Advanced Control And Synchronization Approaches Of Voltage Source Converters For Integration Of Distributed Energy Resources

Voltage source converter (VSC) is an inseparable interfacing fixture for utilizing distributed energy resource (DER) as an AC power supply. This dissertation investigates different control and synchronization techniques for stand-alone and grid-connected DC-AC VSCs. The most common control reference frame for VSCs is the dq reference frame (dq-RF), also known as the synchronous reference frame. The main challenge associated with the VSC control in this reference frame is the strong coupling between d and q axes. In this dissertation, a multi-loop dq-RF control system with a coupling compensation scheme is presented. Then, the droop-based power control technique, which eliminates the need for communication between parallel-connected VSCs and consequently offers a higher reliability, is investigated. A dq-RF-based approach of impedance design, for compensating the inequality of parallel VSCs connecting lines, along with the dq-RF droop control are also proposed. This approach results in an accurate power-sharing among parallel VSCs. The major challenges related to the synchronization unit of a grid-connected VSC control system in the presence of a distorted AC voltage are also briefly investigated in this dissertation. To deal with these challenges, an enhanced complex coefficient filter based PLL is designed and presented. This PLL completely removes the grid voltage imbalance and considerably attenuates the grid voltage dc offset and harmonics while maintaining a fast dynamic response and a simple structure. The VSC-interfaced DER is often required to switch between the islanded and grid-connected operation modes. The VSC integrated into the grid is current-controlled, while in the islanded operation mode is controlled as a voltage source. In the transition between these two modes, first the intended VSC operation mode should be detected, then its control system is reconfigured. To avoid the complexity of the control system and alleviate the drawbacks associated with the control mode transition, a VSC control approach, which mimics the traditional synchronous generator's universal mode of operation, is studied. A method of power-based active synchronization of the VSC-interfaced DER, with the ability of seamless transition between the islanded mode and connected to the grid, is proposed and integrated with this technique of the VSC control.

This thesis addresses the integration of renewable energy resources into the grid-connected and isolated distribution systems using voltage- and current-source converters. Motivated by its promising potential and attractive features, a one-stage current-source converter (CSC) is selected as an effective interfacing device for the photovoltaic (PV) generators to the utility-grid. The operation of the grid-connected CSC-based PV system is investigated under different operating conditions, parameters variation, and control topologies. From the dc-side, it is found that the variation in the weather conditions, e.g., solar irradiance levels, might affect the dynamic performance of the vector-controlled grid-connected CSCs. Small-signal dc-side impedance models for the CSC and the PV generators are developed to investigate the system stability. Active compensation techniques are proposed so that the system performance is well maintained under different operating points. From the ac-side, a susceptible potential to instabilities is yielded under the very weak grid conditions. This potential significantly increases in the vector-controlled converters. The implementation of the phase-locked loop (PLL) is found to have a detrimental impact on the system stability as the grid impedance increases. Two solutions for the very weak grid integration have been proposed. Firstly, the power synchronization control (PSC) scheme is developed for CSCs in PV applications so that the PLL is no longer needed, and hence a seamless integration to the very weak grid is achieved. Secondly, supplementary compensation loops are proposed and added to the conventional PLL-based vector-controlled CSC so that the negative impacts of the PLL are completely alleviated. In both cases, small-signal state-space models are developed to investigate the system stability and provide a detailed design approach for the proposed controllers. A larger system level integration is considered in this thesis by interfacing multiple distributed generation (DG) units to the conventional distribution system. A generalized hybrid alternating-current (ac)/direct-current (dc) system that constitutes ac and dc subgrids is created to efficiently accommodate the dc-type renewable sources (such as PVs, batteries, fuel cells, etc.) to the ac-type conventional distribution system. Both subgrids are interconnected using voltage-source converters (VSCs) to facilitate a bidirectional power exchange via multiple inversion-rectification processes. To achieve an accurate and efficient operation, a supervisory power management algorithm is proposed. The algorithm operates successfully regardless of the control mode of the individual DG units. However, the integration of severe dynamic loads, such as direct online inductions motors (IMs) into the ac-side of the hybrid system reflects very lightly damped modes, which in turns induce instabilities, particularly in the isolated mode of operation. Therefore, active compensation techniques have been proposed to increase the system damping. Throughout this thesis, time domain simulations under Matlab/Simulink® environment for the systems under study are presented to validate the analytical results and show the effectiveness of the proposed techniques.

This book aims to present some advanced control methodologies for power converters. Power electronic converters have become indispensable devices for plenty of industrial applications over the last decades. Composed by controllable power switches, they can be controlled by effective strategies to achieve desirable transient response and steady-state performance, to ensure the stability, reliability and safety of the system. The most popular control strategy of power converters is the linear proportional–integral–derivative series control which is adopted as industry standard. However, when there exist parameter changes, nonlinearities and load disturbances in the system, the performance of the controller will be significantly degraded. To overcome this problem, many advanced control methodologies and techniques have been developed to improve the converter performance. This book presents the research work on some advanced control methodologies for several types of power converters, including three-phase two-level AC/DC power converter, three-phase NPC AC/DC power converter, and DC/DC buck converter. The effectiveness and advantage of the proposed control strategies are verified via simulations and experiments. The content of this book can be divided into two parts. The first part focuses on disturbance observer-based control methods for power converters under investigation. The second part investigates intelligent control methods. These methodologies provide a framework for controller design, observer design, stability and performance analysis for the considered power converter systems.

Presents Fundamentals of Modeling, Analysis, and Control of Electric Power Converters for Power System Applications Electronic (static) power conversion has gained widespread acceptance in power systems applications; electronic power converters are increasingly employed for power conversion and conditioning, compensation, and active filtering. This book presents the fundamentals for analysis and control of a specific class of high-power electronic converters—the three-phase voltage-sourced converter (VSC). Voltage-Sourced Converters in Power Systems provides a necessary and unprecedented link between the principles of operation and the applications of voltage-sourced converters. The book: Describes various functions that the VSC can perform in electric power systems Covers a wide range of applications of the VSC in electric power systems—including wind power conversion systems Adopts a systematic approach to the modeling and control design problems Illustrates the control design procedures and expected performance based on a comprehensive set of examples and digital computer time-domain simulation studies This comprehensive text presents effective techniques for mathematical modeling and control design, and helps readers understand the procedures and analysis steps. Detailed simulation case studies are included to highlight the salient points and verify the designs. Voltage-Sourced Converters in Power Systems is an ideal reference for senior undergraduate and graduate students in power engineering programs, practicing engineers who deal with grid integration and operation of distributed energy resource units, design engineers, and researchers in the area of electric power generation, transmission, distribution, and utilization.

This book covers advancements of power electronic converters and their control techniques for grid integration of large-scale renewable energy sources and electrical vehicles. Major emphasis is on transformer-less direct grid integration, bidirectional power transfer, compensation of grid power quality issues, DC system protection and grounding, interaction in mixed AC/DC systems, AC and DC system stability, design of high-frequency high power density systems with advanced soft magnetic materials, modeling and simulation of mixed AC/DC systems, switching strategies for enhanced efficiency, and protection and reliability for sustainable grid integration. This book is an invaluable resource for professionals active in the field of renewable energy and power conversion. Md. Rabiul Islam received his PhD from the University of Technology Sydney (UTS), Australia. He was appointed as a Lecturer at Rajshahi University of Engineering & Technology (RUET) in 2005 and promoted to full-term Professor in 2017. In early 2018, he joined the School of Electrical, Computer, and Telecommunications Engineering, University of Wollongong, Australia. He is a Senior Member of IEEE. His research interests include the fields of power electronic converters, renewable energy technologies, power quality, electrical machines, electric vehicles, and smart grids. He has authored or coauthored more than 200 publications including 50 IEEE Transactions/IEEE Journal papers. He has been serving as an editor for IEEE Transactions on Energy Conversion and IEEE Power Engineering Letters, and associate editor for IEEE Access. Md. Rakibuzzaman Shah is a Senior Lecturer with the School of Engineering, Information Technology and Physical Science at Federation University Australia. He has worked and consulted with distribution network operators and transmission system operators on individual projects and has done collaborative work on a large number of projects (EPSRC project on multi-terminal HVDC, Scottish and Southern Energy multi-infeed HVDC) - primarily on the dynamic impact of integrating new technologies and power electronics into large systems. He is an active member of the IEEE and CIGRE. He has more than 70 international publications and has spoken at the leading power system conferences around the world. His research interests include future power grids (i.e., renewable energy integration, wide-area control), asynchronous grid connection through VSC-HVDC, application of data mining in power system, distribution system energy management, and low carbon energy systems. Mohd. Hasan Ali is currently an Associate Professor with the Electrical and Computer Engineering Department at the University of Memphis, USA, where he leads the Electric Power and Energy Systems (EPES) Laboratory. His research interests include advanced power systems, smart-grid and microgrid systems, renewable energy systems, and cybersecurity issues in modern power grids. Dr. Ali has more than 190 publications, including 2 books, 4 book chapters, 2 patents, 60 top ranked journal papers, 96 peer-reviewed international conference papers, and 20 national conference papers. He serves as the editor of the IEEE Transactions on Sustainable Energy and IET-Generation, Transmission and Distribution (GTD) journal. Dr. Ali is a Senior Member of the IEEE Power and Energy Society (PES). He is also the Chair of the PES of the IEEE Memphis Section.

This book narrates an assessment of numerous advanced power converters employed on primitive phase to enhance the efficiency of power translation pertaining to renewable energy systems. It presents the mathematical modelling, analysis, and control of recent power converters topologies, namely, AC/DC, DC/DC, and DC/AC converters. Numerous advanced DC-DC Converters, namely, multi-input DC-DC Converter, Cuk, SEPIC, Zeta and so forth have been assessed mathematically using state space analysis applied with an aim to enhance power efficiency of renewable energy systems. The book: Explains various power electronics converters for different types of renewable energy sources Provides a review of the major power conversion topologies in one book Focuses on experimental analysis rather than simulation work Recommends usage of MATLAB, PSCAD, and PSIM simulation software for detailed analysis Includes DC-DC converters with reasonable peculiar power rating This book is aimed at researchers, graduate students in electric power engineering, power and industrial electronics, and renewable energy.

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.

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

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.