Experimental validation of a novel inertia-less VSM algorithm

For years, grid phenomena such as voltage stability, loss of inertia, voltage dips, etc. have been managed by well established solutions. Such solutions include on load tap changing transformers, and synchronous generators. Now, the increased penetration of load and generation interfaced by converter based systems has demanded changes in the way the grid is managed. If the aforementioned issues are not considered, local instability can lead to system-wide instability. One possible solution for the majority of these issues is to change the control logic of the converter, modifying it from the standard vectorial output current theory (DQCl control) to one where the traditional synchronous generator is emulated. These solutions are based within the so called Virtual Synchronous Machine or VSM algorithms. Among them, the so-called Virtual Synchronous Machine Zero Inertia (VSM0H) has been implemented in this paper. Due to its simplicity and efficacy, it has been chosen for experimental testing and analysis. The process to implement the VSM0H algorithm in the lab will be explained; with particular emphasis in the connection process since, with the VSM0H algorithm, the converter becomes a true voltage source, and this procedure can be particularly challenging. An explanation of the blocks added to provide this soft connection will be explained. Additionally, another experiment showing the behavior of the system against changes in the power references will be shown

Design and Control of Virtual Synchronous Machine Based Energy Systems

Conventionally, the operation and stability of power systems have been governed by the dynamics of large synchronous generators (SGs) which provide the inertial support required to maintain the resilience and stability of the power system. How-ever, the commitment of the UK to drive a zero-carbon economy is accelerating the integration of renewable energy sources (RESs) into the power system. Since the dynamics and operation of RESs differs from SGs, the large-scale integration of RESs will significantly impact the control and stability of the power system.This thesis focuses on the design of grid-friendly control algorithms termed virtual synchronous machines (VSMs), which mimic the desirable characteristics of SGs. Although several VSM topologies have been proposed in literature, most of them require further modifications before they can be integrated into the grid. Hence, a novel VSM algorithm for permanent magnet synchronous generator based wind turbines has been proposed in this thesis.The proposed VSM performs seamlessly in all operating modes and enables maxi-mum power point tracking in grid-connected operation (assuming strong grid), load following power generation in islanded mode and fault ride-through during faults. To ensure optimal performance of the VSM in all operating modes, a comprehensive stability analysis of the VSM was performed in the event of small and large per-turbations. The result of the analysis was used to establish design guidelines and operational limits of the VSM.This thesis further evaluates the impact of VSMs on the power systems low-frequency oscillations (LFOs). A detailed two-machine test-bed was developed to analyze the LFOs which exists when VSMs replace SGs. The characteristics of the LFO modes and the dominant states was comprehensively analyzed. The LFO modes which exists in an all-VSM grid was also analyzed. Further, the role of the power system stabilizers in an all-VSM grid was comprehensively evaluated. An IEEE benchmark two-area four-machine system was employed to validate the results of the small-signal analysis.The analysis and time-domain simulations in this thesis were performed in the MAT-LAB/SIMULINK environment

Automatic Text Summarization Using Latent Drichlet Allocation (LDA) for Document Clustering

In this paper, we present Latent Drichlet Allocation in automatic text summarization to improve accuracy in document clustering. The experiments involving 398 data set from public blog article obtained by using python scrapy crawler and scraper. Several steps of clustering in this research are preprocessing, automatic document compression using feature method, automatic document compression using LDA, word weighting and clustering algorithm The results show that automatic document summarization with LDA reaches 72% in LDA 40%, compared to traditional k-means method which only reaches 66%

Multi-rotor virtual machine for grid-forming converter to damp sub-synchronous resonances

Grid-forming power converters (GFMC) have been widely adopted in power systems as an attractive solution against the challenges imposed by the ever-increasing penetration of renewables. Despite its versatility, GFMC is employed only to provide islanded operation, grid regulations, and synthetic inertia. To further extend the use of GFMC in enhancing power system stability, this paper proposes a multi-rotor virtual machine (MRVM) controller to attenuate sub-synchronous oscillations. Driven by the formulation of a virtual synchronous machine (VSM), the proposed MRVM implements a VSM-based GFMC with several virtual rotors whose electromechanical characteristics can be individually adjusted to target specific oscillatory modes in the system. In this work, the MRVM’s working principle is described in detail and tuning guidelines are proposed to simplify the selection of control parameters by using frequency-domain techniques and the eigenvalue locus analyses. To validate the performance of the MRVM, an IEEE benchmark grid model is adopted namely, the three-machine-infinite-bus system. It is evident from the results that the MRVM (i) provides higher degrees of freedom when dealing with sub-synchronous oscillations, and (ii) outperforms conventional GFMC, especially in damping intra-area power oscillations.This work was supported by the European Commission under Project FLEXITRANSTORE-H2020-LCE-2016-2017-SGS-774407.Peer ReviewedPostprint (published version

Analysis and Practical Assessment of Converter-Dominated Power Systems : Stability Constraints, Dynamic Performance and Power Quality

For years, large power systems have been predominantly managed using the very well known synchronous machine on the generation side. With the increasing penetration of load and generation interfaced by converter-based systems, the conventional synchronous machine is being gradually replaced by these new devices. However, this slow and steady change of the generation technology has led to side-effects which can affect to the local stability and, if no remedial action is taken, system-wide stability. The main objective of this thesis is to understand, expose and overcome the weaknesses of converter-dominated grids within a laboratory environment. To do so, a converter of 10kVA has been built implementing the standard D-Q axis Current Injection (DQCI) control and, for rst time to date, the Virtual Synchronous Machine with Zero Inertia (VSM0H). Further developments have been made to this control strategy to implement this theoretical algorithm into a real system. The solutions proposed in this thesis for converter-dominated grids are based on the Grid Forming Nodes (GFN) solution. This composition suggests the usage of a combination of DQCI and Virtual Synchronous Machine (VSM) converters to achieve stability within converter-dominated grids. However, there is limited understanding about VSM functioning. Initially, the application of VSM-style converters was proposed as a natural replacement of the real synchronous machine present in the system without understanding which are the specific weaknesses that affect to this type of grid. Due to the aforementioned reasons, this thesis aims to provide a more scientificc and rigorous explanation of why the GFN solution can provide stability for converter-dominated power systems. Using the converter built, it is possible to recreate scenarios where the technical challenges involved in power systems can be understood

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

“Grid”-Less Power Systems: A Vision for Future Structure of Power Networks

This paper proposes a new paradigm in the structure of power systems to facilitate the large scale move to renewables-based distributed generation necessary to help decarbonize the current electricity networks. Since the design of the incumbent power system topologies is to control large synchronous generators, critical control metrics degrade as the penetration of converter-based units increases. Specifically, the reduction in short circuit level, phase angle movement, and rate of change of frequency limit the wider adoption of converter-based units. This paper proposes structural changes and control that inherently solve such critical performance issues through physically decoupling all synchronous generators from the network. A set of back-to-back AC/DC/AC converters controlled by a universal virtual synchronous machine-based control algorithm, introduced in the paper, allows the repurposing of existing plant to enable the integration of more converter-based units. Despite being physically disconnected, this new structure/control still benefits from inertial capacities of synchronous generators to suppress the oscillations caused by disturbances. Moreover, the method enables further exploitation of synchronous generators as energy storage mechanisms. PSCAD/EMTDC simulations demonstrate the advantages of the proposed structure and control system in different normal and abnormal scenarios

Control of power converter in modern power systems

A la portada consta el nom del programa interuniversitari: Joint Doctoral Programme in Electric Energy Systems [by the] Universidad de Málaga, Universidad de Sevilla, Universidad del País Vasco/Euskal Erriko Unibertsitatea i Universitat Politècnica de CatalunyaPower system is undergoing an unpreceded paradigm shift: from centralized to distributed generation. As the renewable-based generations and battery storage systems are increasingly displacing conventional generations, it becomes more and. more difficult to maintain the stability and reliability of the grid by using only conventional generations. The main reason for the degradation of grid stability is the rapid penetration of nonconventional sources. These new generations interface with the grids through power electronics converters which are conventionally designed to maximize conversion efficiency and resource utilization. Indeed, these power converters only focus on their internal operation despite the grid conditions, which often worsens the grid operation. To overcome such a drawback, the grid-forming concept has been proposed for power converters, aiming to redesign the control of the power converters to enforce more grid-friendly behaviours such as inertia response and power oscillation damping to name a few. Despite the rich literature, actual adaptation of grid-forming controller in real-world applications is still rare because incentives for renewable power plants to provide services based on such advanced grid-forming functions were at best scarce. In the last years, however, several system operators have imposed new requirements and markets for grid-supporting services. In addition, the existing grid-forming controllers require modification to low-level control firmware of a power converter, which is often unrealistic due to the control hardware limitations as well as necessary testing and certifications. To ensure a stable operation of a grid-forming converter under adverse operating conditions, a robust voltage sensorless current controller is developed in this PhD thesis. The proposed controller is able to handle most of the possible abnormal conditions of the grid such as impedance variations, unbalanced voltage; harmonics distortion. These abnormalities of the grid are mathematically represented using equivalent linear models such that they can be used for calculating the controller gains. Linear matrix inequality techniques are also used to facilitate parameter tuning. In fact, the performance and stability of the current control loop can be determined through only two tuning parameters instead of eight parameters for a controller of a similar structure. The existing grid-forming implementations are designed considering that the control firmware of the power converter can be upgraded at will. However, modifications of the control firmware are not straightforward and cost-effective at mass scale. To overcome such a limitation, an external synchronous controller is presented in this PhD thesis. The external synchronous controller uses measurements, which are either provided by the power converter or a dedicated measurement unit, to calculate the actual active and reactive power that should be injected by the power converters in a way that the power plant acts as an aggregated grid­forming converter. As a result, any conventional power converters can be utilized for providing grid-supporting services with minimal modification to the existing infrastructure. Power converters can provide even better performance than a synchronous generator if a proper control scheme is used. In this regard, the final chapter of this PhD thesis presents the multi-rotor virtual machine implementation for grid-forming converter to boost their damping performance to power oscillations. The multi-rotor virtual machine-controller implements several virtual rotors instead of only one rotor as in typical grid-forming strategies. Since each of the virtual rotors is tuned to target a specific critical mode, the damping participation to such a mode can be increased and adjusted individually. The controllers presented in this PhD thesis are validated through simulators and experiments in the framework of the H2020 FlexiTranstore project. The results are throughout analysed to assess the control performance as well as to highlight possible implications.A medida que las generaciones basadas en energías renovables y los sistemas de almacenamiento de baterías desplazan la generación convencional, se vuelve cada vez más difícil mantener la estabilidad y confiabilidad de la red. Estas nuevas generaciones interactúan con las redes a través de convertidores de electrónica de potencia que están diseñados tradicionalmente para maximizar la eficiencia de conversión y la utilización de recursos. Estos convertidores centran su funcionamiento interno independientemente de las condiciones de la red, lo que a menudo empeora el funcionamiento de la red. Para esto, se ha propuesto el concepto de convertidores de potencia formadores de red (grid-forming), con el objetivo de rediseñar el control de los convertidores de potencia para imponer comportamientos más favorables a la red, por ejemplo, la respuesta inercial y la amortiguación de oscilaciones de potencia. No en tanto, la adaptación real del controlador grid-forming en aplicaciones del mundo real todavía es escasa debido a los pocos incentivos para que las plantas de energía renovable proporcionen servicios basados en funciones de formación de red tan avanzadas. Aunque en los últimos años, operadores de sistemas han impuesto nuevos requisitos y mercados para servicios auxiliares, los controladores grid-forming existentes requieren cambios en el firmware de control de bajo nivel de un convertidor de potencia, algo poco realista debido a las limitaciones del hardware de control, así como a las pruebas y certificaciones necesarias. En esta tesis se desarrolla un controlador de corriente robusto, sin sensor de tensión, para garantizar el funcionamiento estable de un convertidor grid-forming en condiciones de operación adversas. Este controlador es capaz de manejar la mayoría de las condiciones anormales de red, como variaciones de impedancia, tensión desequilibrada y distorsión de armónicos. Estas anomalías de la red se representan matemáticamente mediante modelos lineales equivalentes, utilizados para calcular las ganancias del controlador. También, usando técnicas de desigualdad matricial lineal para facilitar el ajuste de parámetros. De hecho, el rendimiento y la estabilidad del bucle de control de la corriente pueden determinarse mediante sólo dos parámetros de sintonización. Las implementaciones de formación de red existentes están diseñadas considerando que el firmware de control del convertidor de potencia puede actualizarse a voluntad. Sin embargo, las modificaciones del firmware de control no son sencillas ni rentables a gran escala. Por tanto, esta tesis presenta un controlador síncrono externo que utiliza las mediciones proporcionadas por el convertidor de potencia o por una unidad de medición dedicada para calcular la potencia activa y reactiva real que deben inyectar los convertidores de potencia, de forma que la central eléctrica actúe como un convertidor grid-forming agregado. Como resultado, cualquier convertidor de potencia convencional puede utilizarse para proporcionar servicios de apoyo a la red con una modificación mínima de la infraestructura existente. Los convertidores de potencia pueden ofrecer mejor rendimiento que un generador síncrono utilizando un esquema de control adecuado. El último capítulo de esta tesis presenta la implementación de una máquina virtual multirrotor para que los convertidores de red aumenten su rendimiento de amortiguación de las oscilaciones de potencia. El controlador de la máquina virtual multirrotor implementa varios rotores virtuales en lugar de un solo rotor como en las estrategias típicas de grid-forming. Dado que cada uno de los rotores virtuales está sintonizado para dirigirse a un modo crítico específico, la participación de la amortiguación a dicho modo puede aumentarse y ajustarse individualmente. Los controladores presentados en esta tesis doctoral han sido validados mediante simulaciones y experimentos en el marco del proyecto H2020 FlexiTranstore.Postprint (published version

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