Download or read book Control and Stability Enhancement of Grid interactive Voltage Source Inverters Under Grid Abnormalities written by Aswad Adib and published by . This book was released on 2018 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Voltage source inverters (VSIs) are an essential interface for grid integration of renewable energy resources. Grid-tied VSIs are employed in power grids to integrate distributed generation units, e.g. photovoltaic arrays, wind turbines and energy storage units, to the utility and extract the maximum energy from the DG units in an efficient manner. However, the stability of VSIs and by extension the entire DG system can be degraded under abnormal grid conditions. In this dissertation, new control and switching techniques for stability and power quality improvement of grid-tied VSIs under abnormal grid conditions are presented. For grids with a low inertia and a low short-circuit ratio, commonly referred to as weak grids, grid connection may make VSIs susceptible to voltage distortion and instability. In this dissertation, through root locus analysis of a detailed state-space model, the design of several circuit and control parameters of the grid-tied VSI are evaluated for improving stability in weak grids. It is shown that grid-side filter inductances can be increased for stable operation of VSIs in weak grids. Accordingly, a virtual inductance emulating the effect of an increased inductance in the grid-side filter is developed in this dissertation, which enables stable operation of VSIs in weak grids without the tradeoffs, i.e. additional voltage drop, increased cost and larger size, associated with a larger inductor. The virtual inductance scheme is realized through the injection of a feedforward current element in the VSI controller through a gain component. The measured grid currents, which are sensed for regular VSI controller operation, are employed as the feedforward component eliminating the need for any additional sensors for the utilization of this control scheme. Furthermore, a direct model reference adaptive control (MRAC) scheme is employed in this dissertation to tune the virtual inductance gain block according to a stable reference model for varying grid conditions. The use of direct MRAC scheme allows tuning of the virtual inductance block without the need for a plant parameter estimation stage. The virtual inductance scheme enables stable operation of VSIs in weak grids without system parameter redesign, thereby maintaining the steady-state performance of the system. The efficacy of the virtual inductance feedforward scheme is verified through hardware tests carried out on a three-phase grid-tied experimental setup. Along with extracting energy from the DG sources, grid-tied VSIs are capable of providing various ancillary services to the utility under abnormal conditions. However, providing ancillary services could drive the inverter voltages beyond the linear modulation region resulting in grid current distortions, which could violate the requirements for grid integration of DGs. An atypical pulse width modulation (PWM) technique is proposed in this dissertation, which maximizes the dc-bus utilization of VSIs, which in turn enables the VSIs to supply the maximum extracted power from the DG units to the grid when providing ancillary services while operating in the linear modulation region. The switching scheme is realized by injecting common mode components in the PWM references, computed based on instantaneous reference magnitudes. The proposed scheme is suitable when providing both symmetrical and asymmetrical ancillary services. In this dissertation, negative-sequence compensation and harmonic compensation are employed as instances of symmetrical and asymmetrical ancillary services. The proposed scheme can be integrated with any control scheme and carrier-based PWM combinations. The efficacy of the proposed atypical PWM scheme is verified through both simulation and hardware tests.

Download or read book Stability Enhancement Methods of Inverters Based on Lyapunov Function Predictive Control and Reinforcement Learning written by Xin Zhang and published by Springer Nature. This book was released on 2022-11-29 with total page 175 pages. Available in PDF, EPUB and Kindle. Book excerpt: This book introduces a family of large-signal stability-based control methods for different power inverters (grid-connected inverter, standalone inverter, single-phase inverter, and three-phase inverter) in practical applications. Power inverters have stability issues, which include the inverter's own instability as well as the inverter's instability in relation to the other power electronic devices in the system (i.e., weak grid and the EMI filter). Most of the stability analyses and solutions are based on small-signal stability technology. Unfortunately, in actuality, the majority of practical instability concerns in power inverter systems are large-signal stability problems, which, when compared to small-signal stability problems, can cause substantial damage to electrical equipment. As a result, researchers must conduct a comprehensive investigation of the large-signal stability challenge and solutions for power inverters. This book can be used as a reference for researchers, power inverters manufacturers, and end-users. As a result, the book will not become obsolete in the near future, regardless of technology advancements.

Download or read book Control of Power Inverters in Renewable Energy and Smart Grid Integration written by Qing-Chang Zhong and published by John Wiley & Sons. This book was released on 2012-11-16 with total page 407 pages. Available in PDF, EPUB and Kindle. Book excerpt: Integrating renewable energy and other distributed energy sources into smart grids, often via power inverters, is arguably the largest “new frontier” for smart grid advancements. Inverters should be controlled properly so that their integration does not jeopardize the stability and performance of power systems and a solid technical backbone is formed to facilitate other functions and services of smart grids. This unique reference offers systematic treatment of important control problems in power inverters, and different general converter theories. Starting at a basic level, it presents conventional power conversion methodologies and then ‘non-conventional’ methods, with a highly accessible summary of the latest developments in power inverters as well as insight into the grid connection of renewable power. Consisting of four parts – Power Quality Control, Neutral Line Provision, Power Flow Control, and Synchronisation – this book fully demonstrates the integration of control and power electronics. Key features include: the fundamentals of power processing and hardware design innovative control strategies to systematically treat the control of power inverters extensive experimental results for most of the control strategies presented the pioneering work on “synchronverters” which has gained IET Highly Commended Innovation Award Engineers working on inverter design and those at power system utilities can learn how advanced control strategies could improve system performance and work in practice. The book is a useful reference for researchers who are interested in the area of control engineering, power electronics, renewable energy and distributed generation, smart grids, flexible AC transmission systems, and power systems for more-electric aircraft and all-electric ships. This is also a handy text for graduate students and university professors in the areas of electrical power engineering, advanced control engineering, power electronics, renewable energy and smart grid integration.

Download or read book Grid Forming Power Inverters written by Nabil Mohammed and published by CRC Press. This book was released on 2023-02-28 with total page 307 pages. Available in PDF, EPUB and Kindle. Book excerpt: Grid-Forming Power Inverters: Control and Applications is the first book dedicated to addressing the operation principles, grid codes, modelling and control of grid-forming power inverters. The book initially discusses the need for this technology due to the substantial annual integration of inverter-based renewable energy resources. The key differences between the traditional grid-following and the emerging grid-forming inverters technologies are explained. Then, the book explores in detail various topics related to grid-forming power inverters, including requirements and grid standards, modelling, control, damping power system oscillations, dynamic stability under large fault events, virtual oscillator-controlled grid-forming inverters, grid-forming inverters interfacing battery energy storage, and islanded operation of grid-forming inverters. Features: Explains the key differences between grid-following and grid-forming inverters Explores the requirements and grid standards for grid-forming inverters Provides detailed modeming of virtual synchronous generators Explains various control strategies for grid-forming inverters Investigates damping of power system oscillations using grid-forming converters Elaborates on the dynamic stability of grid-forming inverters under large fault events Focuses on practical applications

Download or read book Control Contributions to AC Microgrid Inverters written by Daniel Heredero Peris and published by . This book was released on 2017 with total page 463 pages. Available in PDF, EPUB and Kindle. Book excerpt: This thesis is focused on the microgrid control framework. Specifically, it is concentrated on alternative current microgrids. As a result of the author involvement in different industrial projects concerning microgrids during the last years, the goal of the thesis is to apply and expose the acquired experiences for exploring new control possibilities. The followed methodology is detailed in the next lines. Primarily, the thesis presents two chapters whose purpose is to introduce the concept of operation and to analyse the context and challenges posed by microgrids from a top to bottom approach. The first chapter is oriented in general terms while the second one devotes more efforts in the alternative current microgrids particular case. After identifying the different actors that play an key role in the alternative current microgrids it is disclosed that the inverter becomes a key element. In this sense, the first two chapters allows to pave the way addressing how an inverter installed in an alternative current microgrid can be considered and operated. In this direction, the named grid-tied operation mode reflects the most complex configuration. Considering that the scope of the thesis is focused on voltage source inverters, the grid-tied configuration may not only consider the grid-connected or grid-disconnected operation mode but also the transients between them. Thus, important challenges such as the inverter operation itself, the anti-islanding detection capability according to the behaviour of the inverter and the galvanic isolation restriction in a microgrid framework are dealt. The control of the voltage source inverters operated in an alternative microgrid results crucial. For this reason, a full chapter carries out a study proposing and delving into various type of controllers according to different possibilities exposed in the literature. It is conducted a stability analysis for stationary reference frame resonant current and voltage controllers. The chapter concludes with a new hybrid formulation based on fractional calculus and conventional resonant control. This novel formulation allows to enhance the controllable frequency bandwidth range decreasing inter-harmonic excitations and reducing the controller order in respect with other classical formulations. The fourth and fifth chapters are involved into discussing and presenting different operation options when the inverter is considered as a current or as a voltage source. The fourth chapter poses over how to tackle the problem of transients when a current source grid-tied inverter is assumed while grid-connected operated. Two situations are studied; the flying transference from current to voltage source and how to extend the maximum power point extraction when a back-up system supports the inverter operation. A set of different scenarios conducted in simulation and in a real experimental platform complements the two considered situations. The fifth chapter analyses the operation of the inverter from an opposite perspective. The considered inverter is operated by using a hybrid combination between alternative current droop control strategy when grid-connected and voltage-frequency control when grid-disconnected. In this way it is possible to preserve dynamics capabilities in both operation modes meanwhile the inverter is treated as a voltage source in any case. A predominant resistive model applying virtual resistance is considered for the case of alternative current droop control. For both operation modes an LCL-type coupling filter is assumed, complementing the control strategy with an active damping mechanism to mitigate possible resonances. Then, a three-phase four-wire four-leg inverter is proposed to match better with a proper approximation to an AC microgrid with over-load supervisor when grid-connected and sinusoidal short-circuit proof when grid-disconnected operated.

Download or read book Improvement of Stability of a Grid connected Inverter with an LCL Filter by Robust Strong Active Damping and Model Predictive Control written by Seungyong Lee and published by . This book was released on 2021 with total page 338 pages. Available in PDF, EPUB and Kindle. Book excerpt: This study addresses development and implementation of robust control methods for a three-phase grid-connected voltage source inverter (VSI) accompanied by an inductive-capacitive-inductive (LCL) filter. A challenge of current control for the VSI is LCL filter resonance near to the control stability boundary, which interacts with the inverter control switching actions and creates the possibility of instability. In general, active damping is needed to stabilize the system and ensure robust performance in steady-state and dynamic responses. While many active damping methods have been proposed to resolve this issue, capacitor-current-feedback active damping has been most widely used for its simple implementation. There has been no clear consensus regarding design of a control system including capacitor-current-feedback active damping. This is due to the fact that simulation/experiment results are not congruent with the design analyses on which the control is designed. This study explains the incoherence between theory and practice when it comes to a capacitor-currents-feedback active damping system. Proposed capacitor-current-estimate active damping utilizing a developed posteriori Kalman estimator gives coherent simulation results as expected from the design analyses. This reveals that the highly oscillatory capacitor currents containing the inverter switching effects bring about uncertainty in the system performance. The switching effects are not incorporated in the analyses and control system design. Therefore, it is required to remove the switching noise from the capacitor currents in order to yield consistent results. It has been confirmed that the proportional-negative feedback of the capacitor current is equivalent to virtual impedance connected in parallel with the filter capacitor. In a digitally controlled system, the computation delay causes the equivalent resistance of the virtual impedance to become negative in the frequency range of fs/6 to fs/2, which produces a pair of open-loop unstable poles in RHP. This happens when the displaced resonance peak by active damping is in that region. Thus, an a priori Kalman estimator has been developed to generate one-sample-ahead state variable estimates to reconstruct the capacitor currents for active damping, which can compensate for the delay. The one-sample-ahead capacitor-current estimates are computed from the inverter-side and grid-side current estimates. The proposed method provides extended limits of the active damping gain that improve robustness against system parameter variation. It also allows strong active damping which can sufficiently attenuate the resonance. Grid condition is another significant factor affecting the stability of the system. In particular, a weak grid tends to provide high impedance. The system employing the proposed active damping method stably operates in a weak grid, ensuring robustness under grid impedance variation. The developed Kalman estimators offer an effective and easy way of determining the stability status of a system in addition to the functions of filtering and estimation. Stability analysis can be easily made since state variable estimates go to infinity when a system is unstable. As a promising approach, model predictive control (MPC) has been designed for the system. This study suggests that MPC including active damping can be employed for a grid-connected VSI with an LCL filter with good dynamic performance.

Download or read book Control Techniques for LCL Type Grid Connected Inverters written by Xinbo Ruan and published by Springer. This book was released on 2017-07-26 with total page 319 pages. Available in PDF, EPUB and Kindle. Book excerpt: This book focuses on control techniques for LCL-type grid-connected inverters to improve system stability, control performance and suppression ability of grid current harmonics. Combining a detailed theoretical analysis with design examples and experimental validations, the book offers an essential reference guide for graduate students and researchers in power electronics, as well as engineers engaged in developing grid-connected inverters for renewable energy generation systems.

Download or read book Modeling techniques and control strategies for inverter dominated microgrids written by Gkountaras, Aris and published by Universitätsverlag der TU Berlin. This book was released on 2017-02-15 with total page 172 pages. Available in PDF, EPUB and Kindle. Book excerpt: The character of modern power systems is changing rapidly and inverters are taking over a considerable part of the energy generation. A future purely inverter-based grid could be a viable solution, if its technical feasibility can be first validated. The focus of this work lies on inverter dominated microgrids, which are also mentioned as 'hybrid' in several instances throughout the thesis. Hybrid, as far as the energy input of each generator is concerned. Conventional fossil fuel based generators are connected in parallel to renewable energy sources as well as battery systems. The main contributions of this work comprise of: The analysis of detailed models and control structures of grid inverters, synchronous generators and battery packs and the utilization of these models to formulate control strategies for distributed generators. The developed strategies accomplish objectives in a wide time scale, from maintaining stability during faults and synchronization transients as well as optimizing load flow through communication-free distributed control. Die Struktur der modernen Energieversorgung hat sich in den letzten Jahrzehnten massiv geändert. Dezentrale Generatoren, die auf Wechselrichtern basieren, übernehmen einen großen Teil der Energieerzeugung. Ein ausschließlich wechselrichterbasiertes Netz wäre ein realistischer Ansatz, wenn seine technische Machbarkeit verifiziert werden könnte. Die wichtigste Beiträge dieser Arbeit sind: Die Analyse von Modellen und Regelstrukturen von Netzwechselrichtern, Synchrongeneratoren und Batterieanlagen. Die entwickelten Modelle werden verwendet, um Regelstrategien für dezentrale Generatoren in Mittelspannungsinselnetzen zu formulieren. Die erste Strategie ist eine Synchronisationsmethode für netzbildende Wechselrichter. Zweitens wird die Leistungsaufteilung in Mittelspannungsinselnetzen mittels Droop Regelung analysiert. Weiterhin erfolgt die Untersuchung der transienten Lastaufteilung zwischen netzbildenden Einheiten mit unterschiedlichen Zeitkonstanten. Beim Betrieb mehrerer paralleler Wechselrichter wird der Einfluss der Netzimpedanz auf die transiente Lastaufteilung analysiert. Die dritte entworfene Regelstrategie umfasst die Integration der Sekundärregelung in die Primärregelung. Der Ladezustand von Batterien wird mit der Lastaufteilung gekoppelt, um die Autonomie des Netzes zu stärken. Abschließend wird eine Kurzschlussstrategie für netzbildende und netzspeisende Wechselrichter entwickelt. Ziel der Strategie ist die Maximierung des Kurzschlussstromes. Als zusätzliche Randbedingung soll keine Kommunikation zwischen Generatoren stattfinden.

Download or read book Improved Control for Multilevel Inverters in Grid Applications written by Martin Hofmann and published by . This book was released on 2018 with total page 219 pages. Available in PDF, EPUB and Kindle. Book excerpt: Control systems for three-phase grid connected voltage source inverters (VSI) play an important role in energy transformation systems . They are expected to be stable, robust and accurate during steady state as well as different grid faults and disturbances like voltage sags or unbalanced conditions. Caused by increasingly rising grid standards and efficiency requirements the use of multilevel inverter systems in grid connected low voltage applications are getting more and more attention. Nevertheless, the use of these inverter types leads to increased complexity of the control system and the hardware components. This thesis presents an improved control scheme for multilevel inverters in grid applications. The system combines a robust and high-dynamic direct current control scheme called scalar hysteresis.

Download or read book Grid Converters for Photovoltaic and Wind Power Systems written by Remus Teodorescu and published by John Wiley & Sons. This book was released on 2011-07-28 with total page 358 pages. Available in PDF, EPUB and Kindle. Book excerpt: Grid converters are the key player in renewable energy integration. The high penetration of renewable energy systems is calling for new more stringent grid requirements. As a consequence, the grid converters should be able to exhibit advanced functions like: dynamic control of active and reactive power, operation within a wide range of voltage and frequency, voltage ride-through capability, reactive current injection during faults, grid services support. This book explains the topologies, modulation and control of grid converters for both photovoltaic and wind power applications. In addition to power electronics, this book focuses on the specific applications in photovoltaic wind power systems where grid condition is an essential factor. With a review of the most recent grid requirements for photovoltaic and wind power systems, the book discusses these other relevant issues: modern grid inverter topologies for photovoltaic and wind turbines islanding detection methods for photovoltaic systems synchronization techniques based on second order generalized integrators (SOGI) advanced synchronization techniques with robust operation under grid unbalance condition grid filter design and active damping techniques power control under grid fault conditions, considering both positive and negative sequences Grid Converters for Photovoltaic and Wind Power Systems is intended as a coursebook for graduated students with a background in electrical engineering and also for professionals in the evolving renewable energy industry. For people from academia interested in adopting the course, a set of slides is available for download from the website. www.wiley.com/go/grid_converters

Download or read book New Control Algorithms for the Distributed Generation Interface in Grid connected and Micro grid Systems written by Yasser Abdel-Rady Ibrahim Mohamed and published by . This book was released on 2008 with total page 165 pages. Available in PDF, EPUB and Kindle. Book excerpt: Driven by economic, technical, and environmental reasons, the energy sector is moving into an era where large portions of increases in electrical energy demand will be met through widespread installation of distributed resources or what's known as distributed generation (DG). DG units can operate in parallel to the main grid or in a micro-grid mode. The later is formed by a cluster of DG units connected to a distribution network to maintain the reliability of critical loads, mainly when the grid supply is not available. Distributed resources include variable frequency sources, high frequency sources, and direct energy conversion sources producing dc voltages or currents. The majority of distributed resources are interfaced to the utility grid or to the customer load via dc-ac pulse-width-modulated (PWM) voltage source inverter (VSI) systems. However, these interfaces introduce new issues, such as the absence of the physical inertia, wide-band of dynamics, limited overload capability, susceptibility to parameters variation, and switching harmonics generation. In addition, the uncertain and dynamic nature of the distribution network challenges the stability and control effectiveness of a grid-connected inverter-based DG interface. Generally, difficulties appear in the form of grid impedance and interfacing parameter variations, fast and slow grid-voltage disturbances, grid distortion and unbalance, and interactions between the inverter ac-side filter and the grid. On the other hand, a micro-grid system will be dominated by inverter-based DG units. Unlike conventional power system generators, inverter-based DG units have no physical inertia. This fact makes the micro-grid system potentially susceptible to oscillations resulting from system disturbances. Severe and random disturbances might be initiated in a micro-grid system, due to load changes, the power sharing mechanism of the inverters and other generators, and interactions between the DG interface and the network. Motivated by the aforementioned difficulties, this thesis presents new control algorithms for the DG interface that guarantee stable and high power quality injection under the occurrence of network disturbances and uncertainties, in both the grid-connected and micro-grid systems.

Download or read book Enhanced Large Signal Stability Method for Grid Forming Inverters During Current Limiting Preprint written by and published by . This book was released on 2023 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: Grid-forming (GFM) inverters are a promising technology for the widespread integration of renewable energy sources in future power systems. As a key element of GFM inverter control, the primary controller governs the internal reference voltage and angle. During contingencies in the grid--such as faults, voltage drops, or frequency and phase jumps--an inverter can be forced into a current-limiting mode of operation modulating inverter dynamics, and, as a result, it is prone to losing synchronism with the grid. In this paper, we propose a novel GFM primary control method with an additional synchronization term that naturally activates during contingencies to improve the dynamic response. The method allows the inverter to remain synchronized with the grid, which improves the inverter's dynamic behavior both during and after current-limiting grid conditions and enhances grid support, including voltage support using full current capacity. The method is demonstrated for voltage, frequency, and phase jumps both in a single-machine-to-infinite-bus and a network-wide electromagnetic transient simulation of the IEEE 14-bus system with 5 GFM inverters. The simulations provide insights into the proposed synchronization method and confirm the high potential of the method, which robustly secures synchronism under severe contingencies.

Download or read book Dynamic Modeling and Analysis of the Three phase Voltage Source Inverter Under Stand alone and Grid tied Modes written by Faleh A. Alskran and published by . This book was released on 2014 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Increasing energy demand, rising oil prices, and environmental concerns have forced attention to alternative energy sources that are environmentally friendly and independent of fossil fuels. Renewable energy sources (RES) have become an attractive alternative to the traditional energy sources for electric power generation. However, one of the main challenges of RES adaption arises when connecting RES to the electric grid. Voltage source inverters (VSIs), typically, connect RES to the electric grid. Similar to any engineering system, detailed dynamic models of the VSIs are needed for design and analysis purposes. However, due to the non-linearity of VSIs, development of dynamic models that can accurately describe their behavior is a complex task. In this thesis, a detailed averaged-state-space model of the two-level three-phase space vector pulse width modulation VSI and its companion LCL filter is derived. Because VSIs can operate under stand-alone and grid-tied modes, two models were derived for each case. In the derived models, the VSI modulation index m and phase angle ø are initially considered constant. In practice, however, these parameters are considered the main control parameters. To model these parameters as control inputs, small-signal models of the VSI under stand-alone and grid-tied modes were derived. To verify the accuracy of the developed large-signal and small-signal models, Matlab/Simulink simulations were carried out. The simulation results were compared against the models results. Moreover, the models were verified through lab experiments. The developed models can be used as design and analysis tools. In addition, the developed models can be used as fast and efficient simulation tools for system studies, when the modeling of switching transients is not needed. Nowadays, the number of VSIs connected to the electric grid is growing exponentially. The amount of time and computation needed to simulate VSIs using simulation software packages can be significantly decreased by the use of the developed models.

Download or read book Energy Shaping Control for Stabilization of Interconnected Voltage Source Converters in Weakly connected AC Microgrid Systems written by Nadia Lenora Carmita Smith and published by . This book was released on 2017 with total page 324 pages. Available in PDF, EPUB and Kindle. Book excerpt: With the ubiquitous installations of renewable energy resources such as solar and wind, for decentralized power applications across the United States, microgrids are being viewed as an avenue for achieving this goal. Various independent system operators and regional transmission operators such as Southwest Power Pool (SPP), Midcontinent System Operator (MISO), PJM Interconnection and Electric Reliability Council of Texas (ERCOT) manage the transmission and generation systems that host the distributed energy resources (DERs). Voltage source converters typically interconnect the DERs to the utility system and used in High voltage dc (HVDC) systems for transmitting power throughout the United States. A microgrid configuration is built at the 13.8kV 4.75MVA National Center for Reliable Energy Transmission (NCREPT) testing facility for performing grid-connected and islanded operation of interconnected voltage source converters. The interconnected voltage source converters consist of a variable voltage variable frequency (VVVF) drive, which powers a regenerative (REGEN) load bench acting as a distributed energy resource emulator. Due to the weak-grid interface in islanded mode testing, a voltage instability occurs on the VVVF dc link voltage causing the system to collapse. This dissertation presents a new stability theorem for stabilizing interconnected voltage source converters in microgrid systems with weak-grid interfaces. The new stability theorem is derived using the concepts of Dirac composition in Port-Hamiltonian systems, passivity in physical systems, eigenvalue analysis and robust analysis based on the edge theorem for parametric uncertainty. The novel stability theorem aims to prove that all members of the classes of voltage source converter-based microgrid systems can be stabilized using an energy-shaping control methodology. The proposed theorems and stability analysis justifies the development of the Modified Interconnection and Damping Assignment Passivity-Based Control (Modified IDA-PBC) method to be utilized in stabilizing the microgrid configuration at NCREPT for mitigating system instabilities. The system is simulated in MATLAB/SimulinkTM using the Simpower toolbox to observe the system's performance of the designed controller in comparison to the decoupled proportional intergral controller. The simulation results verify that the Modified-IDA-PBC is a viable option for dc bus voltage control of interconnected voltage source converters in microgrid systems.

Download or read book Small Signal Stability Modelling and Optimization of Microgrids written by Simon Eberlein and published by BoD – Books on Demand. This book was released on 2021-08-31 with total page 250 pages. Available in PDF, EPUB and Kindle. Book excerpt: The stability of power systems and microgrids is compromised by the increasing penetration with power electronic devices, such as wind turbines, photovoltaics and batteries. A simulation and optimization environment for such low-inertia systems is created. It is investigated how accurate the models need to be to capture the prevailing modes. An evolutionary algorithm tailored to optimization problems with computationally intensive fitness evaluation is proposed in order to optimized the controller parameters of grid-forming and grid-supporting distributed generators. It becomes apparent that microgrids dominated by grid-forming inverters are very stable systems when well-designed and optimized controllers are used. Model simplifications, such as the neglect of inner control loops of inverters, must be examined carefully, as they can lead to an inaccurate stability assessment.

Download or read book Dynamics Robust Control and Power Management of Voltage source Converters in Hybrid Multiterminal AC DC Grids written by Masoud Davari and published by . This book was released on 2016 with total page 246 pages. Available in PDF, EPUB and Kindle. Book excerpt: The electric energy sector is moving toward extensive integration of clean and renewable energy sources, energy storage units, and modern loads via highly efficient and flexible multiterminal dc grids integrated within the traditional ac grid infrastructure in both transmission and distribution levels. A voltage-source converter (VSC) is the main technology enabling the interconnection of dc and ac grids. In such demanding applications, effective and robust integration of ac and dc grids, in the presence of coupling nonlinear dynamics, parametric uncertainties, and disturbances, is crucial to maintain the stability and robust performance of the overall ac/dc dynamic system. Motivated by this objective, this thesis addresses the dynamics, robust control, and power management of VSCs in hybrid multiterminal ac/dc grids. Firstly, a robust multi-objective dc-link voltage controller is developed for a bi-directional VSC regulating the dc-link voltage of a multiterminal dc grid; i.e., the VSC operates as a dc-voltage power-port. The proposed controller ensures excellent tracking performance, robust disturbance rejection, and robust stability against operating point and parameter variation with a simple fixed-parameter low-order controller. Secondly, the dynamics and control of VSCs considering the instantaneous power of both ac- and dc-side filters and dc grid uncertainties are addressed in the this thesis. The proposed controller ensures excellent tracking performance, robust disturbance rejection, and robust performance against operating point and parameter variation with a simple fixed-parameter controller. Thirdly, this thesis presents a natural-frame variable-structure-based nonlinear control system for the master VSC applied in multiterminal grids to overcome problems associated with conventional dc-link voltage controllers, which are suffering from stability and performance issues, mainly attributed to the small-signal-based control design approach and the use of cascaded control structure based on the power balance framework that yields unmodeled nonlinear dynamics. Fourthly, this thesis presents a robust vector-controlled VSC that facilitates full converter power injection at weak and very weak ac grid conditions (i.e., when the short-circuit capacity ratio is one). The controller overcomes problems related to the stability and performance of conventional vector-controlled VSCs integrated into very weak ac grids (high impedance grids) because of the increased coupling between the converter and grid dynamics, via the phase-locked loop (PLL). As a result, a detailed ac-bus voltage dynamic model, including the PLL dynamics, is developed and validated in this thesis. Then, the model is used to design a robust optimal ac-bus voltage controller to stabilize the dynamics under operating point variation and grid impedance uncertainty. Fifthly, this thesis addresses the challenges associated with a dc-voltage-controlled VSC interfacing a wind turbine into a dc grid, which is gaining widespread acceptance under weak grid connection or isolated operation. Under weak grid connection or isolated operation, the machine-side VSC regulates the dc-link voltage via changes in the generator speed. However, several control difficulties are yielded; important problems are: 1) the nonlinear plant dynamics with a wide range of operating point variation; 2) the control lever is mainly the generator speed, which complicates the dc-link voltage control dynamics; 3) the presence of uncertain disturbances associated with dynamic loads (e.g., power-converter-based loads) connected to the dc grid and wind speed variation; and 4) the presence of parametric uncertainty associated with the equivalent dc-link capacitance due to connecting/disconnecting converter-based loads. Finally, this thesis presents a robust power sharing and dc-link voltage regulation controller for grid-connected VSCs in dc grids applications to overcome difficulties and problems related to the dynamics and stability of a grid-connected VSC with dc power sharing droop control. Major difficulties are: 1) ignoring the effect of the outer droop loop on the dc-link voltage dynamics when the dc-link voltage controller is designed, which induces destabilizing dynamics, particularly under variable droop gain needed for optimum economic operation, energy management, and successful network operation under converter outages and contingencies; 2) uncertainties in the dc grid parameters (e.g., passive load resistance and equivalent capacitance as viewed by the dc side of the VSC); and 3) disturbances in the dc grid (i.e., power absorbed or injected from/to the dc grid), which change the operating point and the converter dynamics by acting as a state-dependent disturbance. A theoretical analysis and comparative simulation and experimental results are presented in this thesis to show the validity and effectiveness of the developed models and proposed control structures.

Download or read book Enhanced Controllers for Voltage sourced Converters Interfaced with Weak Ac Power Grids written by Sushil Silwal and published by . This book was released on 2019 with total page 133 pages. Available in PDF, EPUB and Kindle. Book excerpt: Many distributed energy resource (DER) systems are remotely located and are often interfaced at low or medium voltage levels through power electronics converters such as voltage-sourced converters (VSC). Therefore, a weak-grid situation is encountered where the voltage and frequency at the point of DER coupling can experience fluctuations. A power converter designed to operate in normal and strong grid conditions may not perform satisfactorily during such weak and distorted grid conditions. Hence, considering the full dynamics of the system during weak-grid conditions in the design of converter control is indispensable to ensure the stability of the DER and the grid. For instance, the phase-locked loop (PLL) has been identified as a critical component of the VSC controller that can compromise the DER performance during weak-grid conditions. This dissertation investigates and enhances the performances of inverters connected to weak and polluted grids. It primarily presents a novel approach of enhancing the inverter current controller by including the PLL state variables among the entire system state and use them to optimally generate the control input for the VSC. This mitigates the loop interactions between the PLL and other control loops resulting in a mitigation of the oscillations that could cause system instabilities. The procedure is accomplished using the recently developed linear model of the enhanced PLL (EPLL) for single-phase applications and using a model of the three-phase PLL developed in this dissertation. Extensive simulation and experimental results are presented to evaluate and validate the proposed control approaches. Full practical models of all system components are considered for simulation studies. The experimental tests are done on a practical inverter connected to the utility grid. Significant improvement of the inverter performance in weak-grid conditions is confirmed. This dissertation offers a systematic way of integrating and designing the PLL and controller in a VSC to achieve a robust performance in weak-grid conditions.