Next Generation Inverters Equipped with Virtual Synchronous Compensators for Grid Services and Grid Support

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Virtually synchronous power plant control

During the last century, the electrical energy infrastructures have been governed by synchronous generators, producing electrical energy to the vast majority of the population worldwide. However, power systems are no longer what they used to be. During the last two decades of this new millennium the classical, centralized and hierarchical networks have experienced an intense integration of renewable energy sources, mainly wind and solar, thanks also to the evolution and development of power conversion and power electronics industry. Although the current electrical system was designed to have a core of generation power plants, responsible of producing the necessary energy to supply end users and a clear power flow, divided mainly into transmission and distribution networks, as well as scalable consumers connected at different levels, this scenario has dramatically changed with the addition of renewable generation units. The massive installation of wind and solar farms, connected at medium voltage networks, as well as the proliferation of small distributed generators interfaced by power converters in low voltage systems is changing the paradigm of energy generation, distribution and consumption. Despite the feasibility of this integration in the existing electrical network, the addition of these distributed generators made grid operators face new challenges, especially considering the stochastic profile of such energy producers. Furthermore, the replacement of traditional generation units for renewable energy sources has harmed the stability and the reliable response during grid contingencies. In order to cope with the difficult task of operating the electrical network, transmission system operators have increased the requirements and modified the grid codes for the newly integrated devices. In an effort to enable a more natural behavior of the renewable systems into the electrical grid, advanced control strategies were presented in the literature to emulate the behavior of traditional synchronous generators. These approaches focused mainly on the power converter relying on their local measurement points to resemble the operation of a traditional generating unit. However, the integration of those units into bigger systems, such as power plants, is still not clear as the effect of accumulating hundreds or thousands of units has not been properly addressed. In this regard, the work of this thesis deals with the study of the so-called virtual synchronous machine (VSM) in three control layers. Furthermore, an in-depth analysis of the general structure used for the different virtual synchronous machine approaches is presented, which constitutes the base implementation tree for all existent strategies of virtual synchronous generation. In a first stage, the most inner control loop is studied and analyzed regarding the current control on the power converter. This internal regulator is in charge of the current injection and the tracking of all external power reference. Afterward, the synchronous control is oriented to the device, where the generating unit relies on its local measurements to emulate a synchronous machine in the power converter. In this regard, a sensorless approach to the virtual synchronous machine is introduced, increasing the stability of the power converter and reducing the voltage measurements used. Finally, the model of the synchronous control is extrapolated into a power plant control layer to be able to regulate multiple units in a coordinated manner, thus emulating the behavior of a unique synchronous machine. In this regard, the local measurements are not used for the emulation of the virtual machine, but they are switched to PCC measurements, allowing to set the desired dynamic response at the power plant level.Postprint (published version

Control and stability of Ac/Dc microgrids

The current society is facing several challenges related to the field of energy, such as the high dependency on fossil fuels, the constant increment in the energy consumption and the environmental problems caused by these factors. The integration of distributed generation systems—mainly based on renewable energies—combined with energy storage systems is the most interesting solution to tackle these issues. However, most of these systems are connected to the grid through electronic converters that actively control the power exchange. This fact causes various problems not suffered since the origins of electric grids in the transition from an electric model dominated by synchronous machines to a model where power electronics gain more importance—even being the dominating systems in some cases. The lack of inertial response and primary reserve, the instabilities caused by the interactions of power electronic systems or the premature situation of direct current grids, which are being employed more widely, stand out as some of the most important challenges that we want to address with this thesis. In this context, the main purpose of the thesis is the development of ac/dc microgrid control strategies that improve the dynamic behaviour of the system. In order to achieve this objective we contemplate four main lines that consist of the identification and analysis of different microgrid topologies and control techniques, the study of primary control operation modes of the systems that compose these microgrids and finally the the development and evaluation of various low-level control strategies for ac and dc microgrids. These techniques are based on the concept of operation of classical synchronous generators, enabling their autonomous operation as well as providing inertial response under grid perturbations. Among the contributions of the thesis, we can highlight on the one hand, the analysis and comparative evaluation of synchronous machine emulation techniques for ac microgrids, where we evaluate their behaviour for different types of perturbations and we examine their stability applying the generalized Nyquist criterion. Regarding dc microgrids, on the other hand, we propose novel control techniques that are analogous to the ones analysed for ac grids. We call these techniques virtual-capacitors, as they emulate the behaviour of these passive elements connected to dc grids. In this case, we thoroughly study their transient as well as steady-state behaviour, and we demonstrate that they can be adapted by simply modifying control parameters. Moreover, we analyse the stability of these techniques through parametric analysis of their dominant eigenvalues.Gaur egungo gizartea energiaren arloko hainbat erronkaren aurrean aurkitzen da, besteak beste, erregai fosilekiko dependentzia handia, kontsumo energetikoaren etengabeko igoera, eta faktore hauek eragiten dituzten ingurumen arazoak. Generazio bananduko sistemen integrazioa—bereziki iturri berriztagarrietan oinarritutakoa—metatze sistemekin bateratuta, arazo horiei aurre egiteko aukera interesgarriena bilakatu da. Hala ere, sistema hauetako gehienak bihurgailu elektronikoen bitartez konektatzen dira sare elektrikotara, potentziaren hartu-emana modu aktiboan kontrolatzen dutelarik. Honek, makina sinkronoez menderatutako modelo elektriko batetik, potentzia elektronika garrantzia hartzen hasten den—edo kasu batzuetan mendean dagoen—modelo baterako trantsizioan hainbat arazo eragiten ditu, sare elektrikoak sortu zirenetik jasan ez direnak. Erantzun inertzial eta erreserba primario eza, bihurgailuen interakzioaren ondoriozko estabilitate arazoak edo korronte zuzeneko sareen egoera goiztiarra, geroz eta gehiago erabiltzen direnak, tesi honen bitartez aurre egin nahi diren erronkarik garrantzitsuenetarikoak dira. Testuinguru honetan, tesiaren helburu nagusia ac/dc mikrosareen portaera dinamikoa hobetzen duten kontrol estrategiak garatzea da. Helburu hau lortzeko lau lerro nagusi planteatu dira, besteak beste, mikrosareen topologia eta kontrol estrategia desberdinen identifikazio eta analisia, sare hauek konposatzen duten sistemen kontrol primarioaren untzionamendu motak azterketa, eta azkenik, bai ac eta bai dc mikrosareendako nibel baxuko kontrol estrategia desberdinen garapena. Azken hauek generadore sinkrono klasikoetan oinarritu dira, modu autonomoan eta bananduan aritzeko gaitasuna emateaz gain, sareko perturbazioen aurrean erantzun inertziala ematea ahalbidetzen dielako. Tesiaren ekarpenen artean, alde batetik, makina sinkronoen emulazioan datzan kontrol estrategien analisia eta konparaketa azpimarratu behar dira. Kasu honetan, teknika hauek hainbat perturbazioetarako ebaluatzen ditugu, eta Nyquisten kriterio generalizatuan oinarrituta estabilitatea aztertzen dugu. Korronte zuzeneko sareei erreparatuz, bestalde, kontrol teknika berriak proposatzen ditugu. Hauei kondentsadore-birtual izena ezarri diegu, elementu pasibo horien erantzun dinamikoa emulatzen dutelako. Hemen, teknika hauen erregimen iraunkorreko eta iragankorreko erantzuna aztertzen dugu, soilik kontrol parametro batzuk aldatuta egokitu daitezkela frogatuz. Hortaz gain, sistema hauen estabilitatea aztertzen dugu berezko balio dominanteen (eigenvalue-en) analisi parametrikoak eginez.La sociedad actual se enfrenta a varios retos importantes en materia energética, entre los que destacan la gran dependencia de los combustibles fósiles, el constante aumento del consumo energético y los problemas medioambientales que estos factores conllevan. La integración de sistemas de generación distribuida—principalmente de origen renovable— combinadas con sistemas de almacenamiento de energía, se presenta como la solución más interesante para hacer frente a estos retos. Sin embargo, la mayor parte de estos sistemas se conectan a la red a través de convertidores electrónicos que controlan el intercambio de potencia de manera activa. Este hecho hace que la transición desde un modelo eléctrico principalmente dominado por máquinas síncronas, hacia un modelo donde la electrónica de potencia comienza a cobrar protagonismo—hasta el punto de llegar a ser dominante en algunos casos—acarree diversos problemas que prácticamente no se han manifestado desde los orígenes de las redes eléctricas. La falta de respuesta inercial y reserva primaria, las inestabilidades debidas a la interacción de los sistemas electrónicos de potencia o la prematura situación de las redes de corriente continua, cada día utilizadas en mayor medida, destacan como algunos de los retos más importantes a los que se quiere dar respuesta a través de esta tesis. En este contexto, el objetivo principal de la tesis es el desarrollo de estrategias de control de microrredes ac/dc que mejoren el comportamiento dinámico del sistema. Para la consecución de este objetivo se han planteado cuatro líneas principales que constan de la identificación y análisis de diferentes topologías y técnicas de control de microrredes, el estudio de los modos de operación del control primario de los sistemas que las componen, y finalmente, el desarrollo y evaluación de diversas técnicas de control de nivel bajo tanto para microrredes ac como dc. Las estrategias de control de nivel bajo desarrolladas en la tesis se basan en el concepto de operación de los generadores síncronos clásicos, lo que les permite operar de manera distribuida y autónoma, aportando a su vez respuesta inercial ante perturbaciones en la red. Entre las contribuciones de la tesis destacan, por un lado, el análisis y comparativa de técnicas de control de emulación de máquinas síncronas para redes ac, donde evaluamos su comportamiento ante diferentes tipos de perturbaciones y examinamos su estabilidad aplicando criterios generalizados de Nyquist. En el ámbito de las redes dc, por otra parte, proponemos nuevas técnicas de control que son análogas a las analizadas para las redes ac, y que denominamos como condensadores-virtuales puesto que emulan el comportamiento dinámico de estos elementos pasivos conectados a la red. En este caso, estudiamos en detalle su comportamiento dinámico y en régimen permanente, demostrando que se pueden adaptar simplemente variando parámetros de control, y analizamos su estabilidad llevando a cabo análisis paramétricos de sus valores propios dominantes

Primary control of a microgrid operating in islanded mode

This study focuses in the primary frequency control of a generic microgrid. This microgrid consists of a Photovoltaic (PV), an Energy Storage System (ESS), a battery and a Diesel Generator (DG) which account for the majority of systems installed today, specially in remote areas, such as islands.This thesis addresses the development of the primary frequency control of the microgrid. The battery’s convertor is operated as a grid forming and it’s connected in parallel with the diesel generator. The PV’s converter is operated as a grid feeding. The control is handed as a distributed approach between the different elements of the system. Through this work, this control is analysed, through a simulation model implemented in PLECS® which is a Matlab blockset. The simulation model developed shows the operation of the control droop in different operating conditions, such as load and generation changes

Control and Stability of Residential Microgrid with Grid-Forming Prosumers

The rise of the prosumers (producers-consumers), residential customers equipped with behind-the-meter distributed energy resources (DER), such as battery storage and rooftop solar PV, offers an opportunity to use prosumer-owned DER innovatively. The thesis rests on the premise that prosumers equipped with grid-forming inverters can not only provide inertia to improve the frequency performance of the bulk grid but also support islanded operation of residential microgrids (low-voltage distribution feeder operated in an islanded mode), which can improve distribution grids’ resilience and reliability without purposely designing low-voltage (LV) distribution feeders as microgrids. Today, grid-following control is predominantly used to control prosumer DER, by which the prosumers behave as controlled current sources. These grid-following prosumers deliver active and reactive power by staying synchronized with the existing grid. However, they cannot operate if disconnected from the main grid due to the lack of voltage reference. This gives rise to the increasing interest in the use of grid-forming power converters, by which the prosumers behave as voltage sources. Grid-forming converters regulate their output voltage according to the reference of their own and exhibit load sharing with other prosumers even in islanded operation. Making use of grid-forming prosumers opens up opportunities to improve distribution grids’ resilience and enhance the genuine inertia of highly renewable-penetrated power systems. Firstly, electricity networks in many regional communities are prone to frequent power outages. Instead of purposely designing the community as a microgrid with dedicated grid-forming equipment, the LV feeder can be turned into a residential microgrid with multiple paralleled grid-forming prosumers. In this case, the LV feeder can operate in both grid-connected and islanded modes. Secondly, gridforming prosumers in the residential microgrid behave as voltage sources that respond naturally to the varying loads in the system. This is much like synchronous machines extracting kinetic energy from rotating masses. “Genuine” system inertia is thus enhanced, which is fundamentally different from the “emulated” inertia by fast frequency response (FFR) from grid-following converters. Against this backdrop, this thesis mainly focuses on two aspects. The first is the small-signal stability of such residential microgrids. In particular, the impact of the increasing number of grid-forming prosumers is studied based on the linearised model. The impact of the various dynamic response of primary sources is also investigated. The second is the control of the grid-forming prosumers aiming to provide sufficient inertia for the system. The control is focused on both the inverters and the DC-stage converters. Specifically, the thesis proposes an advanced controller for the DC-stage converters based on active disturbance rejection control (ADRC), which observes and rejects the “total disturbance” of the system, thereby enhancing the inertial response provided by prosumer DER. In addition, to make better use of the energy from prosumer-owned DER, an adaptive droop controller based on a piecewise power function is proposed, which ensures that residential ESS provide little power in the steady state while supplying sufficient power to cater for the demand variation during the transient state. Proposed strategies are verified by time-domain simulations