44 research outputs found

    Uncovering Droop Control Laws Embedded Within the Nonlinear Dynamics of Van der Pol Oscillators

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    This paper examines the dynamics of power-electronic inverters in islanded microgrids that are controlled to emulate the dynamics of Van der Pol oscillators. The general strategy of controlling inverters to emulate the behavior of nonlinear oscillators presents a compelling time-domain alternative to ubiquitous droop control methods which presume the existence of a quasi-stationary sinusoidal steady state and operate on phasor quantities. We present two main results in this work. First, by leveraging the method of periodic averaging, we demonstrate that droop laws are intrinsically embedded within a slower time scale in the nonlinear dynamics of Van der Pol oscillators. Second, we establish the global convergence of amplitude and phase dynamics in a resistive network interconnecting inverters controlled as Van der Pol oscillators. Furthermore, under a set of non-restrictive decoupling approximations, we derive sufficient conditions for local exponential stability of desirable equilibria of the linearized amplitude and phase dynamics

    A New Virtual Oscillator Control Without Third-Harmonics Injection For DC/AC Inverter

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    Grid-Forming Converter Control Method to Improve DC-Link Stability in Inverter-Based AC Grids

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    As renewable energy sources with power-electronic interfaces become functionally and economically viable alternatives to bulk synchronous generators, it becomes vital to understand the behavior of these inverter-interfaced sources in ac grids devoid of any synchronous generation, i.e. inverter-based grids. In these types of grids, the inverters need to operate in parallel in grid-forming mode to regulate and synchronize their output voltage while also delivering the power required by the loads. It is common practice, therefore, to mimic the parallel operation control of the very synchronous generators that these inverter-based sources are meant to replace. This practice, however, is based on impractical assumptions and completely disregards the key differences between synchronous machines and power electronic inverters, as well as the dynamics of the dc source connected to the inverter. This dissertation aims to highlight the shortcomings of conventional controllers and derive an improved grid-forming inverter controller that is effective in parallel ac operation without sacrificing dc-link stability. This dissertation begins with a basis for understanding the control concepts used by grid-forming inverters in ac grids and exploring where existing ideas and methods are lacking in terms of efficient and stable inverter control. The knowledge gained from the literature survey is used to derive the requirements for a grid-forming control method that is appropriate for inverter-based ac grids. This is followed by a review and comparative analysis of the performance of five commonly used control techniques for grid-forming inverters, which reveal that nested loop controllers can have a destabilizing effect under changing grid conditions. This observation is further explored through an impedance-based stability analysis of single-loop and nested-loop controllers in grid-forming inverters, followed by a review of impedance-based analysis methods that can be used to assess the control design for grid-forming inverters. An improved grid-forming inverter controller is proposed with a demonstrated ability to achieve both dc-link and ac output stability with proportional power-sharing. This dissertation ends with a summary of the efforts and contributions as well as ideas for future applications of the proposed controller

    Universal dual-port grid-forming control: bridging the gap between grid-forming and grid-following control

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    We study a dual-port grid-forming (GFM) control for power systems containing ac and dc transmission, converter-interfaced generation and energy storage, and legacy generation. To operate such a system and provide standard services, state-of-the-art control architectures i) require assigning grid-following (GFL) and GFM controls to different converters, and ii) result in highly complex system dynamics. In contrast, dual-port GFM control (i) subsumes standard functions of GFM and GFL controls in a simple controller, ii) can be applied to a wide range of emerging technologies independently of the network configuration, and iii) significantly reduces system complexity. In this work, we provide i) an end-to-end modeling framework that allows to model complex topologies through composition of reduced-order device models, ii) an in-depth discussion of universal dual-port GFM control for emerging power systems, and iii) end-to-end stability conditions that cover a wide range of network topologies, emerging technologies, and legacy technologies. Finally, we validate our findings in a detailed case study

    Advanced Inverter control for mixed source microgrids

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    This thesis focuses on investigating virtual oscillator control (VOC) and applying it to mixed source microgrids to address several stability issues. A detailed comparison between VOC and droop control in a three-phase system is presented in terms of transient responses of a single inverter under small load disturbances and the synchronization speed in multiple paralleled inverters under various inverter terminal voltage amplitude and frequency regulation settings. In the single-inverter microgrid, it is demonstrated in both simulation and experiment that the two control models produce similar transient responses in the output voltage and current amplitudes. However, VOC has a faster instantaneous frequency transient response whilst still maintaining the terminal voltage amplitude transient response of the droop controller. In microgrids with multiple inverters, the synchronization speed of the VOC is faster than that of the droop control when the terminal voltage’s frequency regulation range is allowed to be wide. The conclusion is verified with different types of loads. A virtual inertia design method for the VOC inverter with a mixed source microgrid is presented to improve the frequency stability issues of the system. The per unit inertia constant of a VOC inverter is derived when coupled with a synchronous generator in an islanded microgrid. The control parameters of the virtual inertia are designed via small-signal analysis. A dual second order generalized integrator - frequency locked loop (DSOGI-FLL) is adopted for digital implementation of proposed virtual inertia based VOC. With the use of virtual inertia block, the frequency nadir is improved by 22% and rate of change of frequency is improved by 29% compared with the unmodified VOC inverter during the transient period induced by load disturbances. Simulation and experimental results verify the enhanced transient response of system frequency. A voltage and current dual-loop control structure is added to the VOC inverter to solve the voltage drop issues at the inverter terminals caused by the inverter dead-time effects, non-ideal semiconductor and LCL filter. A complete small-signal model for a multiple-inverters microgrid with the proposed control structure is presented in order to assess system stability using eigenvalue and participation factor analysis. Analytical results show that the parameter related to the frequency regulation and the integral gain of the voltage controller affect the location of the system’s dominant modes significantly. The stability margin is determined by modifying these control parameters. Experimental results on a laboratory test microgrid verify the predication from the small-signal analysis and time-domain simulations. Finally, a method to limit current in the VOC inverter under large disturbances in a mixed source microgrid is proposed. During a large load change in the islanded microgrid, the inverter based sources may get temporarily overloaded until other generations with sufficient power margin take the remaining load burden. The original VOC inverter lacks the ability to constrain the current within limits during the transient period. The dual-loop structure proposed in this thesis can limit the transient current with the use of virtual impedance. Such virtual impedance is presented by the desired maximum current magnitude and virtual voltage drop. Compared with a recently proposed fault ride through VOC inverter, the proposed virtual impedance based current limitation method can effectively constrain the inverter current within the pre-set value under large disturbances, which augments the range of application of VOC and enhances its robustness

    Power Electronics in Renewable Energy Systems

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    Dynamic Incentives for Optimal Control of Competitive Power Systems

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    Technologisch herausfordernde Transformationsprozesse wie die Energiewende können durch passende Anreizsysteme entscheidend beschleunigt werden. Ziel solcher Anreize ist es hierbei, ein Umfeld idealerweise so zu schaffen, dass das Zusammenspiel aller aus Sicht der beteiligten Wettbewerber individuell optimalen Einzelhandlungen auch global optimal im Sinne eines übergeordneten Großziels ist. Die vorliegende Dissertation schafft einen regelungstechnischen Zugang zur Frage optimaler Anreizsysteme für heutige und zukünftige Stromnetze im Zieldreieck aus Systemstabilität, ökonomischer Effizienz und Netzdienlichkeit. Entscheidende Neuheit des entwickelten Ansatzes ist die Einführung zeitlich wie örtlich differenzierter Echtzeit-Preissignale, die sich aus der Lösung statischer und dynamischer Optimierungsprobleme ergeben. Der Miteinbezug lokal verfügbarer Messinformationen, die konsequente Mitmodellierung des unterlagerten physikalischen Netzes inklusive resistiver Verluste und die durchgängig zeitkontinuierliche Formulierung aller Teilsysteme ebnen den Weg von einer reinen Anreiz-Steuerung hin zu einer echten Anreiz-Regelung. Besonderes Augenmerk der Arbeit liegt in einer durch das allgemeine Unbundling-Gebot bedingten rigorosen Trennung zwischen Markt- und Netzakteuren. Nach umfangreicher Analyse des hierbei entstehenden geschlossenen Regelkreises erfolgt die beispielhafte Anwendung der Regelungsarchitektur für den Aufbau eines neuartigen Echtzeit-Engpassmanagementsystems. Weitere praktische Vorteile des entwickelten Ansatzes im Vergleich zu bestehenden Konzepten werden anhand zweier Fallstudien deutlich. Die port-basierte Systemmodellierung, der Verzicht auf zentralisierte Regeleingriffe und nicht zuletzt die Möglichkeit zur automatischen, dezentralen Selbstregulation aller Preise über das Gesamtnetz hinweg stellen schließlich die problemlose Erweiterbarkeit um zusätzliche optionale Anreizkomponenten sicher

    Dynamic Incentives for Optimal Control of Competitive Power Systems

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    This work presents a real-time dynamic pricing framework for future electricity markets. Deduced by first-principles analysis of physical, economic, and communication constraints within the power system, the proposed feedback control mechanism ensures both closed-loop system stability and economic efficiency at any given time. The resulting price signals are able to incentivize competitive market participants to eliminate spatio-temporal shortages in power supply quickly and purposively

    Dynamic Incentives for Optimal Control of Competitive Power Systems

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    This work presents a real-time dynamic pricing framework for future electricity markets. Deduced by first-principles analysis of physical, economic, and communication constraints within the power system, the proposed feedback control mechanism ensures both closed-loop system stability and economic efficiency at any given time. The resulting price signals are able to incentivize competitive market participants to eliminate spatio-temporal shortages in power supply quickly and purposively
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