Nonlinear Dual-Mode Control of Variable-Speed Wind Turbines with Doubly Fed Induction Generators

This paper presents a feedback/feedforward nonlinear controller for variable-speed wind turbines with doubly fed induction generators. By appropriately adjusting the rotor voltages and the blade pitch angle, the controller simultaneously enables: (a) control of the active power in both the maximum power tracking and power regulation modes, (b) seamless switching between the two modes, and (c) control of the reactive power so that a desirable power factor is maintained. Unlike many existing designs, the controller is developed based on original, nonlinear, electromechanically-coupled models of wind turbines, without attempting approximate linearization. Its development consists of three steps: (i) employ feedback linearization to exactly cancel some of the nonlinearities and perform arbitrary pole placement, (ii) design a speed controller that makes the rotor angular velocity track a desired reference whenever possible, and (iii) introduce a Lyapunov-like function and present a gradient-based approach for minimizing this function. The effectiveness of the controller is demonstrated through simulation of a wind turbine operating under several scenarios.Comment: 14 pages, 9 figures, accepted for publication in IEEE Transactions on Control Systems Technolog

Control, stability analysis and grid integration of wind turbines.

In Chapters 2 and 3 of the thesis we propose a self-scheduled control method for a doublyfed induction generator driven by a wind turbine (DFIGWT), whose rotor is connected to the power grid via two back-to-back PWM power converters. We design a controller for this system using the linear matrix inequality based approach to linear parameter varying (LPV) systems, which takes into account the nonlinear dynamics of the system. We propose a two-loop hierarchical control structure. The inner-loop current controller, which considers the synchronous speed and the generator rotor speed as a parameter vector, achieves robust tracking of the rotor current reference signals. The outer-loop electrical torque controller aims for wind energy capture maximization, grid frequency support and generates the reference rotor current. We perform a controller reduction for the inner-loop LPV controller, which is not doable by conventional model-reduction techniques, because the controller is parameter-dependent. In simulation, the reduced order controller has been tested on a nonlinear 4th order DFIG model with a two-mass model for the drive-train. Stability and high performances have been achieved over the entire operating range of the DFIGWT. More importantly, simulation results have demonstrated the capability and contribution of the proposed two-loop control systems to grid frequency support. In Chapter 4 we investigate the integral input-to-state stability (iISS) property for passive nonlinear systems. We show that under mild assumptions, a passive nonlinear system which is globally asymptotically stable is also iISS. Moreover, the integral term from the definition of the iISS property has a very simple form (like an L1 norm). These theoretical results will be useful for our stability analysis of wind turbine systems in Chapter 5. In Chapter 5 we investigate the stability of a variable-speed wind turbine operating under low to medium wind speed. The turbine is controlled to capture as much wind energy as possible. We concentrate on the mechanical level of the turbine system, more precisely on the drive-train with the standard quadratic generator torque controller. We consider both the one-mass and the two-mass models for the drive-train, with the inputs being the deviation of the active torque from an arbitrary positive nominal value and the tracking error of the generator torque. We show that the turbine system is input-to-state stable for the one-mass model and iISS for the two-mass model. Using our abstract results from Chapter 4, we identify the iISS gain of this system. We also propose an adaptive search algorithm for the optimal gain of the quadratic torque controller

From Dynamic Modeling to Experimentation of Induction Motor Powered by Doubly-Fed Induction Generator by Passivity-Based Control

ISBN: 978-953-307-548-8DFIG wind turbines are nowadays more widely used especially in large wind farms. The main reason for their popularity when connected to the electrical network is their ability to supply power at constant voltage and frequency while the rotor speed varies, which makes it suitable for applications with variable speed. We consider in this paper the isolated operation of a DFIG driven by a prime mover, with its stator connected to a load--which is in this case an IM. This paper presents a dynamic model of the DFIG-IM and proves that this system is Blondel-Park transformable. It is also shown that the zero dynamics is unstable for a certain operating regime. We propose and analyse different strategies for the control of the whole system mainly based on Passivity Based Control (PBC). Several of them have been implemented on a 200W DFIG interconnected with an IM prototype available in IRII-UPC (Institute of Robotics and Industrial Informatics - UPC - Barcelona). The main disadvantage of the DFIG is the slip rings, which reduce the life time of the machine and increases the maintenance costs. To overcome this drawback an alternative machine arrangement is proposed which is the Brushless Doubly Fed twin Induction Generator (BDFTIG). The system is anticipated as an advanced solution to the conventional doubly fed induction generator (DFIG). The proposed BDFTIG employs two cascaded induction machines each consisting of two wound rotors, connected in cascade to eliminate the brushes and copper rings in the DFIG. The dynamic model of BDFTIG with two machines' rotors electromechanically coupled in the back-to-back configuration is developed and implemented using Matlab/Simulink

Efficient Control Approaches for Guaranteed Frequency Performance in Power Systems

Due to high penetration of renewable energy, converter-interfaced sources are increasing in power systems and degrading the grid frequency response. Synthetic inertia emulation and guaranteed primary frequency response is a challenging task. Still, there is high potential for application of highly controllable converter-interfaced devices to help performance. Renewable energy sources and demand side smart devices also need to be equipped with innovative frequency control approaches that contribute to frequency regulation operations. First, the wind turbine generator is chosen to represent an example of a converter- interfaced source. An augmented system frequency response model is derived, including the system frequency response model and a reduced-order model of the wind turbine generator representing the supportive active power due to supplementary inputs. An output feedback observer-based control is designed to provide guaranteed frequency performance. System performance is analyzed for different short circuit ratio scenarios where a lower bound to guarantee the performance is obtained. Second, the load side control for frequency regulation with its challenges is introduced. 5G technology and its potential application in smart grids are analyzed. The effect of communication delays and packet losses on inertia emulation are investigated to show the need of using improved communication infrastructure. Third, a robust delay compensation for primary frequency control using fast demand response is proposed. Possible system structured uncertainties and communication delays are considered to limit frequency variations using the proposed control approach. An uncertain governor dead-band model is introduced to capture frequency response characteristics. Guaranteed inertial response is achieved and compared with a PI-based Smith predictor controller to show the effectiveness of the proposed method. Fourth, set theoretic methods for safety verification to provide guaranteed frequency response are introduced. The Barrier certificate approach using a linear programming relaxation by Handelman’s representation is proposed with its application to power systems. Finally, the Handelman’s based barrier certificate approach for adequate frequency performance is studied. The computational algorithm is provided for the proposed method and validated using power system benchmark case studies with a discussion on a safety supervisory control (SSC)

Variable speed wind turbine control scheme using a robust wind torque estimation

This work proposes a robust controller for a variable speed wind turbine system with a doubly feed induction generator. The controller aims at tracking the optimal speed of the wind turbine so that extracts the maximum power from the wind. Also, a robust aerodynamic torque observer is proposed in order to avoid the use of wind speed sensors. This torque observer allows to estimate the aerodynamic torque to be used by the controller in order to calculate the value of the optimal reference speed for the wind turbine. The vector control theory is applied in the present approach, and thereby the stator flux-oriented control is used for controlling the speed of the wind turbine generator. The proposed robust control law is based on sliding mode control theory, which has proved to provide good performance under system uncertainties. The stability of the proposed controller under disturbances and parameter uncertainties has been analyzed using the Lyapunov stability theory. Finally, real time experimental results show that, on the one hand, the proposed controller provides high-performance dynamic characteristics, and on the other hand, this scheme is robust with respect to the uncertainties that usually appear in this kind of systems.The authors are very grateful to the UPV/EHU by its support through the projects PPGA18/04 and UFI11/07 and to the Basque Government by its support through the project ELKARTEK KK-2017/00033

Subsynchronous Resonance in Doubly-Fed Induction Generator Based Wind Farms

The objective of this thesis is to investigate the risk for instabilities due to SubSynchronous Resonances (SSR) conditions in large wind farms connected to series-compensated transmission lines. In particular, the focus is on Doubly-Fed Induction Generator (DFIG) based wind farms. Analytical models of the system under investigation are derived in order to understand the root causes that can lead to instabilities. A frequency dependent approach, based on the Nyquist criterion, has been applied in order to investigate the risk for SSR in DFIG based wind turbines. Through this approach, it is shown that the observed phenomenon is mainly due to an energy exchange between the power converter of the turbine and the series compensated grid. This phenomenon, here referred to as SubSynchronous Controller Interaction (SSCI), is driven by the control system of the turbine, which presents a non-passive behavior in the subsynchronous frequency range. The different factors that impact the frequency characteristic of the wind turbine, thereby making the system prone to SSCI interaction, have been investigated. Through this analysis, it is shown that in a DFIG wind turbine, the current controller in the rotor-side converter plays a major role and that the risk for SSR increases when increasing its closed-loop bandwidth. In addition, it is shown that the output power generated from the wind turbine has an impact on the frequency characteristic of the turbine. Time-domain studies are performed on an aggregated wind turbine model connected to a series compensated transmission line with the objective of verifying the analytical results obtained through frequency-domain analysis. Based on the theoretical analysis, mitigation strategies are proposed in order to shape the impedance behavior of the wind turbine in the incident of SSCI. The effectiveness of the proposed mitigation strategies are evaluated both theoretically through frequency domain analysis and using detailed time-domain simulations

Offshore Wind Farm-Grid Integration: A Review on Infrastructure, Challenges, and Grid Solutions

Recently, the penetration of renewable energy sources (RESs) into electrical power systems is witnessing a large attention due to their inexhaustibility, environmental benefits, storage capabilities, lower maintenance and stronger economy, etc. Among these RESs, offshore wind power plants (OWPP) are ones of the most widespread power plants that have emerged with regard to being competitive with other energy technologies. However, the application of power electronic converters (PECs), offshore transmission lines and large substation transformers result in considerable power quality (PQ) issues in grid connected OWPP. Moreover, due to the installation of filters for each OWPP, some other challenges such as voltage and frequency stability arise. In this regard, various customs power devices along with integration control methodologies have been implemented to deal with stated issues. Furthermore, for a smooth and reliable operation of the system, each country established various grid codes. Although various mitigation schemes and related standards for OWPP are documented separately, a comprehensive review covering these aspects has not yet addressed in the literature. The objective of this study is to compare and relate prior as well as latest developments on PQ and stability challenges and their solutions. Low voltage ride through (LVRT) schemes and associated grid codes prevalent for the interconnection of OWPP based power grid have been deliberated. In addition, various PQ issues and mitigation options such as FACTS based filters, DFIG based adaptive and conventional control algorithms, ESS based methods and LVRT requirements have been summarized and compared. Finally, recommendations and future trends for PQ improvement are highlighted at the end