability of the wind turbine system in power generation sector with optimal performances is one of the important tasks. Especially the wind turbines connected to the grid are subjected to certain electricity grid connection regulations specified in grid codes. A detailed study of the performance of wind turbine systems in various case scenarios is necessary, so that appropriate solutions can be recommended, especially in the converter controls which play the major role in the overall system. In this thesis, the doubly fed induction generator (DFIG) which is still the most widely used wind turbine type is selected for detailed investigation. Its performances during steady state operation in two alternative scenarios, namely, using different pulse width modulation (PWM) types and using different converter topologies, are investigated. The performance criteria include generated common mode voltage at machine side converter (MSC), current total harmonic distortion in the low voltage network, converter power losses and reactive power capability. Additionally, the component counts in the converter and its estimate cost are compared. Regarding the first scenario, the influence of different PWM types on the converter power losses, the reactive power capability and the total harmonic distortion has been investigated in detail, and the most suitable PWM type depending on the optimal performance criteria as well as operational speed range is proposed. In the second scenario, two different converter topologies, namely back-to-back two-level converter and back-to-back three-level neutral point clamped (NPC) converter were implemented in the simulation model, and on the basis of the simulation results their performances in terms of cost against the accruing operational advantages are compared. Finally, a new protection scheme for fault ride-through in back-to-back three-level NPC converter is proposed as an alternative to conventional protection scheme using DC-link chopper. The proposed scheme shows a very similar dynamic behaviors with the conventional scheme when the inner IGBTs of the MSC are designed for about two times higher current rating than the over-current protection limit. Furthermore, it implements simpler operation without higher component count. The need for the inner IGBTs with higher current rating significantly increases the cost of the MSC. However, the total cost of the DFIG system is slightly reduced about 15% by the elimination of the DC-link chopper circuit

Power Converter of Electric Machines, Renewable Energy Systems, and Transportation

Power converters and electric machines represent essential components in all fields of electrical engineering. In fact, we are heading towards a future where energy will be more and more electrical: electrical vehicles, electrical motors, renewables, storage systems are now widespread. The ongoing energy transition poses new challenges for interfacing and integrating different power systems. The constraints of space, weight, reliability, performance, and autonomy for the electric system have increased the attention of scientific research in order to find more and more appropriate technological solutions. In this context, power converters and electric machines assume a key role in enabling higher performance of electrical power conversion. Consequently, the design and control of power converters and electric machines shall be developed accordingly to the requirements of the specific application, thus leading to more specialized solutions, with the aim of enhancing the reliability, fault tolerance, and flexibility of the next generation power systems

Power Quality Enhancement in Electricity Grids with Wind Energy Using Multicell Converters and Energy Storage

In recent years, the wind power industry is experiencing a rapid growth and more wind farms with larger size wind turbines are being connected to the power system. While this contributes to the overall security of electricity supply, large-scale deployment of wind energy into the grid also presents many technical challenges. Most of these challenges are one way or another, related to the variability and intermittent nature of wind and affect the power quality of the distribution grid. Power quality relates to factors that cause variations in the voltage level and frequency as well as distortion in the voltage and current waveforms due to wind variability which produces both harmonics and inter-harmonics. The main motivation behind work is to propose a new topology of the static AC/DC/AC multicell converter to improve the power quality in grid-connected wind energy conversion systems. Serial switching cells have the ability to achieve a high power with lower-size components and improve the voltage waveforms at the input and output of the converter by increasing the number of cells. Furthermore, a battery energy storage system is included and a power management strategy is designed to ensure the continuity of power supply and consequently the autonomy of the proposed system. The simulation results are presented for a 149.2 kW wind turbine induction generator system and the results obtained demonstrate the reduced harmonics, improved transient response, and reference tracking of the voltage output of the wind energy conversion system.Peer reviewedFinal Accepted Versio

Source Grid Interface of Wind Energy Systems

Wind power is one of the most developed and rapidly growing renewable energy sources. Through extensive literature review this thesis synthesizes the existing knowledge of wind energy systems to offer useful information to developers of such systems. Any prototyping should be preceded by theoretical analysis and computer simulations, foundations for which are provided here. The thesis is devoted to an in-depth analysis of wind energy generators, system configurations, power converters, control schemes and dynamic and steady state performance of practical wind energy conversion systems (WECS). Attention is mainly focused on interfacing squirrel cage Induction generators (SCIG) and doubly-fed induction generators (DFIG) with the power network to capture optimal power, provide controllable active and reactive power and minimize network harmonics using the two-level converter, as a power electronic converter. Control of active and reactive power, frequency and voltage are indispensable for stability of the grid. This thesis focuses on two main control techniques, field oriented control (FOC) and direct torque control (DTC) for the SCIG. The dynamic model of induction generator is non-linear and hence for all types of control, the flux and the torque have to be decoupled for maintaining linearity between input and output for achieving high dynamic performance. FOC is used for decoupled control for rotor flux and electromagnetic torque . The stator current is decomposed into flux and torque producing components and they both are controlled independently. FOC uses three feedback control loops generate gating signals for the converter. DTC also achieves high dynamic performance by decoupling of rotor flux and electromagnetic torque without the intermediate current loops. DTC asks for the estimation of stator flux and torque and like FOC has 2 branches which have flux and torque comparators. The errors between the set and the estimated value are used to drive the inverters. The two methods are valid for both steady and transient state. Their validity is confirmed by simulating the systems on MATLAB/Simulink platform and comparing them the results obtained by hand calculations. Further DFIG’s are introduced. The dynamic model is developed using the machines equivalent circuit and is expressed in the stationary, rotor and the synchronous reference frames for evaluating the performance of the machine. The stator of the DFIG is directly interfaced to the grid and by controlling the rotor voltage by a two level back-to-back converter the grid synchronization and power control is maintained. The DTC and the direct power control (DPC) methods are used to control the rotor side (RSC) and the grid side converter (GSC). The RSC generates the 3-ph voltages of variable frequency in order to control the generator torque and the reactive power exchanged between the stator and the grid. The GSC exchanges active power with the grid injected by the RSC with a constant frequency. The steady and transient behavior of the machine is investigated through simulations

Voltage sensorless based virtual flux control of three level NPC back-to-back converter dfigunder grid fault

In this paper, a harmonic elimination of grid and stator currents of doubly fed induction generator (DFIG) in case of grid fault without line voltage sensors is proposed . This can be achieved by compensating power based on virtual flux voltage sensorless technique. Direct power control with space vector modulation (DPC-SVM) is used to control both grid-side (GSC)and rotor-side converters (RSC). To achieve the control objective, compensated active and reactive powers are calculated based on virtual flux technique with balanced and harmonic free current as a control target. A theoretical analysis of active and reactive powers under unbalanced voltage source is clearly demonstrated and the effect of grid fault on the performance of DFIG is profoundly discussed. Simulation results verified the effectiveness of the modified control strategy

A review on DC collection grids for offshore wind farms with HVDC transmission system

Abstract: Traditionally, the internal network composition of offshore wind farms consists of alternating current (AC) collection grid; all outputs of wind energy conversion units (WECUs) on a wind farm are aggregated to an AC bus. Each WECU includes: a wind-turbine plus mechanical parts, a generator including electronic controller, and a huge 50-or 60-Hz power transformer. For a DC collection grid, all outputs of WECUs are aggregated to a DC bus; consequently, the transformer in each WECU is replaced by a power converter or rectifier. The converter is more compact and smaller in size compared to the transformer. Thus reducing the size and weight of the WECUs, and also simplifying the wind farm structure. Actually, the use of offshore AC collection grids instead of offshore DC collection grids is mainly motivated by the availability of control and protection devices. However, efficient solutions to control and protect DC grids including HVDC transmission systems have already been addressed. Presently, there are no operational wind farms with DC collection grids, only theoretical and small-scale prototypes are being investigated worldwide. Therefore, a suitable configuration of the DC collection grid, which has been practically verified, is not available yet. This paper discussed some of the main components required for a DC collection grid including: the wind-turbine-generator models, the control and protection methods, the offshore platform structure, and the DC-grid feeder configurations. The key component of a DC collection grid is the power converter; therefore, the paper also reviews some topologies of power converter suitable for DC grid applications

Prädiktive Regelung und Finite-Set-Beobachter für Windgeneratoren mit variabler Drehgeschwindigkeit

This dissertation presents several model predictive control (MPC) techniques and finite-position-set observers (FPSOs) for permanent-magnet synchronous generators and doubly-fed induction generators in variable-speed wind turbines. The proposed FPSOs are novel ones and based on the concept of finite-control-set MPC. Then, the problems of the MPC techniques like sensitivity to variations of the model parameters and others are investigated and solved in this work.Die vorliegende Dissertation stellt mehrere unterschiedliche Verfahren der modellprädiktiven Regelung (MPC) und so genannte Finite-Position-Set-Beobachter (FPSO) sowohl für Synchrongeneratoren mit Permanentmagneterregung als auch für doppelt gespeiste Asynchrongeneratoren in Windkraftanlagen mit variabler Drehzahl vor und untersucht diese. Für die Beobachter (FPSO) wird ein neuartiger Ansatz vorgestellt, der auf dem Konzept der Finite-Control-Set-MPC basiert. Außerdem werden typische Eigenschaften der MPC wie beispielsweise die Anfälligkeit gegenüber Parameterschwankungen untersucht und kompensiert