Energy Shaping Control For Stabilization Of Interconnected Voltage Source Converters In Weakly Connected Ac Microgrid Systems

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 Connected Converters: Modeling, Stability and Control discusses the foundations and core applications of this diverse field, from structure, modeling and dynamic equivalencing through power and microgrids dynamics and stability, before moving on to controller synthesis methodologies for a powerful range of applications. The work opens with physical constraints and engineering aspects of advanced control schemes. Robust and adaptive control strategies are evaluated using real-time simulation and experimental studies. Once foundations have been established, the work goes on to address new technical challenges such as virtual synchronous generators and synergic inertia emulation in response to low inertia challenges in modern power grids.The book also addresses advanced systematic control synthesis methodologies to enhance system stability and dynamic performance in the presence of uncertainties, practical constraints and cyberattacks. Addresses new approaches for modeling, stability analysis and control design of GCCs Proposes robust and flexible GCC control frameworks for supporting grid regulation Emphasizes the application of GCCs in inertia emulation, oscillation damping control, and dynamic shaping Addresses systematic control synthesis methodologies for system security and dynamic performance

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.

Prosumers, such as energy storage, smart home, and microgrids, are the consumers who also produce and share surplus energy with other users. With capabilities of flexibly managing the generation, storage and consumption of energy in a simultaneous manner, prosumers can help improve the operation efficiency of smart grid. Due to the rapid expansion of prosumer clusters, the planning and operation issues of prosumer energy systems have been increasingly raised. Aspects including energy infrastructure design, energy management, system stability, etc., are urgently required to be addressed while taking full advantage of prosumers' capabilities. However, up to date, the research on prosumers has not drawn sufficient attention. This proposal presents the need to introduce a Research Topic on prosumer energy systems in Frontiers in Energy Research. We believe this Research Topic can promote the research on advanced planning and operation technologies of prosumer energy systems and contribute to the carbon neutrality for a sustainable society.

This thesis addresses the integration of three-phase dc/ac voltage source converter (VSC)-interfaced dc sources and loads into the three-phase ac grids or microgrids under very weak conditions. VSCs are the dominant topologies in high-power electronics due to their excellent performance and their common control method is voltage-oriented control (VOC). Motivated by the VSC industrial popularity on one hand and the issues yielding from dynamic interactions between the VOC-based VSC and the weak grid (WG) or weak microgrid (WMG) impedances, four situations are considered: VSC connected to a very weak grid (VSC-WG) in the inversion mode; VSC-WG in the rectification mode; passive load (PL) connected to a very weak microgrid (PL-WMG); and VSC connected to very weak microgrid (VSC-WMG). The dynamic model of the VSC-WG system is derived in the direct-quadrature reference frame (dq-RF) which serves as the basis for the study. Using the VSC-WG dynamics, the small-signal model of the system is developed in the standard state-space form which makes it possible to obtain the eigenvalue spectrum of the system. Using the eigenvalue analysis, the small-signal stability of the system is investigated under different operating points, grid impedance values, and control parameters variations. It is found that complex unstable modes are present in the eigenvalue spectrum of the system under the nominal conditions of the VSC-WG in both inversion and rectification modes as well as WMG in islanded mode. Linear active compensators are proposed that utilize the VSC output variables as inputs and integrate them into the primary control system of the VSC without adding extra measurement sensors or interfering with the VSC original control design. Moreover, these active compensation methods are lossless and do not change the VSC steady-state values. The design of the compensators is provided in detail using the state-space approach. It is shown that with proper design of the linear compensators, the instabilities are completely resolved under nominal conditions and the system dynamic performance under different conditions is improved. Computer simulations and real-time simulation (using hardware-in-the-loop platform) results are provided to verify the proposed methods and validate the theoretical findings.

Power systems worldwide are going through a paradigm shift from centralized generation to distributed generation. This book presents the SYNDEM (i.e., synchronized and democratized) grid architecture and its technical routes to harmonize the integration of renewable energy sources, electric vehicles, storage systems, and flexible loads, with the synchronization mechanism of synchronous machines, to enable autonomous operation of power systems, and to promote energy freedom. This is a game changer for the grid. It is the sort of breakthrough — like the touch screen in smart phones — that helps to push an industry from one era to the next, as reported by Keith Schneider, a New York Times correspondent since 1982. This book contains an introductory chapter and additional 24 chapters in five parts: Theoretical Framework, First-Generation VSM (virtual synchronous machines), Second-Generation VSM, Third-Generation VSM, and Case Studies. Most of the chapters include experimental results. As the first book of its kind for power electronics-enabled autonomous power systems, it • introduces a holistic architecture applicable to both large and small power systems, including aircraft power systems, ship power systems, microgrids, and supergrids • provides latest research to address the unprecedented challenges faced by power systems and to enhance grid stability, reliability, security, resiliency, and sustainability • demonstrates how future power systems achieve harmonious interaction, prevent local faults from cascading into wide-area blackouts, and operate autonomously with minimized cyber-attacks • highlights the significance of the SYNDEM concept for power systems and beyond Power Electronics-Enabled Autonomous Power Systems is an excellent book for researchers, engineers, and students involved in energy and power systems, electrical and control engineering, and power electronics. The SYNDEM theoretical framework chapter is also suitable for policy makers, legislators, entrepreneurs, commissioners of utility commissions, energy and environmental agency staff, utility personnel, investors, consultants, and attorneys.

The authors here provide a detailed treatment of the design of robust adaptive controllers for nonlinear systems with uncertainties. They employ a new tool based on the ideas of system immersion and manifold invariance. New algorithms are delivered for the construction of robust asymptotically-stabilizing and adaptive control laws for nonlinear systems. The methods proposed lead to modular schemes that are easier to tune than their counterparts obtained from Lyapunov redesign.