Voltage Distortion Mitigation in a Distributed Generation-integrated Weak Utility Network Via a Self-tuning Filter-based Dynamic Voltage Restorer

The dynamic voltage restorer (DVR) is mainly used in a utility grid to protect sensitive loads from power quality problems, such as voltage sags and swells. However, the effectiveness of the DVR can wane under unbalanced grid voltage conditions. Recently, DVR control algorithms have been developed that enable the elimination of voltage harmonics in weak and distorted utility networks. This paper presents a modified control method for the DVR, which can (1) compensate the voltage swell and (2) eliminate the voltage harmonics in a combined utility condition consisting of voltage unbalance and harmonic distortion. A self-tuning filter (STF) is used along with the pq controlmethod to increase the control performance of the DVR. One of the advantages of STF is that it eliminates the need to have multiple filters as part of the control method, and thus reduces the controller complexity. Analysis of the fault ride-through capability of the new DVR revealed an improvement in the voltage stability offered to distributed generation-integrated weak utility networks. The proposed DVR control method is modeled in MATLAB/Simulink and tested in both off-line and real-time environments using theOPALRT real-time platform. Results are then presented as a verification of the proposed system

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

Nonlinear control schemes for extremum power seeking and torsional vibration mitigation in variable speed wind turbine systems

Doctor of PhilosophyDepartment of Electrical and Computer EngineeringDon GruenbacherWarren WhiteThis dissertation presents nonlinear control schemes to improve the productivity and lifespan of doubly fed induction generator (DFIG)-based and permanent magnet generator (PMG)-based variable speed wind turbines. To improve the productivity, a nonlinear adaptive control scheme is developed to maximize power capture. This controller consists of three feedback loops. The first loop controls electrical torque of the generator in order to cancel the nonlinear term of the turbine equation of motion using the feedback linearization concept. The nonlinearity cancelation requires a real-time estimation of aerodynamic torque. This is achieved through a second loop which estimates the ratio of the wind turbine power capture versus the available wind power. A third loop utilizes this estimate to identify the shaft speed at which the wind turbine operates at a greater power output. Contrary to existing techniques in literature, this innovative technique does not require any prior knowledge of the optimum tip speed ratio. The presented technique does not need a dither or perturbation signal to track the optimum shaft speed at the maximum power capture. These features make this technique superior to existing methods. Furthermore, the lifespan of variable speed wind turbines is improved by reducing stress on the wind turbine drivetrain. This is achieved via developing a novel vibration mitigation technique using sliding-mode control theory. The technique measures only generator speed as the input signal and then passes it through a high-pass filter in order to extract the speed variations. The filtered signal and its integral are then passed through identical band-pass filters centered at the dominant natural frequency of the drivetrain. These two signals formulate a sliding surface and consequently a control law to damp the drivetrain torsional stress oscillations caused by electrical and mechanical disturbances. This technique provides a robust mitigation approach compared with existing techniques. These control schemes are verified through holistic models of DFIG- and PMG-based wind turbines. Except for wind turbine aerodynamics, for which an existing simulator is used, the developed models of all components including DFIG, PMG, converters, multi-mass drivetrain, and power line are presented in this dissertation

Dynamic Phasor Modeling of Type 3 Wind Farm including Multi-mass and LVRT Effects

The proportion of power attributable to wind generation has grown significantly in the last two decades. System impact studies such as load flow studies and short circuit studies, are important for planning before integration of any new wind generation into the existing power grid. Short circuit modelling is central in these planning studies to determine protective relay settings, protection coordination, and equipment ratings. Numerous factors, such as low voltage situations, power electronic switching, control actions, sub-synchronous oscillations, etc., influence the response of wind farms to short circuit conditions, and that makes short circuit modelling of wind farms an interesting, complex, and challenging task. Power electronics-based converters are very common in wind power plants, enabling the plant to operate at a wide range of wind speeds and provide reactive power support without disconnection from the grid during low voltage scenarios. This has led to the growth of Type 3 (with rotor side converter) and Type 4 (with stator side full converter) wind generators, in which power electronics-based converters and controls are an integral part. The power electronics in these generators are proprietary in nature, which makes it difficult to obtain the necessary information from the manufacturer to model them accurately in planning studies for conditions such as those found during faults or low voltage ride through (LVRT) periods. The use of power electronic controllers also has led to phenomena such as sub-synchronous control interactions in series compensated Type 3 wind farms, which are characterized by non-fundamental frequency oscillations. The above factors have led to the need to develop generic models for wind farms that can be used in studies by planners and protection engineers. The current practice for short circuit modelling of wind farms in the power industry is to utilize transient stability programs based on either simplified electromechanical fundamental frequency models or detailed electromagnetic time domain models. The fundamental frequency models are incapable of representing the majority of critical wind generator fault characteristics, such as during power electronic switching conditions and sub-synchronous interactions. The detailed time domain models, though accurate, demand high levels of computation and modelling expertise. A simple yet accurate modelling methodology for wind generators that does not require resorting to fundamental frequency based simplifications or time domain type simulations is the basis for this research work. This research work develops an average value model and a dynamic phasor model of a Type 3 DFIG wind farm. The average value model replaces the switches and associated phenomena by equivalent current and voltage sources. The dynamic phasor model is based on generalized averaging theory, where the system variables are represented as time varying Fourier coefficients known as dynamic phasors. The two types models provide a generic type model and achieve a middle ground between conventional electromechanical models and the cumbersome electromagnetic time domain models. The dynamic phasor model enables the user to consider each harmonic component individually; this selective view of the components of the system response is not achievable in conventional electromagnetic transient simulations. Only the appropriate dynamic phasors are selected for the required fault behaviour to be represented, providing greater computational efficiency than detailed time domain simulations. A detailed electromagnetic transient (EMT) simulation model is also developed in this thesis using a real-time digital simulator (RTDS). The results obtained with the average value model and the dynamic phasor model are validated with an accurate electromagnetic simulation model and some state-of-the-art industrial schemes: a voltage behind transient reactance model, an analytical expression model, and a voltage dependent current source model. The proposed RTDS models include the effect of change of flux during faulted conditions in the wind generator during abnormal system conditions instead of incorrectly assuming it is a constant. This was not investigated in previous studies carried out in the real-time simulations laboratory at the University of Saskatchewan or in various publications reported in the literature. The most commonly used LVRT topologies, such as rotor side crowbar circuit, DC-link protection scheme, and series dynamic braking resistance (SDBR) in rotor and stator circuits, are investigated in the short circuit studies. The RTDS model developed uses a multi-mass (three-mass) model of the mechanical drive train instead of a simple single-mass model to represent torsional dynamics. The single mass model considers the blade inertia, the turbine hub, and the generator as a single lumped mass and so cannot reproduce the torsional behaviour. The root cause of sub-synchronous frequencies in Type 3 wind generators is not well understood by system planners and protection engineers. Some literature reports it is self excitation while others report it is due to sub-synchronous control interactions. One publication in the stability literature reports on a small signal analysis study aimed at finding the root cause of the problem, and a similar type of analysis was performed in this thesis. A linearized model was developed, which includes the generator model, a three mass drive train, rotor side converter, and the grid side converter represented as a constant voltage source. The linear model analysis showed that the sub-synchronous oscillations are due to control interactions between the rotor side controller of the Type 3 wind power plant and the series capacitor in the transmission line. The rotor side controls were tuned to obtain a stable response at higher levels of compensation. A real-time simulation model of a 450 MW Type 3 wind farm consisting of 150 units transmitting power via 345 kV transmission line was developed on the RTDS. The dynamic phasor method is shown to be accurate for representing faults at the point of interconnection of the wind farm to the grid for balanced and unbalanced faults as well as for different sub- synchronous oscillation frequencies

Fault Diagnosis and Fault Tolerant Control of Wind Turbines: An Overview

Wind turbines are playing an increasingly important role in renewable power generation. Their complex and large-scale structure, however, and operation in remote locations with harsh environmental conditions and highly variable stochastic loads make fault occurrence inevitable. Early detection and location of faults are vital for maintaining a high degree of availability and reducing maintenance costs. Hence, the deployment of algorithms capable of continuously monitoring and diagnosing potential faults and mitigating their effects before they evolve into failures is crucial. Fault diagnosis and fault tolerant control designs have been the subject of intensive research in the past decades. Significant progress has been made and several methods and control algorithms have been proposed in the literature. This paper provides an overview of the most recent fault diagnosis and fault tolerant control techniques for wind turbines. Following a brief discussion of the typical faults, the most commonly used model-based, data-driven and signal-based approaches are discussed. Passive and active fault tolerant control approaches are also highlighted and relevant publications are discussed. Future development tendencies in fault diagnosis and fault tolerant control of wind turbines are also briefly stated. The paper is written in a tutorial manner to provide a comprehensive overview of this research topic

Supervisory Control of Full Converter Wind Generation Systems to Meet International Grid Codes

This thesis proposes a new supervisory control scheme for full converter wind generators (FCWGs) in compliance with the latest international grid codes. Intermittent behaviour of wind turbines and maximum converter capacity are taken into account in determining the reactive power injection to the grid following severe disturbance. Detailed simulations show that the proposed controller can improve the fault-ride-through capability of FCWGs while also providing support to the network as required by the grid codes

Contributions to impedance shaping control techniques for power electronic converters

El conformado de la impedancia o admitancia mediante control para convertidores electrónicos de potencia permite alcanzar entre otros objetivos: mejora de la robustez de los controles diseñados, amortiguación de la dinámica de la tensión en caso de cambios de carga, y optimización del filtro de red y del controlador en un solo paso (co-diseño). La conformación de la impedancia debe ir siempre acompañada de un buen seguimiento de referencias. Por tanto, la idea principal es diseñar controladores con una estructura sencilla que equilibren la consecución de los objetivos marcados en cada caso. Este diseño se realiza mediante técnicas modernas, cuya resolución (síntesis del controlador) requiere de herramientas de optimización. La principal ventaja de estas técnicas sobre las clásicas, es decir, las basadas en soluciones algebraicas, es su capacidad para tratar problemas de control complejos (plantas de alto orden y/o varios objetivos) de una forma considerablemente sistemática. El primer problema de control por conformación de la impedancia consiste en reducir el sobreimpulso de tensión ante cambios de carga y minimizar el tamaño de los componentes del filtro pasivo en los convertidores DC-DC. Posteriormente, se diseñan controladores de corriente y tensión para un inversor DC-AC trifásico que logren una estabilidad robusta del sistema para una amplia variedad de filtros. La condición de estabilidad robusta menos conservadora, siendo la impedancia de la red la principal fuente de incertidumbre, es el índice de pasividad. En el caso de los controladores de corriente, el impacto de los lazos superiores en la estabilidad basada en la impedancia también se analiza mediante un índice adicional: máximo valor singular. Cada uno de los índices se aplica a un rango de frecuencias determinado. Finalmente, estas condiciones se incluyen en el diseño en un solo paso del controlador de un convertidor back-to-back utilizado para operar generadores de inducción doblemente alimentados (aerogeneradores tipo 3) presentes en algunos parques eólicos. Esta solución evita los problemas de oscilación subsíncrona, derivados de las líneas de transmisión con condensadores de compensación en serie, a los que se enfrentan estos parques eólicos. Los resultados de simulación y experimentales demuestran la eficacia y versatilidad de la propuesta.Impedance or admittance shaping by control for power electronic converters allows to achieve among other objectives: robustness enhancement of the designed controls, damped voltage dynamics in case of load changes, and grid filter and controller optimization in a single step (co-design). Impedance shaping must always be accompanied by a correct reference tracking performance. Therefore, the main idea is to design controllers with a simple structure that balance the achievement of the objectives set in each case. This design is carried out using modern techniques, whose resolution (controller synthesis) requires optimization tools. The main advantage of these techniques over the classical ones, i.e. those based on algebraic solutions, is their ability to deal with complex control problems (high order plants and/or several objectives) in a considerably systematic way. The first impedance shaping control problem is to reduce voltage overshoot under load changes and minimize the size of passive filter components in DC-DC converters. Subsequently, current and voltage controllers for a three-phase DC-AC inverter are designed to achieve robust system stability for a wide variety of filters. The least conservative robust stability condition, with grid impedance being the main source of uncertainty, is the passivity index. In the case of current controllers, the impact of higher loops on impedance-based stability is also analyzed by an additional index: maximum singular value. Each of the indices is applied to a given frequency range. Finally, these conditions are included in the one-step design of the controller of a back-to-back converter used to operate doubly fed induction generators (type-3 wind turbines) present in some wind farms. This solution avoids the sub-synchronous oscillation problems, derived from transmission lines with series compensation capacitors, faced by these wind farms. Simulation and experimental results demonstrate the effectiveness and versatility of the proposa

Power Management Strategies for a Wind Energy Source in an Isolated Microgrid and Grid Connected System

This thesis focuses on the development of power management control strategies for a direct drive permanent magnet synchronous generator (PMSG) based variable speed wind turbine (VSWT). Two modes of operation have been considered: (1) isolated/islanded mode, and (2) grid-connected mode. In the isolated/islanded mode, the system requires additional energy sources and sinks to counterbalance the intermittent nature of the wind. Thus, battery energy storage and photovoltaic (PV) systems have been integrated with the wind turbine to form a microgrid with hybrid energy sources. For the wind/battery hybrid system, several energy management and control issues have been addressed, such as DC link voltage stability, imbalanced power flow, and constraints of the battery state of charge (SOC). To ensure the integrity of the microgrid, and to increase its flexibility, dump loads and an emergency back-up AC source (can be a diesel generator set) have been used to protect the system against the excessive power production from the wind and PV systems, as well as the intermittent nature of wind source. A coordinated control strategy is proposed for the dump loads and back up AC source. An alternative control strategy is also proposed for a hybrid wind/battery system by eliminating the dedicated battery converter and the dump loads. To protect the battery against overcharging, an integrated control strategy is proposed. In addition, the dual vector voltage control (DVVC) is also developed to tackle the issues associated with unbalanced AC loads. To improve the performance of a DC microgrid consisting wind, battery, and PV, a distributed control strategy using DC link voltage (DLV) based control law is developed. This strategy provides simpler structure, less frequent mode transitions, and effective coordination among different sources without relying on real-time communication. In a grid-connected mode, this DC microgrid is connected to the grid through a single inverter at the point of common coupling (PCC). The generated wind power is only treated as a source at the DC side for the study of both unbalanced and balanced voltage sag issues at a distribution grid network. The proposed strategy consists of: (i) a vector current control with a feed-forward of the negative-sequence voltage (VCCF) to compensate for the negative sequence currents; and (ii) a power compensation factor (PCF) control for the VCCF to maintain the balanced power flow between the system and the grid. A sliding mode control strategy has also been developed to enhance the overall system performance. Appropriate grid code has been considered in this case. All the developed control strategies have been validated via extensive computer simulation with realistic system parameters. Furthermore, to valid developed control strategies in a realistic environment in real-time, a microgrid has been constructed using physical components: a wind turbine simulator (WTS), power electronic converters, simulated grid, sensors, real-time controllers and protection devices. All the control strategies developed in this system have been validated experimentally on this facility. In conclusion, several power management strategies and real-time control issues have been investigated for direct drive permanent magnet synchronous generator (PMSG) based variable speed wind turbine system in an islanded and grid-connected mode. For the islanded mode, the focuses have been on microgrid control. While for the grid-connected mode, main consideration has been on the mitigation of voltage sags at the point of common coupling (PCC)